Xfmr sizing

[lume7006]

Hello,

I am looking for some technical references about xfmr sizing, under the following possibilities:

a) This motors would allow an overload of 250% during some seconds, let's say 15.

b) There is another option that one motor to be fed is to have a startinh torque of 2.5 p.u. during 60 seconds.

What we are trying to determine is the procedure to size the xfmr under these possible conditions and mainly knowing if there are some standards or technical recommendations to be followed.

Best Regards,

 

[waross]

Motor starting current is often neglected when sizing transformers. When a transformer is feeding a single motor or two or three large motors you may want to calculate the voltage drop for different combinations of motor starting. If the voltage drop is more than you can live with, select a larger transformer or a transformer with a lower impedance, or consider soft starting or a VFD drive.
Generally, for mixed loads and large numbers of smaller motors the KVA demand is determined with appropriate demand factors from the code. A transformer with equal or greater KVA rating is selected.

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

 

[qqitek]

Dear Warros, can you state the code, where demand factors are determined?
I am from IEC world, so do not know any standard describing demanding factors. It usually comes from designer experience and estimations.
I will appreciate if you point out some codes for reference.
thanks
qqitek

 

[waross]

The first step is to use the sum of the currents of all motors plus 25% of the largest motor, plus other loads.
If all the motors can be shown to not operate simultaneously the capacity may be reduced accordingly.
Loads other than motor loads may be subject to demand factors.
In the Canadian code this is covered in section 8.
One of the most obvious demand reductions allow the air conditioning load to be compared with an electric heating load and only the larger of the two used in service or transformer sizing.

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