short term overloading of a DC motor.

[UASteve]

Does overloading a DC motor result in drawing more than nameplate amps?
My customer is going to operate a DC motor at or slightly above the HP curve at a given RPM. From motor nameplate, 995A 452V, 600HP, 1300RPM. Customer needs 224HP at the load (after a gearbox of unknown efficiency) at 498RPM. So if there are no losses between motor output shaft and final load I am below the HP curve but I think the gearbox has losses so now I am above the HP curve. I believe the result is i will draw greater than 995A from the drive and eventually trip on timed overcurrent.

Is my understanding correct?

My customer only has to run at this point for short periods of time so i think this is do-able as long as i measure the amount of overload and make sure to stay under the timed overcurrent trip period at this overloaded current level and let the motor cool down before attempting another overload.

To give you a time scale they will run for hours well below overload and then they will run in overload for 10-20 seconds and then return to the well under overload level for hours again.

 

[edison123]

A 600 HP, 1300 RPM motor will have about 230 HP at 498 RPM with the same current of 995 A. No OC issues there.

If this is a Banbury motor, they do handle almost twice the rated amps for 15 to 20 seconds fine since the rest of the cycle is low load. Your drive current limit needs to be set for that timed OC.

Muthu

http://www.edison.co.in

 

[UASteve]

Hi Edison
Thanks for the reply. The motor is 1960's vintage GE. It is a tank of a motor. The DC drive was factory set to limit the current at 110% of nameplate current. My hope is i can leave that where it is and just limit the overload duration to not hit the trip point.

I do not know the efficiencies of the mechanical components in the system so my plan is to measure the current as it runs through a cycle and then if we enter
an overload condition i can reduce the time we spend in that condition to keep under the trip curve. If we end up overloaded i think it will only be by 5-10% so i'm pretty sure I can operate there for up to a minute or so.

Thanks again for the help.

Anyone have any experience with 1990s vintage GE DC2000 drives?

 

[waross]

I see two issues:
The effect on the motor.
The effect on the protection devices.
Gògle the "Cowern Papers".
Look for "RMS Horsepower loading".
If you experience nuisance trips set the protection slightly higher.
If you are unable to adjust the trip setting then consider using a resistor to shunt a small percentage of the current past the protection device.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[UASteve]

Thanks Waross
I am reading the RMS HP loading now and this is EXACTLY what I am facing. Long blocks of time well below overload and then seconds long events that may be just above overload.

 

[edison123]

Yup, I have rewound a few of those vintage GE 1000 HP DC motors in Banbury applications. Real workhorses that served for over 35 to 40 years before winding failures. A few of them even had aluminum shunt field windings.

Instead of current based protection, you could use winding temperature based protection. We always fix two RTD's on interpoles as an indirect measurement of armature temperature and set the alarm at 110 deg C and trip at 120 deg C. I have never seen these GE motors hit even the temperature alarm limit let alone the trip limit even though their peak load is about twice the rated armature current for about 10 to 15 sec.

Muthu

http://www.edison.co.in

 

[UASteve]

Thanks again edison
I'll get another look at the motor next week. I suspect i only have motor thermals to alert me of a winding overtemperature but now that you bring it up i will look to see if there are RTDs or thermistors imbedded in the windings.

 

[Gr8blu]

The armature winding will reach thermal equilibrium in roughly 300 seconds. The complete motor may well take several hours, simply due to the mass of material involved. This is why some manufacturers (GE in particular) referred to taking the RMS current loading from a "moving 5-minute window". A lot of industrial process take longer than 5 minutes, when run-up, load cycling, run-down, and dwell time are considered part of the "duty cycle". Keeping the average current below nameplate (or short time overload) is going to necessitate making the calculation over the 5-minute "moving" window.

Given the age of the machine, chances are you might have surface-mount thermostats (on-off switch) or embedded resistance temperature sensors on the main field coil windings. There probably is nothing on the interpole or armature winding. More modern machines - or this one, if it was rewound/updated some time in the last 20-30 years) will have resistance temperature detectors on the interpole winding - with an alarm/trip setpoint roughly 25-30 C below the expected max temperature of the armature coil itself. This is because the cooling of the interpole coil is better than the armature coil - which means lower observed temperatures for the same loading.

DC machines generally have a constant torque profile up to base speed. As waross mentioned, that would be roughly 230 HP at 498 rpm (and 173 V, with full field current applied). Is that enough to compensate for possible gearbox losses at that speed and still deliver 224 HP to the driven load? Quite likely. If not, it will only be a handful of HP short - certainly less than 25 HP, which would be approximately where the 110% current limit would take effect.

Converting energy to motion for more than half a century

 

[UASteve]

Thanks gr8blu
i forgot to grab a set of DWGs the last time i was onsite so i am not sure what temperature monitoring devices are available. if i was to guess i would say the machine has never been rewound and therefore as you say there are probably only thermostats. Looking the cowern papers as waross suggested and doing the RMS HP calc i come up with a RMS HP well below the HP available at that speed.

Here are my operating conditions
RPM Motor HP Load HP Required Duration at this Speed (sec)
341 157 45 120
498 230 224 22
464 214 106 3600 or more

It does seem like this application will work out fine. I will not know until later this month when the customer bolts everything in place before i will get any data on actual current draw. Thanks for all the advice thus far.

 

[UASteve]

Sorry the formatting in my data table got messed up. the table above is just showing the 3 speeds i will operate at and for how long. The bottom line is all points are below the available HP of the motor and only the middle one is close. If i encounter some unforseen mechanical losses i could require slightly more HP from the motor but this is a 22 second event and even with gross mechanical losses i thill think we are talking something like 5% overload.

once again thanks for all the help.

 

[waross]

Credit where due:
It was edison123 who did the 230 Hp, 498 RPM calculation.
Thanks edison.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

22 second FLC in one hour cycle is less than 0.6% of duty cycle. You are quite safe.

Muthu

http://www.edison.co.in

 

[UASteve]

Hi edison
Awesome!
I'm looking forward to running the system now and seeing how well we did for estimates and calculations.

Thanks again all!