I want to know more about variable frequency drives [Archive]

David Castor

03-01-2012, 1:31 PM

You don't mention the horsepower requirements, but if the VFD is adequately sized, it can provide 100% torque at any motor speed. Standard single-phase input (240 V) and three-phase output VFDs are readily available up to about 5 hp. Above that, you could run an oversized three-phase input model - figure on about a 50% derating.

http://www.wolfautomation.com/ProductList.aspx?CategoryID=5103

This link should at least show you some options for single-phase input VFDs. All VFDs are three-phase output intended for use with three-phase motors.

Even though VFDs are sold based on motor horsepower, the key rating is the VFD output current. Compare this with the motor's full load amps on the nameplate.

Dave Cav

03-01-2012, 1:43 PM

You can also go over to OWWM dot org and do a search on VFDs. We use them a lot to run old 3 phase machines on single phase. I have about five of them in my shop.

ian maybury

03-01-2012, 4:33 PM

I used a single phase 4kW Yaskawa VFD to drive a 230V/3phase/4kW Pentz style dust system fan.

The pros and cons I found were (in no particular order):

1. Gives really good control of the start up current - 140% of FLA on a 10 sec ramp on above. Makes it feasible to run quite large motors on low amp supplies, frequent start/stops no problem.
2. Variable speed.
3. Ease of external control. Functions like start/stop, forward/ reverse, speed and so on can be controlled directly using simple manual or radio operated switches via the built in 24V/8mA control circuits. (no relays etc required)
4. Lots of motor protection. Monitors amps and load conditions. Presets to suit the characteristics differing load types like fans, pumps etc.
5. Actually too many built in control functions to mention, but stuff like braking, amps, motor temp sensing, torque limiting, stall prevention, fault display, etc. Can be controlled from a PC or bus system too.
6. Safety functions like e-stop, ground fault detection etc.
7. Full torque available from standstill - but you don't get torque multiplication at lower speeds as in the case of a motor driving through a variable speed gearbox/mechanical gearing.
8. Usually needs to be connected directly to the motor on a machine (so that the VFD's controls are not disturbed), which means the machine is controlled via the VFD controls.
9. Limited to a single load, although it can run more than one motor simultaneously provided they are in the same circuit.
10. Best enclosed in a panel for safety, and needs good air circulation for cooling.
11. Shielded and grounded cables are advisable to avoid electrical noise and/or interference issues.
12. Limited to 230V 3 phase (here anyway) - seemingly because stepping the output voltage up by use of a transformer creates a risk of feedback of electrical noise into the supply and is prohibited by many supply companies.
13. Power output while seemingly very close is not quite a perfect 3 phase sinusoidal waveform - it's synthesised by the electronics outputting pulses of varying voltages.

ian

David Castor

03-01-2012, 5:03 PM

Ian's summary is a good one. Regarding his point #8 - I would add that is never advisable to open up the circuit between the VFD and the motor while the motor is running - this can damage the VFD. So you do want to use the VFD controls to start and stop the equipment.

Another thing that some people find objectionable is the noise the motors make when running on a VFD, especially at lower speeds.

Dave

Mark Burnette

03-01-2012, 5:31 PM

My main concern was with torque. A DC motor offers instantaneous torque and is very responsive to load variation but I thought AC drives & motors had lower torque at low speeds--ie very easy to stall at low RPMs. Do reasonably-priced 1Hp (or less) drives offer closed-loop control? Tach feedback, motor current-sensing, both? If I'm wrong about the low speed torque then I think a 3/4HP motor would be plenty for this size sander.

Van Huskey

03-01-2012, 5:52 PM

Torque is going to be constant down to about 6hz, or 1/10th the normal speed of the motor at 60hz. But, as Ian noted you don't get the torque multiplication of a mechnical drive.

Larry Edgerton

03-01-2012, 7:14 PM

Factorymation cautions running old motors at either end of the speed range for long periods because of heat. The coating on the windings are not up to the task according to them.

I bought one for an old Walker-Turner drill press with a reversing switch. I was too confused what to do with the switch, so I am running it with the converter for now.

Larry

David Kumm

03-01-2012, 9:50 PM

Alan Schaffter

03-01-2012, 10:04 PM

What are your reasons for wanting variable speed on your disk/belt sander?

I think you will be very disappointed with the low speed performance. I added VFDs and (larger HP) 3 ph motors to my lathe and drill press. Until I geared both way down I could easily stall the spindles with my bare hand when turning at low RPM! Once I re-geared the motors with new pulley and sheave combinations, I was able to get decent power at low spindle RPM with the VFD driving the motor at normal 60 hz RPM. However, with this gearing, in order to have the spindles turn at design or mid-range RPM the VFDs had to significantly over-speed the motors- more than double speed!!! In the case of the drill press I must over-speed the motor nearly 400% to turn the spindle at 3000 RPM. Over-speeding is more an issue with the bearings than it is will the windings over-heating.

This is the main reason most woodworking machines with VFD's retain the mechanical gearing for 3 or 4 speed ranges.

And yes all motors will overheat if the VFD runs them at low RPM- less air movement and cooling. My ODP, well vented lathe motor would get hot enough to fry an egg if run at low rpm for too long. Inverter duty motors can handle it better due to higher temp insulation and better cooling design, but they will overheat as well if run at too low freq/rpm, too long.

