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: The better rep will probably be the solution.
1. What could be causing these trips on “pulse missing” ?
///Improper timing. Relaxation oscillator, if present, is less accurate, i.e. fast or slow.\\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 ?)
///This may be set for sensitive protection of the motor, if the motor shaft is not fully loaded. However, the drive protection should be according to the drive spec sheet or manual.\\3. Why this hunting of armature and field voltages in the field weakened operation ?
///Apparently, the motor shaft load is bigger than the drive delivered energy.\\4. The Eurotherm rep has advised to change the existing 4% line reactor to 2% reactor ? Is this ok ?
///This may reduce the voltage drop across the reactor; however, the harmonics will increase on the drive input, if the reactor is on the drive input. If it is on the drive output, then the voltage drop will be reduced; however, the harmonic content will increase.\\\

 

[testtech]

I wonder if a rectifier device on one AC phase is not opening and closing properly. This would cause the other phases to overload as they supply the DC load. Could your drive alarm relate to the input side of the drive, rather than the output?

If can look at the DC waveform, you will see a periodic missing peak when one phase of the AC side is not carrying load.

What you may also find, on the AC side, is a current draw on only side side, i.e., negative or positive. This would confirm a rectifier device fault.

I have encountered this type of fault on several occasions, so perhaps you have something like it.

 

[jbartos]

Suggestion: A systematic analysis including waveforms monitoring as stated in the previous posting, might lead to the quick solution.

 

[edison123]

Thx jb & testech. I will ask my client to check the waveforms as suggested.

 

[jbartos]

Suggestion: Any diagnostics support from Eurotherm would also help (hopefully, it will come from a good tech support)

 

[snoogie]

I have exactly the same problem with the same drive on a much larger system (I have two 300hp Drives) I have tried everything, adding line reactors load reactors, monitoring the supply line with chart recorders the works to avail. It still is a problem today. The customer just has to run the machine slower to stop it from tripping.
I am certain that the problem is at component level on the drive my next move is to get a Eurotherm guy in to strip it down and start swopping out boards.

 

[edison123]

snoogie,

My client just informed me that Euortherm rep disabled the "Pulse Missing" trip. Apparently, he required some password from the HQ in London (?) to do this. Still, he could not explain why the trip occurs in the first place. May be your Eurotherm guy can do better. If so, pls share it in the forum.

 

[ScottyUK]

edison,

I believe the Eurotherm HQ is in Littlehampton on the south coast of England.

 

[edison123]

Thx scotty.

 

[testtech]

Edison:
Do you know what pulse missing means?

Did you obtain current or waveforms to ensure that rectification is occuring properly. You can also obtain vibration readings. Improper rectification can result in high vibration at 10,800 or more likely 21,600 CPM.

I just had a case where a DC motor was reportedly producing a large magnetic field that interefered with instrumentation. My analysis revealed an increase in 21,6000 CPM vibration from .05"/sec to .41"/sec. The DC current waveform revealed extreme peaks of extremely short duration, rather than half cycle duration for each ripple. (Too bad I can't attach the waveforms here, they are very interesting.) In addition, one ripple was actually negative going, suggesting the motor was enduring reverse torque. They had to slow the motor down to keep it on line.

They are now having the drive manufacturers look for a timing or other fault in the drive. The SCR's or whatever is being used in this drive for rectification is clearly not staying open long enough and in the proper sequence.

The bottom line here is that you can learn a lot about the drive operation from both the vibration signatures and the current signature.

 

[edison123]

testtech,

The motor vibration levels at various speeds and loads do not exceed 0.2 mm/sec. By hand, you can feel the motor running very smoothly. Maximum motor temp is only 58 deg C.

Have you had any such expereinces with the drive mentioned ?

 

[testtech]

edison:

That vibration level is so low, you have got to wonder if the machine is running!

In the past, I had a drive that ran heating elements at a computer chip plant. The drive continuously went out on overload. Examination of the input and output waveforms revealed a "pulse missing" in the sense that AC on one phase was not drawn on half a cycle. The resulting waveforms had a "pulse missing". The drive tried to make up the load by increasing the load on the other two phases. This increased the loads sufficiently to trip the drive.

If you go to page 7-12 in the Eurotherm manual,

http://www.eurothermdrives.com/doc/HA470388.pdf

it describes a missing pulse. This condition means that one of the six pulses is missing from the the armature waveform and you probably have a defective SCR or SCR control. This is the condition I was referring to on my first reply to this thread.

Once again, check the DC current waveform using an o-scope.

 

[testtech]

Edison:

Another commment. Since the vibration level is so low, you may not actually have a waveform fault. The page cited above also lists control related faults that may occur even if the waveform is properly formed. It seems that these control related functions might be the place to look. But, the easy starting point is checking the waveform.

 

[edison123]

Thx testtech.

 

[barac]

Does this particular drive have a setting for the front limit firing angle? I have worked with some drives where the firing angle is determined from a separate sync supply where a saftey margin was required when setting this limit. The drive could be trying to fire before the phase crossover point. The field controller may be hunting due to armature reaction when the drive goes into current limit.

 

[electromike]

This dosent look like a bad scr or gate circuit problem.If you had a problem with these your currents would run high at
all modes of operation, not just in field weakening.
The pulse missing fault is most likely caused by some feed-
back element.This raises the question of how the drive gets
speed feedback;tachometer,encoder, or "armature" feedback.
Also could you describe what type of current feed back your system uses.Also I assume you are using a speed control mode of operation as opposed to constant current.
The fault could also be in your "zero crossing" circuit.
This citcuit lets the gate curcuit know when the observed phase is rising from zero volts so the gate can be fired at the calculated angle of the cycle. You could check this with
a dual trace scope connected at the zero crossing test point and the input of the phase to the drive for problems with sync pulses on the z.c. side not matching with the phase zero crossing.
You current limit is set to high at 95% rating of the drive.
You "current limit" should never be set above moter rated full load. Forget about inrush as if you were sizing a curcuit breaker.
I think the hunting of voltages in your armmature and field circuit is normal. Your drive is just tring to keep the speed of you motor constant under demanding conditions.
I dont think the reactor change will help this problem.
Overall though this sounds like poor drive tuning.

 

[cbarn24050]

Hi, it maty be that the armature voltage is set too high on the field controller. If the arm voltage rises to above the peak incoming main, the thyristor does not get forward biased and so does not fire.

 

[buzzmoore]

We have had this happen to us on a 590+ drive system. 480v 30hp. What we found was interference on the feedback and on the signal wire. Solution: Run shielded cable to the motor and reduce the feedback to the drive.

 

[edison123]

Thx barac, electromike, cbarn & buzzmore for your views and time. I will certainly pass your ideas & suggestions to my client.

electromike,

The current limit is set at 95% of motor current of 75 Amps. The drive is rated for 110 Amps. I think the current limit should be set to atleast 110% of motor current to avoid frequenct "Current Limit" operations. Do you agree ?