Optimizing DC drive Bar rolling application

[mash98]

We have a bar rolling mill where a dc motor of following specs is being used while the drive for the same is Mentor II (M1850).

General Electric
Direct current motor
No. 2321820
Type MCF
HP 1250
RPM 500/910
Form_______
Volt 600
Amp 1632
Exc Volt 250
Wound______
Commutator 55OC
Insulation winding and Armature core 40OC
Bare copper winding 50OC
Shunt Field 50OC
Below 625 RPM
Ventilating Air 7000 CFM
CAUTION: Before insulating or operating read instructions GEH-709
Schenectacly NY MAde in USA

The motor and drive were installed and commissioned around 15 years ago and the same were working fine for many years as long as we were rolling flat bar.But recently we changed the sizes and shape of our product and this new product application needs more torque and due to the same we are recently having frequent overload tripping.The above Mentor II drive's rated current is 1850 amperes and it is a single quadrant drive.On analysis of the drive parameters I figured out more torque can be safely produce from this motor as the field was set to below its rated value that is on 200 volts while the rated field is 250 volts.The drive is also controlling the motor field according to the load requirements.Usually we run our application on above base speed that is around 800 RPM where the motor field further decrease around 170 volts or so and when it needs more torque it increases the field voltage up to 200 volts.So I changed the motor field voltage from 200 to 250 volts i.e on its rated value which sure improved the previous condition but still more power was required for the application.Then there were two other parameters OVERLOAD I*t and CURRENT LIMIT were set to 1400 amperes and 1632 amperes respectively.So, I increased the OVERLOAD current to 1632 amperes and the current limit to 2000 amperes but my LT supply system could not support to these increase values and it started tripping from the system.And as it could not support these new values I had to decrease the current Limit to 1650 amperes.They above changes has sure improved the rolling conditions but still we face limited torque issues as the current limit can not be increased due to system's limitations although the current limit function working fine.

The duty cycle of the application is not more than 60% at any condition.As we usually run our motor more than its base speed so can we increase the motor field voltage to around 10-15 percent of it rated value so the motor can produced more torque by compromising speed on these brief instances without overloading our LT system? Can somebody also guide me about the motor field voltage tolerance for such motors as I could not get it from GE Motors or anywhere on the net.

Regards

 

[Skogsgurra]

There should be a test protocol from the manufacturer. If the motor isn't more than 15 years old, the protocol should be in GE's archives.
Such a test protocol usually contains data on field supply, excitation curve and other parameters that you may need.

Sometimes, one just has to realize that there's a limit to a system. In your case it seems that you are getting close to several limits. The choice is usually either refrain from producing that new product or buy new equipment.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[mash98]

thanks Skogsgurra for your reply the motor is more than 15 years old and probably that is the reason i could not get the required information can you tell me from where i can get Instruction GEH-709 indicated in the nameplate information above?
It seems to me that the motor is not being utilized at its rated capacity and the limitations are the input supply system.Replacing the equipment required a lot of money and time so before doing this i want to use all availble options.the option I intent to use is unconventional but seems logical.If I know the standard field voltage tolerance for this size and type of motor than i may try to use this otherwise we can always go to the other option of buying new equipment.

 

[Skogsgurra]

The rated field supply voltage is usually a bit into the "knee" region. That means that an increase in excitation voltage doesn't always lead to the corresponding torque increase for a given current. If you are lucky, there's still some room before you reach saturation.

You can find out what your exitation curve is quite easily. I have done that by using the motor's inertia. Like this:

1. Connect a DC current clamp to the excitation circuit.
2. Connect an isolation amplifier to the armature.
3. Connect a speed pick-up to the shaft (use existing DC tach if available)
4. Connect a recorder to the three signals.
5. Make sure there is a free-wheeling diode in the field supply. Mostly the case.
6. Run the motor at rated speed (not terribly important, but rather high speed).
7. Disable braking (not necessary in your case since the drive is 1Q)
8. Disable (switch off) the drive, but leave excitation on.
8. While the motor is still running near rated speed, switch off field supply.

The excitation current now decays slowly - a motor like this has a few seconds time constant - and armature voltage, excitation current and speed are recorded.

Now, you have characterized the motor and can calculate the excitation curve from your data points. When you think that you are confident in the operation and the results, then try with a higher excitation and see if that makes any difference.

This is definitely a lot simpler than taking the machine to a test stand and if the results are going your way - it may be possible to squeeze maybe 10 percent more torque out of the motor by doing this - especially if it is an older motor that hasn't been optimized to the extremes, then you can hope that the excitation rectifier can deliver the higher current and run your new product. Or increase the rectifier's capacity, which is easy. You could even add an uncontrolled rectifier that delivers the extra current parallel to the existing one.

You should observe several parameters when running with higher torque. Especially temperature of the main coils and, of course the field rectifier. Armature and commutator will not be a problem since the higher excitation is responsible for the higher torque and armature will still run at the current it used to run before.




Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[mash98]

thanks a lot for your guidance let you know when through with the suggested procedure.