Motor Poles and Y connections 1

[kevstar]

I hope someone can shed some light on this for me!

A few weeks ago I had a 3 phase motor burn out at work. I took the old one off and went to the stores to get a replacement. When I looked at the rating plate of the motor it said it was a YYDelta motor, when I checked the new motors windings it appears that there was a circuit between the three windings, apparently there is a star connection inside the motor. I replaced the motor and the machine started working and I sent the old one off for repair.

I decided to phone the re-wind company to find out why there was a star point connection inside the motor. They told me that this was for varrying the speed, If I connect the three terminals up inside the connection box I get a low speed, If I connect the six up in a Y connection I get high speed. This is because with the external Y connection in the circuit the number of poles has been decreased from 4 to 2 and therefore the motor speed has increased.

My questions are:

I know the equation for motor speed against pairs of poles but WHY does the increase in the number of poles slow the motor down. I would have thought that as the number of poles increased so the rate of change of polarity increased in one revolution therefore the rotor speed would change rate quicker and the motor would go faster, I certainly cannot understand why it would slow the motor down.

Also

How does an internal Y connection increase the number of poles anyhow, I have tried drawing this out on paper and really cannot understand how the addition of a Y point inside the motor doubles up the number of poles. It would be very much appreciated if anyone has any drawings of the connections here that they could send them.

Thanks for your help,

Confused
Kev!!!

 

[kabartey]

hi you said you know the equation
the sencron speed of the motor is 120*freq/polenumber
the rotor speed is (1-s)*sencron speed this is why increasing the pole number decreases the speed of the motor

it is called dahlender the way of changing the pole number with some trick like combination of the windings within the motor

 

[kevstar]

thank you for your response, I really wanted to know the physics behind this. I realise that the formulas are correct but I just didn't want to take it for granted that increasing the number of poles slows the motor.

Thanks for your help

 

[ccjersey]

The reason incresing the number of poles decreases the rpm of a motor is that the circumferential distance around the stator that the rotating field moves during each cycle is less. Or in other words it takes more electrical cycles to complete each revolution because each pole is a smaller part of the whole circle.

I don't think the internal star point is necessary or even used for two speed operation unless there are two complete windings with different numbers of poles wound in each. The other type is called consequent pole. I have diagrams to show the connections 2 parallel star = high speed and delta = low speed. The star has to be able to be broken to go to the delta low speed connection. The delta arrangement in this case has twice as many poles as the series star.

The internal star is used for single and dual voltage wye wound 3 phase motors. The circuits are low voltage = 2 parallel star (one star is internal (T10,T11,T12 if they were brought out) and one external T4,T5,T6).

High voltage (or single voltage) = series star where the circuits are T1-T4+T7-T10, T2-T5+T8-T11, and T3-T6+T9-T12.Again the permanent star is what would be called T10+T11+T12 if the leads were external.

Hope this helps,

Jim

 

[kevstar]

Thank you that was just what I wanted to know.

re motor poles, I think I am missing a fundamental point here, does the speed of the rotating field within the stator slow down then because of the number of cycles it has to make?

If you have those diagrams of the windings that would be very helpfull. The diagram I have here shows a Y connection and a 2//Y connection. I think that the Y connection is broken but on the outside of the motor (i.e. in the control panel) there is only 1 hard wired Y point inside the motor. Here is the connection diagram that I was sent.


o o o
= (Y) Low Speed
o o o
l l l


o----o----o
= (2//Y) High Speed
o o o
l l l

o = terminal
l = supply connection

The terminals connected together in the //Y connection must be the Y point that is broken during low speed operation. So if we are saying that one Y is low speed and 2//Y is high speed, I would have thought we have just connected more windings with the 2//Y connection and therefore increased the number of poles!!!!!???????

Thanks again for your help, this has really got me baffled!

Kev

 

[itsmoked]

Does more poles also equal less rotational torque variation?

 

[SteveKW]

It slows down because of Time and Distance. A 2 pole motor goes faster since point A & B are farther apart. Double the poles and the distance is now half but you still take the same time.

http://www.usmotors.com/Connections/Connections.htm

The pole changing doesnt come from connecting "more" windings. It's a function of reversing half the poles so that the number of poles doubles!

http://www.eng-tips.com/viewthread.cfm?qid=113713

 

[ccjersey]

"So if we are saying that one Y is low speed and 2//Y is high speed, I would have thought we have just connected more windings with the 2//Y connection and therefore increased the number of poles!!!!!???????"

It depends on how the windings are arranged inside whether there will be more poles or not. In the 2 voltage motor, the speed is the same when switching voltages so poles don't change, magnetic field strength doesn't change, only voltage, resistance and amperage.

I cannot tell from your diagram if the top 3 terminals are connected to anything on the low speed Y connection.

Does this motor have 2 hp ratings on the nameplate? or just one? How about amperage?

"Does more poles also equal less rotational torque variation?"

Don't know. Maybe. Motors have a smooth torque compared to an engine, but have never considered that the torque might have peaks other than those related to rpm. For example the speed-torque curve for a specific motor will show the startup,"breakdown" and rated torque related to the rpm.

Jim

 

[kevstar]

To be honest, the motor has now been sent away for rewind, but I can remember that it had two speed ratings they were 1390 and 2700. I do have a diagram of how the windings have been wired but not sure how to get it on this message board!! When wired up as the slow speed then the other terminals are not connected to anything at all.

 

[edison123]

kevstar,

Your winding seems to typical dahlender winding.

See if this is helpful.

http://www.joliet-equipment.com/ter...tions-3ph_motors-two_speed-single_winding.htm

 

[DickDV]

kevstar, to understand the relationship between the number of poles and shaft speed, the first fundamental thing to grasp is that no matter how many poles are in a motor's stator, it takes one cycle of the AC power to move the rotor from one pole to the next pole.

Second, in a two pole motor, one complete rotation is divided up by only two poles so there is 180 degrees between poles. Each cycle then would produce 180 degrees of rotation.

Now, in a four pole motor, one complete rotation is divided up by four poles so there is now 90 degrees between poles. Each cycle then would produce 90 degrees of rotation. Or, to say it another way, it would take two cycles to produce one complete shaft rotation.

This process continues thru 6,8, even 24 poles to slow the motor speed down at any given frequency.

As for torque, it seems obvious to me that if you have only two poles pulling on the rotor, you would have half as much torque as if you had four poles pulling on the rotor. As so on, the torque goes up as the poles go up and the shaft speed goes down.

Hope this makes it clearer.

 

[kevstar]

Thanks DickDV

I now understand it. I hadn't realised your funamental point. I thought that one cycle of power was 360 deg rotation in the stator I didn't realise that one full cycle of power is completed between poles, this would account for the motor slowing down. I know see, thanks very much.

Edison123, thanks for that weblink, I am going to checkout the motor today and see if those diagrams make sense now.