How Can I Choose a Correct Power Transformer in our Existing Workshop?

If I understand correctly, your 380/3 loads total only to 12kw, which would be about 15kva (assuming 0.8 pf). This is considerably less than the already existing 220/3 loads. Therefore, I would consider a 15kva 220/380 step-up transformer (if there is enough "headroom" on the overall 220/3 system). The 380v main breaker would be on the order of 30A, but I haven't actually calculated it.

Given the relatively small power requirement for 380V I would look very closely at the cost of 4 new 220V motors against the cost of a new transformer along with it's associated equipment.

I have to agree with TonyS on this one. If you can find the same frame size, HP/KW 220/3P motors. I think it would be a much easier job and more cost effective to swap out the motors than change your electrical infrustructure of your current system.

Thanks sir Jimh, but I think we need to use a 380volts supply for our machines instead of 220volts (posible to insist) because it must be used in heavy loads. Furtunately, we will used the 380volts machines when we are going to transfer into our new workshop permanently which has a power supply of 380volts. Therefore, we need to used a 380volts transformer(step-up). So, is it posible to used 15kw/380volts transformer?

Thanks!..

just step up only for the 4 new machines which based on what you have told me is about 5 anps each. so a 100amp 380v 3 phase panel will do.

so 4 X 3KW = 12KW

12KW is the smallest transformer. i would go up one size to a 15KVA. this would be the minimum for your machines.

If you have a transformer 3X220v make sure that the primary is delta configured and change it into Y configuration. Then reconfigure the secondary in Y and measure voltage it should be 380 V. That is all.

First, a quiestion. Is the existing supply 220V line to line three wire, 220V/127V three phase with neutral or 380V/220V three phase with neutral?

The first seems unlikely although a shipboard standard;the second has 127V for light loads and overvolts easily obtained appliances and the third would run the hydraulic valve test machines as specified. Please tell uswhich you have at your works.

For your valve test machines; my thought goes to assuming most power goes to a 380V L-L 3kW motor, a little to some contactors , and some for low power electronics. If this is true, it is very likely that the motor is star connected and has six terminals so you can reconnect it to run on the 220V L-L. All the remaining (control) circuitry will run on a single phase via a 'control transformer'.

To make machines for world markets it is usual to run all the control items at consistent voltage(s) in all markets, via the "control transformer'. That transformer will have the ability to accept many of the worldwide range of supplies.

At worst, you will have to either provide a 220 to 380V autotransformer; or a 220V primary control transformer which has similar ratings to the machine's. With luck, you will find a primary configeration in the manual to run on 220V.

Saudi must have plenty of problems like yours, you should be able to find an industrial electrician's firm with enough competence to support you.