Quote:
Originally Posted by cbenham
Hi Bob, it is a viable idea and has been used for dynamic braking of the reel motors of tape recorders by AMPEX and others. I've never seen it done by superimposing the DC on the AC power feeding the motor. The usual method is switching the AC and DC voltages to the motor windings through a relay so only one of the two is applied to the motor at any moment.
The recent post showing the wheel set on Jim Hawes website uses two motors to spin and regulate the wheel speed. There are pictures showing the mechanical configuration. This is the set I showed at ETF in 2006.
The main motor has AC applied through a rheostat and spins the wheel slightly faster than it's synchronous speed. The second motor is coupled to the shaft of the main one and has only a varying DC voltage of about 8 volts applied to it from the servo amplifier. The DC varies across the second motor acting as a dynamic brake to slow the wheel and keep it spinning in sync and in phase with the red field I.D. pulses so the picture is correct.
I used this method to sync this first wheel set I ever built because I had a very difficult time making the saturable reactor servo work. I got the idea for using the second motor from a technical book on VTR and VCR circuitry.
When I get some time, I'll investigate simultaneous application of AC and DC
to a motor and find out if it might work for wheel sets.
I know this will work with a universal motor, one with field coils and a wound rotor with brushes and a commutater, but so far I've used only split phase-capacitor run induction motors in the sets I've built.
The one thing I've never done is to build a set using a DC motor with a phase lock loop chip like the 4046 or a pulse width modulated servo.
I still have much to learn... #;^)
Cliff
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The CBS/Motorola "EVR" film video player in 1970 used an induction motor with a simple SCR control circuit, so (IIRC), it actually had pulsed DC going to the motor. At first it used a shaded-pole motor, but that operated so poorly that I convinced them to spend the extra money for a capacitor-run motor. The pulsed DC was not very kind to the motor, resulting in low efficiency and lots of wasted heat.
The film had sync windows down the middle that were sensed by a fiber light pipe source shining through onto a phototransistor that triggered the SCR, so the SCR itself was the phase detector between the trigger pulse and the phase of the AC line.