How to re-rate a motor from 440 v/60Hz to 380v/50Hz

[JoeNx]

We are shipping a motor from this country, where it was rated at 100 Hp at 60 Hz/440 volts, to Saudi Arabia where it will run at 50 Hz/380 volts. What horsepower will the motor be "re-rated' to, for use in that country? (I am a mechanical with an electrical problem) Thanks in advance.

 

[Marke]

Hello JoeNx

This has been covered a number of times before and a search of the forum will bring up appropriate threads.
If you reduce the frequency applied to an induction motor, you have to reduce the voltage by the same ratio. i.e for a reduction of 60 Hz to 50Hz, you multiply the voltage rating by 5 divided by 6 to give the voltage rating at 50 Hz.
The shaft torque remains the same, so the power reduces by the speed ratio also.
If your motor has a maximum continuous voltage rating of 440 voltas at 60Hz, it is suitable for 366 Volts at 50 Hz. For 380V 50Hz operation, the motor would need to have a voltage rating of 456 V at 60Hz.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Skogsgurra]

Yes. So it is.

380 V is only 3 - 4 % more than 366 so it will probably be OK to run it directly off your 380 V mains.

But, as you probably know, there are no 380 V grids in Europe anymore. It is 400 V nowadays. So, you better check what voltage you really have. Outside EU 380 V still exists.

Gunnar Englund

http://www.gke.org

 

[jraef]

And is that motor really rated 440V? That is somewhat unusual now unless it is an older motor. Most new motors in the US are now rated 460V. A 460V 60Hz motor works fine on a 380V 50hz supply, other than the 20% loss of speed. Even if the supply goes to 400V it is still within the +10% tolerance of the motor design.

If the motor nameplate actually says 440V then you will be overexciting it and it will run hot, as well as slower.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[JoeNx]

Thanks, it is a 460 volt motor, I checked.

 

[DickDV]

On 380V or 400V 50Hz, your motor will now be derated to 100 x 5/6 = 83.3hp. This is entirely due to the loss of speed since the available torque will be the same at either V-freq service.

 

[petronila]

joeNx,

If you want to know about output motor due to changes in frecuencie and voltage you can use this formula:

KW= (N-m x RPM)/9549. where KW: Kilowatt (HP/1.34)- RPM: Speed and N-m : Torque. Assuming you have a 4 Pole Motor, the Synch RPM is 1800. RPM Synch = 120 x F / Poles
In your case the Voltage/Frecuencie Ratio will remain constant and N-m (Torque)too.For it the 60 Hz Torque will be N-m (60 Hz)= 75 x 9549/1800= 397.875

For 50 Hz operation the new RPM will be 1500 then the New KW = 397.875 x 1500/9549 = 62.5. So this confirm why you have to re-rated the motor´s power using a factor of 5/6 (0.833).Some important aspect Due to the speed reduction is the motor´s cooling, at 50 Hz the motor will run slower and the motor´s fan too, so wach up the operating temperature.

Regards

Petronila