Gear motor needs to rotate at much lower than its rated speed constantly. What should we do? 4

[M.Barzegar]

Hi everyone
We have a 0.55KW gear motor in our factory that because of process reasons needs to work at a much lower than its rated speed constantly. The rated electric motor speed is 4Poles/1500RPM and the installed VFD needs to set the frequency to about 20Hz constantly. This motor’s winding got burned the other day and my supervisor says the reason is because this motor worked in 20Hz frequency for a long time
1. Why working in 20Hz frequency would cause damage to an electric motor?
2. Given the circumstances, we decided to rewind the electric motor in 8Poles/750RPM to lower the motor rated speed. But after that, the no load current of the motor has exceeded the rated current on the motor nameplate. Why is that?
3. What’s your suggestion in this situation? What would work best here?
Thanks

 

[Gr8blu]

M Barzegar
1. The motor is (probably) designed to adequately cool itself when operating at rated speed, usually the nameplate speed. If it is designed for gear motor applications, it may also be rated for a duty cycle that is not continuous (i.e., 30 or 60 minutes on, followed by enough dwell time to cool back down to near-ambient temperatures). If the motor is turning too slowly (i.e., at lower-than-rated frequency), the internal and/or external shaft-mounted fan will not move enough volume of air at enough pressure to properly remove the heat through convection - leaving all the heat to be dissipated through radiation, which is not ideal.
2. The amount of steel in the stator and rotor is determined by a level of magnetic flux, which is dependent on both applied voltage and - to some extent - speed. I say "to some extent" because higher-torque machines (which typically come from either increasing the electrical power output while holding speed constant, or by reducing the operating speed while holding power constant). By reducing the speed by a factor of two, you have increased the torque requirement by the same factor of two (assuming power throughput was held constant). There simply isn't enough metal there to magnetize properly - which means you are now severely saturated, which implies very high no-load losses and correspondingly high current draw.
3. Your best bet is to:
a) obtain a machine that is rated for the desired power output at the desired speed, or
b) obtain a different gear reduction unit to change motor speed to the desired output value so it can run at or near design frequency, or
c) install a separately powered fan assembly to force-cool the motor's exterior by providing external convection. Note that the fan power may actually higher than the gear motor draw, given the rating you initially described.

Converting energy to motion for more than half a century

 

[edison123]

At 20 Hz, a 50 Hz motor output will be 20/50 of rated output. Similarly if you rewind it for half the speed, the output will be halved. In such small motors, multipolar winding is going to take a large no-load current.

If you need the same 0.55 KW at 20 Hz, use a 1.375 KW, 50 Hz motor.

Muthu

http://www.edison.co.in

 

[zlatkodo]

M.Barzegar
1.- Motor cooling at 20 Hz (600 rpm) is drastically reduced and that is why the motor overheats.
2.- The outcome of the redesign from 4 to 8 poles, if done correctly, is half the power, so the motor is overloaded.
3.- The simplest solution is to use an existing 4-pole motor with an additional powerful external fan, or just use a new 10-pole motor without a VFD. If it is hard to find such an 10-pole motor, take a 4-pole motor of at least 1.5 kW and make a redesign to 10 poles, if feasible.
Winding Redesign and Motor Repair

 

[TugboatEng]

A TENV motor should be more tolerant in this application as the cooling is primarily through fins on the frame as opposed to a fan. Or, use a motor with a higher rating and use the parameters of the VFD to derate the motor.

 

[davidbeach]

VFDs used for constant speed applications are almost certainly not the best solution. A different, or additional, gear box and a normal starter would be more efficient and wouldn't have cooling problems. If you need a range of speeds, but they're all well less than 60Hz, get a new gearbox that puts the highest process speed at 60Hz and then work down from there. Supplemental cooling of the motor may still be required if there's too much speed reduction at the low end.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[iop95]

Add cooling to motor (external fan at constant speed).
Airflow of internal fan is 10-15 times lower at 20Hz than at 50Hz... so cooling is far away from nominal.

 

[jraef]

Is the motor 3 phase or single phase? If it is single phase, you can all but forget the idea of a VFD.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[LionelHutz]

You have to start at the basics. You need to know the power required at the motor shaft when it is turning 600rpm. Then, you can size the drive.

If the load will only run at that speed then another 2.5X gear reduction would likely be a better solution.

The motor may have reduced in power with the re-wind but the speed reduction is less. Both the original and rewind are capable of 0.22kW at 600rpm assuming enough cooling. You actually gained nothing.
Original - 0.55kW x 2/5 = 0.22kW
Rewind - 0.275kW x 4/5 = 0.22kW