other than swapping two of the three leads...

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nosparks

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I understand how to reverse a three phase motor by simply swapping two of the three leads. I have spent the better part of the last two nights researching the issue theoretically, only to have the majority of text books basically tell me that by swapping two of the three leads to the stator, I will displace the phases in a manner to cause the stator's magnetic field to rotate in the opposite direction. Ok, I believe this but, as I stated the examples are just that, basic. Can somebody help me with an illustrated example that is somewhere between basic and to much detail.
As always, thanks for your help in advance! :cool: :confused:
 
Re: other than swapping two of the three leads...

There is two methods for reversing a three phase motor.

#1. Reverse any two leads.

#2. Reverse the rotor in the stator.
 
Re: other than swapping two of the three leads...

The rotor doesnt care which way it is oriented unless you mean you will be looking at it from a different direction? The stator "polarity" and by that I mean the phase orientantion is the only way to reverse it.
 
Re: other than swapping two of the three leads...

nosparks,
As you probably know, the currents in the three phase conductors supplying the motor are 120 electrical degrees out-of-phase.
That is, if the phase sequence is A-B-C, the current in Phase B reaches maximum value 120 deg later than Phase A, 360 degrees being one cycle, or 1/60 of a second.

The sketches below represent the windings of a two pole, three phase motor, simplified. In a two pole motor, electrical degrees in the current waveform are equal to mechanical degrees of rotation.

Each "view" (1, 2, 3, etc) shows the magnetic field produced, and the sine wave of the three currents at that instant. Each "view" is 30 degrees later in time.
Note that in a three phase system, when current is maximum value in any phase, it is approximately half of maximum in the other two phases, at that instant.

In Diagram A, the phase sequence is A-B-C, (red-black-blue, please excuse the Canadian color coding) resulting in CCW rotation.

In Diagram B, two supply conductors have been interchanged, the phase sequence is now A-C-B, and rotation is now CW.

Ed


3Ph%20Motor1.gif


3Ph%20Motor2.gif


[ March 27, 2004, 08:17 PM: Message edited by: Ed MacLaren ]
 
Re: other than swapping two of the three leads...

My reference to changing the rotor direction in the stator, is when the make up box needs to be changed to the opposite side.

By turning the stator 180 degrees, the make up box is on the the other side.

The motor will be reversed, the motor leads will have to be changed to make the motor run in the correct direction.
 
Re: other than swapping two of the three leads...

Hey Bennie I always found it easier to take my armature out and turn it instead of the stator. :)

[ March 28, 2004, 03:43 PM: Message edited by: ronaldrc ]
 
Re: other than swapping two of the three leads...

Originally posted by pierre:
Ed
You are good :) :cool:

Ed I do have a question here, I do not understand this.

Note that in a three phase system, when current is maximum value in any phase, it is approximately half of maximum in the other two phases, at that instant.
I would have thought it would be 1/3 or 2/3 not 1/2.

Why does it come out to 1/2? :confused:

[ March 28, 2004, 05:26 PM: Message edited by: iwire ]
 
Re: other than swapping two of the three leads...

No problem with the photo, Bob. It reminds me of the "good old days". :D

I would have thought it would be 1/3 or 2/3 not 1/2.
Why does it come out to 1/2?
In order to obey Kirchoff's Law.

It states in effect, that the vectorial sum of the currents in a circuit is zero. This the same as saying that, at any instant, the current flowing to the load must be equal to the current flowing back to the source.

Take an example (Note-the sketch is not-to-scale) of a three phase circuit with a balanced load of 10 amps (rms).

If we look at the instant when Phase A is at maximum value, the A phase conductor would be carrying 14.14 amps, let's say, towards the load.
At that instant in time the other two phases are equal and opposite in polarity, so the B and C conductors would have to be carrying 7.07 amps back to the source.

Try this sometime. Put your clamp-on ammeter around each wire of a three phase circuit with a balanced load, such as a motor branch circuit.
Then put it around any two wires at a time.
Finally, put it around all three circuit wires. Of course, these will all be rms values.

Ed

3Ph%20Motor3.gif


[ March 29, 2004, 08:16 AM: Message edited by: Ed MacLaren ]
 
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