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The Artful Bodger
12-19-2010, 12:46 AM
I have a small electric motor which I believe is two phase.. it came from an old radar installation and I believe drove the 'nodding dipoles' on a GCA system.

The motor is about 3" diameter and maybe 4" long, it has three connections one of which has two of the internal wires going to it, the other two connections have one wire each. According to my meter there is 150 Ohms betweens each of the single connections and the double connection and 300 Ohms between the two single connections. This makes me think it is just two windings in serial.

The only markings are the markers name Evershed & Vignoles, London, and a hand written date of 1942.

There are no brushes etc.

100V AC across one of the windings and an unknown capacitor across the other gets the motor to run nicely, I increased the volts to 200V AC with no sign of heating though it was a lot noisier.

Peeping through the ventilation holes shows very fine windings, maybe 28 gauge or so.

Now the question is, knowing the resistance of one coil and the frequency of the mains, how to calculate an appropriate capacitor?

Thanks.

Mark McGrath
12-19-2010, 03:50 AM
Could be start and run windings.I thought it might be a tachogenerator as that was one of their main products,but no brushes rules that out.

J Tiers
12-19-2010, 10:59 AM
IF the motor is a capacitor start/run, i.e. a "PSC" type motor, the correct capacitor is generally the one that results in the LEAST motor current under load.

It's a compromise, and the unloaded best capacitor may not be the same as for the loaded case.

MaxHeadRoom
12-19-2010, 11:10 AM
Evershed and Vignoles made instrumentation equipment, I believe the famous AVO meter?
It is possible it is an AC synchronous stepper motor, these require a capacitor to produce a split phase as is done in a split phase induction motor.
These motors are identical to a DC bi-polar stepper motor but are wound to be used on 120 or 240 ac.
If this assumption is right they will run at low rpm due to the high pole count, also if this is the case they run at syncronism to the incoming supply.
Otherwise they are probably ordinary split phase induction motor,
Make sure when experimenting with caps you use AC continuous rated types.
Max.

squirrel
12-19-2010, 11:50 AM
I might be a Selsyn motor.

The Artful Bodger
12-19-2010, 01:12 PM
I might be a Selsyn motor.

It has only three terminals and apparently only two windings so I doubt it is a Selsyn.

The Artful Bodger
12-19-2010, 01:19 PM
Evershed and Vignoles made instrumentation equipment, I believe the famous AVO meter?
It is possible it is an AC synchronous stepper motor, these require a capacitor to produce a split phase as is done in a split phase induction motor.
These motors are identical to a DC bi-polar stepper motor but are wound to be used on 120 or 240 ac.
If this assumption is right they will run at low rpm due to the high pole count, also if this is the case they run at syncronism to the incoming supply.
Otherwise they are probably ordinary split phase induction motor,
Make sure when experimenting with caps you use AC continuous rated types.
Max.

It ran quite fast on 50Hz, maybe 3000rpm which would mean two poles.

Also, the motor has a cooling fan attached which indicates it was expected to run at a reasonable speed.

I think it is a split phase induction motor, as you say, but I wonder why the two windings appear to be identical?

MaxHeadRoom
12-19-2010, 02:20 PM
You will often find on smaller motors that use one start/run cap, the windings are the same.
Max.