Disconnecting the VFD from the motor is not the problem- reconnecting it when the motor is still spinning IS. Reverse EMF created as the motor decels can fry the VFD circuitry. You are better adding the usually optional braking resistor and using the VFD for emergency stops- it will be much quicker than disconnecting power!

Larry Edgerton

03-02-2012, 6:23 AM

Larry, I had understood the danger relating to old motors was only the 440 volt ones. Have not heard of problems with 220. As to the reversing switch, I have a drum switch on my Millrite and just leave it on forward. A manual switch will not hurt a vfd if left on. It generally won't kill a vfd if shut off while the vfd is on. Not good to do very often but as an emergency stop it is better than not having one. eventually is supposed to cause failure but over time, not right away. Dave

The problem is David I "want" to use the reversing switch. I bought a bunch of left hand bits cheap, and this drill press has a threading attachment that I want to use occasionally that requires a reversing switch. I would like to be able to run this without the phase convertor. May just buy a reverseable single phase motor. Will my three phase reversing switch work with single phase? I am an electrical illiterate in case you haven't figured that out.

Larry

Van Huskey

03-02-2012, 6:29 AM

Larry, wiring a on/off switch, reversing switch and speed pot are easy on a VFD. They have low voltage, low current, control circuits just for this purpose. many of the paper manuals (thus the ones you can download) don't fully explain it but the CD-ROMs that come with the drives have all this info in DETAIL.

David Kumm

03-02-2012, 8:35 AM

The problem is David I "want" to use the reversing switch. I bought a bunch of left hand bits cheap, and this drill press has a threading attachment that I want to use occasionally that requires a reversing switch. I would like to be able to run this without the phase convertor. May just buy a reverseable single phase motor. Will my three phase reversing switch work with single phase? I am an electrical illiterate in case you haven't figured that out.

Larry

Larry, most if not all vfds have reversing switches built in. If for some reason yours doesn't you can still use the vfd and drum switch. Just don't use the switch when the vfd is running the motor. My machines are set to run with either a vfd or RPC. I keep the manual starter in place- regular or reversing so I can use the RPC. When using the vfd I just leave the switch in the on position and plug the machine into the vfd. The vfd doesn't know a switch is in between it and the machine unless you use it to disconnect. Dave

ian maybury

03-02-2012, 9:40 AM

On torque. It seems that some recent VFDs using specialised starting current and voltage control ('vector control' as they call it in one case) can deliver up to 150% of the rated motor torque without causing over current problems.

Don't forget though that HP = torque x RPM. This means that even though the torque is constant over the full range of RPM that the available HP will rise and fall in proportion to the RPM. Meaning that if you start with a 1HP motor it's only going to deliver 0.1HP at 10% of its normal running speed. (say at 345rpm for a 2 pole motor) Or 0.4 HP at 40% of it's normal speed etc.

A gear drive in comparison is constant HP. As in if as a result of gearing down the gearbox output shaft is running at 10% of the input motor speed, the available torque will be x10 that of the motor. (this is the 'torque multiplication' mentioned above)

While a VFD can theoretically run a motor at any speed the mechanical limitations of the motor means the useful range is pretty restricted in practice - probably about ± 10% of the plated RPM. Motors vary in what they can handle as the guys have said above - the likes of ABB for example do motors designed for VFD use, while e.g. an eastern made low spec/down to a price model may cry enough rather more quickly.

Running below speed at anywhere near to the available HP the reduced cooling available from the motor fan and issues to do with changing inductive effects (as i understand it leading to increased currents) intrude pretty quickly, as does the fact that the available power falls away pretty quickly as above.

Running above speed at near the available HP bearing life etc is reduced, and anyway cooling will likewise become an issue as the HP rises above that the motor was designed for.

How suitable a VFD is to run a sander will depend among other stuff on the characteristics of the load - how the torque varies with the load; and on the required speed range.

If the torque requirement (literally the force needed to skid the sandpaper across the timber) increases significantly with reducing RPM then i'd be cautious (not to say it can't do it, but it'd need careful looking at), likewise if the required speed range is much wider than the ±10% mentioned above.

Against that dust system fans are a great example of a load pretty much perfectly suited to the characteristics of a VFD drive...

ian

Mark Burnette

03-02-2012, 11:10 AM

In my own mind I can totally justify varying the speed of a stationary sander--slow it down to reduce burning or gumming the abrasive, slow it down to sneak up on a line, etc. But why in the world would you want to reduce the CFM of a dust collector? Or is the VFD being used in this application to over-speed the impeller to increase flow?

Regarding low speed torque which would be the better approach: use a 3600rpm motor with the largest ratio pulley set that will fit or use a 1800rpm and just don't slow it down as much with the drive?

and Ian--you obviously have some expertise in this subject and your comments are well-considered. I wonder though if ±10% is perhaps way too conservative? Why are VFDs capable of 10%-400% rated speed if it's so detrimental?

