Hidraulic Motor vs Electric Motor VFD driven

[petronila]

Hello friends,

I would like to know how to make correct electric motor power calculation when you replace a hidraulic motor with one electric vfd driven.

If we have 7 KW 300 RPM load requirements: Can I use a 750 RPM(8 poles-50 Hz) motor and decrease the speed up to 300 rpm ( 20 Hz), then for constant torque aplication calculate the new KW at 750 RPM like KW = 7 x 750/300 = 17.5 KW?

Thanks for the inputs

petronila

 

[Skogsgurra]

The principle is right. But you may need to look at cooling at the 40 % of rated speed that you will be running at. Some induction motors, and I think that an eight-pole motor is more sensitive than a four-pole motor, need full cooling across the whole speed range and a shaft driven fan will not work well at 300 RPM. A separately driven fan will be needed.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[potteryshard]

Or use a four-pole motor with a 5:1 in-line gearbox. The desired 300 rpm output will put thte motor at synchronous speed, so cooling will be effective, and the VFD can still fine-adjust speed as needed.

 

[ScottyUK]

Bear in mind that a hydraulic motor can produce very large torque at standstill and maintain it through the speed range. The speed-torque curve is much flatter than that for an induction motor.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

For a given HP the motor physical size and the price both increase as the number of poles increases. You will be using a motor that would be large and expensive even before the size was increased (750/300) 2 1/2 times.
I heartily second the suggestion for a gear-box.

Bill
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"Why not the best?"
Jimmy Carter

 

[petronila]

Dear all

Thanks for the inputs

A 8 pole motor and a gearbox are spensive, then I will propouse a solution including a 4 pole vfd driven motor with a load beltdriven with pulleys witha 1:2 ratio

Carlos