TWO PHASE MOTOR

BQW · 6 Apr 2019

I have been repairing a machine of American origin that uses a 240v two phase motor.
Never knowingly worked on any thing with a two phase motor before, and after reading up, on the principle basically understand it, but this particular example had a 15uF start cap in series with one winding.
Nothing I have read mentions about a start cap being needed for a two phase motor, and I cant see the need either.
The motor is obviously being fed direct from its voltage supply not through a inverter .
Informed replies most welcome.

bernardgreen · 6 Apr 2019

BQW said:
a machine of American origin that uses a 240v two phase motor.

American single phase domestic supplies are 110 volt which means higher currents that for the same load on 230 volts UK supply.

To reduce the current and wiring cost US domestic supplies are often "2 phase" allowing for 110 volt appliances and 220 volt appliances to be used.

These "phases" are 180° out of phase with each other ( often from a single phase centre tapped transformer secondary ) unlike a 3 phase supply where the phases are 120° to each other.

Therefore your motor is in reality a single phase ( 220 volt ) motor and hence the start capacitor is needed.

Jackrae · 6 Apr 2019

Standard USA domestic supply is/was 120/208 volts. What exactly is printed on the motor name-plate

Normally, powering across 2 phases of a 3-phase 120v supply will give 208 volts

winston1 · 6 Apr 2019

Standard USA domestic supply is split single phase 240v with a neutral centre tap at 120v. 120v is used for most things with 240v for high power loads. It is NOT two phase. So yes a start cap could be required. Frequency is also 60Hz which may or may not matter.

NB 110v supplies have not been the norm for well over 50 years.

bernardgreen · 6 Apr 2019

winston1 said:
NB 110v supplies have not been the norm for well over 50 years.

There are still places where the supply is rated at 110 and can be as low as 100 at times of peak load.

JohnW2 · 6 Apr 2019

winston1 said:
Standard USA domestic supply is split single phase 240v with a neutral centre tap at 120v. 120v is used for most things with 240v for high power loads. It is NOT two phase. ...

We've had this terminological discussion before ...

... as far as BS7671 is concerned, 'split phase' (180° between phases) IS "2-phase", as are systems with 90° and 120° differences between two 'phases' in a 3-wire system ...

Kind Regards, John

winston1 · 6 Apr 2019

JohnW2 said:
We've had this terminological discussion before ...

... as far as BS7671 is concerned, 'split phase' (180° between phases) IS "2-phase", as are systems with 90° and 120° differences between two 'phases' in a 3-wire system ...

View attachment 162265

Kind Regards, John

So would you refer to 110v site transformers (55-0-55) as 2 phase transformers?

winston1 · 6 Apr 2019

bernardgreen said:
There are still places where the supply is rated at 110 and can be as low as 100 at times of peak load.

Where?

bernardgreen · 6 Apr 2019

winston1 said:
Where?

I will ask an American aquaintance, he is a service engineer aquaintance who roams the Eastern Seaboard.

bernardgreen · 6 Apr 2019

winston1 said:
So would you refer to 110v site transformers (55-0-55) as 2 phase transformers?

Input single phase hence the transformer can only be single phase.
The output therefore is and and can only be single phase.
The output has a centre tap that can be connected to Ground or Earth,
There are two live conductors to the appliance.

There maybe no grounded Neutral to the appliance.

If there is no grounded Neutral to the appliance then the appliance sees only one phase at 110 volts.

JohnW2 · 6 Apr 2019

winston1 said:
So would you refer to 110v site transformers (55-0-55) as 2 phase transformers?

I don't think that many people would, even though they are no less "2-phase" than is a 240-0-240 'split-phase' supply.

I think that the way we speak is influenced by how the supply is used. If the L's of a 240-0-240 supply were used to supply a single 480V load (ignoring the neutral), then that would be analogous with the site transformer case, and there would be no intuitive reason to call it '2-phase', since it would essentially be single-phase - but that, of course, is not what we do with a 240-0-240 supply.

On the other hand if the neutral and one or other of the Ls of a 240-0-240 supply are used to provide 2-wire supplies to (240V) loads, then some loads will be getting one phase, and other loads will be getting a phase with is 180° different - which sounds very much like '2-phase' to me. There's nothing particularly magic about 180° (or any other phase difference) which makes it conceptually different from 120°.

Kind Regards, John

JohnW2 · 6 Apr 2019

bernardgreen said:
Input single phase hence the transformer can only be single phase. The output therefore is and and can only be single phase.

I'm not sure about that as a generalisation. A 'black box' could have a single-phase input and an output of as many phases as you wanted.

As I think we have both said, I think it really depends on how the supply is used. As I've just written, if one has a 3-wire 240-0-240V supply and connects loads between '0' ('N') and one or other of the 240V lines ('L') then loads are getting one of two (240V) 'supplies', which are 180° apart. I see that as no less a '2-phase' situation than if the voltages on the two Ls were 120° (or any other angle) apart.

Kind Regards, John

BQW · 6 Apr 2019

bernardgreen said:
American single phase domestic supplies are 110 volt which means higher currents that for the same load on 230 volts UK supply.

To reduce the current and wiring cost US domestic supplies are often "2 phase" allowing for 110 volt appliances and 220 volt appliances to be used.

These "phases" are 180° out of phase with each other ( often from a single phase centre tapped transformer secondary ) unlike a 3 phase supply where the phases are 120° to each other.

Therefore your motor is in reality a single phase ( 220 volt ) motor and hence the start capacitor is needed.

BQW · 6 Apr 2019

I understand about the 220v 2 phase USA system, however I thought as there is a phase shift in the two supply phases it would not need the cap, but I think what you saying is, as the phase difference would be 180 deg, it then needs a start cap.
Incidentally, the Motor has 3 connections marked 240v - N- 240v and earth.
Or is it because the machine is powered from a UK 240V single phase supply not a 240v American 2 phase supply. and both the" live connections" would be in phase ?
There is a 240v to 110v step down transformer in the machine , but as far as I can tell it does not feed the motor just the motor contactors A1 A2 and other components .
Incidentally I always understood a 110v site Tx was 55-0-55 and technicialy two phases.

JohnW2 · 6 Apr 2019

BQW said:
Incidentally I always understood a 110v site Tx was 55-0-55 and technicialy two phases.

As I've been saying, in terms of the 'pedantics of terminology', I think it really depends upon how one uses it ...

... if one connected some (55V) load(s) between the "0" and one of the "55s", and other (55V) load(s) between "0" and the other of the "55s", then I personally think it is difficult to escape the view that various loads are seeing two different 55V 'supplies', which are 180° apart - hence, to me, fairly convincingly '2-phase'.

However, that's not what we usually do. Instead, we usually connect a 110V load between the two "55s" and (as far as supplying power to the load is concerned) 'ignore' the "0V" output. In that case, the load is just single a single-phase 110V supply.

Kind Regards, John

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TWO PHASE MOTOR

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