Not Another VFD Question

[Richard Ralph]

I know this question has been asked and answerd before. I have read several posts on VFD's. I want to know if I am on the right track. I have an opertunity to buy a table saw with a 3 HP 3PH motor. Since I only have 220V at the house I will need a VFD. I have a basic understanding of what a VFD does. Not sure on how to spec one out. If I am going to be using to power a 3 HP motor do I need one that has 3 HP on the input? I am looking at the TECO FM50-203-C http://www.factorymation.com/s.nl/it.A/id.198/.f?sc=2&category=32 . Is this the correct unit?

[Carroll Courtney]

TECO FM50 AC Drive, 3 HP, 230V 1 / 3 PH Input, 230V 3 PH Output, 10.5FLA, IP20
Richard I'm no expert but it says 3hp then it says 1 single phase input or 3 phase input.But what you really need to look at is your motor's FLA's rating and see what VFD you need by looking at its FLA rating which is 10.5
What is nice about factorymation is you can always call them and ask any questions and you will get answers without someone guessing.Keep us posted so we all can learn alittle----Carroll
By the way Welcome!!!

[David Kumm]

Generally you need to derate vfds if listed as 3 phase in but that isn't necessarily the case with small ones. Check the output amps. You need a vfd that accepts about 20 amps in on single phase to get 10 amps out. Most vfds that state 3 phase in can be run with single phase input but again you need 20 amps in. Dave

[M Toupin]

Richard,
For a table saw, were you're not interested in variable speed, the FM50 series, which are simple volts/hz VFD, will be the economical solution. At 60hz you'll get full torque & hp. If you're after variable speed and low end torque (say on a lathe), then a sensor-less vector drive, which can compensate for torque loss at lower speeds, would be more appropriate. The down side is SV drives are more expensive.

The TECO FM50-203-C is marketed as a "3hp", in actuality, VFDs are rated in FLA, the 203-C is 10.5FLA which is the "average" 3hp 3ph motor give or take. When selecting a VFD check your data plate on the motor for the FLA. You want a VFD rated for the FLA of the motor or there about. To muddy the water a bit, VFD's generally handle 150% rated FLA for a given time period, usually 60 seconds or so. If you don't plan on pushing the motor to the limits, or only for short periods of time, then you could get away with a slightly under sized VFD. Table saws are good candidates as you very seldom run a TS at 100%, much less for more than 60 seconds at a time. Now, if you have a stock feeder and frequently saw really hard and thick stock, I wouldn't recommend it, but for the average hobby shop, you can get by with a amp or so under sized.

The TECO FM50-203-C is rated for single phase input, you do not have to worry about "derating" it. The only caveat I would add is make sure your 3ph motor is rated for 220/230v. Some 3ph motors are 200 or 208v, not 220/230. The FM50 series can not compensate for output voltage. If you have a 200v motor, then you'll need to step up to the 7200 or 7300 series which can be adjusted for output voltage.

Mike

[Stephen Cherry]

Don't forget to get the braking resistor for fast stops.

[Mike Henderson]

Generally you need to derate vfds if listed as 3 phase in but that isn't necessarily the case with small ones. Check the output amps. You need a vfd that accepts about 20 amps in on single phase to get 10 amps out. Most vfds that state 3 phase in can be run with single phase input but again you need 20 amps in. Dave
I'm not sure that's correct. The problem with single phase input and three phase output is that the single phase goes to zero twice each cycle, but since the output is three phase, at least two of the phases will be above zero at the times of the zero crossing. For a VFD to operate on single phase, there must be some storage devices which provide power to the output during the times when the input is zero crossing.
To do this, a VFD will convert the input to DC. For single phase input, the DC stage will have large capacitors to store power. The DC will be converted to three phase AC to be fed to the machine. The output is not a sinusoid but is pulses which approximate a sinusoid.
When the input is three phase, it's not necessary to have those large storage devices in the DC stage because power is always being supplied - when one phase is zero crossing, the other two phases are not zero crossing, by definition. Some storage devices are used, but they are sized for use with three phase input and not single phase input.

So if the VFD is designed to operate with three phase input (only) it may not work with single phase input.

