VSD control on old motor- cooling problem

[Pieterv]

Hey,

We have an application in which an old motor will be foreseen of a VSD control. Local consultants say we have to put another motor because it cannot be cooled sufficiently at lower speed (<50hz). Is that true, or can we put extra cooling to the motors at low speed; in case yes, how is that done in practice???

Thanks for any reactions

 

[Skogsgurra]

There might be more problems than just the cooling.

It is true that external cooling usually is needed below 30 - 40 Hz. It depends on how heavily loaded your motor is. A fan can usually be taken lower before extra cooling is needed because the load torque goes down with speed.

But, you should also consider the insulation. Modern motors are almost universally made with insulation that can withstand the steep edges (and ringing with overvoltage) that the VSD produces (if it is a PWM frequency inverter, most are). Older motors usually have a problem with voltage distribution in the winding - first turns take more stress than they were made for. Motor reactors can help reduce the stress on the windings.

I think that buying a new motor is wise.

Motor reactors and an external blower could work. But be prepared to have some problems. There is also the bearing EDM issue.

 

[Pieterv]

Thanks for your answer,

What exactly is a motor reactor and the "bearing EDM isue"?
Do you have any idea how I can determine the cooling needed for this problem?

Bey
Pieter

 

[Skogsgurra]

Sorry to be so cryptic.

A motor reactor is a three-phase inductor that is connected at the inverter's output. It "breaks the wave" so that the edges that reach the motor winding are less damaging. There are also du/dt filters (they reduce the rise-time of the edges even more) that are more efficient than simple reactors. And, finally, there are also sine filters that (in principle) let only the fundamental frequency through. They cannot be used with vector drives, but are really nice to the motor in case your dynamic needs are limited. Like they are in fan and pump applications.

EDM - Electric Discharge Machining - is a well-known technique in tool shops. It is used to remove metal using a wire or an electrode that erodes metal using electric arcing. The same thing takes place in bearings in some inverter-fed motors. There is a lot said about this on the net. Google for EDM or "bearing current".

Regarding extra cooling; it is very difficult to say anything without having seen the installation or knowing motor size, load etc. It is something that you will have to consult a drive expert about.

 

[bentov]

We've installed a lot of VFD's on existing motors, some very old - mostly they're on pumps where the load is much lower at reduced speed (affinity relationship), so that certainly helps with cooling. We always use line reactors to provide some protection for the drive (not the motor) and to minimize nuisance tripping from power transients.

We figure it depends on how critical the application is (i.e. what are the consequences of downtime?) - if it's no big deal, and the motor can be repaired or replaced easily when it does fail, why not use it up? I point out there's an increased risk of stress or damage to the drive with a motor failure, so it's a gamble if you expect it will happen, but mostly the motors seem to survive, and the drives generally trip off and survive when there is an occasional winding failure. Our experience with the low cost "low road" (use the old motor, load reactor ONLY if the motor run length exceeds mfg spec, no special cooling, etc.) has been generally quite positive in a fairly harsh climate with several different brands of VFD's . . .

 

[Pieterv]

Thank you for the helpful information,

In our application it also concerns a pump, however critical in a cooling process for production.
If we have budget to put new engines, I think replacing them gives us the minimum risk. On the other hand Each pump is double to avoid production problems in case of failure, so in fact the risk is maybe small...


Bey

 

[Marke]

Hello Pieterv
For pumping applications, additional cooling is not necessary except for positive displacement pumps when it may be.

As discussed above however, there can be problems with using an inverter on older machines where the insulation may not be as good as it should be.

Best regards,

Mark Empson

http://www.lmphotonics.com