Hello all,

I have a rather peculiar doubt. Someone asked me whether it was possible to use an MV drive instead of a soft starter and obtain a soft start? The basic reason for this query is that he wants to use a de-rated MV drive and save on the costs. (the drive will be bypassed once start is obtained)

Some background of the application:

The load is a positive displacement pump. And it will always be started in no load condition. the rating is 11kV, 7.5 MW
Motor GD2= 9856 kg M2
Pump GD2= 70254 KG M2 (I dont know how relevant this data is?)
4 pole motor at 50Hz

Basically, some one has claimed that if the drive acc. time is 60 sec, the motor and pump can be started with a 3.6MW drive.
How can he arrive at this figure? (he above mentioned data is all that is available) And more importantly, will the application work?? I am very skeptical that such a system will be a good option to go for. I think that since the drive will have to run at a lower frequency, this will reduce its efficiency.

Any comments???

P.S. the same person has also claimed that if we increase the acc. time of the rive, the rating can be further reduced to around 1.8 MW.

Hello MaVericK

You can certainly use a drive to accelerate the motor and pump, but the de-rating of the drive needs to be questioned.

As the pump accelerates, the work load will increase up to a maximum at full speed.
The minimum drive size is going to be determined by the work output of the pump at full speed.
If the pump is producing 7.5MW of power at full speed, then the drive must be capable of providing that power.
If the drive only produce that power for a short time and is then bypassed, then you could size the drive so that this maximum power is provided by the overload capacity of the drive.

I the load was purely inertial, then the slower you accelerate the load, the lower the level of torque that is required. In this case, you size the drive for the torque output and you can reduce the drive size considerably.

Be careful bypassing the drive.
You need to disconnect the drive from the motor and then reconnect the motor to the supply. You must not backfeed the drive.
Refer to the manufacturers instructions before attempting this.
Best regards,

To further Marke's response, this is done quite often. People who have experience doing it and the engineering knowledge of ALL of the potential pitfalls are very successful at it. People who try to do it on their own because they saw someone else do it usually end up with very expensive repairs to the drives.

If the "someone" who has made the claims is going to do the engineering, ask them for references of SUCCESSFUL installations, then call those people and ask if they had any problems. If he has never done it, I would not recommend being his guinea pig. Assume that a MV drive is going to cost you a minimum of US$200,000.00. If that is a paltry sum to you, then go ahead. If it seems like a big financial risk, treat it as such.