Large DC Motor Tripping current

[mash98]

Dear guys we have a dc motor with the following Specs.

Rated KW: 1000
Rated Volt: 700
Rated Ampere: 1700
Frequent Max Current: 225%/Min of rated(mentioned on original electrical Drawing)
E Max current: 275%/Min of rated(mentioned on elect drawing)
RPM : 30/80
Exitation : Seprate
Rated Torque: 32.5 Ton-m
Frequent Torque: 73.8 Ton-m
Occ Torque: 89.5 Ton-m
Type SMP290
FORM PCPV
pOLE 12
Insulation Class B
Make Toshiba
Now can somebody interpret the following terms.
1- Frequent motor torque
2- Occ torque
3- Frequent Max current
4- E Max current

Thanks

 

[EChristenson]

You need to learn a little Japanese/English here (grin)
The key is:
Frequent --> Repeated
(that is, these frequent things can be done without damage to the motor. I think these things can be done for a minute at a time)
The Occ torque is clearly the maximum torque before damaging the bearings.
I'll guess that E max current means the extreme max current.

What's your application? My employer, Cableform, makes drives for such motors.

 

[mash98]

Thanks Echristenson for ur adivice about japanese and english these two seem even difficult for their kids and can be witnessed without being very keen observer(grin).Sir you interpret frequent for repeated that is fine but can you further elaborate this as I want to know how many times these things can be done in an hour? you seem right about guessing E max current for extreme max current which was what I thought.Actually this motor is being used for plate rolling mill and the same was commissioned by toshiba's engineer in 1970.The drive system is Motor generator set( walt leonard system) and the motor tripping was being actuated by High Speed Circuit Breaker and the original current was set at 3750 ampere and was working nice ever since but a couple of years ago the high speed circuit breaker completely damaged at tripping and the cause was arc chute failure.Then we replaced this damaged breaker with MM74 high speed dc circuit breaker of Whipp& Bourne UK
this new breaker had the setting range of 4000-8000 amperes and dont have any other setting like time etc and neither we had these in our original breaker.So we set the same at the minimum i.e at 4000 amperes but witnessed more frequent trippings even at low loads. while earlier the same system (original breaker) used to sustain more load for comparatively longer time as we have no means to detemine the response time especially for the original breaker so this is all we are guessing and this later frequent trippings seem nuisance.Beside this nuisance tripping the HSCBs are very expensive and these trippings reduce their life.So we designed a system where we measure motor current from shunt and then the controller after comparing the input current and the set values actuate an isolated relay which we use to inactivate the generator field reference which in turn reduced the motor voltage to zero without tripping the High speed circuit breaker and is working satisfactorily.But here I want to know the safe current values and the time duration the circuit should sustain the overload conditions without compromising too much on motor's health and also avoiding above nuisance trippings.
Kindly advice on above.

regards

 

[Skogsgurra]

Are the more frequent trippings caused by a difference in actual current? Or, which is a very probable scenario, because the new breaker's trip setting is more accurate?

Your approach is ambitious - and impressive - and I think that you can be quite successful if you 'divide and conquer'. Dividing means that you look at all aspects of a DC motor's life and death. These are the more important ones, with the deadliest first:

1. Commutator flashover. The Germans call it 'Rundfeuer', literally 'Fire Around' and it will destroy the commutator in a split second. Flashover is more critical at high speeds and if you are running with field reduction. A flashover can also destroy the armature winding if severe enough. Remember that your Ward-Leonard genset also has a commutator that is at risk at high current levels and needs to be checked for limits.

2. High currents at very low speed or stillstand will heat the commutator locally so that it swells and develops runout, which, when you increse speed, will cause arcing and that is also bad for the commutator and brushes.

3. Mechanical damage to shaft, couplings, keys etcetera is usually caused by very high currents. Quite a bit higher than currents causing flashover. At low speeds, however, you can have such problems without flashover on the commutator.

4 Thermal problems in excitation and armature winding. The excitation winding is usually never a problem and easily covered with a simple temperature sensor or Klixon. The armature winding is trickier but a couple of IR sensors can be used to check temperature and trip the breaker (or reduce output from motor) when temperature gets high.

5. Overspeed, vibration, lack of excitation, dirt, water, debris and tools and other well-known DC motor killers are probably not your first concern. But should also be considered when there are DC motor problems.
Brush position and correct connection of compensation and interpole windings are other things to consider.

In view of this, I think that 'Frequent Max Current' is the current that the motor is designed for when reversing the mill and that E Max current is the maximum allowed current 'all categories' where 'E' may stand for Emergency or Extreme, hard to say which. Rated current is, of course, the maximum continuous current and also the RMS of the current taken over a certain time span, usually over an hour for larger machines with air cooling.

Do you have any recordings showing the current profile and what current you actually have when the breaker trips? That would be very helpful in determining what is happening in your drive.



Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[EChristenson]

Nice answers, Gunnar, especially about how motors die.
I would take E max current to be an absolute limit, that is absolute maximum current.

As for how many times per hour you can reach the "frequent max current", I think the rule is probably that whatever pattern of current is applied to the motor armature should have an RMS average value below the rated current. I'm thinking the /Min means for one minute; a motor that large is going to have a thermal time constant on the order of hours. But I'm guessing; my primary experience is building DC drives, not deciphering nameplates.

Given what you have said about the breaker, I would recommend you set that breaker current high enough not to nuisance trip on overloads, and use a separate system to deal with the longer term thermals like you are doing.

P.S. Where in the world is this rolling mill? I'm thinking you might have a DC hoist that needs a cableform control, too!

 

[waross]

Look at the Cowern Papers

http://baldor.com/support/literature_load.asp?LitNumber=PR2525

And scroll down to
RMS HORSEPOWER LOADING
This may give you an indication of the effect of your overloads on motor heating.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Thanks E. I noticed that you are quite new in these fora.
FYI, there are rules that we try to follow. It is about behaviour and explicit marketing. There is a text about it just under the text box when you post a message. Read the Posting Policies, you can benefit from it.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[mash98]

Dear Skogsgurra thanks for your reply and interest in an effort to solve my problem.As we dont have any current recorder in my system so to provide a precise current profile is difficult but I have some details of actual tripping current and the last two tripping values are 4031 and 4068 amperes as now the the mill has gone into some preventive maintenance so I cant give some more recent values for the time being.The most recent current settings for my above developed control system where we inactivate the generaor field reference are as under.

Set Point: 3950 amperes
Tripping time : 0.3 sec
Max Current: 4200 amperes

The control wait for 0.3 sec before tripping if the current is 3950 ampere and trips the circuit immediately without delay if the current touches 4200 i.e max current and in between the tripping time would be somewhere 0.3 - 0 sec depending on the amount of current, the higher the current the lesser the time.
Now both systems that is HSCB and controlller are in circuit and there is a race between two of them and 70 percent of the time the HSCB wins.
So I would like you guys to provide your value input based on this data.

Best regards

 

[Skogsgurra]

Did you record the current in your system? Would that mean that you have signals available that could be recorded? I think that a simple hundred dollar device could help a lot. Like a LabJack or similar. Terrible user interface, but does a decent job. Use it to see currents and voltages and get a better understanding of the problem.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[mash98]

Thanks as soon as I get the current profile I will post the same.

best regards