Motor type - synchronous or not?

[Leon Crampin]

A synchronous motor without a wound field (which would usually need brushes) will "cog" when you turn it by hand unenergised due to the action of the permanent magnet rotor's flux. You can exploit this to determine the number of poles without dismantling the motor.

Connoisseur made a budget range of turntables using synchronous motors(s), the first being a 2 motor job (33/45) with very soft idler wheels to decouple the jerky drive from the motor and the second having a belt drive, one motor and a dual pulley. I think it was the BD1.

With a heavy platter and compliant belt, it worked quite well. The actual motor was cheap and cheerful.

Leon.

[Kat Manton]

Originally Posted by Leon Crampin

A synchronous motor without a wound field (which would usually need brushes) will "cog" when you turn it by hand unenergised due to the action of the permanent magnet rotor's flux.

It's probably unsurprising now, but my EMT motor doesn't "cog", it spins freely when turned by hand.

This seems to match the rotor's construction:



I think, to avoid getting into an argument with another EMT enthusiast (now seemingly backed-up by an EMT specialist), I'll conclude that my specific EMT 927A is definitely not fitted with a synchronous motor.

Now to devise a suitable PSU. Without the felt brake components, the turntable runs slightly fast. I don't feel particularly inclined to track down and re-instate the missing parts as it's somewhat crude and is apparently noisy. That'd be objectionable in a domestic environment - but then, this wasn't designed for domestic use.

[Alistair D]

Hi Kat
A company I worked for used to have to control small 240V 3 phase motors as part of the automated test equipment.
We used an off the shelf controller called an Altivar. This was a 240V single phase in 240V 3 phase out mdule with remote stop start. The interesting point here is that it also had a speed control with a wde range of adjustment which I assume varied the frequency while still maintaining the phase angles.
Not sure if there is much ont tinternet but it may be worth a look.

Al

[julie_m]

How pure a sine wave do you need?

If you don't mind it being a bit "steppy" (and the inductance of those windings should smooth the steps out nicely anyway), you could do it all digitally using logic ICs. A Johnson counter with a multiple of 3 flip-flops would give you a multiple of six states, from which point on you can take advantage of spin symmetry to get your three phases.

If you could derive a feedback signal from a steady LED reflected from the strobe dots, you could even build the whole thing into a PLL. (For an extra touch of the unusual, you could have the speed changer work by mechanically shifting the sensor up and down to pick up on a different row of dots.)

[ex seismic]

An Altivar is merely the brand name of Telemechanique standard industrial inverters. They are most commonly three phase 400v input and whatever output voltage and frequency you programme it for, within certain limits. They are also available with 240v three phase input and, in smaller Kw sizes, for 240v single phase input. But you can only get the output volts that you put into it, so you cannot get 400v output with only 240v input.

You can usually alter the output frequency manually or by an analogue input, 0-10v or 4-20mA.

The more up market versions have clever electronics and include things such as PID control of the output frequency.

Gordon

[Synchrodyne]

Originally Posted by Kat Manton

The 928 is a later belt-drive turntable introduced in 1968. It's a heavily re-engineered Thorens TD-125 (few common parts, and it sold for rather more than the TD-125.) So I'm assuming EMT carried over at least the oscillator/amplifier motor drive of the TD-125, if not the motor itself.

I hadn’t realized the connection between the EMT 928 and the Thorens TD125, but I have since found a picture of the former in HFYB 1975; the similarity is obvious.

As I recall the original TD125 had a single-phase oscillator-amplifier (with germanium PNP output transistors) with a post-amplifier capacitor phase-shift to provide a two-phase drive to the motor. I had always assumed that the motor was two-phase, but there is no reason why it couldn’t have been three-phase, with the capacitor phase-shift set accordingly. The TD125 Mk II I think had a separate amplifier for each of two phases.

So possibly EMT retained the original motor – if it was of the three-phase type – and substituted its own three-phase oscillator-amplifier.

Cheers,

[Kat Manton]

Hi,

Originally Posted by Alistair D

A company I worked for used to have to control small 240V 3 phase motors as part of the automated test equipment.
We used an off the shelf controller called an Altivar. This was a 240V single phase in 240V 3 phase out module with remote stop start.

I've had a look at those. One thing which concerns me is the output waveform they produce.

Without knowing very much about them, I'd guess they rectify incoming mains to produce DC, then use PWM to drive the output pass devices. Turning MOSFETs hard on or hard off would make for good efficiency and small heatsinks.

(They must use PWM, mustn't they? One I looked at was claiming almost full torque at 1.5 Hz - that'd be about 43 rpm from a 1450-ish rpm motor. I can't see a motor intended for 50 Hz operation being too happy with 1.5 Hz sine wave drive...)

But I doubt the intended applications have a moving-coil magnetic pick-up about a foot away from the motor. If the outputs are PWM, there'll be a lot of harmonics; these could be radiated then picked up.