Mike

[David Kumm]

Mike, Many manuals explain which inputs to attach for single phase in on 3 phase input vfds. Some don't address it but the tech guys will let you know. I have 8 or 10 vfds of all sizes and brands, all are 3 phase in and all work on single phase. Not implying someone should buy based on my experience but that it may not be necessary to exclude all units. Best to verify. Dave

[Mike Henderson]

Mike, Many manuals explain which inputs to attach for single phase in on 3 phase input vfds. Some don't address it but the tech guys will let you know. I have 8 or 10 vfds of all sizes and brands, all are 3 phase in and all work on single phase. Not implying someone should buy based on my experience but that it may not be necessary to exclude all units. Best to verify. Dave
I assume that if the manual specifies how to hook it up for single phase, the VFD is designed to work with single phase, perhaps at a reduced capacity because of the amount of storage in the DC stage. My concern was that a VFD designed for only 3-phase input might not work with a single phase input.

Mike

[M Toupin]

Don't forget to get the braking resistor for fast stops.

Not necessarily true. All VFDs have some built in ability to handle resistive or degenerative loads. The only time you need a breaking resistor is when your load is larger than the ability of the VFD to handle it. If your load is light, like in the case of the table saw, (blade & arbor etc) you won't need a breaking resistor. The inertia isn't high enough, even if you set the spin down at 1-2 seconds. On a lathe were you're turning big heavy blanks or a big BS with heavy CI wheels and you're trying to stop all that inertia in say 2 seconds, yes you'll need a resistor. On the other hand, if you want to stop that big load in say 20-30 seconds, the average VFD would likely be able to handle the load without a breaking resistor.

For example, I have a VFD on a Delta 12" HD lathe with no breaking resistor. I have the ramp down set fairly aggressively at 3 seconds & it has handled everything I've turned. I also have a 3hp shaper on a VFD with no breaking resistor. The shaper, which even has a large, 10"+ dia cast iron double sheave pulley and running 4"+ panel raising cutters, is set with the ramp down set at 2 seconds. It too works fine.

My recommendation is to try the VFD and set your ramp down times to what you're trying to achieve. If it handles the load without tripping, then you're fine. If not, then get a breaking resistor. If the VFD can't handle the resistive load it won't damage anything, it'll just go to fault mode and coast to a stop. VFDs are fairly smart, they try and keep you from doing something stupid by faulting out before any damage is done.

Keep in mind that VFDs are industrial equipment, they were never designed or intended for the home use. Not that they don't work for our purposes of converting 1ph to 3ph, they do, but keep in mind what they were designed for. In a industrial environment were stopping very heavy inertial loads (100lbs+) is not unusual. For the most part, these VFDs aren't even cracking a sweat on our little stuff.

[Stephen Cherry]

Not necessarily true. All VFDs have some built in ability to handle resistive or degenerative loads. The only time you need a breaking resistor is when your load is larger than the ability of the VFD to handle it. If your load is light, like in the case of the table saw, (blade & arbor etc) you won't need a breaking resistor. The inertia isn't high enough, even if you set the spin down at 1-2 seconds. On a lathe were you're turning big heavy blanks or a big BS with heavy CI wheels and you're trying to stop all that inertia in say 2 seconds, yes you'll need a resistor. On the other hand, if you want to stop that big load in say 20-30 seconds, the average VFD would likely be able to handle the load without a breaking resistor.

For example, I have a VFD on a Delta 12" HD lathe with no breaking resistor. I have the ramp down set fairly aggressively at 3 seconds & it has handled everything I've turned. I also have a 3hp shaper on a VFD with no breaking resistor. The shaper, which even has a large, 10"+ dia cast iron double sheave pulley and running 4"+ panel raising cutters, is set with the ramp down set at 2 seconds. It too works fine.

My recommendation is to try the VFD and set your ramp down times to what you're trying to achieve. If it handles the load without tripping, then you're fine. If not, then get a breaking resistor. If the VFD can't handle the resistive load it won't damage anything, it'll just go to fault mode and coast to a stop. VFDs are fairly smart, they try and keep you from doing something stupid by faulting out before any damage is done.

Keep in mind that VFDs are industrial equipment, they were never designed or intended for the home use. Not that they don't work for our purposes of converting 1ph to 3ph, they do, but keep in mind what they were designed for. In a industrial environment were stopping very heavy inertial loads (100lbs+) is not unusual. For the most part, these VFDs aren't even cracking a sweat on our little stuff.

Incorrecto!

Actually, the drives are good for about what they are rated for, and you really want to reduce the thermal load on the active components as much as possible. Every heat cycle results in additional thermal stress (or is it strain?) due to differences in the rate of thermal expansion of the material of the components. The effect is cumulative, the more the heat and the greater number of cycles, the quicker it checks out.

So even if the drive can go overcurrent for a short time to stop a load fast, the price is degradation of the components of the drive and a measurably shorter lifespan.

