Hi

Back again with another question for the great minds on this forum. I have been studying a logic diagram for a rotor resistance starter and am confused about the operation of carbon brushes. The logic i have been given says

if START and NOT FINAL ROTOR CONTACT and BRUSHES DOWN then set RAISE BRUSHES


if START run a 70sec timer, use the output of this to set LOWER BRUSHES

START - the command to start the motor
FRC - the final conactor i.e the SC contact

now i am confused particularly with the first line of logic, i dont understand why the brushes would want to be raised during the motor starting period, i assumed that the starting resistances are wired directly onto the brushes, so if the brushes are raised the resistances will be open circuit.

ofcourse i might have raise and lower brushes confused i.e. raising the brushes may actually bring them into contact with the rotors slip rings.

could anyone please clarify that i have either misunderstood the logic, or explain the actual physical process here?

Thankyou in advance.

Not sure of your exact requirement however the basic operation should be as follows:

1. Confirm brushes down.
2. If so apply start.
3. Motor accelerates to speed.
4. Shorting contactor closes.
5. Brushes raised.

Followed by:

6. Apply stop.
7. When stop completed brushes down (ready for next start)

Hope that helps.

Regards,
GGOSS