Effects of Low Field Current on DC Shunt Wound Motor 1

[jktwn]

We have had 3 50HP DC motors fail in the same location in the last 18 months. It has also been noticed that commutator brushes wear out quickly on this motor (3 mo). Failure modes on last 2 motors are shorted field windings and shorted armature windings on most recent failure.

Drive checks show that field current is 0.5 Amps versus 2.0 amp nameplate rating. Also field volts are 70V versus 300V nameplate. Armature current is 73A versus 83A nameplate. Field windings are separate supply - 1/2 wave rectified dc for low ripple.

Question is - Will low field voltage/current result in premature motor failure and rapid brush wear? No eveidence of brush sparkiong is seen while running at normal speed.

 

[ScottyUK]

Weak field operation is usually associated with running above base speed in the constant power region of operation. Is your motor speed normal? Operation above base speed would account for faster than expected brush wear.



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

 

[Dopler]

If the field is reduced the motor power is reduced also:
T is proportional to Ia x If

HP = (T x N) / 5250

N is proportional to CEMF / If

If you have less field you will have more speed but less torque for the same load

On the 1/2 wave field rectifier, do you have control or adjustments?
The type of brushes has changed? when you have high brush wear it means that the brush is too soft, be aware of getting too hard, you will have your conmutator wear down.

 

[jktwn]

The motor is driving a 2.5" extruder screw in a plastics extrusion process. It is a variable speed dc drive control with a pressure control outer loop.

Speed for most products is set to about 80-90%% of base (1800rpm) and controls ok. However, the operators have complained of drive trips when the process is upset. The field supply is adjustable but we just discovered the low current and have not tried to adjust it yet.

I am most concerned about getting to the root cause of the previous motor failures and am trying to understand if the low current on the field may be the smoking gun I'm looking for.

 

[AFab]

The drive system may be a little under sized for a 2.5" extruder. The 2.5" in my plant have 75hp motors and drives on them. Therefor the weakend field on your system will compound the problem with less torque from your 50hp system.

 

[Dopler]

Are you also experiencing more vibration than other times? In some cases if an SCR is not working properly (switching on or off time) it can induce a harmonic distortion that is reflected in rapid brush wear, mechanical vibration of the motor, heat of an isolation transformer (if it is the case).

You could see, with an oscilloscope, the armature DC wave forms, A1 to ground or Voltage feedback, DC current feedback and they must be a complete signal with no parts missing. There you will see if one is not working well.

 

[jktwn]

All SCRs are firing as indicated by uniform Ia waveform on scope. No excessive vibration is present.

 

[DickDV]

The low field current reduces the torque/armature amp production resulting in higher armature amps for a given torque level. That is likely causing the brush wear.

Get the field current up to nameplate. This is essential. Then retune the speed loop and external loop and your performance should be much better.

Whoever commissioned the drive/motor system with field current that low has made a serious mistake. It is a wonder that the motor has not run away into overspeed with an exploded armature resulting in total loss of the motor. The only thing preventing this calamity is the tach speed control in the drive, fortunately.

 

[Dopler]

I agree with DickDV, low If increases Ia, you need to increase If to nameplate, may be the field module was changed 18 mounts ego or more, some body could forget setting the adjustments to the specific motor rates.

 

[aolalde]

Low field current reduces the motor torque and increases the speed if the armature voltage is kept constant. In your case, probably the controller is reducing the armature voltage and probably the speed is under the design value.
The motor is running out of design parameters and into an unsteady zone.
You should work as close as possible to nominal field intensity. The half wave rectifier bridge is very poor, try at least a full wave rectification and if possible three-phase full wave rectification. The lower your ripple the bether your motor performance and life expectancies.

 

[jktwn]

Thanks aolalde,
I agree with your response up to the point where you state that full wave rectification is better than half wave for the field supply. The controller manual states:

"It is recommended that the field controller be used in the half-controlled mode rather than fullcontrolled,
unless the application is one that demands that the field be capable of very rapid weakening (suppression), or if the time-constant of the motor is unusually long. In general, less current ripple is produced by a half-controlled output, and therefore torque ripple is less."

Although not intuitive, the waveforms confirm that this is indeed correct.

 

[ScottyUK]

You are both right - just at cross purposes.

Full wave rectification and half wave rectification are very different in meaning to fully controlled and half controlled rectification. Both of the latter terms relate to full-wave rectification: in fully controlled rectification all rectifying devices are SCRs while in half controlled rectification, 3 SCRs and 3 diodes are used plus a freewheel diode bypassing the bridge.

Fully controlled rectifiers achieve their faster repsonse by being able to actively regenerate stored energy into the source, but at the expense of poorer ripple performance.




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

 

[jktwn]

I am not sure what the standard setup looks like but our controller uses four SCRs interconnected to form a single phase symmetrical bridge. Operating in half-controlled mode, only the positive half-cycle of the AC supply voltage is able to pass. If the SCR's were fired for the full 180 degree of the half-cycle, the mean value of the output voltage would be 0.318 of the AC voltage peak amplitude.

 

[aolalde]

I am confused.

To me, the field has constant current excitation.

The armature voltage is controlled to match the load requirements.

 

[ScottyUK]

Hi aolalde,

I agree with your comment, but by making the field controllable, it gives the options to either increase starting torque by boosting the field above base level, or to allow field weakening in the constant HP area above base speed.






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

 

[aolalde]

Hi Scotty. Thanks.

Original post reports field current of 0.5 amperes vs. 2.0 amperes nameplate.
Is not that motor too close to unsteady operation? I suspect that the controller reduces the armature voltage to keep the speed into acceptable limits.

wxashcra could you report the motor operating speed range (rpm) and the nameplate speed.

 

[jktwn]

Nameplate:
50 HP,500 VDC,1750 RPM,328AT FRAME,
DRIPPROOF,83 AMPS,300 FIELD VOLTS,2.25 FIELD AMPS,
CLASS F.

Speed range varied by product but is between 40%-100% normally.

You are correct in that the dc drive varies armature voltage to control speed. Field current is regulated as previously mentioned.

 

[aolalde]

wxashcra:

Thanks for your response.
Three motors failing in a row means something wrong in the application.
Discuss the controller set ups with the technical support of your control provider.
I strongly recommend for 20-100% speed range that the field be kept constant and very close to 2.25 amperes, the speed being controlled only by armature voltage. That makes your motor strong and steady.

 

[ScottyUK]

Hi aolalde,

Your observations are entirely in line with the information in the original post. Keep posting - I learn a lot from your comments.



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

 

[jktwn]

Followup:

We increased the field current from 0.7A to 2.25A with the following results:

Motor speed = 825RPM
Armature current dropped from 77A to 37A!

We expect this will take care the rapid brush wear problem and future motor failures due to winding insulation premature failure.

Thanks for the posts. I will use this web site in the future. The help from experts in the field is quite valuable.