Medium Voltage FVNR Starter to be Replaced by MV VFD

[hanksmith]

I am currently working on a proposal to replace three (3) medium voltage (MV) full voltage non-reversing starters (FVNR) with VFDs.

The motors are 1250hp, 4000V, induction machines connected to large blowers. The client would like to reuse the power cables feeding the motors but I have a feeling they will be too short as the new drives can't be located in the same position as the FVNR starters.

The biggest issue I have thought of so far is to ensure the motors can handle being powered by a VFD (i.e. ensure they can handle the additional heating that can occur and that problems with induced current on the bearings won't be an issue).

One issue the Client mentioned, which I am not familiar with, is to ensure the mechanical blower can handle a VFD. He mentioned the natural frequency of the blower and I am assuming he wants the drives set so that they don't operate at this frequency.

Couple questions I have are as follows:

1. Can anyone tell me if there are any other technical issues I should be aware of when putting this proposal together?

2. Would it be better to replace the power cables in this situation?

3. Thoughts on the clients issue on the frequency of the mechanical load?

Thanks

 

[DickDV]

Regarding your last item on the fan's natural frequency, it will only be an issue if the natural frequency is within the range of planned operating speeds. In that case, the drive will have to be programmed to avoid operating continuously at that speed. Not to worry, probably every drive out there has frequency avoidance (sometimes called critical frequencies, or skip frequencies).

If the natural frequency is above the expected speed range, don't worry about it.

As to the motor leads, the principle issue is the length and whether or not the motor will need additional filtering between it and the VFD. If it does, it might be useful to locate this filter where the old starter was. You said the old leads may be too short so that way the filter serves as a splice box as well. Your VFD manufacturer will be able to advise you on the need for filtering. Give them the lead length and the motor nameplate data.

 

[dpc]

The other issue on the motor cable is that this conductor is potent source of RF energy when on the output of a VFI. If the existing cable is shielded, you are probably OK. If it is old unshielded cable, I'd recommend replacing it with shielded cable. If the cables are enclosed in a steel conduit, this is less of a concern.

It would be good to get an idea on what the desired speed range of operation is expected to be. If someone is thinking they can crank up the blowers to 2x the current speed, there may be trouble.

I assume you're considering the increased harmonic distortion on your entire 4160 V system due to the VFDs. It can be dealt with, but it needs to be covered in the drive spec or in some fashion. If these have captive transformers, this issue is not quite as big a concern.

David Castor

http://www.cvoes.com

 

[waross]

In the early days of VFDs I saw 3" or 4" holes blown out of the scroll cases of centrifugal pumps when the PID control inadvertently ran the pumps at the systems natural frequency. They did in at least two and possibly more pumps before realizing that they had encountered a critical frequency.
For water this is not a vibration frequency but an instance where the timing of the pressure pulses from the impelor blades matches the timing of the reflected pressure pulse returning from the first bend or elbow in a long straight discharge line.
I am not sure if there is a similar effect with air but it may be worth discussing this with the client.

Bill
--------------------
"Why not the best?"
Jimmy Carter