Running a motor at 0.6Hz

AJZ · May 27, 2009

A small roller conveyor is controlled by a small 1/3hp TEBC motor driven by a PowerFlex 4 drive. A PLC selects the speed via the two digital inputs of the drive. Fast speed is set to 83Hz and slow speed target is 0.6Hz. The conveyor runs at high speed for about 5-10 seconds, then at the slow speed for up to 50 seconds and then idle at a speed of 30Hz. This cycle is repeated as often as every 90 seconds.
Slip compensation is set to 3Hz and boost select is set to 5%CT (Default). THe motor and drive will work as described above but is it OK to run the motor that slow without affecting the durability of the drive/motor? Any changes that will improve the durability?

Christoff84 · May 27, 2009

I didn't think it was possible to run a motor at that speed. Seems like 1 full sine wave every 3 seconds wouldn't even turn the motor.

At 0.6Hz I would suggest putting a seperate blower on the motor. The cooling fan won't do anything at that speed. I'm suprised you haven't burnt up the windings already.

hd_coop · May 27, 2009

That should be fine, you pay a big premium up front for the TEBC motor to be able to do just this....although I am a little suprised that the Powerflex 4 is able to control it that slow.

I've got a couple applications where I need to vary my speed from ~0.5 to 120 Hz and I am using a Powerflex 70 to do so.

ojz0r · May 27, 2009

AJZ said:
THe motor and drive will work as described above but is it OK to run the motor that slow without affecting the durability of the drive/motor? Any changes that will improve the durability?

Well ofcours its not as good running it on 0.6Hz as it is running it at say 20Hz but if you use a separate blower as said above that would be better then nothing.

You might want to change the gearbox if you have one so you can maybe go up 20Hz.

hd_coop · May 27, 2009

ojz0r said:
Well ofcours its not as good running it on 0.6Hz as it is running it at say 20Hz but if you use a separate blower as said above that would be better then nothing.

You might want to change the gearbox if you have one so you can maybe go up 20Hz.

His motor already is Blower cooled. Changing the gearbox probably wouldn't work since he also needs to go up to 83 Hz

Tom Jenkins · May 27, 2009

First, I assume this is a 1800 rpm nominal motor. You need to verify that the motor can handle 83 Hz (2,490 rpm) structurally. If it is a 3600 rpm motor you are probably in trouble over 75 Hz or so.

Second, at 10 Hz and below you will probably experience cogging - instead of smooth rotation you will get motor rotation as a series of jumps. If your application can handle this you might be OK, but it will put extra strain on the entire drive train.

I suggest you contact the VFD supplier and see if they have options to give smooth control at that low Hz.

The Plc Kid · May 27, 2009

Tom just for my knowledge why would you be in trouble with a 3600 rpm motor above 75 hz if a 1800 rpm could possible handle 83 hz?

Why do you typically want to run a motor over 60 hz?

Why not just get a motor with the correct rated rpms or rpms rated higher than the application needs i.e if you need it to run 3400 rpm then gat a 3600 rpm motor and run it below 60 hz to get your 3400 rpm.

I just don't understand why running above 60 hz is needed but i am still very green at motor control methods.

Thanks

hd_coop · May 27, 2009

Tom Jenkins said:
Second, at 10 Hz and below you will probably experience cogging - instead of smooth rotation you will get motor rotation as a series of jumps. If your application can handle this you might be OK, but it will put extra strain on the entire drive train.

I suggest you contact the VFD supplier and see if they have options to give smooth control at that low Hz.

This is exactly the reason I was suprised a Powerflex 4 could do this. I needed a Powerflex 70 in FVC control mode with a bit of tuning to get smooth rotation at those low speeds. The Pflex 4 can't even do sensorless vector.

TConnolly · May 27, 2009

Since you say it works as described I'll take that at face value.

You might gain some improvements with vector control, or if you aren't getting the performance you need then perhaps a servo motor. If you don't need any improvement then sleep well at night knowing that the TEBC motor will be just fine.

TConnolly · May 27, 2009

The Plc Kid said:
... why would you be in trouble with a 3600 rpm motor above 75 hz if a 1800 rpm could possible handle 83 hz?

Why do you typically want to run a motor over 60 hz?

If its a 3600 RPM motor (2-pole motor) then at 83 hz the motor will be turning 4980 RPM. Thats too fast for the bearings and will create high centripetal forces in the rotor.

For an 1800 RPM motor (4 pole motor) the motor will turn at 2490 RPM. Thats fine for the bearings and rotor forces.

However turning a motor above rated speed has a tradeoff. Below speed the motor develops a constant torque and has a variable horsepower. Above speed the horsepower is constant so torque has to fall off.

hd_coop · May 27, 2009

The Plc Kid said:
Tom just for my knowledge why would you be in trouble with a 3600 rpm motor above 75 hz if a 1800 rpm could possible handle 83 hz?

Why do you typically want to run a motor over 60 hz?

Why not just get a motor with the correct rated rpms or rpms rated higher than the application needs i.e if you need it to run 3400 rpm then gat a 3600 rpm motor and run it below 60 hz to get your 3400 rpm.

I just don't understand why running above 60 hz is needed but i am still very green at motor control methods.

Thanks

Tom will be able to provide a better answer than I, but in my application I needed a very wide speed range. The majority of the time my drive is running less than 2-3 Hz to maintain pressure, but while filling or circulating material through the lines, we may need to run quite a bit faster, I take it up to a max of 120 Hz. I made sure to verify before hand that the motor and gearbox were both rated to run @ double speed mechanically.

plc noob · May 27, 2009

Alaric i guess i just don't understand the relaionship between torque and hp i thought they were pretty much the same just a different way of expressing motor capacity.

I will have to do some more reading and digging on this.

Thanks for your reply the speed part makes a little more sense now.

TConnolly · May 27, 2009

Torque is measured in ft-lbs. Horsepower is ft-lbs per minute, 1 HP=550 ft-lb/min. Put simply, horsepower is torque * speed - but in terms of how fast the motor is able to do work. A 1 HP motor can move 1 lb 550 feet in one minute, or 550 lbs 1 ft in one minute.

AJZ · May 27, 2009

THis small roller conveyor suppose to move parts at a speed of 6 inch/minute. A 4-pole motor is used with a gearbox and a timing belt. The gear ratio is maxed out (unless changing gearbox size) and the timing belt drive pulley is as small as the shaft allows.

Since you say it works as described I'll take that at face value

I only had it running at 0.6Hz for about half an hour and I did not check for smoothness of running. Next time I am on site I may check that.

I suggest you contact the VFD supplier and see if they have options to give smooth control at that low Hz

I have asked my AB supplier but I am still waiting for feedback. I also checked the AB Knowledgebase but did not find anything.

I may try a PowerFlex40 using Sensorless Vector Control.

DickDV · May 27, 2009

In my view, the TEBC motor is completely suitable for the application and is probably an inverter duty four pole motor capable of running up to 120hz. Even if it isn't, a four pole commodity motor with a cast aluminum rotor would be good for 90-100hz easily.

The drive is a complete mistake. To run smoothly under varying load conditions under 3hz requires sensorless vector capability at minimum. If you are committed to the AB product line, that means at least a PF40.

I would not expect that an encoder would be necessary. A reasonably good sensorless vector system is adequate.

Running a motor at 0.6Hz

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