Hi,
I'm no motor expert, but I'm fairly sure that the motor driving my EMT turntable is a 3-phase induction motor - and is
not a synchronous motor. But there seems to be some confusion; there are some who claim it
is a synchronous motor.
Photos of the rotor and stator taken while it was in bits can be seen attached to
this post.
While it was apart, curiosity got the better of me and I made a few checks. There are three wires (red, yellow, blue) emerging from the stator assembly. The DC resistance between any pair of them is the same. One EMT diagram shows the motor as having three windings connected in star configuration, which agrees with my measurements. I also connected a signal generator and used a coil and 'scope to figure out the disposition of the magnetic field.
I came to the conclusion that it's a 3-phase, 4-pole induction motor.
Fine speed control of the turntable is achieved by an oiled felt friction brake (or it would be if these parts weren't missing from mine.) Without the brake, it runs slightly fast. A neon (fed from 50Hz AC) and strobe dots around the periphery of the platter are provided for correct adjustment of the felt brake.
Is it possible to mechanically slow a synchronous motor? What I know of them is that the rotor spins at
exactly the same speed as the rotating magnetic field produced by the stator coils; for a 4-pole motor this would be 1500 rpm.
A couple of quick measurements and some arithmetic:
- Internal diameter of platter = 396mm
- Motor shaft diameter (33⅓ step) = 9.1mm
- Motor speed = ( 396 / 9.1 ) x 33⅓ = 1450 rpm
This motor is also inherently self-starting, whether run from a single-phase supply with a phase-shift capacitor and series resistor (the original arrangements) or run from a 3-phase supply (an experiment I tried to see if it could be made to run more smoothly.) It's quite torquey, able to get the platter moving from a standstill fairly rapidly. The mechanism engages the idler wheel first, then applies power to the motor, so it's started under load.
I can't see how it can be a synchronous motor, yet be slowed down by a mechanical friction brake to 1450 rpm (which would surely defeat the object of using a synchronous motor in the first place.)
But, as I said, I'm no expert on motors. Apply power, it goes round and round... exactly how they work isn't usually something I need to think about
Cheers, Kat