DC motor terminals wrongly labelled for Armature and Field

[digitrex]

I had 2 DC motors,from same manufacturer and installed at the same time, found with wrong labels for Armature winding(terminals A & F) and Field winding (terminals J & K).

These motors runs from 110Vdc battery banks and applied as emergency lub oil pumps. During the commissioning, we did not notice the problem because it was test run as a standalone pump. However, recently before the problem surfaced, it was somehow started and pumped against high luboil pressure because there was another AC motor luboil pump running. This time the higher motor current(due to parrallel operation of 2 screw pumps) of DC motor caused the Field series resistor burnt off. The starting resistor was only used in start-up and it did not help in reducing the field voltage.

We confirmed that the Field and Armature terminals were wrongly labelled by measuring the winding resistances (actual armature winding has lower resistance and field winding has higher value). After this problem was discovered the armature resistance was measured about 4ohms while the datasheets shows 0.213ohm. I afraid the operation of the DC motor on wrong connections (incorrect resistors connected) has cause some problems on the motor windings.

Can someone suggest what I need to check on this motor?

 

[ScottyUK]

digitrex,

Are you saying that the each winding had its terminals swapped, or that the windings were transposed i.e. the armature connected to the field controls, and the field connected in place of the armature?

If the latter, I can understand why the field weakening resistor burnt out. I am surprised that the wiring error occurred - armature windings are usually in much heavier gauge than field windings.

I would be very surprised if you have damaged the motor. In this application the field is almost certainly rated for direct connection across the 110V DC, in which case it will be ok. It is definitely worth checking that the field is rated for direct connection: if not, it may be toast.

The armature will have seen less than rated current because of the presence of the field weakening resistor in series, so it is highly improbable that it will be damaged. If you are measuring armature resistance from the motor terminals, you are very probably getting measurement error because of the non-linear resistance characteristic of the commutator at low current and voltage. You need to employ a higher test voltage and current, and you should get a more accurate measurement of the resistance value.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[digitrex]

ScottyUK,
The windings were transposed, resulted in armature current flowing through the field resistor continuously. With a resistor in series with the armature, the pump hardly turned.

Anyway, we have already tested again that the motor run normally after rectification. However, after that test run, we dismantled the motor and bring it to workshop to be opened up for inspection. Your suggestions on what to check are welcomed.

 

[digitrex]

For the 2nd DC motor, we haven't encounter burnt resistor yet but our follow-up actions on it will depend on what we will observe on the first motor in the workshop tommorrow.

 

[ScottyUK]

Hi digitrex,

Opening motors up to repair them isn't my field of expertise - hopefully Edison123, aolalde, or ElectricPete will add their weight of knowledge to the inspection.

My opinion, for what it's worth, would be to spend most of the time checking the field winding. The armature will have seen very little voltage and a small current, so the commutator should be immaculate. If you have original measurements for the field winding - the OEM should have test data for such a new motor - you can do a comparative measurement. Have you identified the rated voltage of the field winding? If it is 110V, then I honestly don't think you have anything to worry about.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[edison123]

digitrex,

I assume this is shunt field motor with the same voltage for both field and armature. So, even if the leads were mixed up, both the armature and the field would have seen the same 110 V. So, no damages due to overvoltages there.

Because of the lead mix up, the field resistor (with small ampacity) got into armature circuit with probably high starting currents (which is normal for the armature) and blew up. So the resistor had acted like a fuse and the armature circuit was opened after the resistor blew up. So, no damages to the armature due to overcurrent.

As for field, in the absence of field resistor, it would have seen more than the normal current seen with the series resistor in circuit. But then, field windings are rated for full current (without the field resistor. So, no damages to the field winding either due to overcurrent.

In short, you blew the series resistor. Chalk it up to learning expenses and re-identify the leads properly for both the motors.

 

[digitrex]

Dear all,
The motor is opened today. The armature and field windings are all in good conditions. Therefore, we don't have any concern except to correct the drawings to reflect the actual connections.

 

[digitrex]

Thanks a lot to ScottyUK and edison123 for the advices.

 

[aolalde]

Normally a DC motor does not need a resistance in series with the field winding. By doing that, the magnetic field is weakening and the motor lose torque and increases the inrush armature current.

A resistance makes sense in series with the armature winding. This will reduce the inrush current. After a few seconds that resistance must be bypassed allowing full voltage to the armature under load operation.

Check the motor manufacturer wiring diagram. Hopefully the field and armature Voltages of your motor are the same.