Motor type - synchronous or not?

[Kat Manton]

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

[AlanBeckett]

It is synchronous in the sense that its speed is governed (sorry) by the number of poles and the supply frequency. However, it will never run at true synchronous speed as there must be some 'slip' to generate torque - See http://en.wikipedia.org/wiki/Induction_motor.
Loading it increases the slip.

Alan

[Sean Williams]

Sounds very close to basic AC generator theory there Kat,

4 pole 3ph alternators need to run at 1500 rpm to make 50Hz

Sounds synchronous to me

[Mullard]

The motor you have is an induction motor. Induction motors rotate at a synchronous speed only when unloaded. In order to provide torque it is necessary for there to be slip; i.e. rotation less than the synchronous speed. For more details see http://www.plantservices.com/articles/2002/48.html

[amp_mangler]

Hi Guys,

I dont think it will be 3 phase!!! more likely a split phase motor, this would normally have a capacitor ( VERY generally about 2uF ) connected to the tap the other end of the cap going to either neutral or active.

a 3 phase motor WONT run on single phase!!
to adjust speed with such a motor I made an audio oscillator ( wein bridge) and drive that into a 20 watt mosfet amp, which in turn drives a normal mains toroid transformer ( 240 volts to 12 volts ) which is connected backwards,i.e. the 20 watt amp is connected to the 12volt winding, and the 240 side is connected to the motor. I have a normal "volume" control on the input of the amp to "set volts", and a variable dual pot in the wein bridge oscillator to adjust frequency ( speed control) its VERY low noise and drives my Garrard perfectly Garrard 401,s also have a motor big enuf to drive a cement mixer

regards

Joe

[Sean Williams]

Ahem,

Sorry Joe, small 3 ph motors will run on single phase, with a phase shift cap.

It is the basis of most small phase converters used for machine tools as well.

[amp_mangler]

3 phase in australia is 415 volts I agree between neutral and any phase its 240 volts.
How do you get any torque? even in a TT motor? hmm maybe I have missed something here. Machine tools in Aus are all 4 wire input as well, to comply with MEN system ( Main Earth Neutral) ( regardles of weather its Wye or Delta wound). Is there a text I can read regarding your comments ?

[Kat Manton]

Sorry Joe; I've already established that it's a 3-phase motor.

Mains is applied to two of the connections (the windings are star connected) and the third is fed via a capacitor and resistor in series; quite a well-known technique for operating 3-phase motors from a single-phase supply.

[amp_mangler]

OK fair enough I stand corrected. BUT !!! isnt this known as "split phase" ?


Joe

[AlanBeckett]

Originally Posted by Sean Williams

It is the basis of most small phase converters used for machine tools as well.

The 3hp motor on my Chipmaster is entirely happy with this technique, although it does need a very hefty start capacitor switched in briefly to get it rolling.

Alan

[Kat Manton]

An excerpt from the 930 schematic (the 927 and 930 are similar):



As expected, it runs more smoothly (with more torque and less heat) when run from a 3-phase supply, 100V per phase, produced with three 100V-line power amplifiers, a multi-tap delay line and an audio oscillator set to just below 50 Hz. Hence I'm already designing a more compact 3-phase supply which will fit under the deck.

Some Empire turntables used a 3-phase motor (TT PSU article part 1 and part 2) too; the EMT isn't unique in that respect.

[Leon Crampin]

You have a split phase induction motor. The rotor is a "squirrel cage" of shorted turns under the skewed lams - you may see the ends welded or soldered up.

It's not synchronous because the rotor flux is derived from the stator by transformer action which gives rise to "slip", meaning that the phase relationship between primary and secondary (stator and rotor) will vary with load. That's why your motor will not rotate under load at its theoretical speed which would be governed solely by the number of pole pairs and the applied frequency.

A synchronous motor will have either a permanent magnet rotor (eg. clock motor) or a wound rotor carrying a field winding to which DC is applied (eg. car alternator). When you load a synchronous motor the speed does not change until the load reaches a critical value - then it stops.

Leon.

[kalee20]

It does sound like an induction motor - as others have said, this runs slightly below synchronous speed when loaded.

However, the Garrad SP25 mk 4 turntable (and possibly others) does run as a true synchronous motor, ie at synchronous speed - make sure your motor isn't like this! The Garrard motor has a squirrel-cage rotor - like an induction motor - and the stator has a couple of thick copper single-turn links part way through the pole pieces (so-called shading links). This gives an extra field, phase-shifted from the main field, and drags the rotor round in a definite direction when the stator is energised from a single-phase supply. The rotor runs up as an induction motor. However, the rotor also has a permanent magnet bonded to it, and as the speed nears the synchronous speed, this 'takes over'. The squirrel-cage internal currents drop to zero, and the steady field of the permanent magnet ensures the rotor is dragged round in step with the rotating field created by the stator.

It's instructive to brake thie motor spindle between fingers - you can feel the torque keeping the speed constant, and after a certain point, as synchronism is lost, the torque has a step change as the motor reverts to induction mode.

In principle, there would be nothing against having a 3-phase version of this, or using a capacitor to give the phase-shifted field instead of the shading links.

The deciding factor for your motor is, does it have means to provide a steady feild for the rotor, such as a permanet magnet? If so, it is synchronous.

