Squirrel cage motor windings

[AlwaysConfused]

Hi, I heard once from a very clever electrician that the windings of a 3 phase induction motor can be wired up backwards and burn the motor out (i think slowly). What i mean by this is if you have 6 cores coming out of your motor all black, none labelled, then simply testing for each winding and taking one core from each for a star point and adding 3 phases to the other isn't good enough (assuming star connection is good enough). And that a further test has to be done to ensure the cores are correct, ie u1 v1 and w1 must be found. Has anyone else heard of this, because I always thought that as it was ac and the current was at 50hz, that the winding couldn't possibly be backwards.

 

[jraef]

No such thing as forwards and backwards when referring to winding connections. Your electrician friend is proof as to the fact that electricians don't necessarily know what they are talking about.

If you connect a 3 phase motor incorrectly there are 3 things that can happen (assuming IEC configurations for a dual voltage motor);

1) it runs fine, but in the wrong direction.
2) it doesn't run at all, i.e. no complete connections
3) it gets connected in star instead of delta or vice versa. In this case, if the motor were designed for star connection at your given voltage, and you connected it in delta, your applied voltage would be too high for that configuration and the motor would saturate, overheat and die eventually if left like that. If your motor were designed for delta connection at your given voltage, and you connected it in star, your voltage would be too low and the motor would stall from lack of torque, which may burn the motor out if left in that condition.

 

[Skogsgurra]

But, if one of the windings is connected 180 degrees wrong? You would then have three field vectors spaced 60 degrees apart instead of 120 degrees and the sum of them would not be a rotating vector with constant length, but one rotating and one pulsating, I think. I guess that that could increase temperature a bit. Perhaps too much?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[AlwaysConfused]

Thanks, that confirms what I thought, he said he learnt it in an advanced motor principles course, he also said that not many people were aware of it and so consequently burn out motors. Maybe he was pulling my leg.

 

[stardelta]

I think your friend may be talking about what I know as a "Reversed Phase". The 1 and 2 (or start and finish if you like) of a phase, be it u v or w become reversed. The condition is very obvious, particularly on larger machines. Symptoms include: Lots of vibration, unbalanced current demand, loss of power and a reduced running speed. If the fault is not recognised or the motor protection devices are not up to the job the motor will ultimately overheat and burn out in a very short time.

 

[AlwaysConfused]

I think i see, i suppose if the coil is reversed then the current induced in the squirrel cage will be in the opposite direction even though the phase rotation is still correct.

 

[AlwaysConfused]

And if that is true then my next question is how do you find the correct ends of your windings.

 

[Skogsgurra]

A rather simple way is to connect all three windings in series and leave one corner open. Connect a sensitive AC voltmeter to the open corner and turn the rotor.

You will then get a three-phase sine voltage (a few volts or less) induced in the three windings. If all windings have the correct polarity, the voltage will be very low since three 120 degree voltages cancel. If you do not get almost zero volts, then change one winding and see if you get close to zero. If that winding didn't do it, then try next.

Depending of remanent flux in the rotor, you will get a higher or lower voltage. You may also get a non-zero voltage if the flux doesn't produce a perfect sine voltage in the windings. But you will always get the lowest voltage when the connection is correct.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[petronila]

Always Confused,

For a standard 6 Lead Motors (Not for Dahlander or Part winding starting) the external connections could be Wye or Delta,6 Leads NEMA Motors are labelled with numbers from 1 to 6.The thread No. 237-153197 can help you with procedure to identify the leads, for low voltage squirrell cage motors. Normally For a wye connection we conect L1--1, L2--2 and L3-- 3 and Joint 4-5-6.I think when you are talking about "wired up backwards" you are refering to perform the Wye connection like L1--4,L2--5 and L3--6 and Joint 1-2-3.

Wondering, when you have 6 numbers missed leads you have to identify each one. When you have all numbers you can connect the motor in Wye or delta and energize with Voltage, but take account you are not shure if the new assigned numbers are the original lead labels,but the motor Runs,and the No load current can tell you if is working in the proper connection.

For Delta conection the thinks are not differents but you have two ways to perform Delta Connections the First is L1--1&6,L2--2&4,L3--3&5 Second: L1--1&5,L2--2-6 And L3--3&4.We have experienced two problems with air compressor motors when the starting method is star-delta, we noted the motor start proper in Wye but it trips the breakers when perform the change to Delta,to solve the problem we twisted the DELTA Connection (From fist to second or second to first), the problem dissapears.

Regards

Petronila

 

[ACY]

Practical soloution.
Try each different connection - that is in Star connection - and apply power.
Label each lead with a,b,c a1,b1,c1.
connect start point, apply power to relevnt cable and listen to motor. If not good, interchange one of the phases. IE interchange a & a1.
apply power and try again.
If no good, cotinue interchangin process.
If power only applied for a short time, and motor has no load, you wont burn it out.