Electrical Question(s)

[Andy Howard]

I have a few questions for the electrical gurus out there!

1. On a three phase motor, and assuming a constant voltage. When the hertz are increased to the motor (via a Variable frequency drive) what will happen to the amps? Will they stay constant no matter what hertz you are running the motor at?

2. Can you damage a three phase motor by running it at a lower amperage than the rated FLA(full load amps)? Assuming I was to have a VFD that output 4 amps, and a motor rated at 5 amps. Would it be damaging to the motor to limit the output of the VFD to 4 amps? I realize that the motor would not develop full power in this situation, and I am ok with that, but I do not want to damage it. In reality it would be more like limiting the motor by .2 amps or maybe .3 amps.

Thanks
Andy

[Dev Emch]

O.K. First of all, there are two basic modes of operation you need to be aware of. Constant torque and constant horse power.

When the motor starts up, the VFD SOFT STARTS the motor by beginning at DC and ramping up the frequency to 60 cycles. Once the motor is running at 60 cycles, it is at its sync speed. The actual RPM will be dependent on the pole count of the motor. From 0 RPM to sync RPM, your at constant torque. As mechanical load on the motor is increased, the amperage will go up to meet this requirement and maintain motor slip. So at no load, your motor is at idle and your amps are at idle amperage even though your at sync speed.

As you load up your motor, your amperage draw increases. Usually, the heaters or overload relays are sized slightly less than full load nameplate amperage. Exceed this value, and the relays trip taking the motor off line. Nothing is stoping you from backing off on that full nameplate rating. If you install heaters that are rated at only 50 percent full load, then your motor will trip off line at 50 percent horsepower output. Not sure why you would wish to do this.

The VFD replaces the heaters and overload relays. Thus, if your VFD will overload trip at say 75 percent motor full load, then your VFD will effectively derate your HP nameplate output by say 25 percent. You simply cannot overload the motor because you have already overloaded the VFD in the process and it will trip out.

The second mode of operation is constant horse power which to me is a bit of an oxymoron. As I increase my frequency from say 0 to 60 cycles, I also ramp up my RPM from 0 to syncronous speed. Now, if I increase my frequency from 60 cycles to 120 cycles, I will effectively double the speed of my motor above and beyond its designed sycnro speed. For example, a 3600 RPM motor can now run at 7200 RPM ignoring slip effects.

But this is not free. The torque output from syncro speed to 2 x Syncro speed is RPM dependent. For small cutters running almost engraving like work in metal, this is not an issue. For a planer or jointer with a massive steel cutter head, this can be a major problem. Any application that needs the torque to work may be hindered by the substantial loss of torque at the higher speeds. But the horse power is constant. A router rated at 3 HP is much smaller than a 3 HP induction motor. The router gets its power from spinning at 20,000 RPM whereas the induction motor gets its power from having more torque at a much lower speed. For example, at 1750 RPM I still get my 3 HP. At 1750 RPM, the router is lost in the back forty!

So how much this torque loss means to woodworking machines remains a mystery to me. I have considered using a VFD to bump my jointer up to 4500 RPM from its 3450 RPM DMD speed. But I dont yet know if the torque loss will turn out to be a kill joy and make the change over worthless.

Hope this helps...

[Andy Howard]

Dev,

Thanks for your input.

A little bit more about my project....I am converting a jet mini lathe from 6 stepped pulleys and an underpowered motor to a VFD and a real 3 phase motor.

I was able to find a siemens VFD that accepts 230V single phase input and outputs 230V three phase. The VFD is only rated at approx 3/4hp, or 550Watts. I am having more luck finding a varienty of 1hp 3 phase motors on ebay very inexpensively. The 3/4hp or 1/2hp motors seem to be a bit more scarce and thus the reason for asking if I can run the 1hp motor at a lower amperage and rating. I dont want to fry my drive!

I know most people would consider this project somewhat useless, but what I hope to gain is Electronic Variable speed, reversing capabilities, a bit more power, and a whole bunch of knowledge about 3 phase motors/power and variable frequency drives.

Thanks for the help.
Andy

[Mike Henderson]

It's been a lot of years since I took the motors course in college but here's what I think would happen.

The kind of three phase motor you're talking about is an induction motor. In an induction motor, the magnetic field in the stator rotates at the line frequency, which is going to be variable because of your VFD (depending upon the number of poles but let's simplify and make this assumption). The rotor will rotate a bit slower than the magnetic field of the stator. This difference in speed of rotation (called "slip") is what induces current flow in the rotor, which, in turn, generates a magnetic field which interacts with the rotating field in the stator to give you a rotating motor.

When you load down the motor, the rotor slows down relative to the rotating field in the stator, which induces more current flow in the rotor, which in turn makes the rotor magnetic field more powerful. But more current in the rotor means more current in the stator.

