1. Home
  2. ⏚ Electrical Engineering
  3. VFD's effect on the output power rating of a motor??

VFD's effect on the output power rating of a motor??

I have a 3750rpm, 460V, 60hz, 7.5kw motor that is rated for a VFD. Through the use of pulley's, the final drive speed is 7000rpm, without the use of a VFD. I need to reduce this final speed to 2000rpm. I have been told that with of a VFD, running the motor at 20hz, I should be able to achive this but the motor's output power would be reduced to

2.5kw. This does not seem possible, if a motor is rated at 7.5kw then it will stay a 7.5kw. I realise that I will acheive more torque with the frequency reduction and the amount of power needed to acheive my target torque output will be less but the actual rated output of the motor should not change?? Can anyone shed some light on this subject???
Reply to
nayjdk
Loading thread data ...

You have been advised well! You do not appear to understand why. I will not try to educate you much. At 20Hz, your VFD should apply 1/3 of the rated voltage or about 153V. Torque will be about the same as at 460V 60Hz.

Bill

-- Ferme le Bush

Reply to
Salmon Egg

When using VFD control to reduce the speed, the controller has to also reduce the voltage. Most are designed to maintain a constant Volt/Hz ratio down to almost dead stop. But the amount of torque that can be developed in an induction motor is related to the magnetic flux in the air gap. As the speed control is run down, the air gap flux stays about the same. So the amount of torque that can be generated stays about the same.

You would *like* the torque to go up with a reduction in speed, but that isn't how induction motors work on VFD.

1/3 the speed, yet the same amount of torque means about 1/3 the power.

daestrom

Reply to
daestrom

I worked around a office of an important man. The air handler/vfd was running at

20-30% of speed due to the noise of the air handler. Running the motor at these speeds created a howl in the motor that was worse than the air handler at 100%. The FIX you say. Some rocket scientist put a box over the motor. Then the motor burned up.

My solution, change the pulley ratio so the fan ran slower. The motor would run closer to 100%.

Noise when away, replacing the motors went away. Big shot was happy.

Change the pulley ratio and run the drive above 55% you will be a lot happier.

Reply to
SQLit

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.