[David Kumm]

A problem with braking a motor, whether it be dynamic, regenerative, or DC injection, is how to dissipate the heat and energy buildup. Most vfds can not send the power back to the line as in regenerative braking so it is stored in the unit. Larger injection brakes have a heat sink to help with that which is what a braking resistor kind of does with a vfd. Some brakes use the rotor for that purpose. The ability to absorb or dissipate heat determines the amps that can be applied and the duration before something gives. Injection brakes have a curve that defines the number of stops allowed per hour at a given load and duration. Exceed those and the brake - or vfd - fails. That is a big issue in Europe where stopping is often mandated. Much better selection of brakes over there. Dave

[M Toupin]

Every heat cycle results in additional thermal stress (or is it strain?) due to differences in the rate of thermal expansion of the material of the components. The effect is cumulative, the more the heat and the greater number of cycles, the quicker it checks out.

Ok, you got me... at least in theory. On the other hand I assume the engineers who design VFDs are smart guys and have already taken this into account and set the fault values accordingly. If the engineers were concerned about it, they'd set the fault levels lower or at zero. I stand by my statement that VFDs are industrial equipment designed for an industrial environment. The average home shop is not going to stress them anywhere near what a industrial environment would.

Mike

[Stephen Cherry]

Ok, you got me... at least in theory. On the other hand I assume the engineers who design VFDs are smart guys and have already taken this into account and set the fault values accordingly. If the engineers were concerned about it, they'd set the fault levels lower or at zero. I stand by my statement that VFDs are industrial equipment designed for an industrial environment. The average home shop is not going to stress them anywhere near what a industrial environment would.

Mike

The wear on the drive is a normal result of use and not a fault, and a big current rush at the end of a on off cycle would result in higher temps. I agree, even the low end drives are pretty robust, but why not make them last, particularly in a table saw, which could see lots of ons and offs.

The whole thing is similar to stopping a car-- it seems like it is better to use the 20 dollar brake pads than downshifting and wearing out the 2000 dollar transmission. Maybe not that extreme, but it's the same idea.

I think that the fast stop in a table saw is a big safety plus, particularly if it makes some sound in the process, which would signal the mind that the dangerous part is over. Also, I really like the sound of using a magnetic starter for the run permit for the drive; the clunck of the contactor pulling in tells the operator that it's time to pay attention.

[M Toupin]

Also, I really like the sound of using a magnetic starter for the run permit for the drive; the clunck of the contactor pulling in tells the operator that it's time to pay attention.

A motor starter for a motor starter. That's a bit redundant not to mention expensive, but hey, it's your money and setup. Whatever makes you feel good.

Mike

[Stephen Cherry]

A motor starter for a motor starter. That's a bit redundant not to mention expensive, but hey, it's your money and setup. Whatever makes you feel good.

Mike

My guess is that the machine already has a nice contactor. Just wire to the two legs that run the button, and The third, unused contact could be used as the run permit. If not, these things can be picked up cheap. For me, the sound is worth a few extra bucks. The last thing I would want is a quiet table saw; I think that the sounds make a great marker for the brain that something has changed and it is time to pay attention.

[David Kumm]

Stephen, would that be a good application for a bandsaw? With several kill switches wired into the starter, could you just wire the starter into the vfd, leave the vfd on the wall and out of the way, and use the original starter? Dave

[Rod Sheridan]

David, you cannot use a contactor to disconnect the motor from the VFD, the VFD must remain connected to the motor.

I would simply use the existing interlock switches to control the VFD.....................Regards, Rod.

[Dick Strauss]

Rod is exactly right! You don't want to disconnect the load while the VFD is running if you want to keep the VFD from self-destructing.

However, you can use a remote switch to cut the low voltage control signal that then triggers the VFD to stop...

[David Kumm]

I understand that the vfd must be connected directly to the motor. My question was to Stephen's point and about whether the mag starter could be used as to control the run and stop command to the vfd. Dave

[Rod Sheridan]

Sorry Dave, I was mistaken.

Yes you could use a contact from the mag starter to start/stop the VFD..............Regards, Rod.

[Stephen Cherry]

The contactor does not disconnect power from the drive, it supplys only run permit contact.

Disconecting power to the drive, as Rod points out, is a real no no.

You certainly could use the contactor if you were converting a bandsaw. You would likely wire power to the two of three legs of the contactor that supply the power to the contactor. This would maintain the use of the run interlocks. The unused leg of the contactor would then supply the run command to the drive.

