We need to install a 100 hp motor 380 volts using the existing 13.8kv powersupply.

I have a spare200kva stepdown 13.8kv/380 volyts Dyn11 transformer which i wish to use it for this project.

i will be using a softstart.

My question is about sizing of X-mer , whether 200 kva transformer can take care of 100 hp motor or is it undersized?

chaterpilar

It Is UNDERSIZED IF You Are Going To USE a Soft-Start AND you cannot set the current limit below 400% of motor FLA.

Neglecting losses in the transformer and wiring, calculations show you need about 365 kVA to support 400% current. That doesn't mean that you cannot overload the 200 kVa transformer for short time demand. BUT, as you increase the load on the transformer, you will reach a point where the transformer output voltage sags and will impair the operation of the Soft-Starter. How much will it sag ?? That depends upon the impedance of the tranformer and the copper losses in the wiring.

Below 400% current, you may not develop sufficient motor torque to start the motor + mechanical load ( you havent't supplied any application detail) ;

If you have to use that transformer, you might want to consider a VFD instead of a Soft-Start.

200 kVA would be more than adequate for a 75 kW 380v motor supplied by a VFD.

Given that softstart is the only option then what would be the recommended transformer size...( 500 kva?)

chaterpilar

First, I'd suggest that you select the supplier of the Soft-Starter.
In that way, you'll know what the limitations of the unit are.
You can discuss the application with them, and determine the
actual starting torque needs.

Second, once you have done the above, and selected the
particular manufacturer and model Soft-Start, you can ask
them for their recommendation on how much kVA you need
to have for the application.

Seems like a "solid" approach to an application problem to me.

Good luck.

I would go about it in a different order,
First establish what the driven load is and how much torque for how long is required to start it.
Second, I would determine the motor characteristics and then calculate the start current required to develop the required starting torque.
Once you know the start current and time figures required to start the application, you can then ensure that you select a soft starter capable of achieving these figures. You can now determine the suitability of the transformer for your application.

If you exceed the transformer rating during start, provided that it is only for a short time, the only consequence will be an increase in voltage drop.
Provided that you know the start current requirements and the transformer impedance, you can calculate the voltage drop during start.
NB Some soft starters are not very tolerant of voltage drop more that 10% during start.

Best regards,

Thanks Jomega and Marke for your valuable inputs.

The motor is starting a centrifugal pump for irrigation.

We have an existing setup which is similar and is drawing 390 amps at startup and settles to 120 amps while running.

Will 200 kva X-mer be able to take 390 amps for 3 secs?

chaterpilar

Hello chaterpilar

Typically a submersible pump will require 250 - 300% start current and a surfacepump of this size will require 350 - 450% start current depending on the starting characteristics of the motor.

If we assume a round figure of 400% current, then you may have a start current as high as 480A.
The 200KVA transformer would probably have an internal impedance of around 5% giving a voltage drop of 5% at rated load.

I am assuming that your supply is 400V 50Hz. The 200KVA transformer has a current rating of 290A
If your start current is 480A and the transformer rating is 290A at an impedance of 5%, then the voltage drop during start will be in th order of 5 x 480 / 290 = 8.37% which I would not expect to be a problem.

Make sure that this overload will not make problems with the transformer protection. It would not normally be a problem.

Best regards,

Thanks Marke for your prompt reply.

Keep up the good work.

Chaterpilar