So now, let me see if I can answer your questions:

1. When you increase the frequency, the motor will run faster. Assuming no load, the amps will stay about the same, maybe going up a bit to account for the increased friction because of the higher speed. With any kind of load, the power needed to drive that load will probably increase at the higher speed, which means that the amperage will increase.

2. If you limit the amperage to the motor, it will not harm the motor but the motor will not develop the name plate power - it will stall below where it would if it were not current limited.

One additional thing I would be concerned about is that the motors you are talking about are designed for 60 Hz operation. You should not have a problem with operation at higher Hertz but there might be a problem at lower Hz because of insufficient iron for the magnets - you might see excess current and heat when operating at 20 Hz, for example. I would tend to worry more if you were operating at full load at low speed than if you were operating at a very low level of load at those speeds. If your VFD is current limited, you'll see lower torque at these lower frequencies but you won't burn the motor up.

But in any case, if I were you, I wouldn't worry about destroying a small three phase motor. They are so cheap, the biggest cost is shipping.

Warning: I spent my career in communications and semiconductors and not power systems. I could be wrong in my analysis. See what some of the other engineers on the board say. Barry - are you reading this?

Mike

[Dev Emch]

Dev,

Thanks for your input.

A little bit more about my project....I am converting a jet mini lathe from 6 stepped pulleys and an underpowered motor to a VFD and a real 3 phase motor.

I was able to find a siemens VFD that accepts 230V single phase input and outputs 230V three phase. The VFD is only rated at approx 3/4hp, or 550Watts. I am having more luck finding a varienty of 1hp 3 phase motors on ebay very inexpensively. The 3/4hp or 1/2hp motors seem to be a bit more scarce and thus the reason for asking if I can run the 1hp motor at a lower amperage and rating. I dont want to fry my drive!

I know most people would consider this project somewhat useless, but what I hope to gain is Electronic Variable speed, reversing capabilities, a bit more power, and a whole bunch of knowledge about 3 phase motors/power and variable frequency drives.

Thanks for the help.
Andy

Hi Andy...
This scheme of variable speed control is becomming the standard in smaller lathes these days. For example, the top end general lathe, the 260, is powered by this very scheme. In the older days, the trick rig was to use a DC motor which supposed to have constant torque but variable speed done through variable voltage. My buddy's modern sliver eagle Oliver patternmaker lathe is done this way. When I slow that beastie down, no mortal man can stop the spindle!

The top end option on the old conover lathe was also done using a DC motor. My dream lathe is a RED EAGLE OLIVER 20-C patternmaker's lathe with the LIMA drive option. I have heard the Zimmermans are better but I have never seen a photo of one.

In the Lima Drive, there is a four speed lima inline transmission attached to the motor. This gives you FOUR speeds. But the drive belt is flat and comes up from below to a four step conehead pulley. So I get FOUR pulley speeds. That gives me a total of 16 discrete speeds. Now, if I can change the motor over to use a VFD, that gives me infinite speed control within each of these 16 speeds. But now you can look at each of these 16 speeds more as speed ranges. It pays to write this stuff down as there will be some speed overlap between the 16 original speed settings and the VFD speeds.

Clearly this may be complex and a bit overkill for most applications. But, it does have the ability to change the torque characteristics for the low end speed settings thereby eliminating the need to drive the motor at very low frequencies.

As for the VFD. The application you have outlined is not a current limited application. In other words, when the motor load exceeds the ability of the VFD to supply voltage and current, the VFD *WILL* attempt to exceed the trip point current output. This excess output will continue for a programable number of seconds. You can setup to make this instant or delay the trip out by say 5 seconds. Some can go for quite a while before the VFD duty cycle cuts in and delines the motor. The point is, your not going to ramp up to a predetermined current output and remain at this level indefinintely. All your doing is backing up the safety trip out point on the motor. You can do this with even greater ease by just changing out the heaters on say your Allen Bradley 709 or 509 starters.

Its the other boundary case you need to be concerned about. If your VFD is rated at 10 HP and your running a 1 HP motor on it, you need to reprogram your VFD trip out for a 1 HP motor inlieu of 10 HP or you will fry your 1 HP motor. No worries. This is done all the time. Likewise, you can do this with your application. But hear this. Your wasteing resources in that the last 0.25 HP of your motor will never be used.

[Rob Will]

I run a VFD on a covveyor. In my application when I speed it up, the machine does more work and amps naturally increase.

I would call the motor manufacturer and inquire as to what min / max RPM's are recommended when running that particular motor on a VFD.

Rob

[Barry O'Mahony]

Mike,

Yes, I read this. You and Dev covered it; besides, my experience with magnetics has mostly been confined to designing transformers, and school was a long time ago. ;)