DC Motor Drive Tripping Often 8

[edison123]

Our client has a 440 V, 40 HP, 75A, 400 RPM base speed DC Motor driven by Eurotherm 590 + drive with 110 Amps capacity. The motor drives a rubber calendaring mill with a speed ranging from 400 RPM (field current 5.0 Amps) to 700 RPM (field current 2.4 Amps). The armature current as measured by an AC/DC clip on ammeter was found to vary from 20 Amps to 65 Amps. The current limit of the drive has been fixed at 95% of rated 75 Amps by the drive manufacturer’s rep.

This DC drive has tripped many times since its recent installation on “pulse missing”. In the field weakened range of about 650 to 690 RPM, the drive often goes to current-limit, the armature voltage is seen to vary from 200 to 570 Volts, and the field voltage from 70 to 270 volts. The drive indicates a speed error of 1.2 to 1.5%. At lower speeds, the armature and field voltages are stable.

1. What could be causing these trips on “pulse missing” ?

2. Is the current limit of 95% of rated current (as set by the drive rep) correct ? (Wouldn’t 125% be the norm for over current limits ?)

3. Why this hunting of armature and field voltages in the field weakened operation ?

4. The Eurotherm rep has advised to change the existing 4% line reactor to 2% reactor ? Is this ok ?

The same motor was previously working fine with a Siemens drive with 4% line reactor. The motor was also thoroughly checked after this tripping problem and found to be alright in all aspects.

 

[jbartos]

Suggestion: Rather than to measure the motor by ammeter (obtained values from 20A to 65A stated in the original posting), it will better to record the current. 95% setting of 75A gives 71.25A. Therefore, the motor shaft load could easily cause to exceed 71.25A since there are other factors to take into consideration, e.g. voltage fluctuation, manufacturing tolerances of motor drive, flaws in the motor drive, etc.

 

[barac]

The current limit setting is dependant on your motor design and application. When I worked in a steel mill our engilsh electric 'mill motors' had a current limit of 2 per unit. Since the application required very quick screwdown movements and as they were not continous the overall motor heating effect was ok. The important point to consider is not to exceed the i2t of the motor or drive.

 

[RadarRay]

Edison, the fluctuations in both current and voltage are highly unusual, even if the pulses are misfiring. Is the field weaking loop tuned properly? Noise on the tach, as mentioned by buzzmoore could be a contributing factor. Monitor the output of the speed regulator with a scope. I assume it is quite eradic. For troubleshooting purposes, the drive can be detuned to see if this has any affect.

To anyone else with a missing pulse, if it is possible, can you monitor the current feedback waveform? Preferably with a storage scope. I would like to know exactly how many pulses are missing, if in fact they are missing.

Also, make sure all of the gate leads which go to the Scr's are secure. I have seen in one case where this was a problem on a drive mounted in a high vibration area.

 

[RadarRay]

Testtech,
Just in case you are not aware of this, if one Scr is open or not firing, for example, you will not be missing 1 pulse on a 3phase, 6 pulse system. You will, in fact, be missing two consecutive pulses.

 

[electromike]

edison125,

Your operating current limit should be a percentage of the maximum current of the drive. Which in this case is 110A. So at 95% of 110A your drive will allow your motor to run at 104.5A. Most drives have a parameter that limits the time a drive will run in limit. The term current limit also seems to be getting confused with what should be the current trip of the drive. Some drives allow you some range in setting current trip. Keep in mind when you hit current trip your drive shuts down instantly.(At least your drive stops instantly, your conducting scrs will try to carry the high current until the holding current drops below the given value for the scrs. Not good.) The reason for the instant shut down is to protect your scrs,,semi-fuses, and load. This is the way the systems I know operate and I suspect this one is no different. You can exceed the full load current for a very short time without heat and commutation problems in D.C. motors. I would check armature current when the drive is in limit to investigate what the current is realy doing.

 

[jbartos]

Comment: It is not clear from the original posting how old is the DC motor drive. If the drive is aging, the capacitor values are adrift and so are resistor values. This may cause various irregularities in the electronic triggering circuits.
Also, the ambient temperature is not mentioned in the original posting. The high ambient temperature can cause electronic circuits to have changed operating points and quiescent points, which can have detrimental impact on the gate triggered switching devices.

 

[edison123]

Thx everyone. I passed your views to my client. Eurotherm rep by bypaased the "Pulse Missing" signal after getting some special password from UK and the current limit has been set at 100% of rated motor current. The drive is working fine now.

Thx again for your time and views.