WR motor failure 2

[jbartos]

Suggestion: Many types of dusts of organic origin can also cause flashovers.

 

[electricpete]

It seems to me that rotor voltage proportional to slip is a good approximation if we are comparing two cases where rotor is open-circuited in both cases.

If instead the motor is overloaded as I think is Gus' case, then there will be high current through the stator series leakage reactance would tend to decrease the rotor voltage below the proportional-to-slip.

Back to pierre's problem, I would think we might expect even slightly higher than proportional (higher than 1635V=1090V x 900RPM/600RPM) if the 1090v is based upon full load current through primary reactance, while only magnetizing current flows through reactance in the the 300rpm reverse rotation, open-circuited rotor configuration.

 

[pierrec]

Hi:

Humidity and environmental dust are definetly not the root cause.

I learned that similar motors are designed for 200% rotor voltage plus 1000 volts. In my case that means 3.2 KV isolation.

Normally the motor is powered off by opening the HV breaker.
This cuts the power to both stator and rotor. In my case, the rotor first stage contactor was opened with power still applied to the stator. The motor winded down and then went into reverse rotation at 300 RPM when the flashover occurred.

When a coil is disconnected a transient voltage can be calculated from:

V = L * dI/dt

I is equal to about 400 A. t is in the range of a few ms.
I do not know what the rotor inductance is (yet) but I suspect the resulting transient voltage can be large and exceed 3.2 KV.

Can 4 or five of these transients damage the insulation?

 

[UKpete]

pierrec - I would say it is a possibility. There may have been some low energy flashover/tracking at the instant of opening the rotor circuit, although a good proportion of the energy will be dissipated at the opening contacts which were breaking 400A.

But it sounds like it is too late to say for sure, as you have now had a big flashover which has destroyed the evidence on the slip-ring insulation.

 

[electricpete]

We have said that clearly the final failure did not occur at time of breaking of rotor circuit, but the damage may have occurred earlier as a result of spikes from breaking the rotor ciruit.

One would think that such spikes could attack the rotor turn or ground insulation. But doesn't appear from what we know the failure initiated there. I don't believe that surges will typically initiate carbon tracking across surfaces in the area of slip ring which would leave it weakened. I believe normally it takes current flowing over time to establish that kind of track.

Just my opinion. I could be wrong of course...

 

[pierrec]

Hi:

I saw the motor yesterday. It is vertically installed. The lower ring looks good. The arcing occurred between the two upper rings. The middle one is all blacked out and there are grooves in it up to 1/8" deep. 60 degrees clockwise from the brushes there is a black spot on the middle ring that looks like the foot print of a brush.

Something else, the motor protection relay did not open the HV breaker. A human did manage to hit a mushroom about 5 seconds after the beginning of the event. The stator is OK I'm told.

This motor will be repaired quickly. The application is critical and enough pumps must be available. They are now down to three pumps and *cannot* risk to operate with only two. I will keep you posted if I can learn anything from the repair shop.

 

[GusD]

HI Pierrc

I Hope you find some answers to your flashover problem.
It is a plesure just to read through the posts and admire the amount of knowledge and expertize that reveals itself by all who answered these not so easy questions.We all benefit from this knowledge bank.Thank you all for that.
I have a couple of points that I like to clear up regarding some of my own flashover problems.

Electropete.=You mention that probably we had an overload.
Some failures ocurred while starting empty conveyors.
As for your belief that you need current for carbon to track, I agree.Obviously we have to have a combination of Voltage and current.Highly conductive mediums,high Humidity and a little carbon can be very explosive when voltage stressed.

To JBartos'.=Your formula for the peak voltage
1635V=1090V x 900RPM/600RPM\\\.Is it correct to assume that if the 600 rpm is reduced to 200,we could have 3x 1635v ? Is it right ?

We did experienced a lot of rotor "shorting contactor"
failures, before we added a 1.5 sec delay to open whenever
the system tripped with heavy loads.This time feature did eliminate the contactor problems.
IN our systems we cannot have a rotation reversal
condition.

Thank you Gents

GusD

 

[jbartos]

Suggestion to pierrec (Computer) Apr 30, 2003 marked ///\\
When a coil is disconnected a transient voltage can be calculated from:

V = L * dI/dt

///Initial conditions also have to be considered, since the inductor has stored energy inside with certain voltage entering the initial conditions.\\

 

[Farkel]

One does not open the circuit on a current transformer, and the wound rotor motor is operated like a transformer, where the secondary or armature or rotor changes values as the resistance increases or decreases the amount of current, which it see's, thereby increasing and decreasing the speed and torque.

 

[don01]

I have to give farkel a star here.
I think we've all seen CTs after being open circuit. The wound rotor is the similiar in that it is low resistance(impedance?)and relies on the external resistors (short links) to provide voltage regulation. Open circuit it and there is no current flow ===>voltage rising vey high. I suspect if the rotor is reversed the transformer effect will be increased and the voltage will be increased more.

Now if I'm wrong we could confirm it,-- if someone finds a circuit for a "salt bath" starter. ie the one where the plates are lowered into a conductive solution. My memory says the unit we worked on had a resitor bank to give the max ohms for the rotor and then was progressive lowered into the solution until shorted at max speed. The unit I recall was supplied by Seimens
The resistors were to stop open cct on the rotor

Regards
Don

 

[electricpete]

Sorry to be picky, but IMHO, an open-circuited ct secondary is not a good analogy.

Yes, there is v=L*di/dt in both cases. The similarity stops there.

First: a ct steps up the voltage from primary to secondary by 10:1 or more. No such stepup ratio in a w/r motor... I'm guessing effective stator and rotor turns ratio is not too far from 1.

Second: The primary circuit of a ct in a power system acts like a current source. Specifically the series impedance of the power system load is much higher than the impedance presented by the ct (ct secondary ckt impedance is generally low and is further reduced by turns ratio squared when seen on primary). The power system load impedance controls the primary current, and primary current will flow regardless of what we do on the secondary ckt. There is nothing comparable in a motor.

 

[jbartos]

Suggestion: Wound rotors have certain voltage rating, e.g. medium voltage wound motor has low voltage rotor winding rating. The rotor is not exactly the current source. However, its voltage is changing during the motor startups. The rotor could be referred to as a current varied voltage source.

 

[GusD]

Hi again.Pierrec.

In a previous post I reffered to our WRMotors and its problems,that we had introduced a TDelay of 1.5 to 2 sec before we open the shorting contactor.
This time factor allows the stored energy in the rotor to dissipate as heat thru the resistor grids .
Our original control systems did not have this TDelay feature, causing many failures of the shorting contactors.
Possibly other failures as well.

GusD

 

[jbartos]

Suggestion to the previous posting. Apparently the failing contactors used for shorting are not adequately rated.

 

[GusD]

Hi JBartos

As for the shorting contactors rating I don't think that was a problem .We were still using the same contactors sizes until 2 or 3 years agf late, all of the new WRM
Rotor stepping contactors are of the Vacuum Bottle types .
The shorting contactor failures stopped many years ago,long before we started using the newer Vac contactors.
JBartos, It is true that if the contactors were larger ,we would have minimized our failures.

Thanks JB

GusD

 

[jbartos]

Suggestion to the previous posting. It is necessary to know more about the time delaying. Evidently, if the time delay allowed for the rotor energy dissipation in the wound rotor motor starting resistors then the current shorted by the contactors will be smaller.

 

[Shortstub]

I am sure that the frequency of starts has an appreciable impact on contactor life.