[M Toupin]

TYou certainly could use the contactor if you were converting a bandsaw. You would likely wire power to the two of three legs of the contactor that supply the power to the contactor. This would maintain the use of the run interlocks. The unused leg of the contactor would then supply the run command to the drive.

Yes, you could use the original motor starter to provide power to the VFD, but let's get something straight, a motor starter is NOT a contactor. Contactors have no overload protection were a motor starter does, different animal for a different purpose. Now, if you re-purpose the original 3ph motor starter and you're lucky, you'll at a minimum need to change the heaters. 1ph pulls more amperage per leg than 3ph, your VFD needs approximately 2X the amperage per leg as the original 3ph motor required. Depending on the size of the original motor starter, you may have to get a new one if the original is rated for less than your 1ph VFD needs.

The idea of using the 3rd unused leg on the motor starter is just plain crazy, never mix high voltage and low voltage control circuits. That aside, keep in mind that the typical low voltage control, which is what your VFD uses to input the start stop commands, normally consist of a NO (normally open) start button and a NC (normally closed) stop button. On a motor starter All 3 legs are tied together, you can't just close and open them independently. Even if you managed to power up your VFD and send the start command to the VFD simultaneously through the original motor starter, how do you intend to send the stop command without completely powering off the VFD? And you are aware that completely powering off the VFD defeats all the FVD controls to include dynamic deceleration etc?

The bottom line is you'll need some way to engage the original motor starter to provide power to the VFD. That could be from the original motor starter if it has a built in start/stop buttons or from a separate low voltage start/stop station. You'll then need some method to send start/stop commands to your VFD. You could use the buttons on the from of the VFD, or a separate low voltage start/stop station mounted somewhere convenient on the machine.

[Stephen Cherry]

Yes, you could use the original motor starter to provide power to the VFD, but let's get something straight, a motor starter is NOT a contactor. Contactors have no overload protection were a motor starter does, different animal for a different purpose. Now, if you re-purpose the original 3ph motor starter and you're lucky, you'll at a minimum need to change the heaters. 1ph pulls more amperage per leg than 3ph, your VFD needs approximately 2X the amperage per leg as the original 3ph motor required. Depending on the size of the original motor starter, you may have to get a new one if the original is rated for less than your 1ph VFD needs.

The idea of using the 3rd unused leg on the motor starter is just plain crazy, never mix high voltage and low voltage control circuits. That aside, keep in mind that the typical low voltage control, which is what your VFD uses to input the start stop commands, normally consist of a NO (normally open) start button and a NC (normally closed) stop button. On a motor starter All 3 legs are tied together, you can't just close and open them independently. Even if you managed to power up your VFD and send the start command to the VFD simultaneously through the original motor starter, how do you intend to send the stop command without completely powering off the VFD? And you are aware that completely powering off the VFD defeats all the FVD controls to include dynamic deceleration etc?

The bottom line is you'll need some way to engage the original motor starter to provide power to the VFD. That could be from the original motor starter if it has a built in start/stop buttons or from a separate low voltage start/stop station. You'll then need some method to send start/stop commands to your VFD. You could use the buttons on the from of the VFD, or a separate low voltage start/stop station mounted somewhere convenient on the machine.


Mike, I should have explained this with a little more detail. Most of these three phase machines will come with a packaged motor control circuit. This includes a box with a contactor and thermal overload, a normally open start switch, a normally closed stop switch, aux contact, etc. These parts are usually very robust NEMA style devices, built to last a long time.

The concept is to supply 230 V (or 220, whatever it takes) to the two legs of the contactor that run the coil. The motor is unhooked from the thermal overload, and the thermal overload can be removed, or left in. If you supply 220 volt single phase to two of the contactor inputs, and leave last unconnected, one combination of the two of three inputs can be used to still run the switch and contact. When you press the start button, the contactor will pull in, and when you press the stop, the contactor will release. This will leave one contact of the contactor as a run permit for the drive. Nothing is connected to the outputs of the contactor that recieve the 220 v power.

At this point you have a machine with 220 v single phase supply, and you can push the start and hear a click, and push the stop and hear another click, but that's it.

Now the drive gets powered by its own 220v supply. The contactor does not interrupt this supply. I like the idea of a hand switch to remove power from the contactor and drive. THe output of the drive goes to the three phase motor, and the run permit of the drive goes to the unused contact on the contactor. THis may not be strictly correct for some reason, the contacts are rated for much higher than the control voltage. ANd it is very normal to mix 24v with the motor voltage in a starter box (for example 24vac and 230 v).

