Wound Rotor Motor

[DanielElias]

Dear Gentelmen:
I am the going to take part of a start up of a big Wound Rotor Motor and this is my first time with 700HP motor).
I will appreciate so much if anybody of you can give me some values according to a similar experience and application. What I really need is the time of every timer that will eliminate every step of the resistor.

t1= ??sec (will eliminate R1-R2, see data below)
t2= ??sec (will eliminate R2-R3, see data below)
t3= ??sec (will eliminate R3-R4, see data below)
t4= ??sec (will eliminate R4-R5, see data below)
t5= ??sec (will eliminate R5-R6, see data below)

RESISTOR
R1-R2 .73 ohms, 142.8 amps
R2-R3 .40 ohms, 163.2 amps
R3-R4 .24 ohms, 204 amps
R4-R5 .16 ohms, 224.4 amps
R5-R6 .08 ohms, 244.8 amps

700 HP – WOUND ROTOR MOTOR
FS: 1.15
Duty: continuous
Manufacturing Standard: NEMA
Voltage Stator: 4160V,
Current: 106.6A.
Voltage Rotor: 1140V
Current Rotor: 272 A.
Rated frequency: 60 HZ
Start Torque: 180%
Max. Torque: 250%
Starter per hour: 3
Load: Crusher.

The load is a very critical part of the facility.

Thank you in advance for your valuable support.

Daniel

 

[waross]

Wound rotors are one of the most forgiving motors to set up by trial and error.
Energise the motor on the first step. Monitor the line current. Check the time until the current stabilizes and the acceleration drops off. Set the first stage timer. Restart and check the current on the second stage.
Repeat as required.
Bear in mind your 3 starts per hour. Use some common sense.
Starting on one or two steps and then stopping is not a full start. However be prepared to go for a long coffee break once or twice to allow the motor time to cool down.
Even with several "cool-downs" this is a lot quicker than calculating the acceleration on each step and you haven't provided the information required for the starting calculations.
respectfully

 

[DickDV]

Or, if in doubt, set each timer at the long end and keep reducing the time settings until you get a nice steady accel without huge current surges at transition.

The cool down period for multiple starts still applies.

 

[Marke]

I agree with waross and DickDV.
The optimum time is a function of the torque produced and the load inertia. Without this information, it is impossible to calculate the individual steps.

Remeber however, when you are starting, the slip energy is dissipated in the resistors. Be mindfull that these will probably not be continuously rated and if you leave in circuit for too long, you could damage them. - you will not cause problems with the motor.

One thing that you could do, is to get an estimate of the expected starting time of the machine and divide this by the number of steps and increase by say 50% as a starting point.

The start current should usually be in the order of 250% initially on each step and drop as the motor accelerates. Once it gets down to rated current, it should be safe to swithc to the next stage. - it may settle lower than this if you leave it longer. If the current steps to higher than 250%, you have probably stepped too soon.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[DanielElias]

Dear gentlemen
Thank you for your valuable responses.I appreciate all the experience you have shared with me.

Best Regards

Daniel

 

[DanielElias]

Just a couple of things

WAROSS(ELECTRICAL)
If my understanding is correct, after setting all timers to maximum and starting up the motor, I have to watch out the moment in which the line current stabilizes (nominal value?). This will give the value of t1
Then, I have to re-start the motor with t1 adjusted. Now the current line will reach Inom in a new time greater than t1. Therefore, t2=new time-t1
and so on.
Is it correct?
Does it mean that I have to start the motor 5 times?. Of course taking into account cool downs

MARKE(Electrical)
Lets suppose that I have assumed that starting time is 40 sec. If my understanding is correct. Do I have to set every timer 40/5*1.5=12sec to have an starting point?

THank you again

Daniel

 

[waross]

Hi Daniel;
Yes, 5 starts would be required.
Check your circuit to be sure when timing initiates for each step. The timers may start timing all together or each timer may start timing when the previous timer times out.
If you are nervous, do 5 starts. If you are feeling comfortable with the proccedure, find your first time setting and then set the other times at about 125% and run through a start. Readjust the timers and do a final check start. If you find that the timers still need a little adjustment, take a 20 minute break and then recheck your settings.
Considering the time-temperature characteristics of natural cooling, 20 minutes of cooling should allow one more start.
I mis-spoke in my previous post. Don't wait for the current to stabilize, use the time at which it falls to rated current.
In regards to Mark's method. If your starter moves to the second step before the current has dropped sufficiently, terminate the start cycle immediately. Reset the timers to longer periods and try again. If I had any idea of the expected starting time of the machine I may use Mark's method. It would depend on my confidence level of the estimated or calculated starting time.
respectfully

 

[LionelHutz]

Do you have a current datalogger? If so, logging a start would make the set-up much easier. You just set the timers long and start. As the current falls in each step you crank the time dial down and time-out the timer. Then, go back and look at the log to see what time you need for each step to drop to 100% current and set the timers accordingly.

You could also do the above but manually time each step. The time isn't really critical. Just get the timer set to a long enough time the current drops to or below 100% before stepping. Lingering a few seconds than necessary at each step won't hurt anything.

Otherwise, the description Waross gives of finding the step time for the first step then setting all the others just a little longer would work fine. The resistors should be sized to provide fairly equal acceleration steps making this approach reasonable.

Knowing more info on the load - ie what type of crusher, how heavy any flywheels are etc - could help with some starting suggestions.

One thing to consider is if the load can change. If it could be harder to start some times compared to others then setting the timers to hang at each step a little longer than necessary is probably a good thing.

I'd wonder what criteria the manufacturer of the motor used to determine that 3 starts per hour was the limit? It is quite possible to run at part speed continously with that motor. It's also quite possible to have a start that does not exceed the rated current of the motor by using a liquid reostat.

 

[DanielElias]

Thank you everybody for your great technical support. Now everything is clear.
Best regards

Daniel