DC motor commissioning problem 2

[7465]

Hi,
I need urgently your help.Some 3 years ago our DC drive (thyrsitor leonard drive) failed.The basic data of the motor is (95 kw,arm current274A,Field voltage180v SEPA??,380/1200 rpm) I immediately replaced with a abb ACS 800 drive with a new AC motor and run the plant without any problem.Now since the DC motor kept ideal,I ordered a digital DC drive of ABB and try to commission it.(DCS502-0450-51).Itis a four quadrant drive.The original analogue drive was a single quadrant drive with dynamic braking resistor.The original Dc motor was shinko,japan.They have not given the field current in the name plate.I approxmated the field current data in the Dc drive and it is not matching between the actual and set value.I called ABB and they told that I should get the field current and without that it is difficult and approximation of field current will not work.Well,I am sure there must be a way to find the actual field current originally it was designed.By the way,shinko is not doing this drives anymore and the details to contact them were futail.The application is a wood veener lathe main drive.( to peel the log).As per the machinery manufacturere the maotor will run in constant torque mode from 0-380 and constant power from 380-1200 rpm.
The motor is a compound motor as the marking indicates with 4 leads on the armature with markings as (A+ F-) on the outer terminals and A- and F+ joined together on the inner terminal. The shunt field is j+ and k-.
Now my questions are as follows
1.The chosen drive is ok for the application? (intead of 1q with dynamic resistor to 4q without dynamic resistor)?
2.How to find the field current in the absence of data from manufacturer in a Scientific way ?
3.what I have to take as base speed? 380 or 1200?

Pl give your replies at the earliest.

regards and thanks in advance.

7465

 

[edison123]

2. If you know the field voltage, measure field resistance across j & k. Field current = Voltage/Resistance

3. Base speed - 380 RPM


* Basically, I would like a full-time job on part-time basis *

 

[Skogsgurra]

Your first question: If the voltage and current is OK, the drive is OK. 4Q is always better than 1Q and braking resistir. Shoudn't be a problem at all.

Second question: As edison says. But remember that field resistance may be about 20 percent higher when winding is warm.
If there isn't any voltage either on the nameplate, there is always the possibility to run the motor at some convenient speed (using a small motor and some coupling - perhaps a pulley and a belt) and then increase field current and record voltage. When the armature voltage is 10 or 20 percent above the "knee", you are close to rated field current.

You do not need the exact rated value.

Gunnar Englund

http://www.gke.org

 

[7465]

Dear edison 123and Gunnar,
Thanks for the base speed advise.but I dont think the the direct relationship for the field current is correct.because,the resistance of the field winding is 5.4 ohms.the voltage is 180v.As per yours it is then 180/5.4=33.3A.It can not be.The resistance I measured was incold stateof the motor which was not even rotated for 3 years.By the way what is the "knee point" amps of the armature?I am not familiar. pl explain.What is SEPA? ( the name plate is giving for the field voltage 180v SEPA)
I have a similar motor of 110kw from a different manufacturer.It states the field current as follows.It is also having 200/300/1200 rpm.@200 rpm the arm curent is 380A and arm voltage is 330V and the field current is 12A.@300 rpm the arm current is 280A and arm voltage is 440v and anfd field current is again 12A.and @1200rpm arm current is 280A and arm voltage is 442V and field current is 3.2A.so it si clear from this the motor is runnig on constant power after 300 rpm.Once i know what is the knee point then I will do that experiment.

 

[edison123]

Unless you have a field regulating resistance in series, the equation still stands subject to resistance rise due to temperature. A designer can design the field current to whatever he wants it to be. It is ampere x turns that matters for flux. I have done machines with 100 Amps field current for a 1500 KW DC Motor.

You cannot compare one manufacturer with another in design matters.


* Basically, I would like a full-time job on part-time basis *

 

[7465]

dear edison,ok.i got it.I measured the value at the motor terminals.Unless the feield regulating resistance were given inside the motor,which I don th ave the idea.But what is the knee point so that I can find with the little experiment the field current.From your explanation i can make out you must be good in Dc drives.pl help me.
regards
7465

 

[Skogsgurra]

About the "knee":

A DC motor is also a DC generator. So the easiest way of seeing the magnetizing aka excitation) curve is to run the motor at some constant speed. It doesn't matter what speed, as long as it is constant and not too slow. one hundred RPM and up is OK.

Get yourself an adjustable DC supply that can produce the voltage and current expected (say 180 V and 30 - 40 A). Rotate the motor, measure armature voltage and excitation current. Increase excitation from zero and upwards in some well defined way (in your case the sequence 0 - 5 - 10 - -- 40 A seems to be just right) and plot voltage vs current. You will get a straight line going up until it bends and gets rounded and then straight again. That is where your "knee" is. The nominal excitation usually (always) is on that latter part of the curve, not far from the knee.

Gunnar Englund

http://www.gke.org

 

[edison123]

7465

I am not a drives man. If you can explain the problem with your drive, I am sure experts like Gunnar, jraef et. al. will give you some excellent advice.

SEPA - Probably means separate voltage source (180 V) for your field winding since your armature voltage is around 350 V.




* Basically, I would like a full-time job on part-time basis *

 

[7465]

Thanks Mr.gunar to give a valuble feedback.I will do the experiment.ow this motor being run is both constant torque and constant power,it should have two field current values as I have given the other motor name plate.One is in constant torque mode ( which will have a higher field current value) and the other one in field weekening mode( constant power mode) which will have a lesser field value.Now this nominal field current corresponds to which one? the field for constant power or constant torque? If it is c.p then how to find the c.t value? and vice versa?
any idea?
sorry for the botheration.

regards
7465