Two speed motor getting a VFD retrofit

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salm10

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Can an existing two-speed motor accept a new VFD? I talked to one person and they thought it could, provided that the windings were set up for the high speed. Another person was not so sure. Anyone have any experience with this?
 
I've don'e it on wye-delta motors. Just pick the high speed winding. Is that what you're talking about? It runs in my mind that there's a torque issue with high starting torque applications that might have to be addressed. In my case, this wasn't an issue. The motor started basically unloaded.
 
I have heard of problems with using a VFD on a motor not designed for a VFD. The problems were caused by current leakage through the bearings, and resulted in very early failure of the bearings. I have also heard that the only ways to prevent this problem are to use ceramic bearings or to use some sort of ground strap/brush on the shaft. Is this no longer a consideration? :-?
 
charlie b said:
I have heard of problems with using a VFD on a motor not designed for a VFD. The problems were caused by current leakage through the bearings, and resulted in very early failure of the bearings. I have also heard that the only ways to prevent this problem are to use ceramic bearings or to use some sort of ground strap/brush on the shaft. Is this no longer a consideration? :-?
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I've heard the same thing, but havn't seen it evidenced in the field. I do notice some replacement motors have lots of decals and marketing literature that say "VFD Ready". I do know that Danfoss, for instance, wants to see certain megger values for old motors that are being fitted with a VFD beforehand to keep the warranty good on the drive.
 
Charlie,
I haven't see the issue in the field either. It is very common for us to put a VFD on a older motor that is not rated for "inverter duty". We have not seen the bearing or winding failure problems and our cables between the drive and the motor almost always exceed 150'. This seems be to another case where most of the information on these types of problems come from a company with an economic interest in helping you solve the problem.
Don
 
don_resqcapt19 said:
I haven't see the issue in the field either. It is very common for us to put a VFD on a older motor that is not rated for "inverter duty". We have not seen the bearing or winding failure problems
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I certainly believe you entirely:smile: , at the same time I have first hand experience with the bearing failure issue. The motors I was working with had 15-20 amps of current measured threw the bearings, however they where 'VFD rated' motors so I suspect there is a lot more to this then one size fits all answer.:smile:

I put a VFD on a very old motor that had been controlled with a large drum switch and resister banks. That worked fine for many years. :cool:
 
having a two speed motor is certainly not a problem in installing a VFD, especially involving an OLD motor. we say OLD motors as these things usually are built to last, unlike some of the newer motors I have seen today
 
the old motors and VFD rated motors will have higher rated winding insulation. The bearing issues are typically a fluke and are a case by case situation.

If you need higher starting torque as Mark suggested, get a VFD with a higher amp rating. Variable torque drives are typically rated at 110% of FLA. Constant torque are rated at 150% of FLA. If more is need to overcome inertia, get a larger VFD and change the overload rating after the ramp up time.

Good luck
 
... overheating

... overheating

I suggest that you make sure the prevailing speed is high enough to let the motor's fan cool the windings.
i.e.... If the older motor were to run at 10% rated speed all day, the windings would likely overheat.
An eralier mention that VFD rated motors have a higher (temp) insulation rating is entirely true.

JM
 
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