Old DC motors and drives

[dmeunier]

Hello
I'm working on an crane electrical rebuilding with existing DC motors from general electric.
These motors will be conserved and are supposed to be controled by new thyristor drives (from abb or rockwell).

Existing motors are about 30 years old.

My question is to know if that kind of motors can be controled by thyristor drives without problems or if I have to be careful to something specially.

It would be kind if someone would share his knowledge.

Thank you.

 

[yoryolo]

To control old motors, of that amount of years, is not very recommendable. Anyway, which is but affected, is aislacion of the motor. Depending of the power and the existing distance of drive to the motor, he would be recommendable to use exit filters. Consultation with the manufacturers of the equipment that is what they recommend.

Regards

 

[dmeunier]

I have some dificulties to find general electric old motors specifics.
The customer doesn't have the documentation.
Does anyone know where I can find informations about GE old motors.

I have heard that depending of poles technology it is not sure that old dc motor can be controlled with thyristor drive.

Any idea or link ?

Thank you

 

[niallnz]

i once had to investigate the excessive noise from an old DC motor on an extruder, which had recently been up graded to thyristor drive control. The basic problem was that the armature construction did not has inductive as a similar sized modern day motor. Hense the ripple current was very significant and casued considerable noise at frequency that was particularly annoying to people. It was probable causing additional heating in the armature circuit but in the aplication was not causing any problems.

The solution was to add a series inductor into the armature circuit or a tuned filter (tuned to the ripple frequency) across the armature circuit.

You could put this problem back onto the bidding suppliers an see what they recommend.

 

[jbartos]

Suggestion: Compare the quality of DC power supply between the one motor is receiving now with respect the power quality output the motor will receive from the DC motor drive. Select the better one. An oscilloscope or a power quality meter will be needed.

 

[edison123]

I have clients who have many 40 to 50 year old dc motors retrofitted with modern dc drives and working perfectly fine.

I do not understand how the inductance of a dc motor will affect the normal operation with dc drive (unless ofcourse the drive itself was producing severe ripple content).

 

[electromike]

A 30 year old motor design is not much different than what is produced today, but the motor is most likely worn out.
If a thyristor drive cannot run this motor than nothing can.
Modern thyristor drives will have no problems with a motor with different armature inductance since all motors armature inductance varies and the drives have parimeters that allow you to tune your drive for this.
On the question of low armarture inductance causing ringing,
I have never heard of this. The reason I would install reactors on the armature side of a system woud be to correct
for discontinuous current,not as a filter. Almost nobody
bothers filtering the 360Hz ripple that comes from a Full wave 3 phase bridge due to the low amplitude of the ripple.
What I woud do is test the motors to EASA standards, and if they pass I wouldnt think twice about using a modern thyristor drive with them.

 

[TomG33]

Hi,
From my experience with old GE Motors under the control of SCR Equipment, The main area of concern is the pole face windings ( compensating windings ). The 300 or 360 Hz torque ripples tend to cause the poleface bars to "Rattle" in the holes in the poles, causing Chaffing of the insulation and eventual insulation failure.

These types of windings are only included in the larger motors, and I am thinking that these are most likely not used on any Crane application I have seen. Could be wrong here.

SCR Drives have been used to drive Motors for very close on 30 years now so maybe your motors are built in an era when the design takes into consideration the possibility it could be powered by an SCR Drive. I have worked on GE motors powered by SCr's that were commissioned prior to 1976 and there appeared to handle the supply Ok.

Tom

 

[UKpete]

Speaking from a different perspective (rail traction rather than cranes) I don't think there were any significant changes to the insulation systems etc when we made the changeover from resistor control to thyristor. The waveforms to the motor were pretty awful, but not the high frequency pulses that caused the problems with ac induction motors.

The only problem I came across was with closed-loop speed control systems, the old design of dc motor had a thick solid steel frame that carried the main field flux. The flux couldn't change quickly in response to field current changes (an eddy current effect), this caused closed-loop stability problems which were cured by laminating part or all of the motor frame. In addition to this there is additional heating in the frame due to the eddy currents because of the supply waveform.
This is the reason that modern dc designs have fully laminated frames - usually recognizable as a square rather than a round frame e.g.:

http://www.reliance.com/pdf/catalogs/imc_2004/dc_motors_med.pdf

In summary, you may have a problem with additional heating in the frame of the motor, and if you are operating in closed loop (with a tacho) there may be control stability problems. It may be worth trying one out first if possible.