Electric motors being overdriven and freewheeling 1

[matthew109]

Hello, I have 2 queries of 2 parts of our machine we are looking at redesigning from currently hydraulic to electric.

freewheeling - we are considering replacing our hydraulic motor with a Variable Speed Drive connected to an electric motor (400VAC) for example a 12.5kW 3 phase 4 pole aluminum framed motor, this would have closed loop control to maintain a speed. When the system is shutdown and the power is turned off to the motor, as the motor is connected to the shaft of a large rotor, it would continue to freewheel for up to 30 minutes.
Would this cause a problem to the electric motor with the motor continuing to spin without power applied to it?

Second query is overdriving an electric servo motor,
We have a hydraulic motor connected to a turntable that rotates it 90 degrees in either direction every few seconds. The environment causes this turntable to move as well so we also have an independent auxiliary hydraulic brake circuit connected to this same turntable that does braking and reduces its maximum velocity when required. This brake circuit has an inline cooler that takes the heat out of the braking system. We are looking at replacing the hydraulic motor with an electric servo drive and retaining the hydraulic auxiliary braking circuit (as we have a kW of braking and heat generated that we can easy remove with our oil to water cooler).
My query is if we don’t what this electronic servo drive to do any braking, when it is being over-driven, can a servo drive cope with this condition and what would be the best way to minimise the heat generated from the electric servo drive and leave that to the auxiliary brake circuit?

Appreciate any help or comments.

 

[mikekilroy]

query 1: if you meant freewheel when you turn off power to the DRIVE, then should be no problem at all. You did not say motor would freewheel FASTER than its max mechanical limit speed rating - which would be a problem.

query 2: too little info. most servo drives have pretty good control parameters for tweaking braking limits, but too many undefined things to say 'no problem.' but why not let the servo do the braking too? it is really really good at that! I'd strongly suggest you contact app engineers at the servo mfgr of your choice (or a few for more opinions since not all servos are built alike) and let them guide you in choices they have. I would not sell you a servo system for this based on the info stated so far, even if you told me the torque and speed you wanted. Too many unknowns.

 

[electricpete]

If the motor is restarted while rolling, it might require some some attention in vfd setup to address the starting conditions (not a typical ramp up from initialzero speed)

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(2B)+(2B)' ?

 

[waross]

I believe that it takes a VFD with special circuitry to restart a spinning motor "on the fly".

Bill
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"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Be aware that an electric motor and a hydraulic motor have totally different torque-speed characteristics. This applies to both driving a load and braking a load.

An induction motor's speed-torque curve is symmetrical with the axis of symmetry at the synchronous speed, so you can easily extrapolate how the motor will behave with an overhauling load.