# what are the consequences of running large electric 60 Hz motors (more than 100 hp) on 50hz current?

Relevance
• EE68PE
Lv 6

The motor needs to be connected to the proper voltage for 50 Hz operation. That voltage is five sixths (83%) of the nameplate voltage for 60 Hz operation. If it is a 460 volt, 60 Hz motor, it should be connected to 380 volts for 50 Hz operation.

The motor will run at about 83% of the nameplate speed for 60 Hz operation. If it is a 1750 RPM motor, is will run at about 1450 RPM at full load. The synchronous (no-load) speed will be 1500 RPM instead of 1800 RPM or 3000 RPM instead of 3600 RPM or 1000 RPM instead of 1200 RPM depending on the number of poles the motor has.

The motor will be able to provide the same full-load torque at 50 Hz as it does at 60 Hz. The current at full load will be about the same at 50 Hz as it is at 60 Hz. The power produced at full load will be about 83% of the 60 Hz nameplate rating because of the speed reduction. Power is proportional to torque X speed.

Using a 60Hz motor in 50Hz duty.

1) It will turn 20% slower.

2) Cooling will drop dramatically.

2) The load's horsepower requirements will drop, possibly dramatically.

4) V/f will increase possibly causing an overexcitation of the motor windings and an increase in current draw (see below).

Make sure the load will still do what is required of it when running slower. For instance, a fan load would now move less air, a pump will pump less fluid etc. Still enough?

The motor cooling will reduce dramatically. Is it enough to matter? This depends on how the load has decreased. If the hp requirement dropped because the load has decreased the current will drop and less heat will be released internally. You would want to take temperature measurements until the motor reaches a steady state temperature running in its new frequency application.

The motor's hp will drop because hp is a function of speed x torque. The motor's torque doesn't change but its speed has dropped so it is now a lower hp motor. If you change pulley sizes to return the load to its original speed your motor will likely be undersized, possibly seriously. Example: A 10 hp motor is now an 8 hp.

The most serious issue is the V/f issue. The V/f will increase. Likely enough to cause a large increase in the motor's current. This coupled with the reduced cooling may cause rapid overheating. However the V/f problem can be fully mitigated! You reduce the voltage to the motor by the amount required to return the V/f back to its original value. This removes the hazard of excess current from an increased V/f.

Example: A 60Hz 460Vac motor is going into 50Hz service.

V/f = 460/60 = 7.6

So if V/f x f = V then by plugging in the new frequency of 50 we see:

7.6 x 50 = V

V = 380

Running the motor at 380V at 50Hz will remove the V/f problem

• 5 years ago

For most AC motors, you will need to de-rate the applied voltage magnitude since at a lower applied frequency the magnetic flux in the machine increases which can increase currents in the windings. Some motors are designed for a 50/60Hz range and work fine on both frequencies, but check with the motor nameplate or manufacturer. So, for a machine designed for 60Hz running on a 50Hz system, set the applied voltage to (50/60)*rated_voltage. OR, there are solid-state induction motor drives available that can take single phase AC at 50Hz and convert that to a variable frequency voltage at the output (I think you can vary the voltage magnitude as well). So just adjust the output to 60 Hz. This might be a viable solution, albeit a little costly and there will be some power losses within the drive.

• Anonymous

The motors will run slower 1750 rpm for 4pole motors will rotate 1450 rpm.The motor current will come out higher than what is required. 1Hp = 0.746 kw I = 150[0746]/V{1.732}for a 50 Hz motor since voltage will also be 380 volts only.

• .
Lv 6

the motors will run slower