Phase shift motor control?

[throbscottle]

I had an interesting thought whilst I was munching my sandwich the other day. Supposing I have some kind of motor and I feed it a phase to each lead. So, if the 2 leads are 180 out of phase, the motor will run at full power. If the 2 leads are in phase, that brakes the motor because it acts like a short. But if I can adjust one of the phases so it is some other amount then a variable amount of current can flow through the motor. If I do it using a square wave, the current starts to look like an AC version of PWM.

So the question is, is there a useful application for this? Is it actually done anywhere? Or did I just have a sandwich-induced brain fart?

 

[audioguru]

Some AC motors synchronize to the mains frequency so their speed cannot be controlled by phase shift. Other AC motors are universal and are actually AC/DC and can have their speed controlled by phase shift.

It takes a powerful complicated circuit to adjust the phase shifts.

Have you forgotten about a cheap and simple light dimmer to control the speed of a universal motor?

 

[throbscottle]

Since I don't have an application in mind, light dimmers didn't occur to me, the phase shift idea was just something that occurred to me out of the blue, as it were and I thought I'd see what anyone has to say about it.

I like the phrase "powerful complicated" - makes it sound like mysteriously and intractably complicated

Thanks for the reply though

 

[dr pepper]

A sort of similar method is/was used in aicraft instrumentation motors, 2 phase drag cup motors, they have a phase constantly energised, and another phase that is shifted to speed up/slow down/stop the motor.
They were designed to reduce inertia so a steam guage style aircraft instrument could have a rapid response.

 

[throbscottle]

Fascinating! I suppose the shifting phase would be related to other electromechanical devices that produce it inherently???

 

[dr pepper]

Depends what the display is for, but yes a lot of old planes use syncro's which work on that principle, 'cept they tend to be 3 phase.

 

[shortbus=]

Don't remember the details or what it was called, but didn't they use some thing similar to this to aim the big guns on battleships? One motor phase shifted to all of the other motors so the aim point was synchronized.

 

[throbscottle]

Surely the big guns would need something more? You can tell them all to go the same way, but supposed one gun got knocked out of sync, it would swivel around with the others but always be off. Can't say I like talking about guns *shudder*

 

[dr pepper]

Thats correct, only there is 2 synchro's, one on the sights and the other on the weapon, a control transfomer and amplifier produces a output to go to the servo that aims the weapon, quite clever, and it had to be seeing as processors were not in common use then.

 

[throbscottle]

I'm assuming synchro here means synchronous motor and not something else?

 

[dr pepper]

Nope, something else.
A synchro is a rotary transformer and or a rotary actuator pair, the actuator follows the position of the transformer, down to zero speed.
Have a look here if your interested:

 

[throbscottle]

Ah, that makes more sense. I watched the video with sound off, interesting all the same. I'll watch it with audio another time...