Motor With VFD Slows Down When Frequency Above 150

I have a Teco 3HP VFD, and I am switching it from a 2HP motor to a 3HP motor. The machine is a belt grinder. I have the VFD set to 167 Hz max because this gives me the my chosen maximum FPM.

Everything works fine until I get up to 150 Hz. Then the motor slows back down again.

Any idea what could cause this?

The motor is a nice American Reliant 1745-RPM job. It's rated at something like 7.5 amps. It's not an inverter-duty motor.

Steve H. Graham said:

I have a Teco 3HP VFD, and I am switching it from a 2HP motor to a 3HP motor. The machine is a belt grinder. I have the VFD set to 167 Hz max because this gives me the my chosen maximum FPM.

Everything works fine until I get up to 150 Hz. Then the motor slows back down again.

Any idea what could cause this?

The motor is a nice American Reliant 1745-RPM job. It's rated at something like 7.5 amps. It's not an inverter-duty motor.

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Hem's sake! It is simply TRYING to not commit suicide and wound the bystanders!

If a motor was wound for 50-60 Hz, it has to be acting like a low-pass filter and BLOCKING current flow at 150-170 Hz frequencies. Most can do 90 Hz, but not all motors will stand even 120 Hz.

Then, too, if the motor is 'not even' inverter-duty rated, it probably lacks a Max RPM figure on the nameplate.

Which max exists - just back in the motor-maker's tech data that they didn't see a NEED to put onto the nameplate for the duty as was expected of it.

Search on "squirrel-caging" of an over-speeded motor. It is not a pleasant event.

Running at 150 Hz ++ you have been B****y fortunate not to be a victim of it.

You need to either resolve the need for speed with geared or belted ratio change, ELSE seek a different sort of motor - Dee Cee, even - that is designed and INTENDED for the range and max RPM you want and sez so right on its nameplate.

They certainly do exist.

Whether is makes sense to even run a belt sander at the RPM sought I leave to others. Thankful it isn't a grinding WHEEL wishing to do homage to the late Henry Shrapnel.


Bill

You could do what you are trying to do if.....

the motor can stand mutiples of the base Voltage at multiples of the base frequency

You have a voltage rated VFD and can supply that voltage from mains power.

But, as Bill has eluded to, Driving Triple frequency at base voltage results in a fraction of the rated torque. And in your example, not enough torque to over come friction. The motor slows.

You are trying to do something the drive and motor combination is not meant to do.

Steve H. Graham said:

The motor is a nice American Reliant 1745-RPM job. It's rated at something like 7.5 amps. It's not an inverter-duty motor.

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You typed it wrong, should read as:

"The motor was a nice......"

Mechanically the motor may not take that much increase, but the reason is that as frequency goes up the voltage to the motor must also go up. At 150 hz the motor is probably at the minimum voltage to excite the field.

Tom

Thanks for the help. You learn something new every day. I thought the bearings were the big consideration.

Any hope I can make it work with a bigger drive wheel? right now it's 4". I suppose I could get down to 120 Hz with a 6" wheel.

Put an 8 inch pulley on it!

Or...put a 3450 motor on it.

Stuart

Let's do the math.

The motor is designed to put out full rated torque at 60Hz. 3HP, = 9ft-lbs of torque (assuming 1750RPM design speed). Also assuming 230V, the torque rating is based upon the V/Hz ratio being 230/60 = 3.833:1. When you increase the Hz to 167, but you cannot increase the V, the ratio becomes 230/167 = 1.38:1 1.38/3.83 = 0.36, so your motor is only able to produce 36% of it's rated torque, so a little over 3.2 ft-lbs. In addition, the PEAK torque capacity of the motor, the torque it uses to accelerate or RE-accelerate a load, is reduced by the SQUARE of the change. So your motor is only producing 13% of the PEAK torque it is capable of (peak torque is usually 200% of full load torque), so essentially you have even LESS torque as soon as you load the motor, about .74 ft-lbs. of torque. So you have gone from 9 ft-lbs to 0.74 ft-lbs of torque by pushing the motor to 167Hz and it can't even accelerate itself, let alone another load.

The issue is, just because the drive CAN make the motor spin faster, doesn't mean you will be able to use it. That feature is for allowing the use of specially designed motors, often called "spindle motors". They will be designed around a higher V/Hz ratio.

A "trick" you can do however, IF your motor is designed as a "dual voltage" 230/460V. Wire up the motor as 230V, but get a transformer to boost the line to 480V, then go buy a 7-1/2HP 460V rated VFD (15HP if you have single phase power). You program the drive to put out 60Hz at 230V, then you can run it up to 120Hz and give the motor 460V, which is still 3.83 V/Hz, just like it wants.

Steve H. Graham said:

Thanks for the help. You learn something new every day. I thought the bearings were the big consideration.

