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new sync machine controls for lab

new sync machine controls for lab

new sync machine controls for lab

(OP)
We are in the process of overhauling our 45 year old electric machines lab.  Our lab has three sets of 7.5HP GE synchronous machines (brush-type) coupled to 5kW compound DC machines.  The sync machines are 1800RPM, 15A, 240V, with a field excitation of 2.7A at 125VDC.

The present controls are simple contactors.  The field circuit for the synchronous machines just had a small relay and a giant varistor to clamp the transients from switching the field on and off.  A large resistor load bank was used for loading the machine used as a generator.  A motor-generator set provides a 125VDC bus for all the DC machines and sync field circuits.

The relays, contactors, and starters are all in various states of flakyness, so we would like to move up to modern solid-state controls.  The catch is that nobody makes small synchronous machines like these anymore, so there isn't much market for controls.  Our budget doesn't really accommodate a custom system from someone like Kinetics, so we are trying to piece something together.

For the DC machines I've found Emerson Quantum II regen drives that will both drive them as motors and load them as generators in place of the old load banks.

My question (you thought I'd never get to that :)) is what we can do about the synchronous machines.  Bringing the motor up to speed and switching the field excitation on and off involves some pretty high currents and voltages for brief periods, and I can't afford to smoke these irreplaceable motors.  I'd like suggestions on the best direction to go for switching and transient suppression for this application.  The best idea I've come up with is to make an unregulated DC supply with a variac, isolation transformer, and bridge rectifier, provide a series field discharge resistor, and use a pair of solid state relays to apply the excitation or discharge the field.  Gas discharge and TVS devices would be needed to protect the SSR.  Am I on the right track?  Better ideas welcome.

Thank you,
-- Ralph
Walla Walla University
 

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