Auto_transformer rating

[friend81]

Hi Guys,

I would like to to know if the calculation for dimensioning a 3 Phase Auto-transformer is same as that of a power transformer
ie (Volts x Amps X 1.73)/1000 ?

i am thinking of installing a 30HP,380V pump set in 220V,3Phase,AC power supply by employing a step-up auto-transformer, so the calculation should be (380 X 52 X 1.73)/1000 = 34.184kVA & the nearest available rating should be a 40KVA transformer .

I would also like to know how to calculate required generator capacity for this situation?

 

[waross]

For 30 HP use 3 X 30 KVA = 90 KVA for the generator.
OR
Go to the Cat site or the Onan site or the F. G. Wilson site or another manufacturers site and use their software.
A prime power set has 10% more capacity for motor starting than a standby set.
The PMG option (Permanent Magnet Generator) may help motor starting.

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

 

[podobing]

Friend81,
Regarding the sizing of an auto-transformer, the formula to determine the kva of the auto-transformer is:

KVA(Auto) = (1 - (Vin/Vout))* KVA of the load. The auto-transformer is only transforming the difference between the input and output voltages. This equation applies if it is a step up auto-transformer. If you are using a step down auto-transformer, simply interchange Vin and Vout.

You can see that if the input and output voltage are significantly different from one another, the size of the auto-transformer approaches the size of an isolation transformer, so it does not make sense to use an auto-transformer in such an application.

Keep in mind that there is no electrical isolation between the primary and secondary, and be careful with grounding/ground fault relaying.

Dave

 

[automatic2]

go with waross. An auto is much smaller than an isolation of the same kva rating.

 

[waross]

I would first be looking for a 220/380 Volt star delta motor.

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

 

[friend81]

Thanks Guys,

Podobing
as per the formula if i work out the Auto-transformer rating for this 30HP motor, the working will be as follows

(( 1- (220/380))* (22kw*0.83PF) = (1-0.58)*18.26KVA = 7.67kVA.

The auto transformer can be a minimum of 10KVA rating with safety margin.

To calculate the KVA of the load, motor output power is considered in the above calculation. Is it correct or motor input power to be taken into account instead of output power to calculate KVA of the load ?

 

[friend81]

Waross,

The motor is 220/380V dual voltage motor. The idea of running the motor in 380V is to reduce the cost in Power cable & control panel components. The pump set is for deep well application hence there is space restriction in using a bigger diameter cable.

The control panel will be equipped with a Soft-starter or an Auto-Transformer starting.

 

[podobing]

Friend81,
Using the formula that I gave earlier, kva(load) is the kva, not kw or kw*pf. In your example, I would use 30 kva for the load. Applying the formula, you would get (1-0.58)*30 kva = 12.6 kva. That would be the (minimum) size of the auto-transformer that you would need to do the job. As you can see, if the input and output voltages get farther from one another, the part inside the parentheses approaches 1.0, and the size of the auto-transformer approaches the size of an isolation transformer.
Dave

 

[friend81]

Podobing

oh ya there was a blunder mistake in my kva calculation. For calculating the kva of a 3Ph motor the formula is = (1.73*I*V)/1000 = (1.73*45*380)/1000 = 29.6kva.

Based on the same calculation if i to need step-up the Voltage for 460V instead of 380V, then the Auto-transformer rating increases to 15.7kva.

Is the same formula applicable to 1Phase system also ?

 

[podobing]

Friend81,
Yes, the same formula can be used for single phase or three phase auto-transformer sizing calculations.
Dave

 

[friend81]

podobing
Thanks for throwing some light

 

[waross]

Three phase formula: (1.73*I*V)/1000
Single phase formula: (I*V)/1000

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

 

[friend81]

Thanks Waross,

Actually i was asking if the formula KVA(Auto) = (1 - (Vin/Vout))* KVA of the load, is same for 1Ph & 3Ph.