Size 5 NEMA Starters

[Hileg]

We have a size 5 starter operatinga 225 h.p. motor. The starter has c.t.'s (300:5)installed in the back of it to monitor the current going to the motor. The secondary makes up to heaters installed in the front of the starter. The motor normally runs anywhere from 40 amps to 135 amps throughout the day. The current on the secondary can be calculated by cross mutiplication. My question is, is there a mathematical way to figure out what the voltage should be across the heaters. Thanks for any help.

 

[dpc]

I don't think you should really care what the voltage is.

To size the overload heaters, take the motor FLA and divide it by the CT ratio (60:1). This will give you the motor full load amps in secondary amps. The overload heaters are rated in amps.

If you measure the dc resistance of the heater (out of the circuit), you can get a rough idea of what voltage to expect - just I^2*R. The ac impedance will be a little different than dc resistance, but it will be close.

But maybe I didn't really understand your question.

 

[jraef]

dpc is right, but what would that info be useful for?

"Venditori de oleum-vipera non vigere excordis populi"

 

[aolalde]

v = I * Z

Z = SQRT( R^2 + Xl^2 + Xc^2)

Xl & Xc ----> 0