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# Problem In A Dc Motor

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Dear All

We have a machine which has two DC motors 93.8kw and 5.5kw. One motor (M1) drives the corrugating rolls which produce the board and the second motor (M2) pulls this board. The specs of both motors are as following;

M1

Kw = 93.8

Va = 400

Ia = 256

Vf = 330

If = 3.16

M2

Kw = 5.5

Va = 400

Ia = 16.5

Vf = 110/no other information

If = 0.95/0.70

Rpm= 2700/3200

There is only one DC drive which controls the speed of both motors. M1 is the master motor which has a tacho generator for sending feed back to the drive while M2 has no tacho generator. If the speed of rollers driven by both motors is not the same we have to adjust the speed. For this reason we only regulate the field voltage of M2, presently set at 35vdc while it is designed at 110vdc.

Now I am facing a problem in M2 that as I increase the speed of machine and the motor reaches at 2650-2700rpm, a spark produces on commutator and the circuit breaker (16A) trips. The machine works well bellow 2300-2400rpm and the motor draws 8-12A. I have checked the motor without load but found the same problem. I have generally checked the motor and its carbon brushes and found ok.

Please help me if any one has any idea.

Regards

"Don't assume any thing, always check/ask and clear yourself".

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Hello AB2005

Is this a new machine that you are building? or is it an older machine that has been workin correctly and now just started to play up?

Best regards,

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Thank you Mark for your responding.

Actually the corrugator, machine which I have recently erected, has some old units and some new units not a complete new. The unit name “Single Facer” is also a 15 years old, part of machine in which I am facing the problem. Two motors with one drive also states the old technology. During commissioning of machine, we haven’t found the problem. But now, when machine is in production, this problem is generated.

"Don't assume any thing, always check/ask and clear yourself".

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Hello AB2005

we only regulate the field voltage of M2, presently set at 35vdc while it is designed at 110vdc.

This concerns me.

You must be very careful in reducing the field excitation.

It is normal to regulate the voltage applied to the armature, not the field. The reason for this is that the shunt wound DC motor has a rated speed and a rated field voltage. If you apply full armature voltage and full field voltage to the motor, it will not exceed it's rated speed. This is because at the rated speed the voltage induced in the armature, by the action of the armature rotating within the field flux, is equal to the armature voltage. If you reduce the armature voltage, the speed will reduce. If you reduce the field voltage, the speed will increase because the induced voltage will reduce.

The field voltage is normally kept constant at the rated field voltage. In some situations, the field is reduced on a motor when it is operating at full speed to enable it to run a little above rated speed. This is known as field weakening.

I believe that in this case, you are reducing the field voltage to reduce the drive torque. For a fixed load, the motor will slow until the torque produced equals the torque required by the load.

The effect of reducing the field and operating the motor at reduced speed, is that the voltage induced in the armature is reduced. This results in much higher commutator voltages and will cause commutator sparking.

If you are wishing to reduce the torque, I would recommend that you reduce the armature voltage, not the field voltage.

Best regards,

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Thank you Mark.

I am sorry to say that this is originally designed technique. The manufacturer has given a regulator in the panel for this purpose. We have another unit same that in the machine which has the same control. I have checked and found that its field winding (M2) has also adjusted at 35v. Normally both motors of both units draw 8-10A (armature). But as I increase the speed of faulted unit and motor reaches at about 2400rpm, spark generates on commutator and at last a huge spark produce and breaker trips. But the motor in other unit don’t trip.

"Don't assume any thing, always check/ask and clear yourself".

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Hello AB2005,

I suspect that the problem is that the induced voltage is low and the result is that the commutator voltage is high and at the higher speed, possibly with some commutator/brush wear, the increased voltage is causing a spark. Once you get some arcing, the ionized air can cause an insulation breakdown that can blow fuses etc.

I would suggest that you get the commutator checked. Also check the brushes and spring tension.

Best regards,

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Ok, I have checked the all but found everything ok in the motor.

I have also found that as the apeed of machine crossed 225M/m (above 2300rpm of M2) a little speed variation observed. There are two tacho generators on master motor M1 TG1 and TG2. Both are the same rating. TG1 is for feed back to drive and TG2 is for displaying speed on control desk. I have interchanged the wires of both tacho generators and found no physically speed variation and no problem in speed meter and most interesting thing was that the problem in motor M2 (tripping at full speed) was also disappeared. I don’t know how could that possible that tripping was ocured in only M1 as the speed variation problem was generated in both motors (M1 and M2) because both are powering via same drive.

For now, the fault is removed but I am sure it will appear in second time because theoretically it is not possible what happened.

Any how, thank you Mark for your comments.

"Don't assume any thing, always check/ask and clear yourself".

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Ok, I have checked the all but found everything ok in the motor.

I have also found that as the apeed of machine crossed 225M/m (above 2300rpm of M2) a little speed variation observed. There are two tacho generators on master motor M1 TG1 and TG2. Both are the same rating. TG1 is for feed back to drive and TG2 is for displaying speed on control desk. I have interchanged the wires of both tacho generators and found no physically speed variation and no problem in speed meter and most interesting thing was that the problem in motor M2 (tripping at full speed) was also disappeared. I don’t know how could that possible that tripping was ocured in only M1 as the speed variation problem was generated in both motors (M1 and M2) because both are powering via same drive.

For now, the fault is removed but I am sure it will appear in second time because theoretically it is not possible what happened.

Any how, thank you Mark for your comments.

maybe the breaker is faulty??

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maybe the breaker is faulty??

No Carl.

If the breaker is faulted then why the sparking on commutator when at full speed? Breaker was triped when high spark generated on commutator.

"Don't assume any thing, always check/ask and clear yourself".

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• 1 month later...
No Carl.

If the breaker is faulted then why the sparking on commutator when at full speed? Breaker was triped when high spark generated on commutator.

carl you may check the vibration level for each speed equivalent i have a doubt that the sparking occur during full speed and probably theres a high vibration causing the spark

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