Any hope I can make it work with a bigger drive wheel? right now it's 4". I suppose I could get down to 120 Hz with a 6" wheel.

Click to expand...

Others have commented on reduced torque at 150 Hz, similar with bigger drive pulley. Maybe whoever designed the sander used the slow motor because they thought the torque was necessary. Was the original motor 1745 RPM?

CarlBoyd

I feel like I've heard some BS about grinder FPM and 3-phase motors. People keep bouncing numbers like 5000 FPM and 7000 FPM around, and they say 1750 RPM motors are the way to go. From what I'm hearing here, it sounds like they're not the most reliable sources.

Maybe the best thing is to shoot for around 3500 FPM and see how it goes. I can get that right now at 120 Hz, and I would not need it most of the time.

I really felt like I had researched this. I checked the bearings and confirmed they were not an issue, and I did some math and calculated the additional force on the armature. It didn't seem crazy; about 1.7 times what you would expect at 3600. Now I find out the frequency itself is a problem. I did not see that coming, and apparently I'm not alone.

I can make a bigger wheel. I can't even guess what 8" aluminum round rod costs, though. It would be fun drilling and threading the hole for the set screw. I would have to crown it manually; no CNC.

I couldn't find information on squirrel-caging. I Googled, and all I saw was a motor with a squirrel nest in it.

Anyway, it's fixable. Thanks for the help.

I wonder if someone could answer one more question.

I decided to change the VFD so the maximum frequency was 120 Hz. I found out there are two settings. One is "frequency limit," which is the top frequency the VFD will allow you to enter using the potentiometer. The other is "maximum output frequency." These values were set at different figures.

Does anyone know what the difference is? It doesn't seem to make sense. You would think a maximum output frequency of 120 would limit the frequency to 120, but apparently it does not.

Max output feq is the final word on the several VFD's I use in the shop.
The POT is subordinate.

I can't figure out why there would be two separate settings which appear to control the same thing.

Steve H. Graham said:

I can't figure out why there would be two separate settings which appear to control the same thing.

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I would expect one to 'range' wotever potentiometer was being used.

The other to guard against unintentional overspeed if the potentiometer or the wires to it failed.

Makes sense to ME anyway.

I can't understand why my 2HP motors had no problems with 167 Hz. Electrical problems, that is.

Steve H. Graham said:

I can't figure out why there would be two separate settings which appear to control the same thing.

Click to expand...

They are not the same thing. You have to look at these things in context.

"Max Frequency" only applies to when you are setting up your own custom V/Hz pattern. In the previous two parameters, you selected one of 6 pre-defined V/Hz patters, then if you select "pattern #7" you are telling the VFD that you want to customize your own special V/Hz pattern by defining the minimum, maximum Hz, voltages and a couple of deflection points in between. NONE OF THIS MATTERS if you are in Sensorless Vector Mode, it ONLY relates to when you are in V/F (V/Hz) mode.

So why you would use that: If you bought a spindle motor that is designed for 400Hz operation, 230V, you need to customize a V/Hz pattern that tells the VFD to provide 230V when you get to 400Hz. Otherwise it will get to 230V when you get to 60Hz and the motor will be seriously under-fluxed.

Confusing? This is why it's not as simple as just "telling the motor to go 120Hz". You are dabbling in things that veteran drives people take a long time to learn.

Steve H. Graham said:

Thanks for the help. You learn something new every day. I thought the bearings were the big consideration.

Any hope I can make it work with a bigger drive wheel? right now it's 4". I suppose I could get down to 120 Hz with a 6" wheel.

Click to expand...

It might benefit you to post a couple of pictures of your pulley drive arrangement, the teco model, and the nameplate on the motor(s).

Jraef said:

They are not the same thing. You have to look at these things in context.

"Max Frequency" only applies to when you are setting up your own custom V/Hz pattern. In the previous two parameters, you selected one of 6 pre-defined V/Hz patters, then if you select "pattern #7" you are telling the VFD that you want to customize your own special V/Hz pattern by defining the minimum, maximum Hz, voltages and a couple of deflection points in between. NONE OF THIS MATTERS if you are in Sensorless Vector Mode, it ONLY relates to when you are in V/F (V/Hz) mode.

So why you would use that: If you bought a spindle motor that is designed for 400Hz operation, 230V, you need to customize a V/Hz pattern that tells the VFD to provide 230V when you get to 400Hz. Otherwise it will get to 230V when you get to 60Hz and the motor will be seriously under-fluxed.

Confusing? This is why it's not as simple as just "telling the motor to go 120Hz". You are dabbling in things that veteran drives people take a long time to learn.

Click to expand...

Should be over-fluxed.

Tom

Oops, yeah. Fluxed that up...