Mark Burnette

03-02-2012, 11:17 AM

Disconnecting the VFD from the motor is not the problem- reconnecting it when the motor is still spinning IS. Reverse EMF created as the motor decels can fry the VFD circuitry.
Do AC motors generate back EMF? I don't understand how if there's no magnetism. I'm not very knowledgeable on AC systems--in my younger years I worked with DC drives & motors.

Van Huskey

03-02-2012, 11:30 AM

Do AC motors generate back EMF? I don't understand how if there's no magnetism. I'm not very knowledgeable on AC systems--in my younger years I worked with DC drives & motors.

All motors AC or DC produce back EMF, except for the curious case of salient pole synchronous motors.

David Kumm

03-02-2012, 12:53 PM

I run my disk sander and all the drilling machines on a vfd. Because you don't usually have them running for long the slower speed isn't going to be an issue. My old Greenlee borer usually runs at 20-30 hz or less- direct drive 3500 rpm scares the crap at full speed. I've not had the bit bog down yet although a 4 hp motor at low speed still has some grunt. My disk sander never runs over 50 hz. As for dust collection you generally increase the speed but it depends on the pipe resistance. If you have a 12 amp 5 hp motor running an 8" main with two ports open you might find the impeller pulls so much air you need to scale it back to FLA. Happens with mine when I use the shaper right next to the cyclone but the sander 50' away needs 63 hz to reach 12. Dave

ian maybury

03-02-2012, 3:31 PM

I'm no expert Mark, it's only what i picked up on the way through on VFDs while sorting out my dust system - plus an engineering background.

It's possible I overstated the ±10% thing, in that it's not a hard and fast limit, and whatever does apply is determined not by the VFD but by what the load can tolerate. RPM aside it depends greatly on the duty cycle and how heavily loaded the motor is. Care is certainly needed though in the case of the sort of continuous close to full load running you can get with e.g. a fan. I'm sure there's applications that can use a much wider range of speeds though...

I fitted a vfd for several reasons - it's a Pentz system and the blower was developed to use a US 5hp motor at 60Hz/3,450rpm. Our power (Ireland) is 50Hz, and my shop supply is limited to 80A. The VFD gave me the option to bump the speed back up to 3,450, to use a 4kW 230V 3 phase motor since 5HP single phase is not normally available (too much start up current), and to hold the starting current right down. Less fundamental side benefits are the ability to tune the RPM to avoid noisy patches if needed, and to raise or lower the pressure depending on the specific need.

I've never checked it out, but presuming that both your motors (3,600 or 1,800 rpm - presumably the actual RPMs are a little different?) deliver the same HP then barring other differences in the motors i think the available torque should be pretty much the same. (since HP = torque x rpm) i.e. for the 1,800 rpm motor to produce the same HP as the faster running one it's got to deliver the same torque as the output shaft of the faster one that's geared down to give the same speed. If you know what i mean...

ian

Van Huskey

03-02-2012, 5:27 PM

I've never checked it out, but presuming that both your motors (3,600 or 1,800 rpm - presumably the actual RPMs are a little different?) deliver the same HP then barring other differences in the motors i think the available torque should be pretty much the same. (since HP = torque x rpm) i.e. for the 1,800 rpm motor to produce the same HP as the faster running one it's got to deliver the same torque as the output shaft of the faster one that's geared down to give the same speed. If you know what i mean...

ian

The key here being a 2 pole motor (3600 rpm) will have less torque at lower speeds than a 4 pole motor (1800 rpm). So if one's plan to use a VFD to REDUCE machine speed a better choice for this would be the 1800 rpm motor if torque or HP is an issue. (note actual speed will be lower than theoretical). If one was buying new and mainly worried about slow speed I would look for a 6 pole motor (1200 rpm) sometimes you see them at the motor surplus sites pretty cheap.

In a perfect world we would have the ability to buy comparable priced motors with higher base frequencies. This would allow for 2 pole motors (and their benefits such as higher power factors and smaller form factors) with a base frequency of 120hz to run at 1800rpm @60hz. This all exists, but at custom prices.

David Castor

03-02-2012, 6:16 PM

Disconnecting the VFD from the motor is not the problem-

Sorry - this is incorrect. Opening the motor circuit from a VFD to a motor while the motor is running can damage the VFD unless the manufacturer has specifically provided features to protect against this and not all VFDs have this protection. Trying to "catch" a spinning motor can also be a problem, but after a few cycles the back EMF of an induction motor will decay to low levels and many newer drives can deal with this - it is a specific feature. All starting, stopping, and reversing should be done through the VFD controls.

Whatever you buy, you will want to make sure to read the manufacturer's instructions and make note of the specific capabilities and limitations of that particular drive.

M Toupin

03-02-2012, 6:59 PM

It's disappointing that VFD threads always bring out so much half truths and misinformation...

HP = torque x RPM / 5252

At the risk of over simplifying; at or below rated Hz (60hz in the US) a motor operated on a VFD is in constant torque mode. Above rated Hz the motor is in constant HP mode. Think of it this way, below the rated Hz horsepower drops off, above rated Hz torque drops off. Increasing Hz will NOT increase the motor's HP above the name plate rating. As the RPM increases, torque drops off. Theoretically you can get to a point were the RPM is so high the motor no longer has enough torque to turn the rotor. HP = torque x RPM / 5252. On the inverse, if you slow the motor enough you'll reach the point were there is zero HP to turn the rotor.