[Leon Crampin]

An induction motor fitted with copper shorting turns across part of the stator poles is called a shaded pole motor.

The purpose of the shading coils is to delay the build up of flux across part of each pole face to give a rotational effect to the flux build up and hence make the motor self-starting. By Lenz's law, the flux in the shaded part of the pole will be delayed because the shorted turn produces an opposing flux until the iron saturates. It's not a very efficient system (shorted turns in machines seldom are) and these motors consequently run rather hot, but it serves well enough for small sizes.

The capacitor split-phase system is a far better idea but more expensive to make.

Leon.

[Kat Manton]

This is what I thought after some reading; a defining feature of synchronous motors seems to be that the rotor turns at exactly the same speed as the rotating magnetic field, 1500 rpm for a 4-pole motor run from 50 Hz.

Given this motor runs at around 1450 rpm (with fine speed adjustment achieved by means of a friction brake), I couldn't see how it could be a synchronous type. That's apart from its construction.

I'm 100% certain it's a 3-phase rather than split-phase, though.

I expected it to be a split-phase motor, due to the presence of the phase-shift capacitor (and was intending to build a PSU along the lines of a beefed-up Linn Valhalla.) But dismantling the motor, resistance measurements, how it's shown on the schematic, determining the disposition of the magnetic field in the stator and eventually running it from a 3-phase supply all eventually convinced me I was wrong (and needed a more complicated PSU!)

[kalee20]

Originally Posted by Leon Crampin

An induction motor fitted with copper shorting turns across part of the stator poles is called a shaded pole motor.
...
The capacitor split-phase system is a far better idea but more expensive to make

Definitely!

Shaded-pole motors are only used at low powers, and they don't give much starting torque anyway. Fan motors are often of this type.

The capacitor split-phase system does have the drawback that it needs a capacitor - and from other recent threads, this can have significant failure rates. So the shaded-pole motor, while not really elegant, can be more reliable and long-lasting. And this would also apply to the shaded pole induction-cum-synchronous motor.

[Kat Manton]

Originally Posted by kalee20

It's instructive to brake the motor spindle between fingers - you can feel the torque keeping the speed constant, and after a certain point, as synchronism is lost, the torque has a step change as the motor reverts to induction mode.

It just gets slower, there's no step in torque.

I also couldn't figure how the friction brake fine-speed adjustment could work in conjunction with a synchronous motor. I figured that, as the braking friction is increased, a synchronous motor would maintain constant speed until a point where it loses synchronisation (and stalls); which doesn't fit with how it works in practice.

Varying the speed of a synchronous motor could only be achieved by varying the frequency of the supply, I assume?

[kalee20]

I'm with you on both counts, Kat - feeling the torque, and the method of speed control. It wouldn't work with a synchronous motor. It might not stall, but it would give very uneven running as the rotor gets further and further behind the rotating field and then suddenly slips a cycle giving a jump to the rotor movement - only for this to repeat.

Yur motor can't be a synchronous type.

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.

[Synchrodyne]

Some indirect evidence, perhaps: Hi Fi Year Book 1973 lists both the EMT 930st and 928, the latter appearing to be a newer model, as it was not listed in HFYB 1972, whereas the 930st was. Anyway, the 928 is described as having a three-phase synchronous motor fed by an electronic three-phase generator. The same description does not apply to the 930st, from which one might (circumstantially) infer that it did not have a synchronous motor.

Anyway, as Kalee20 has already said, 1450 rev/min and the speed brake pretty much outrule a synchronous motor. The use of induction motors with speed brakes (usually of the eddy current type) seems to have been reasonably common for “transcription” turntables in the 1960s; the Garrard 301 & 401, and Thorens TD124 were of this type. Thorens moved to an oscillator/amplifier-driven low-speed synchronous motor (16-pole) with the TD125 circa 1969.

Three-phase motors have higher starting torque than capacitor split-phase motors, which may have been a reason for EMT’s choice. In TV studios at least three phase supplies might be available as I think that some telecine units, for example, have or had three-phase drives, and Selsyn evidently works better with three-phase. Thus a three-phase motor could be used as such, or in a capacitor split-phase mode where only a single phase supply was available. Normally capacitor split-phase motors seem to have two-phase windings, but as the phase shift is not that exact, I think that the technique could be applied to pretty much any polyphase winding.

Cheers,

[Kat Manton]

The 927 was introduced in 1951 and was joined by the smaller 930 in 1956, aimed at customers who didn't require the 16" disc capability of the 927. The two machines are similar in design, construction and performance; one of the manuals I have covers both models. The 930 motor is smaller (but still wouldn't look out of place on a small lathe!)

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've gone through some documentation looking for clues; I may have found where the 'synchronous motor' idea originated. An article in 'Sound Practices' issue 16 (.pdf) (late nineties?) states:

"The motor is massive. It looks like an industrial motor, 13.5cm in diameter and 20cm long! It is a 3 phase synchronous type with phase shifter."

This may be where some have got the idea that it's a synchronous motor from; an article which isn't entirely error-free. This statement is obviously incorrect:

"A precision manufactured 3-step pulley is mounted on the motor axis [...]"

As can be seen from my photo of the rotor, the rotor shaft itself is stepped. (I was wondering how it came off before I dismantled the motor, I couldn't find any grub screws...)

So, I guess it must be a synchronous motor because it says so in the 'Sound Practices' article...