3450 RPM Motor at 1750 RPM

[PUMPDESIGNER]

Is there any long term problem running a 3450 rpm submersible motor permanently at 1750 rpm with a VFD, so long as sufficient water is moving to cool the motor?

2 Pole motors are much less expensive than 4 Pole.
I have to have the VFD anyway, why not save money on the motor?

PUMPDESIGNER

 

[mc5w]

Your main problems will be lubricant circulation in the bearings and cooling oil circulation within the motor. The latter transfers heat from the rotor to the stator.

At 30 Hertz you will only have 50% horsepower and 50% voltage.

You best be is to ask the pump manufacturer and motor manufacturer if reduced speed running will impair anything. There are likely some people who are running their submersible pumps variable speed to reduce water hammer when starting and stopping and to reduce the number of on/off cycles.

Mike Cole mc5w at earthlink dot net

 

[aolalde]

An induction motor must keep the Volts/Hertz ratio close to a constant in order to avoid saturation of the magnetic circuit. Then at 30 HZ you have to reduce the voltage one half. That will produce constant torque “T” (Lb-Ft) but your power (HP) will drop proportional to the shaft speed.

HP= T*rpm/5250

If a pump demands 50 HP at 1750 rpm, a standard 50 HP, 4 Poles, design B, induction motor, 60 Hz will handle that load; however you will need 100HP, 2 poles, 60 Hz to provide those 50 HP with 30 Hz.

I think you will not save a penny.

 

[dbyres]

Can anyone tell me what happens to the amperage draw as you slow the motor down in an application like this? We have a centrifugal pump requiring 10 HP at 2370 RPM. So we selected a 15HP 3450 rpm motor to run at 41 Hz. However the amperage draw is much higher than expected about 21 Amps at 460 volt.

 

[aolalde]

dbyres (Mechanical)

Did you reduced the voltage proportional to frequency?

example; if the original motor was 460 Volts- 60 HZ
at 41 HZ Vn = 460 *41/60 = 314.3 Volts

Other way the magnetic circuit reaches saturation increasing the magnetizing current and finally the total current increases too.

 

[dbyres]

We are using a Baldor VFD, do you know if these automatically reduce the voltage or do we need to program it for reduced voltage output.

 

[aolalde]

Baldor has a wide variety of inverter drivers. Most of them have a microprocessor to set up the operating parameters. Look for V/Hz ratio, linear (or squared reduced), base frequency, output voltage, minimum frequency limit, maximum frequency limit etc.
Better check the instruction book.

 

[aolalde]

The Motor current should be proportional to the HP and inversely proportional to the voltage.
The input to the motor for 10 HP:
I ~ 10*746/(1.732*314.3*.8) ~ 17 amps at the motor input.
(+/- 1 or 2 amps due to harmonics)
The input to the driver, considering driver EFF*PF= 0.95, should be around:

I1 ~ 10*746/(.89*1.732*460*0.95) = 11.07 Amps at 460 V, 60 HZ

 

[DickDV]

If you have sized the motor to operate the pump properly at 3450rpm, then you can surely operate the pump-motor system with that same motor at 1750rpm.

At half speed the torque is one-quarter that of full speed and the horsepower has dropped to one-eighth of full speed.

The bigger question is whether the pump and motor package is designed to operate at half speed. That info must come from the pump manufacturer.