A VFD is n not a do all, be all, it needs to be incorporated in a sound design using both the VFD and mechanical torque multiplication (gear ratios) to achieve the desired performance envelope. Then there's different types of VFDs such as Volt/Hertz, sensorless vector and vector drives, all of which have different characteristics and appropriate applications which also need to be taken into consideration.

Mike

ian maybury

03-03-2012, 5:17 AM

;) it's nice to be gently set right every now and then Mike.... Luckily i'm not an expert.

ian

David Kumm

03-03-2012, 8:31 AM

Chris Rosenberger

03-03-2012, 8:51 AM

This has been a very interesting thread.
I had always read on forums that a VFD was the magic bullet for all things motor. I had my doughts until recently. But now I am not so sure again.

I have a large hollow chisel mortiser that has a 1 1/2 hp, 3 phase. 3450 rpm motor. I want to slow the speed of the motor down. I will still be running it on 3 phase.
If I use a VFD to slow the motor to say 1800 RPM. Will it still have enough power to run the bits bit. I cut up to 1" mortises.

When I bought the mortiser new, I did not have 3 phase power in my shop. I ran it a couple of years on a static converter. I had to be careful when cutting motises in hardwood because it would stall out with a 1/2" bit.
Will I end up with the same problem with a VFD?

David Kumm

03-03-2012, 9:34 AM

ian maybury

03-03-2012, 10:36 AM

I'd appreciate hearing some more too Mike - there's lots of stuff that's not set out in the manual or the sales material with these things. Just to be clear - mathematically speaking HP is proportional to torque x rpm - replacing the proportional sign is the source of the equal sign and the (1/5252) constant in the full formula you quoted - there didn't seem much point in complicating a conceptual discussion with it.

On reducing the speed of the morticer Chris. It sounds like with a VFD delivering constant torque that the motor could end up at about half its rated HP at the 1,800 RPM you want. (something better if the VFD has ways of doing better than constant torque)

It'll depend on what actually happens to the torque requirement when morticing as the RPM reduces (the nature of the load, and nothing to do with the VFD), but since it's for use with the largest tools it sounds like it could be very marginal unless the motor is much larger (x2?) than is needed at full speed. Which seems unlikely unless these machines are for some reason well over motored. (?)

A quick look at the spec sheets on a few morticers suggests that they quietly avoid specifying the motor RPM. Which seems to suggest that the drill in the tool may be running rather too fast for its own good at the higher diameters, and is no doubt why you want to step the speed down.

My instincts would be to make a trial on the large chisel at the stock 3,450 rpm - if the motor is getting anywhere close to reading full load amps then the above could well be true. Maybe then have a word with a knowledgeable VFD supplier - maybe they have ways around the issue.

The torque available from the motor using a static phase converter (less so in the case of a rotary/idler motor equipped converter) is also reduced, this because their output is a pretty approximate version of three phase power. They are not normally recommended for full power/continuous heavy duty cycles/high starting torque/variable load applications - it seems because they tend to produce one phase that's low in voltage, and is also often mistimed. How much it might be off gets beyond my knowledge, but i've seen mention of avoiding situations that load the motor past about 75% of its plated amps. The basic scenario seems to be that the capacitors deliver a proper phase at only one load and RPM situation, and that for the rest of the time the situation can be far from optimal.

ian

PS added after reading David's reply which crossed mine - looks like his experience more or less confirms the above cautions. (i.e. a VFD may be OK in principle, but only with more motor than it looks like you have and if you don't drop the rpm too much)..

Chris Rosenberger

03-03-2012, 9:00 PM

Chris, what is your mortiser. Not to steal Van's thread but you are asking a lot of a 1 1/2 motor. In the old days those were considered the light duty models and rated up to 1/2-5/8 mortises. To get to the 1" you had to step ip to the 5 hp units. I have an old Fay and Egan 509 with a 3 hp motor. I've found that 50hz is my best all around speed and have plenty of power for 3/4 mortises although I go easy. I believe the vfd is an answer on a mortiser but don't know if you have enough reserve in the motor to slow it much for that size mortise. Dave

David, I have a General 220 mortiser. It is rated as a 1" mortiser. Weighing over 1000 lbs, I do not consider it light duty, but it may be. I have cut many 1" & 3/4" mortises in red oak & maple. It never had a lack of power. It just runs to fast for the larger bits. Someone else pointed out to me that the current General 220 comes with a 1800 RPM motor. That is what got me to thinking about a VFD to reduce the RPMs. I will check into a new 1800 rpm motor before going the VFD rout.

David Kumm

03-03-2012, 10:23 PM

Chris, General makes a great mortiser and I'm not belittling it. While the machine has the capacity for 1" mortises I was questioning whether the motor had enough grunt to handle those at half speed. In the day of old floor standing mortisers the ratings were often listed for softwood at 1" or 1 1/4' but less for hardwoods. The Oliver 92 was rated at 3/4 hardwood with the standard 2 hp motor and offered a 3 and a 5. A vfd for your purposes is pretty cheap and would have other uses so I would give it a try. You might find as I did that you only need to slow it down a little for a great benefit. Dave

Stephen Cherry

03-03-2012, 11:00 PM

[QUOTE=ian maybury;1887261]I'd appreciate hearing some more too Mike - there's lots of stuff that's not set out in the manual or the sales material with these things. Just to be clear - mathematically speaking HP is proportional to torque x rpm - replacing the proportional sign is the source of the equal sign and the (1/5252) constant in the full formula you quoted - there didn't seem much point in complicating a conceptual discussion with it.

