Induction motor

[gar]

171026-1038 EDT

An induction motor operates by inducing a current in the rotor from the stator. This is a very basic definition of an induction motor.

To be able to induce this current there must be relative motion between the inducing source and the rotor. This is call slip.

Synchronous speed of a motor is defined as f*60*2/N where f is the frequency in Hz and N is the number of stator poles. For 60 Hz and two poles this is 3600 RPM.

To produce any output shaft mechanical power there must be slip. This means that a real world induction motor can never produce any output power at or above synchronous speed.

There is nothing that can be done to the input waveform that can make the motor run at synchronous speed.

Anyone trying to sell you something to do this is ignorant of electrical circuit theory, and is simply a "snake oil" salesperson. Anything else that person says is certainly suspect.

.

.

 

[junkhound]

aha - I'd been waiting for your response to the query on BS type power quality claims I'd started a few days ago.. appears mods closed it before you weighed in hmy:

I was waiting for the apologist of the subterfuge ad to tell us how his magic device could make an induction motor run at sync speed with zero losses, but he flapped his advertising gums once to often and got canned.

 

[Jraef]

Seems obvious, but in light of recent activity in this and other sources on the internet, I think it was worth repeating and clarifying. Thank you.

One basic tennet if a lot of nefarious activities is to prey on ignorance. In the case of a lot of electrical "energy saving" devices and schemes, ignorance of the details of how and why electrical things work the way they do is how the perpetrators attempt to make their case for how THEIR "revolutionary" or "new" discovery solves problems that you didn't know you had. In the vast vast majority of cases the reason you didn't know you had a "problem" is simply because it isn't a "problem" and never was. It was just something about the way things work that you were not aware of.

So thanks again gar for reiterating this basic fact.

 

[zbang]

In the case of a lot of electrical "energy saving" devices and schemes, ignorance of the details of how and why electrical things work the way they do is how the perpetrators attempt to make their case for how THEIR "revolutionary" or "new" discovery solves problems that you didn't know you had.

And when you're trying to explain things to electrical PEs, saying "it's complicated" without detailed factual backup info just doesn't work.

Perhaps the real ignorance is not knowing the audience .

 

[Besoeker]

Perhaps the real ignorance is not knowing the audience .

And perhaps that's a more profound statement than you realise.
:thumbsup:

 

[Sahib]

To produce any output shaft mechanical power there must be slip. This means that a real world induction motor can never produce any output power at or above synchronous speed.

.

At above synchronous speed the induction motor produces electrical power output.

 

[retirede]

Snake oil has been peddled since the beginning of time. And I don't expect it to stop anytime soon - as long as there are potential buyers who don't know any better.

 

[Besoeker]

At above synchronous speed the induction motor produces electrical power output.

In such circumstances it would be more correctly termed an induction generator.
I played around with this in the 1980s for off grid generation at local level. I got a 30kW unit up and running based on a submersible pump running as a turbine.
It would have been good for third world countries. A smattering of interest but no investors. Another one left to gather dust.

Life moves on. Except sometimes it doesn't.

 

[Sahib]

In such circumstances it would be more correctly termed an induction generator.
.

Even during course of normal operation, an induction motor may generate power.

 

[Besoeker]

Even during course of normal operation, an induction motor may generate power.

Your esteemed words of practical experience are greatly appreciated.

 

[kwired]

Even during course of normal operation, an induction motor may generate power.

It may have generating periods but the inertia of the driven load was put there by the motor in the first place, so you never get more energy back then what you put in without another outside force being involved.

 

[Besoeker]

It may have generating periods but the inertia of the driven load was put there by the motor in the first place, so you never get more energy back then what you put in without another outside force being involved.

You'd have to have an external source to get it above synchronous speed. Or a variable frequency drive during deceleration of a high inertia load.

 

[winnie]

You'd have to have an external source to get it above synchronous speed. Or a variable frequency drive during deceleration of a high inertia load.

I have been told (but have no direct evidence) that pumpjack oil wells ('dipping donkeys') will drive the motor above synchronous speed during part of the pump cycle. The cycles are slow enough that the power meter will run backward for a portion of the time.

This would make the _cyclic_ mechanical load the 'external source' that you mention to drive the motor above synch speed.

-Jon

 

[Besoeker]

I have been told (but have no direct evidence) that pumpjack oil wells ('dipping donkeys') will drive the motor above synchronous speed during part of the pump cycle. The cycles are slow enough that the power meter will run backward for a portion of the time.

This would make the _cyclic_ mechanical load the 'external source' that you mention to drive the motor above synch speed.

-Jon

Yes, I can see that as a possibility. Nodding donkeys is what I have heard them called.

 

[Sahib]

a variable frequency drive during deceleration of a high inertia load.

Presence of VFD is not required for an induction motor to generate power and feed it into grid, for example, during the descent of an elevator.

 

[GoldDigger]

Presence of VFD is not required for an induction motor to generate power, for example, during the descent of an elevator.

B. did say "an external source OR a VFD", so why bother disputing that?

 

[Sahib]

B. did say "an external source OR a VFD", so why bother disputing that?

I don't think B meant that way. I think my last edited post corrects him.

 

[Besoeker]

I don't think B meant that way. I think my last edited post corrects him.

"You'd have to have an external source to get it above synchronous speed."
Are my actual words.
A descending elevator is an external source.

 

[GoldDigger]

"You'd have to have an external source to get it above synchronous speed."
Are my actual words.
A descending elevator is an external source.

And I would not call it a high inertia load in the sense that you meant it. Although that arguably applies.
Even elevator motor deceleration from rapid movement will not drive the motor above sync speed, unless the elevator is designed to move with and against gravity asymmetrically.

 

[Sahib]

"You'd have to have an external source to get it above synchronous speed."
Are my actual words.
A descending elevator is an external source.

Very clear.