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

Author

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

nayjdk@juno.com

2006-04-27, 1:21 pm

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???

Salmon Egg

2006-04-27, 5:21 pm

On 4/27/06 9:07 AM, in article
1146154022.500697.81000@u72g2000cwu.googlegroups.com, "nayjdk@juno.com"
<nayjdk@juno.com> wrote:

> 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???
>
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

daestrom

2006-04-27, 6:21 pm

<nayjdk@juno.com> wrote in message
news:1146154022.500697.81000@u72g2000cwu.googlegroups.com...
>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???
>

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

SQLit

2006-04-27, 11:21 pm

<nayjdk@juno.com> wrote in message
news:1146154022.500697.81000@u72g2000cwu.googlegroups.com...
> 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???
>

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.

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