The torque available from the motor using a static phase converter (less so in the case of a rotary/idler motor equipped converter) is also reduced, this because their output is a pretty approximate version of three phase power. They are not normally recommended for full power/continuous heavy duty cycles/high starting torque/variable load applications - it seems because they tend to produce one phase that's low in voltage, and is also often mistimed. How much it might be off gets beyond my knowledge, but i've seen mention of avoiding situations that load the motor past about 75% of its plated amps. The basic scenario seems to be that the capacitors deliver a proper phase at only one load and RPM situation, and that for the rest of the time the situation can be far from optimal.

QUOTE]

If you look at a manual for a good drive- like the hitachi drive from driveswarhouse.com, you will see torque curves that tell the story. The el cheapo drives that I have seen really do not have documentation for anything other than hobby use.

As for static phase converters- basically all they do is trick the motor into starting with a capacitor, then single phase the motor. While running it's on only two of the three input wires.

Van Huskey

03-04-2012, 12:45 AM

I think a VFD can be a magic bullet but it can also be the bullet that blows up in the chamber.

In the industrial world the motor and drive would be fitted as part of a system both of them picked to produce the best results. MOre often than not "we" are trying to use an existing motor and mechanical drive system and simply add a VFD to accomplish different parameters of operation for a system that is already in place. This opens a lot of room for error and less than steller results.

We must keep in mind that the motors we generally deal with are designed and built to operate in a very narrow performance range. They usualy are single speed induction motors, when we ask them to operate outside that narrow band we need to be aware fo their limitations.

When we want to accomplish variable speed with out machines and want to use a VFD for it we have to examine what this will do to the performance of the system. As pointed out up to the base frequency (60hz for our motors) we will have constant torque, well about a certain threshold ususally domething in the 6-10hz range but because of the relationship of torque and horsepower the horsepower will be reduced as speed reduces. We have to be aware of this.

Take any machine and begin to reduce the speed, at some point it will no longer have the ability power wise to accomplish the task we desire. Part of the problem for us designing that system requires some knowledge of the power needed to accomplish the task, in most cases we only have a general idea and are left with trail and error.

Often the best way to adjust speed on out machines is mechanically, this allows the motor to always run at its base speed so it keeps the HP and torque constant at the shaft and at speeds below 1:1 you get torque multiplication. The problem is multiple pulley arrangements may be difficult or expensive to install and reeves drive would be even more expensive and difficult, both are make impossible with direct drive machines.

The bottom line here is the VFD will work for speed reduction BUT it may require trial and error to determine how much speed reduction can occur before the machine can no longer do its job. In general direct drive machines are poor candidates unless the motor has significantly more power than the job requires at normal speed. On the other end of the spectrum are machines that already have multiple speed via belts but just not full VS. This way you can have different ranges of VS and keep the motor running at high enough speed to do the job at hand. Probably the best candidate machines are lathes, and obviously many of them come with VFD drive, but most of them still have 2-4 speed ranges via belt drive. Keep the limitations in mind and the VFD can be a magic bullet, ignore them and you will have failure.

Van Huskey

03-04-2012, 1:00 AM

David, I have a General 220 mortiser.

The 220 is an EXCELLENT mortiser but in the grand scheme it is a light duty mortiser, but a super heavy duty in hobby and light commercial terms.

I think Dave's point is you have two data points, 3600 is too fast and 1800 is slow enough, that was General's and other mortiser builders 2 cost effective choices. However, you don't know what speed would be slow enough, it might be 2000 or 3400. Point being a 2hp VFD is a relatively cheap option to see if you can find that magic number and still have enough HP to get the job done. The fly in the ointment is you mainly want the reduced speed for larger bits, exactly where you need the increased horsepower.

ian maybury

03-04-2012, 6:59 AM

I wonder if an 1800RPM motor and (if you need the higher speeds) a VFD might not be the option....

ian

Chris Rosenberger

03-04-2012, 9:05 AM

The 220 is an EXCELLENT mortiser but in the grand scheme it is a light duty mortiser, but a super heavy duty in hobby and light commercial terms.

I think Dave's point is you have two data points, 3600 is too fast and 1800 is slow enough, that was General's and other mortiser builders 2 cost effective choices. However, you don't know what speed would be slow enough, it might be 2000 or 3400. Point being a 2hp VFD is a relatively cheap option to see if you can find that magic number and still have enough HP to get the job done. The fly in the ointment is you mainly want the reduced speed for larger bits, exactly where you need the increased horsepower.

Thank you Van.

Since the motor on the General 220 is a special purpose motor, I am sure that it would be a very costly change, I will try a VFD first.

This may be another lower cost option if the VFD does not work out. I do not know if this was done because the original motor failed, they needed to change phases or RPMs.