Though it makes the motor PSU less efficient, I think I'd prefer to drive the motor with something resembling sine waves.

OTOH, I'm still interested in these variable-speed drives. I've just bought an old 3-phase motor to play around with. One reason is I'd rather experiment with PSU circuits connected to something other than the EMT motor. The other reason is I'm thinking about getting a small lathe; the motor is likely to come in handy for that and a variable-speed drive would save mucking about with belts and pulleys.

Originally Posted by ajs_derby

How pure a sine wave do you need?

I think, for the reasons above, fairly clean. If I haven't got harmonics of 50 Hz (and/or some switching frequency) present in the supply or the motor windings, there's less chance of undesirable noises ending up superimposed on the music.

Originally Posted by ajs_derby

If you don't mind it being a bit "steppy" (and the inductance of those windings should smooth the steps out nicely anyway), you could do it all digitally using logic ICs. A Johnson counter with a multiple of 3 flip-flops would give you a multiple of six states, from which point on you can take advantage of spin symmetry to get your three phases.

Before I started the thread I'd started sketching something out in my schematic capture software. It's probably no surprise; I've already got a Johnson counter (with a couple of gates to ensure it behaves) as that seemed like a good way to get a three-phase output from a single-phase clock signal. I'm intending to follow that with low-pass filters (op-amps), then class B output stages.

I think that should be good enough (and probably will produce a cleaner waveform than the mains tends to be these days anyway.)

Originally Posted by ajs_derby

If you could derive a feedback signal from a steady LED reflected from the strobe dots, you could even build the whole thing into a PLL. (For an extra touch of the unusual, you could have the speed changer work by mechanically shifting the sensor up and down to pick up on a different row of dots.)

That's something I've been thinking about. It couldn't use the strobe dots, they're on the upper acrylic platter (they're also transparent and horizontal, lit from below by a neon lamp.) The upper platter is braked by solenoid-operated braking system for quick starts (less than 0.4 seconds for zero to 33, 45 or 78 rpm!)

The main platter needs to keep rotating at the correct speed when the upper platter is braked, so I'd need to pick up a tacho signal from that. There's a lot of space under the platter; I could attach a strobe disc under the platter and use a reflective IR sensor to produce a tacho signal. Part of the fun would be to devise ways of attaching the disc and sensor such that they could be removed later and not leave any trace, but I already have some ideas about that.

Cheers, Kat

[Studio263]

Originally Posted by kalee20

In fact to me a friction-brake is a bit of an inelegant method anyway to control speed. I've seen medium-price B & O turntables with an eddy-current brake to control speed of an induction motor - the gearing and pulley sizes chosen such that 'free' speed (ie nearly synchronous) gives a turntable speed slightly too fast. Then the braking brings it down to the correct speed. The eddy current brake is just a thick copper or aluminium disc on the rotor, with a coil and pole-piecss (having variable DC shoved through it) close by.

I think that's just one series, the Beogram 1100 / 1900 / 3400 and the Beocenter 1800. These all use a copper disc on the motor shaft and a permenant magnet. I rather liked it, previously they had felt brakes acting on a centrifugal govenor (42V & VF, early (three speed) Beogram 1000, 1500, 1800) which was clever but noisy, then thin idler wheels that could be moved up and down a conical motor spindle (Beogram 1200, 1202, 2000, 3000, Beocenter 3500 etc, OK until the idler gets bent!). All these models used asynchronous shaded pole mains powered motors but the Beogram 4000 (and early examples of the 4002 and 6000) have synchronous ones, they are low voltage and driven by an on-board Wein bridge oscillator (complete with bulb) and power amp. After that it's all Matsushita FG DC servo brush motors except for the "Tangential Drive" Beogram 8000, 6006 and 8002.

[ricard]

A small note: a synhronous motor will run below its synchronous speed if heavily loaded. I've noted this in the Bang & Olufsen Beocord line of reel-to-reel recorders, which use Papst synchronous motors. That the motor is in fact synchronous can be determined if nothing else by putting a stroboscope sheet on the motor; when it is running, the stroboscope lines are absolutely still. However, in fast wind mode, especially at the end of the tape, the load is significant, and the motor speed drops. Nothing really surprising; after all, a synchronous motor must be able to run below its synchronous speed if nothing else just to get started!

/Ricard

[kalee20]

That's just it - pure synchronous motors tend not to self-start. (If the rotor is really low inertia, it may pick up enough speed during a half-cycle before the next half-cycle causes it to reverse. Microwave oven turntable motors are of this type and start in a random direction).

However, I can imagine a synchronous motor running at below synchronous speed if occasional heavy loads are put on and then almost imediately removed - the rotor may slip a cycle and re-synchronise (a bit like a phase-locked loop losing lock and regaining it, or a TV timebase losing lock and regaining. The motion would have seen a brief jerk, and the long-time average speed would have dropped fractionally.

The type of motor you refer to almost certainly has a rotor which operates in induction mode to allow it to run up to speed.