At this point, if you had a braking resistor, that could be wired in, but, other than a few parameters in the drive, that's about it. Works great, can be wired up in just a few minutes, and you are good to go.

[Alan Schaffter]

Stephen, Mike, it is a bit hard to follow your descriptions, but it sounds like even though it is not needed you are suggesting the OP can re-purpose the motor/starter that came with the machine- use the contactor without the thermal overload as a latching relay to control the VFD start permit circuit- solely for the purpose so there is a nice solid "click" as the contacts mate. You sure are making things a lot harder than they need to be. Don't you think the sound the machine makes on start-up is sufficient? You shouldn't need the latching protection either- the start permit on most VFDs is low voltage (5V?) and can be wired a number of ways with either a SPST (on/off) switch or momentary start and stop buttons and programming so there won't be auto re-start after a power interruption.

Unlike the current generation of mag starters, most older starters used Low Voltage Control (LVC). They have a small multi-tap transformer that takes various input voltages (120V, 208V, 220V, 440V, etc.) and provide 24V to the contactor coil. Yes, the older Delta LVC units (I have 3) mixed line voltage and 24V control voltage in the same box- for protection, they added a fuse to the control circuit (mounts right on the transformer)- and used one set of relay contacts as the latching circuit. One of my machines has a 3 phase contactor- with four pairs of contacts - 3 line voltage and 1 24V control voltage. You can get nearly the same results from a smaller (noisy?) 24V relay, but again it is not needed.

I use the original mechanical latching buttons on my Delta DP to power the VFD and just a small SPST toggle switch (below red knob) for the "run" permit circuit. The red knob is the remote speed pot (I also added a tach):

207281

[Stephen Cherry]

Alan, that's about the idea. I like the sound, and the idea of a magnetic seal in. The best part of all this is that it only takes a couple of minutes and it is cheap; with most machines the parts will already be there. Of course, others will see their own benifits to doing things some other way. For "vintage' machines, this retains the look and feel of the original buttons.

With your machine, I would have used the start/stop buttons to start and stop the drive. That way the functions are properly labeled. Then a switch could be used to power the drive on and off. I worked for a while as a control system engineer, and designed operator interfaces for a bunch of machines so I guess I'm pretty particular about machines being intuitavely easy to operate.

As for the sounds, this is just my opinion, but I think they make for a safer machine by providing a little bit of a heads up signal.

[Alan Schaffter]

Alan, that's about the idea. I like the sound, and the idea of a magnetic seal in. The best part of all this is that it only takes a couple of minutes and it is cheap; with most machines the parts will already be there. Of course, others will see their own benifits to doing things some other way. For "vintage' machines, this retains the look and feel of the original buttons.

With your machine, I would have used the start/stop buttons to start and stop the drive. That way the functions are properly labeled. Then a switch could be used to power the drive on and off. I worked for a while as a control system engineer, and designed operator interfaces for a bunch of machines so I guess I'm pretty particular about machines being intuitavely easy to operate.

As for the sounds, this is just my opinion, but I think they make for a safer machine by providing a little bit of a heads up signal.

The buttons are mechanical, so don't have a nice neat action like the LVC ones. Someday maybe I'll find button covers that say "on" / "off" that I can use with the DP. Or I might replace the toggle switch with a "push on", "push off" button, maybe one that is lighted. I normally run my DC and blast gates, in the full auto mode (machine triggers both autogate operation and starts the DC) but when I want to manually open a gate I use a "push on", "push off" button with a separate pilot light since I could find a button that did both:

207294

I use the Delta-style momentary start/stop pushbuttons in about 6 stations located around my shop, and connected to a 3-wire, low voltage, circuit to control my DC manually when needed:

207295 207296




lighted on

[Stephen Cherry]

The buttons are mechanical, so don't have a nice neat action like the LVC ones. Someday maybe I'll find button covers that say "on" / "off" that I can use with the DP. Or I might replace the toggle switch with a "push on", "push off" button, maybe one that is lighted. I normally run my DC and blast gates, in the full auto mode (machine triggers both autogate operation and starts the DC) but when I want to manually open a gate I use a "push on", "push off" button with a separate pilot light since I could find a button that did both:



I use the Delta-style momentary start/stop pushbuttons in about 6 stations located around my shop, and connected to a 3-wire, low voltage, circuit to control my DC manually when needed:




lighted on

Nice. You need a small plc to control all of that. (half joking)

I just noticed, you are in wash, nc. I travelled many times for work to Wilson, nc. I really liked the bbq, with greens. I'm in Maryland, and our best bbq is really not that great.