I recently saw a picture of a General 220 that had a second motor mounted to the side of the standard motor. They had removed the fan from the top of the standard motor & installed a pulley in its place. A belt ran from the standard motor to the second motor.

I could mount a 1725 RPM motor to the 220 like they did or stack it on top & use a coupler, keep power to both. Then I could run the higher speed motor for smaller bits & the slower speed motor for larger bits & still have close to full power.

Van Huskey

03-04-2012, 9:19 AM

I thought I might add to this, my thought process for sizing a motor/VFD to make a VS machine.

My plan is based on a drill pres I plan to convert. The DP comes stock with a 3/4 hp motor which I find sufficient for all my drilling needs but I do not want less. The problem with this situation is the small bits which use higher speeds need less HP and the large boring tooling needs lower speed and more HP, the exact opposite of the trend when using a VFD, so you gotta get creative.

First, I have to pick the motor, a nice 2hp face frame motor can be gotten fairly cheap via surplus sites and I will start with a 1725rpm motor. Most good motors you can track down the full manufacturer specs and most list maximum RPM which will mainly be based on the bearings, bearing lube and rotor balance. Many good motors, even non-inverter duty, in the size range will be in the 6,000 rpm max range. So my motor will be a 2hp face frame, 1725 plate rpm, 6000rpm max.

My low end rpm requirement is 250 rpm, this is for circle cutters and a couple of other specialty items. All my forstners (1/2"-2 1/4") are fine at 500 rpm, fast for some but Colt Maxi-cuts are fine at the slightly higher speed.

Speed range #1 will have a low speed of 250rpm.

If I run the VFD down to 23hz I get approximately 650rpm on the motor and the design spec .75 hp, so I do not want to go slower at the motor.
So my low speed range will use the closest mechnical reduction to 2.6:1 to get the low end of 250 rpm
So at 60hz in this range I will get a spindle speed of 660 rpm
But I can easily run the motor up to 120hz/3750 rpm so the effective speeds of range #1 is 250 to 1320 rpm, this covers 95% of my drilling in wood with that highest speed being 1200rpm for 1/8" brad point in hardwood.

Speed range #2 will have a low speed of 1200 rpm

If I run the VFD down to 23hz I get approximately 650 rpm on the motor and approximately .75hp again the lowest HP I want
So my high speed range will be the closest mechanical reduction to 1.3:1 so I have a low speed of 500 rpm
So at 60hz I will have a spindle speed of 1320 rpm
At 120hz I will have a high spindle speed of 2640 rpm as high as I usually ever use

Please note all my numbers were rounded and not by any standard rounding just what felt right!

In the end I have a drill press that on a single set of pulleys I can do the vast majority of my drilling. If I need more speed I can cover it with 1 other speed range and if I happen to be in the high speed range I can use the vast majority of my tooling without having to change range as well. If I need more horsepower I can just run the VFD up to 60hz (full HP) and move the belts to the correct speed. Given you can have a mechanical 10:1 ratio on the DP in question I could run the spindle at 65rpm have 3/4hp and a pretty massive amount of torque, on the other end I wouldn't exceed the normal max speed of DP for a number of reasons, not the least of which is I know of no reason why I would need to.

David Kumm

03-04-2012, 9:21 AM

Yes Chris, a motor swap will likely cost about what a used mortiser does. I think you will be surprised what a difference a $150 vfd will make on the existing machine and what an improvement just a 5-10 hz reduction will make. You didn't say what bits you are using but I'm assuming the large ones are high quality. Makes quite a difference in power needed. Dave

Van Huskey

03-04-2012, 9:25 AM

I could mount a 1725 RPM motor to the 220 like they did or stack it on top & use a coupler, keep power to both. Then I could run the higher speed motor for smaller bits & the slower speed motor for larger bits & still have close to full power.

And if it weren't for all those pesky perpetual motion grimlins once you got one started you could use it to power the other one... and only have to use a couple of seconds of power each time you needed to change speeds.

Stephen Cherry

03-04-2012, 10:40 AM

But I can easily run the motor up to 120hz/3750 rpm so the effective speeds of range #1 is 250 to 1320 rpm, this covers 95% of my drilling in wood with that highest speed being 1200rpm for 1/8" brad point in hardwood.

Speed range #2 will have a low speed of 1200 rpm

At 120hz I will have a high spindle speed of 2640 rpm as high as I usually ever use

.

One option, if you want to make a science project, would be to run a 60 hz 230v motor with a 400 volt drive with 120 hz base frequency. It seems this would allow constant torque over a much wider range, and double the hp at the 120 hz speed. It's still approx the same torque as the 60 hz 230 volt operation.

You would need a drive to produce the nameplate current rating of the motor, so if it's a 1.5 motor, you would need a 3 hp drive. Plus, you would need the approx 480 volts for the drive. Conceptually, this could be done with a 480-230 volt transformer, and some recitifers from digi-key. Some drives have a separated input section, where the rectifier and capacitor output is fed back to the drive as an input (maybe b and b+ terminals). I think Hitachi has this. This would allow the feeding of the rectified 480 volts into the drive. People like to do this sort of thing for electric car experiments, etc.

