Transformer voltage drop based on impedance
Transformer voltage drop based on impedance
(OP)
Hello,
I would appreciate a sanity check. I have a 1.5MVA transformer, 13.8kV to 480, 5.67% impedance. I have a 600Hp motor (720 FLA) to start. I am trying to make sure my voltage drop is less than 20%. I have assumed 6 times FLA for starting. From this information, I have calculated a voltage drop of 13%. Does this seem reasonable?
thanks,
EE
I would appreciate a sanity check. I have a 1.5MVA transformer, 13.8kV to 480, 5.67% impedance. I have a 600Hp motor (720 FLA) to start. I am trying to make sure my voltage drop is less than 20%. I have assumed 6 times FLA for starting. From this information, I have calculated a voltage drop of 13%. Does this seem reasonable?
thanks,
EE





RE: Transformer voltage drop based on impedance
Rafiq Bulsara
http://www.srengineersct.com
RE: Transformer voltage drop based on impedance
However, the impedance is probably mostly reactance and the starting current is mostly reactive so calculations based on impedance and starting current alone may be surprisingly accurate. Not exact but you may introduce more error by ignoring the cable impedance.
When you include the cable impedance you should calculate two voltage drop values.
One at the transformer terminals to evaluate the effect on other loads and one at the motor terminals to evaluate the effect on motor starting.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Transformer voltage drop based on impedance
RE: Transformer voltage drop based on impedance
RE: Transformer voltage drop based on impedance
"It's nice when actual conditions agree with the simulation."
A designer with infinite faith in his computer simulation.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Transformer voltage drop based on impedance
The frequency also could be 60+/-1Hz.The transformer copper losses may be 15-20 kW .Also, the transformer impedance has a tolerance of 7.5-10%. So the voltage drop could be at least 10% more in these conditions.
This is only a theoretical calculation, indeed.
RE: Transformer voltage drop based on impedance
Normally transformer voltage drop depends on the transformer regulation which is heavily influenced by the power factor of the load.
A rigorous solution of voltage drop requires knowledge of the load power factor and the transformer X:R ratio as well as the impedance.
However, in this instance, given the information available and the fact that the load (motor starting) is highly reactive, the simple calculation may be surprisingly accurate.
By dint of offsetting assumptions and conditions the original calculation is probably within acceptable limits of error.
Bill
--------------------
"Why not the best?"
Jimmy Carter