# Why does this three phase induction motor have two kW ratings?

I came accross a three phase induction motor with two kW ratings written on its nameplate against the two frequencies 50Hz and 60Hz.

• Does the kW rating increase with frequency?
• Shouldn't the kW rating depend upon the design of the motor?
• Why does current rating change, too?

• It does show star & delta figures so design is covered, which changes the current. Dec 18 '20 at 5:20
• Motors are limited by peak torque. Output power is proportional to torque * speed. More speed = more output power. So it would make sense that the same motor would be rated for more power at 60 Hz compared to 50 Hz. However, I can't see the 50 Hz power rating in the picture so I am not sure I am correct. Dec 18 '20 at 5:52
• Same current = same torque; higher F = higher speed. Same torque * higher speed = higher power. Dec 18 '20 at 12:58

## 3 Answers

Here are 2 charts, for 50 Hz and 60 Hz, derived from the name plate for clarity.

The nominal kW rating at 50 Hz is not visible on the name plate (appears to be covered by the rivet).

The following comparison is the result of new data for the power rating at 50 Hz (11 kW).

The higher power input would be required at 60 Hz to obtain around the same torque at the higher speed.

• Those are output power ratings, I do believe. Therefore, your comment about iron losses seems to be out of place. Dec 18 '20 at 23:03
• @mkeith, Yes, they are indeed output power ratings. However, input power at 50 Hz = √3 * 400 * 20 * 0.88 = √3 * 690 * 11.6 * 0.88 = 12.2 kW and at 60 Hz = √3 * 460 * 19.8 * 0.89 = 14 kW. Dec 19 '20 at 3:19
• so the efficiency at 60 Hz is 12.2/14 = 87 percent. Likely efficiency at 50 Hz is similar. So probably the output power at 50 Hz, 400 V delta is around 0.87 * 12kW = 10.4kW. If, indeed, the efficiency at 60 Hz is lower, then it could be a BIT higher than 10.4kW. But not too much. Dec 19 '20 at 3:23
• @mkeith, To be exact, the efficiency at 60 Hz would be 12.6 / 14 = 90% and at 50 Hz could be even higher (the efficiency of an induction motor could be as high as 97%). Many thanks. Dec 19 '20 at 4:45
• The hidden KW rating is 11kW, I found it after removing the rivet Dec 19 '20 at 5:20

The strength of the motor's magnetic field is determined by the voltage divided by the frequency. If the frequency is increased by 20% from 50 Hz to 60 Hz, the magnetic field will have the same strength if the voltage is also increased by 20%. That is approximately what is shown on the nameplate. Operating the motor at a higher frequency with the same magnetic field strength should result in the same torque capability at the higher frequency. That should result in the same current and the same slip RPM at rated torque at 60 Hz vs 50 Hz.

The motor's synchronous speeds are 3000 RPM at 50 Hz and 3600 RPM at 60 Hz. The rated speeds are 2940 and 3540 RPM for 50 and 60 Hz resulting in 60 RPM slip at either frequency. That is another indication that the rated torque is the same for either frequency.

The power marked on the motor nameplate is the mechanical output power.

Output power (kW) = Torque (N-m) X Speed (RPM) / 9.549

The rated speed at 50 Hz is 83% of the rated speed for 60 Hz, so the rated power at 60 Hz should be 83% of 12.6 kW or 10.5 kW.

The nameplate indicates that the motor has a normal operating range of voltage variation resulting in a range of operating current variation. That would suggest a similar range of normal operating torque and power.

At 60Hz the rated voltage and current is higher than at 50Hz that will increase the torque.

The 60Hz motor will rotate 20% faster than the 50Hz motor.

Power = $$\ NT\over 5252\$$ So increasing torque and increasing speed will increase the power.

• At 60 Hz, only voltage speed are higher, current and torque will not increase. Dec 18 '20 at 18:24