Homework Help: A question about transformers

1. Aug 19, 2008

herich

There are primary and secondary coils in a transformers. So, what hat are the difference between when the secondary coil is in 1)open circuit, 2)closed circuit with a load, and 3) shorted circuit?

When can the ideal transformer equation be used? It seems that not only is it related to whether the transformer is ideal or not, but also related to which case (1, 2, or 3 above) the secondary coil is in.

2. Aug 19, 2008

cabraham

1) open circuit - primary current is the magnetizing current plus the hysteresis and eddy current components. The total is called "exciting current". Vp/Np = Vs/Ns, as the P & S voltages are directly proportional to the turns ratio. Open secondary should only be used with voltage transformers, never with current transformers.

2) loaded - secondary current established with load and Ohm's law. If primary is constant voltage driven, then secondary displays constant voltage behavior. Vp/Np = Vs/Ns, and NpIp = NsIs.

3) short circuit - must never be used with voltage transformers, but is ok with current transformers. NpIp = NsIs. This is used when measuring current across an isolation boundary. The secondary is isolated and allows connecting an ammeter so that large currents can be safely measured. If the turns ratio is 50 to 1, then 50 amp line currents can be measured with a 1.0 amp meter.

Does this help?