WRM with VFD

[W@ttson]

Hello,

I have a crane hoisting application that has really large inrush and starting torque requirements. The existing system is a pair of WRMs being controlled by old thyristor drives. The two motor/thyristor drive pairs are setup to be synced to lift a heavy load evenly.

As it is being increasingly hard to get replacement parts for the thyristor drives, I want to replace them with VFDs for about a 2 year duration until we can get the infrastructure in to replace the motors and gearboxes.

I know the insulation rating of the WRMs is likely not rated for the CIV of the drives but I only need them to last a couple more years.

I plan to keep the fixed resistance in the rotor that the existing thyrister drive uses so that I can get a more stable low speed operation with the VFD and WRM combo.

I plan to directly connect the two VFDs so that they can communicate with each other.

Anything else to be concerned about?

 

[winnie]

You might consider output filtering on the VFDs to protect the motor insulation.

The WRMs could be re-used as 3 phase variable autotransformers. List them here before you scrap them

-Jon

 

[zbang]

How big are these motors? IMNSHO scrapping a good WRM is a crime; controls come and go but those just seem to keep working.

 

[W@ttson]

How big are these motors? IMNSHO scrapping a good WRM is a crime; controls come and go but those just seem to keep working.

They are 50hp.

We work with a few motor manufacturers that still make new ones if you ever need one.

 

[Jraef]

If it's for hoisting, you need VFDs that have "torque proving" capability.
Even for 3 years, I would invest in some sine wave filters, my experience with old WRIMs on VFDs is that the insulation can't handle it. At he very least, DV/DT filters.

Also, you will have to short out the rotor windings, and you will want to do that on the rotor, not after the brushes. So that means pulling the motors and sending them to a motor shop in most cases.

 

[W@ttson]

If it's for hoisting, you need VFDs that have "torque proving" capability.
Even for 3 years, I would invest in some sine wave filters, my experience with old WRIMs on VFDs is that the insulation can't handle it. At he very least, DV/DT filters.

Also, you will have to short out the rotor windings, and you will want to do that on the rotor, not after the brushes. So that means pulling the motors and sending them to a motor shop in most cases.

Why on the rotor itself? I was reading somewhere that you may want some resistance on the rotor to have better control at lower speeds for high inertia loads. I was thinking about shorting out by the resistors.

 

[zbang]

Why on the rotor itself?

So you don't have to maintain the brushes, I expect.

 

[Jraef]

So you don't have to maintain the brushes, I expect.

Exactly. Plus as the brushes wear, it changes the rotor circuit resistance and the VFD tuning becomes invalid. VFD tuning is critical on WRIM retrofits because the VFD does not have direct control of the rotor current, and stator current can be really high if everything isn’t perfect.

 

[Jraef]

Why on the rotor itself? I was reading somewhere that you may want some resistance on the rotor to have better control at lower speeds for high inertia loads. I was thinking about shorting out by the resistors.

That usually applies to retrofitting a Soft Starter to a WRIM.