Lots of effort, yes. maybe too much for a drill. It seems to me that this would be perfect for a lathe, where you want good torque at low speeds, as well as the ability to spin. The practical machinist site has local gurus for this type of thing.

Van Huskey

03-04-2012, 10:46 AM

An interesting idea Stephen. This is a practical application though and as is I am spending more than I should just to avoid changing belts!

Stephen Cherry

03-04-2012, 10:52 AM

An interesting idea Stephen. This is a practical application though and as is I am spending more than I should just to avoid changing belts!

I hear you-- I have lots of experience with drives, lots of test equipment, etc., and about zero interest in experimenting.

Tom Stenzel

03-04-2012, 4:18 PM

One option, if you want to make a science project, would be to run a 60 hz 230v motor with a 400 volt drive with 120 hz base frequency. It seems this would allow constant torque over a much wider range, and double the hp at the 120 hz speed. It's still approx the same torque as the 60 hz 230 volt operation.

You would need a drive to produce the nameplate current rating of the motor, so if it's a 1.5 motor, you would need a 3 hp drive. Plus, you would need the approx 480 volts for the drive. Conceptually, this could be done with a 480-230 volt transformer, and some recitifers from digi-key. Some drives have a separated input section, where the rectifier and capacitor output is fed back to the drive as an input (maybe b and b+ terminals). I think Hitachi has this. This would allow the feeding of the rectified 480 volts into the drive. People like to do this sort of thing for electric car experiments, etc.

Lots of effort, yes. maybe too much for a drill. It seems to me that this would be perfect for a lathe, where you want good torque at low speeds, as well as the ability to spin. The practical machinist site has local gurus for this type of thing.

Another formula for horsepower is 746 watts = 1 horsepower. To double the motor horsepower you need to double the watts driven into it. In your example I doubt if you're going to get 3 HP out of a 1.5 HP rated motor without some smoke.

Also the drives that have been mentioned here are all PWM (pulse width modulated) type drives. There's a reason for that- they're the cheapest type of drive to build. They work by taking the incoming AC, rectifying it into a fixed voltage DC bus, then using transistors to reinvert the DC into AC.

On a 220 VAC volt drive the bus is about 330 volts. On a 440 VAC drive the bus is about 660 volts. To deliver a specific voltage, say 150 volts, The 220 VAC drive will have to have the inverter section deliver a 45% on-time duty cycle. The 440 VAC drive will do the same thing with a 22.5% duty cycle. But can your 220 volt motor handle the 660 volt spikes the 440 volt drive will constantly give it? For how long?

As far as selection of drives being carefully matched to the motor, at work we have replaced Reliance UniMac drives with Toshiba drives, both GE AF400 variable voltage drives and Louis Allis current source drives have been replaced with Robicon 454GT drives, CEGELEC load commutated inverters have gone away for Robicon Perfect Harmony drives. These applications ran from 30 kVA to 2000 kVA. The overriding engineering concern when the replacement was made: Low Bid. In all cased the motors have been fine. And some of the motors have seen constant service since the '70's.

-Tom Stenzel

Alan Schaffter

03-04-2012, 5:43 PM

All this VFD stuff aside and if cost were not an issue, the best variable speed rig for a lathe or drill press would be a DC motor and controller/rectifier- all the low-end hp you would likely ever need. However, just like VFD prices, DC controller/rectifier and DC motor prices start going up really fast and available surplus becomes much scarcer when you get above fractional horsepower.

The price of the surplus 1/4 hp DC gear motor and controller/rectifier I installed on my drum sander conveyor were right at the limit of what I thought reasonable for a DIY project.

Stephen Cherry

03-04-2012, 6:18 PM

Another formula for horsepower is 746 watts = 1 horsepower. To double the motor horsepower you need to double the watts driven into it. In your example I doubt if you're going to get 3 HP out of a 1.5 HP rated motor without some smoke.

On a 220 VAC volt drive the bus is about 330 volts. On a 440 VAC drive the bus is about 660 volts. To deliver a specific voltage, say 150 volts, The 220 VAC drive will have to have the inverter section deliver a 45% on-time duty cycle. The 440 VAC drive will do the same thing with a 22.5% duty cycle. But can your 220 volt motor handle the 660 volt spikes the 440 volt drive will constantly give it? For how long?

-Tom Stenzel

That is a consideration Tom. There are basically two effects. The first is the heating of the windings, basically i^2r heating. Notice, this heating is not a function of the voltage, just current, and the voltage is what is giving the extra power. Since the current is not going up, neither will the heating. Except maybe the heating of the high frequency component of the output of the drive.

The second effect is the insulation capability of the windings. Can't say for sure, but my guess would be that a dual voltage motor 220-440 would have OK insulation between windings and ground because it is made to handle the 440 when wired for it. Just guessing though, and that is what would make it a science project.

It seems to me that this could give a nice setup for a machine that has a high rpm range. For example a lathe, particularly if you start with an 1800 or lower rmp motor (at 60 hz). This would allow constant torque up to about 3600, then reduced torque after that. It would take a little doing though.

Van Huskey

03-04-2012, 10:18 PM

motor prices start going up really fast and available surplus becomes much scarcer when you get above fractional horsepower.

