How does this motor speed control circuit work?

[maxhifi]

This is a single phase induction motor circuit, from a USSR made record player (circa 1977). This type of motor is known as a permanent split capacitor type. More common in European equipment versus our shaded pole induction motors.

R16 is used for fine speed control. I can't make sense of this circuit, why the diodes are there, etc. and I have never seen it done this way anywhere else. It looks like a strange form of non variable phase angle control, but I don't know what is the function of R14.

And more to the point, can I increase the resistors to make this thing work on 60Hz, or is it better to machine the pulley down?

[dieseljeep]

Originally Posted by maxhifi

This is a single phase induction motor circuit, from a USSR made record player (circa 1977). This type of motor is known as a permanent split capacitor type. More common in European equipment versus our shaded pole induction motors.

R16 is used for fine speed control. I can't make sense of this circuit, why the diodes are there, etc. and I have never seen it done this way anywhere else. It looks like a strange form of non variable phase angle control, but I don't know what is the function of R14.

And more to the point, can I increase the resistors to make this thing work on 60Hz, or is it better to machine the pulley down?

All AC motors are designed for the mains frequency. The circuit is a rather interesting approach to speed correction. It probably will only vary the speed, 10% +or-. The line frequency in some areas, isn't as precise, as areas using a power grid.
The only remedy is reducing the diameter of the pulley, by remachining.

[old_tv_nut]

This control works by creating a DC current component due to the full conduction by the diode with no resistors compared to reduced current on the opposite half cycle with the resistors in series with the other diode. This both reduces the AC current and induces eddy current braking due to the DC component. The eddy current braking dissipates the braking energy in the motor and is therefore both inefficient and causes heating of the motor. It can only be used over a small percentage range of adjustment. If you tried to slow the RPMs by 17% to go from the 60 Hz speed to the correct speed, you also would reduce the torque, possibly too much for reliable operation, and also quite possibly overheat the motor.

I speak from experience, as I worked on a similar speed control for a video film player in the early 70s that used essentially the same circuit except that the diode was replaced by an SCR triggered by frame sync pulses. In that case, it conducted not for a half-cycle, but for a time depending on the phase of the pulses. So, the excess dissipation was somewhat different than it would be in this circuit, but the principle still applies.

Machining the pulley is the only reasonable approach.

EDIT: R14 appears to be simply reducing the (AC) voltage across the capacitor winding to get the nominal speed to coincide with centering of the fine control.
EDIT #2: The large value of R14 makes me wonder if the capacitor winding contributes much to running torque - maybe mainly starting torque?

[maxhifi]

Thank you for the excellent explanation, the idea of adding a DC component makes sense. I have long wondered how this circuit works.