Resistor in the field winding circuit of a brushless exciter

[sngpl]

Can anyone please let me know the purpose of a 22 ohms resistor in the rotor field circuit?

 

[jbartos]

Suggestion: Reference:
Robert W. Smeaton "Switchgear and Control Handbook," 2nd Edition, McGraw-Hill Book Co., 1987.
Figure 6 "Generator and exciter control panel with diagram" depicts different resistors. One of them controlled by device 41 normally closed contact is the generator field winding energy dissipating resistor.

 

[sngpl]

jbartos
I don't have access to the handbook. Can you please explain it?

 

[jbartos]

The exciter output powers the generator field over ammeter shunt resistor, two-pole circuit breaker (ANSI device no. 41 "Field Circuit Breaker&quot, and in parallel to the field winding is the magnetic energy dissipating resistor that is in effect when the circuit breaker 41 opens, i.e. the circuit breaker normally closed contact is in the series with the field winding deenergizing resistor. The exciter output has another resistor, which is on at all times.

 

[ScottyUK]

sngpl,

If I read between the lines correctly, you are describing a 22R resistor connected in series with the field winding of a brushless exciter.

A circuit with resistance and inductance has a time constant given by R/L. L is effectively the field winding inductance neglecting any stray L that may be around. R is the field winding resistance.

The ability of the generator to change its excitation, especially under fault conditions, has important consequences for the stability of the machine. A fast-acting AVR allows the capability of the generator to be maximised while remaining stable under transient conditions. The field time constant plays a dominant role in the determining how fast the AVR can respond.

If you wanted to speed up the time constant, you could increase L (but to do that you have to re-design the exciter) or you can increase R by adding a resistor. The 22R resistor serves exactly this purpose. Of course if you add resistance to the field, you must increase the voltage driving your field winding in order to get the same current through it. The resistor dissipates a lot of energy, which is the downside of this type of system.

The added resistance allows your AVR to produce a faster response from the generator, at the expense of wasting a lot of energy as heat. It is not unusual for the AVR to have an output voltage capable of driving far more current than the exciter or generator field can withstand continuously. This allows a very rate of change of field current by operating on the early, steep part of the exponential time constant curve. The current is limited from reaching its final value by a monitoring circuit.