To clear this up for anyone that made the same comprehension error I did when, you say fractional horsepower you mean it as in less than 1 hp not the NEMA term of art which means any size 42, 48 or 56 frame despite the fact it may be 1hp or more. I don't even know if the term of art is applied to DC motors, but when I read it the first time I was thinking is that woodworkers (in a home/light commercial shop) hardly ever have motors larger than factional, then I realized what you meant and my mistake. They also do get expensive and I usually don't see them above about 1.5hp, the 3/4 to 1.5hp DC motors usually run in the 200-250 range new and they are generally longer than AC induction motors so their form factor could be an issue with some machines but probably fine for the best places like boring and lathes. I have seen them used on several lathe conversions.

Tom Stenzel

03-05-2012, 11:39 AM

That is a consideration Tom. There are basically two effects. The first is the heating of the windings, basically i^2r heating. Notice, this heating is not a function of the voltage, just current, and the voltage is what is giving the extra power. Since the current is not going up, neither will the heating. Except maybe the heating of the high frequency component of the output of the drive.

The second effect is the insulation capability of the windings. Can't say for sure, but my guess would be that a dual voltage motor 220-440 would have OK insulation between windings and ground because it is made to handle the 440 when wired for it. Just guessing though, and that is what would make it a science project.

It seems to me that this could give a nice setup for a machine that has a high rpm range. For example a lathe, particularly if you start with an 1800 or lower rmp motor (at 60 hz). This would allow constant torque up to about 3600, then reduced torque after that. It would take a little doing though.

You’re right about the heating effect on the stator, as long as the current doesn’t go above the max motor rating the motor should be OK. Also I don’t believe that your concern about the higher frequency is a problem. Even at low frequencies the motor is getting hit with pulses at the drive’s carrier frequency. In the case of the TECO drives the default carrier frequency is 10 KHz. Going over 60 Hz doesn’t change that unless you program a different frequency which is in effect whatever speed you request from the drive.

So on paper you’re right. I’m still concerned about the motor stator with all this going on. I’ll thumb through my Kosow’s Motor book and if I find anything I’ll pass it on.

-Tom Stenzel

Mike Henderson

03-05-2012, 1:17 PM

You’re right about the heating effect on the stator, as long as the current doesn’t go above the max motor rating the motor should be OK. Also I don’t believe that your concern about the higher frequency is a problem. Even at low frequencies the motor is getting hit with pulses at the drive’s carrier frequency. In the case of the TECO drives the default carrier frequency is 10 KHz. Going over 60 Hz doesn’t change that unless you program a different frequency which is in effect whatever speed you request from the drive.

So on paper you’re right. I’m still concerned about the motor stator with all this going on. I’ll thumb through my Kosow’s Motor book and if I find anything I’ll pass it on.

-Tom Stenzel
You're correct about the heating - the rate of heat generation will remain constant. The problem is the cooling fan. Most "ordinary" induction motors are designed to cool with the built in fan (the fan is mounted on the shaft). As the shaft speed decreases, the amount of air moved decreases and the motor overheats if you run it at rated torque (current). This is especially a problem with TEFC motors. You can fix this by rigging up a separate fan that blows on the motor to keep it cool.

Mike

Salem Ganzhorn

03-05-2012, 6:54 PM

Alan Schaffter

03-05-2012, 8:26 PM

Alan, I must be missing something. A VFD running at 60hz should be identical to running the motor on a "normal" 3 phase supply. I don't see how you could stall a 3hp motor with your hands in this case without some serious mechanical advantage. Why would you have to run 400% RPM? I can almost see 200% if the motor was 2x the speed of the motor it originally replaced, but even then you only have a 2x mechanical advantage. Have you see the YouTube video "I fought the lathe and the lathe won"? Muhahahaha!
Salem

My post may have been a bit confusing, but the first motor ran at 3450 RPM and was not geared down enough mechanically, so when I slowed it with the VFD to achieve 350 spindle RPM for drilling large holes in steel, there wasn't enough motor RPM left to generate decent hp, hence I could stall it easily with my hand. You need to run the numbers to wrap your brain around this issue.

In my updated setup I have a 1725 RPM motor and approx. 3:1 gearing (I removed the intermediate idler pulley on my drill press to make room for other stuff.) So when the motor is turning its "normal" 1725 (VFD set to 60 Hz) the spindle is turning one third of that- 575 RPM. Now, to get even slower spindle speed- 350 RPM for drilling large holes in metal- the motor RPM must be 1050 RPM (60% of its normal 60 Hz RPM). To achieve that, the VFD is putting out and the motor running on only 36 Hz which seems to work just fine for drilling large holes is steel.

But look what happens at the other end of the RPM scale, most drill charts indicate small twist drill bits should turn around 3000 RPM in wood and metal. Keep in mind my gearing is 3:1, so the motor must turn 9000 RPM!!!! That is actually 500% of its "normal" RPM and requires the VFD to put out and motor to run at 300 Hz. I plan to study my drilling needs and habits more closely and may play around with my gearing and "sweet spot."

Again, I go back to my original comment- a VFD and mechanical gearing are both required in order to achieve the RPM range woodworkers need for most drilling and lathe applications. A VFD is definitely not a be all, end all solution to variable speed.