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MMD > Archives > March 2001 > 2001.03.20 > 03Prev  Next

Variable Speed Induction Motor
By Tom Lear

I was wondering if one the MMD resident electrical engineers could
enlighten me about the theory and operation of the Dynavoice roll motor
speed control circuit.  What's going on with this circuit, how does it
work, and why?

The motor is a simple AC fan-type motor: a dozen or so laminated
steel plates in a open square with a coil of wire around one side
of the square, the rotor placed in a hole through the plates on the
opposite side.

There are two leads to the coil: lead #1 (neutral conductor) is direct
to the coil; lead #2 (hot - 120 V AC) is interrupted by a 200 ohm,
1/2-watt rheostat which is paralleled (bridged) by a "top hat" silicon
rectifier, then it continues on to the other coil connection.

1.  I have done some experiments and determined that if high enough
additional resistance is added to the rheostat portion of the circuit,
that the motor will come to a dead stop, even though it's still
receiving DC power through the rectifier, which leads me to believe
that the motor is an AC motor only.

2.  At top speed, with no resistance, the rectifier is effectively
removed from the circuit.

3.  The motor pulls about 40 watts.  Without the rectifier in the
circuit, the rheostat on any setting, other than full on, will get

My question is, how does this simple little circuit work to allow a
small-1/2 watt potentiometer to control 40 watts of motor?  How is the
rectifier protecting the pot?

Tom Lear
San Francisco, CA

 [ Editor's comments:
 [ This simple variable-speed motor was in production for many years
 [ and was often used to drive the turntable in inexpensive phonographs.
 [ The motor is a simple 2-pole high-slip squirrel-cage induction
 [ torque motor such as used in small electric fans.  Normally, with no
 [ load, it runs at near synchronous speed (3600 rpm on 60 Hz current).
 [ When the load is increased it runs slower.
 [ The half-wave rectifier adds direct current (DC) to the alternating
 [ current flowing through the coil, but the DC creates an opposing
 [ torque in the rotor, just as does a fan.  The more direct current,
 [ the more load and the motor slows down accordingly.  Only one-half
 [ of the AC flows through the rheostat, so it doesn't overheat; the
 [ large direct current component flows only through the diode.
 [ It was a nice, cheap speed-control method for a constant-torque load
 [ like a music roll.  The fly-ball governed motor of the Ampico B is
 [ terribly expensive, but in 1927 Ampico had few choices.  Nowadays
 [ the modern permanent magnet DC motor and a solid-state controller
 [ are used.
 [ -- Robbie

(Message sent Tue 20 Mar 2001, 10:14:40 GMT, from time zone GMT-0500.)

Key Words in Subject:  Induction, Motor, Speed, Variable

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