Speed up motor

[eskim]

Dear All,

I have very basic question, but make me inconfidence regard to speeding up induction motor. The existing nameplate is 600kW, 3.3kV and 2965rpm. We have to upgrade the feed pump system by installing VSD to control this motor up to max 3400 rpm. (operating will be 3100rpm)

The question is what is the effect of over-speed ?
Can I use the existing or have to replace with other motor?

For detail please see attachment.
Thank for your recommendation.

Regards,
Eskim

 

[jraef]

Using a VFD to over speed the motor will not increase the kW capability of the motor, you are limited to 600kW. Over speeding a centrifugal pump motor WILL increase the LOAD on the motor, by the cube of the speed increase. So from 2965RPM to 3100RPM isn't much, roughly 4.5%, but that means the load power requirement will increase by about 14%. Assuming you had some head room in the original design and your calculations in the attachment are correct, you are only increasing the motor load to 3% over it's maximum rating. But you are also running on an inverter, which itself is increasing the thermal stress on the motor.

•Required pump power at 3100 RPM and 142 m3/hr is 618 Kw. The existing motors are 600 Kw. We need to evaluate the long term affects of motor overload of ~ 3%. Since the motor is turning at a faster speed the cooling air flow will be increased, this increased cooling will offset some of the impact of higher motor load.

I'm not sure the last part of that is valid. Some motor cooling fan designs are somewhat flat, so at the increased speed I'm not sure that you will have any significant added cooling. So increased load beyond rated, inverter operation, no additional cooling... smells like eminent failure to me.

One thing you could try, albeit expensive, would be to boost the line voltage by enough to give you that 3400RPM speed. A motor needs a constant V/hz ratio in order to provide constant torque, your problem as I described earlier is that once you reach full voltage you can no longer keep the V/Hz ratio constant as speed increases. But if you COULD continue to increase the speed and voltage together, you can get more power from the motor. So if you can increase the line to about 3800V, and the VFD and switchgear etc. can take that, then you can program the VFD to provide 3800V at 3400RPM, and the torque will remain constant, the kW output will track and you can run your motor at 3100RPM easily. How you would get there is problematic, but that's a possible route that is likely less expensive than changing the VFD and motor.

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[petronila]

Good point Jraef

The voltage increase could be possible, the motor is feeded from one reducer transformer and the output ratio can be increased moving the tap changer, maybe taking a look of the transformer load conditions this voltage rise could be done.

Regards

Carlos

 

[powerjunx]

agreed to petronilla..tap change will do.


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