field ground relay

[blackseaman]

http://img269.imageshack.us/img269/2697/adszxs.jpg

hello, I wonder how a scheme is this.?f the injection is made through, so wont the rectifier d.c voltage be short circuited or complete its path from the ground brushes?

 

[davidbeach]

If all is well with the rotor there is no current through the ground brush. It would only be during a rotor winding to iron fault that any current would flow.

 

[blackseaman]

no no, look at the scheme pls, if the minus pole is connected to the rectifier plus pole so the rectifier current will return to the source through grounding brush in even without any fault, is there any misunderstanding ? this brush is the brush which let the d.c current to circulate the field and it is grounded? or it is something else and i see wrong the way the injection current is short circuited through it?

 

[davidbeach]

The negative end of the field voltage will be at some positive offset from ground, no problem. There is no connection or reference between the field voltage and ground, so moving it around doesn't much matter. The worst thing about this scheme (which is quite commonly used) is that instead of the field settling in around +/- 1/2 voltage to ground it is now all positive with respect to ground.

So if you had a 250V field, instead of it floating between about -125 and +125 it now is +50 (say) to +300. A bit more stress on the insulation but not much.

The field DC source and the ground fault detection DC source are independent of each other and form different current loops; the field ground source being open circuited except during a field ground.

The brush in question is a shaft ground brush, has nothing to do with the field brushes.

 

[rcw retired EE]

The grounding brush shown in the diagram only grounds the rotor shaft. It has no intentional connection to the exciter DC winding. The DC brushes that carry the field current are not shown.

The Vdc source at the lower left of the drawings has a single connection to the exciter/field winding. No sensing current can flow through the 64G relay until a ground fault occurs in the winding and completes the circuit: From +Vdc through the 64G relay to the negative field lead to the ground fault then back through the rotor frame, rotor shaft, grounding brush, ground and to the -Vdc connection.

Current only flows in the sensing circuit when a ground fault occurs (ignoring initial energization when some current flows through the winding-to-ground leakage capacitance).

 

[blackseaman]

you are gorgeous thank you ...