change Wound rotor motor to VFD fed induction motor

[ePOWEReng]

There are existing 1600HP wound rotor motors driving centrifugal pumps at a water pumping station. There is a push to convert the motor to a VFD driven induction motor.

The VFD cost is high. The thought is that reliability would be improved. Also maintenance on the slip rings would go away.

I was wondering if anybody has data on reliability for a VFD-induction motor compared to a wound rotor motor? Is converting a wound rotor motor to inductor motor worth the cost?

Thanks for the input.

 

[dpc]

I'm not entirely clear if you are talking about buying a new motor or not. The wound-rotor motor is an induction motor and can be run from a VFD if the slip rings are shorted. We have done these conversions in the past.

The voltage stresses on the motor windings might be a concern, but the old motors could run a long time on a new VFD.

 

[ePOWEReng]

I am referring to using the existing motor and shorting the slip rings. My question is trying to justify the cost of the VFD.

thanks.

 

[jraef]

Compared to what? Electro-mechanical resistor control or liquid rheostat control? Most people find that it pays to replace the resistor control schemes because of the long-term reliability issues of not having to deal with the resistors and switching devices. Most people with liquid resistance controls tend to stick with them, as long as they have people on-board who understand them.

One can also make an argument that VFD controls will save a little extra energy as well.

But here we go... 3...2...1... launch objections to that last statement!

 

[dpc]

The efficiency question is going to depend on how the existing pumps are operated and how much speed reduction the rotor resistance is doing. If the motors are burning up a lot of heat in the resistor, the VFD operation could be much more efficient. If the motors spend most of their time at full speed, not much saving.

The VFDs are getting pretty reliable - but 1600 hp is a big drive. Is this a 4000 V motor?

I agree with jraef that most installations where money is a big issue stick with the rheostats or resistors until the thing just falls apart or they can't make them work anymore.

 

[roydm]

I seem to recall that slipring motors have a superior starting torque when the rotor resistance aproaches reactance, would you loose that with a VFD and shorted rotor? Perhaps starting torque is not an issue in your case.
Some of the sliprings I worked on as an apprentice had a hand switch to short out the rings once up to speed thus reducing wear on the brushes. I wonder if it's possible to get a solid state device to do that.
I agree with Jraef, liquid starters are cool!

Roy

 

[jraef]

You can't use soft starters on WRIMs without having some resistance in the circuit when starting, but because a VFD controls everything, you can get it to perform just as well by permanently shorting the rotor connections, preferably on the rotor itself so that the brushes become irrelevant.

I should have qualified that savings issue a bit more, dpc is absolutely correct. If you are running at full speed all or almost all of the time, a VFD will actually cost you MORE than a sorted WR controller.

 

[ozmosis]

I would check the insulation class of the motors as a matter of course. Class F insulation is advised. If you are looking (as you shouldbe) at reliability and lower maintenance, then the last thing you want is the motor to fail due to the introduction of a VFD.
If the calculations on ROI are done with the choice to use a BFD on the application, it may be viable to investigate some level of output filter between VFD and motor if the insulation is suspect or simply rewind (yeah, 'simply'!) the motor to higher insulation class. Do your ROI calcualtions on each scenario, including just replacing the motors.

 

[ozmosis]

oops, "BFD"? meant VFD of course..

 

[jraef]

Hey, no BFD...

 

[ScottyUK]

1600HP is a BFD...

The thought is that reliability would be improved.

Do you have reliability problems now? What are they? Big drives bring their own maintenance requirements: the cooling fans and their filters, especially in a dusty or dirty environment, are a maintenance overhead; electrolytic capacitors are a relatively short lifetime item; a large unit may require auxiliary cooling plant.


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

 

[QualityElectrical]

Is this existing motors 2000hp?