Overloading DC motors

[Skogsgurra]

Hi,

I visited a mobile container crane the other week. DC motors and thyristor controllers fed from a diesel generator. Customer complains about repeated undervoltage tripping.

I immediatley felt an acrid smell around the crane. Smell came from cooling air exhaust for hoisting motor. Found out that current was around 1200 A when lifting 30 ft containers. Motor rated current is 636 A. Motor had been replaced. Was stronger and needed less current before.

Customer worried. Me too. Crane manufacturer had a motor with 750 A and 4.3 Nm/A installed before. New motor has 3.3 Nm/A. So, original motor needed less amps to lift the container and new motor (with lower rated current) needs more current.

Case closed as far as I could see. Repair original motor and put it back in service is what I recommended the customer. That will also take care of the undervoltage tripping caused by generator overload and deep/broad commutation notches.

But I had not taken the crane manufacturer into account. He now says that he knew about the overload all the time and that the new motor (somehow?) was designed to take the abuse.

I have some difficulties accepting this. What are your opinions?

To recap: Acrid smell around motor, close to twice rated motor current, undervoltage trips. What do you think?

Gunnar Englund

http://www.gke.org

 

[ScottyUK]

What does the motor manufacturer say? I suppose they have already said it once on the nameplate, but they would give the best answer.

My opinion... you are right in your diagnosis. The motor appears to be grossly undersized for the task. In a lifting application I'm surprised that the manufacturer is willing to stress the motor so far beyond its namplate rating. I'm not a crane expert, so I could be way off target with that last thought.



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

 

[DickDV]

I'd go with your observations and judgement, Gunnar. While it is true that 200% overload for a DC motor is not usually damaging for short periods, a container lift probably lasts longer than "short-term" and starts becoming damaging to the motor.

Plus the generator problems. Poor choice of motor, I agree.

 

[Skogsgurra]

Thanks,

The crane manufacturer is the one that installed this "tiny" motor in the crane. He now maintains that it SHOULD work. But he is very cautious to say anything about life of that motor. The truth is that the original motor had to be replaced because of sudden death. Probably due to overload.

My feeling is that the manufacturer is afraid that he shall have to compensate the customer for low productivity or something. So he tries the "stiff upper lip" routine - for a while.

Gunnar Englund

http://www.gke.org

 

[UKpete]

Skogsgurra, from what you say it is fairly obvious that the motor is overloaded based on the smell. It is very easy to spot an overloaded motor once it is stripped down, and it may also be visible from the condition of the brushes (and pigtail connections) and commutator. But I would like to make the following points.

The crane motor is a similar duty to a rapid transit traction motor, short periods of heavy duty interspersed with short periods of light duty or rest. I once did a survey of 13 different series dc traction motors rated between 41kW and 268kW (not particularly large machines, rated currents 160A to 440A), and found that the average maximum starting current (approx. 2 mins rating) was 2.12 x max continuous current. The one hour rating was typically 1.18 x max continuous current. What really determines the motor insulation life is the RMS current taken over a period of around the thermal time constant of the motor, say one hour. This assumes the maximum current isn't exceeded.

It is also possible to design a motor with improved short time current rating by increasing the length of the commutator and increasing the carbon brush area, without changing the continuous current rating. So it is possible that the manufacturer is telling the truth when they claim that an alternative motor can take more abuse.

 

[mc5w]

The two different sizes of motors are going to draw about the same amount of current for a given load. The 750 amp motor will be more robust. The hoist motor will have rest periods when the other axes of the crane are moving.

However, I would be worried about the SCRs and the gearbox. The SCRs do not have much in the way of thermal capacity and need to be rated the full 1200 amps. Likewise, the gearbox needs to be rated for the actual peak torque so that oil is not sqeezed out from between gear teeth, what is known as lubrication failure. In fact, the gearbox could be ruined by now because of the overloads.

Also, Cummins application and engineering information recommends that the generator be rated at least 200% of the actual current drawn by a 6 pulse rectifier such as a 3-phase in, DC out phase controlled drive. The generator also should be sized a little larger to account for the low power factor when the motor voltage is low and the SCRs have maximum phase angle from the voltage. Regenerated power needs to be limited to about 10% of generator rating. If regenerated power is greater than that then there has to be permanently connected lighting OR an electric heater that is connected to an overfrequency relay.