Hi all,

I'm very new to electronics, and I'm looking to understand this motor assembly which I have removed from a record player.

The motor I believe to be a synchronous motor (Sev Litovel M303 16V 2VA 300rpm).

I have a few questions on this circuit:

- My very limited understanding of synchronous motors is that they require 3 phase power supply to the stator, and I understand in principle how these types of motors work. However, clearly this is a small motor supplied by AC power not 3 phase. How is a rotating electromagnetic field set up using only AC current?
- What is the function of the two capacitors?
- What is the arrangement of the wires likely to be inside the motor?
- Is there some kind of controller built into the motor?

I have tried drawing a circuit diagram, but without an understanding of how the wires are arranged inside the motor I am really struggling to understand how the whole system works.

If anyone could be kind enough to offer any explanations, or point me in the direction of anything that could help me answer the above questions I would really appreciate it!

Many thanks,

M

 

Not

- My very limited understanding of synchronous motors is that they require 3 phase power supply to the stator, and I understand in principle

Not necessarily, before digital, all wall clock motor were 1ph synchronous motors.
Max.

 

Two coils in the motor, one fed directly and the other fed via a capacitor so the currents in the two coils have a phase difference giving a rotating field. Probably.
No idea what the capacitor with a switch across it is doing.

 

Early spin-to-start synchronous clocks ran without any rotating field.
I took one apart when I was a kid to figure out how it worked.
It was a type of reluctance motor.
They had a magnetic plate rotor with many square teeth on the periphery that were matched by similar teeth on the stator (stator picture below).
You manually started the clock by spinning a small knob on the back of the clock.
Once it settled to the synchronous speed it moved exactly one tooth distance per half cycle.
(It's also possible that the teeth in the two halves of the stator were 1/2 tooth pitch displaced so that it moved 1/2 tooth pitch per half cycle, I'm not sure.)
Inertia carried the rotor towards the next tooth as the AC went through zero and as the AC voltage rose it attracted the rotor teeth to the next stator tooth.

An interesting characteristic was that they would run in either direction so, if you spun it in reverse, the clock would run backwards.

 

The cap across the switch is most likely a snubber to limit the pop when the switch opens.

 

Two coils in the motor, one fed directly and the other fed via a capacitor so the currents in the two coils have a phase difference giving a rotating field. Probably.
No idea what the capacitor with a switch across it is doing.

Thanks!

For the benefit of anyone else reading this thread, with your reply I came across these two videos which really helped me understand:

I guess then the rotor must be a permanent magnet...

 

Those videos pertain to induction motors, but looking at some of the Litovel motors for turntables (Gramaphones!) they indicate a PSC AC induction motor, which doesn't make much sense, as these motors are not synchronous.

 

Those videos pertain to induction motors, but looking at some of the Litovel motors for turntables (Gramaphones!) they indicate a PSC AC induction motor, which doesn't make much sense, as these motors are not synchronous.

Yes actually it might be a bit confusing that I posted as it is showing an induction motor.

However, if the rotor was a permanent magnet, rather than an induced coil, would this not "lock" the rotor to the magnetic field, therefore making it synchronous?

Again I'm completely new to this so your patience is appreciated!

Thanks

 

Yes BLDC, ECM (electronically commutated motors) or P.M. rotor motors are popular but usually do not involve capacitor fed winding's.
Max.