DETERMINING LOAD ON DC BATTERY SYSTEM

[powergage]

I need to establish a load profile for our DC battery system at a U.S. power plant. The profile is being used to evaluate the adequacy of size of our battery system. I have two questions:

1) What is a typical estimate for DC current needed for 2400V switchgear to trip? I know that the tripping circuit does not require very much power. I was using 5 amps for a duration of 1 minute as my estimate. I was guessing that actual load would be more like 1-2 amps for a split second. Is my estimate adequate?

2) What is a typical estimate for inrush current on DC motors? I have two 125VDC motors tied to the system. One is a 7.5 HP, 3500 rpm motor rated at 53 amps. The other is a 140 HP, 3500 rpm motor rated at 140 amps. My guess was to assume an inrush current at 3 times the nameplate rated amperage for 1 minute. Is this a good estimate. (I realize that the inrush will not continue for 1 minute, but that is how long IEEE 485-1997 recommends that you estimate inrush current for.)

Any input into intuition would be great!

powergage

 

[advidana]

Usually the mfg of the switchgear has all this information. Check with them.

 

[powergage]

I'm not really sure how a SWGR manufacturer would have information about DC motor inrush. More than likely, I will just have an electrician clamp the input to the motor during the next outage when we can start up these Emergency DC motors and get accurate measurements, but until then, I would like to put together a good estimate of the situation.
Most of you guys probably know this stuff off the top of your head, but since I am obviously a new engineer, I don't have very much intuition to make a good guess.

Thanks!

powergage

 

[Lakey]

Hi Powergage,

1. Dunno. Can you measure the current with the aid of a shunt and scope?

2. Do you know the running currents when the motors are running normally under load? Generally speaking, I would expect the in-rush currents to be 2 to 5 times the running current. But bear in mind that the amplitude and duration of the in-rush will be dependant upon how long motors take to get up to speed.

Also, you should consider the stalled motor condition. You can get a rough idea what this will be, using Ohms law, if you measure the total resistance of the field and armature (connected in series).

Regards,

 

[ScottyUK]

Hi powergage,

The details of your second motor look a little strange: 140HP does not appear to be a rating, as this would equate to approx 105kW and draw a current of ~850A @ 125V DC. More likely this is a frame size. Your motor will probably be in the region of 17kW (25HP) based upon the quoted current of 140A.

The inrush would normally be limited by either an electronic controller, or by a resistor timestarter. As this sounds like an emergency standby system, I suspect it will be the latter, chosen for simplicity and robust construction. Timestarters are normally designed to keep the peak current within reasonable boundaries, and 400 - 500% of FLC is fairly normal. I think a period of 1 minute would be abnormally long for a timestarter unless specially designed for a particularly awkward load.

If you are not using some means of limiting the inrush current, then expect very high levels of current. The larger of your two motors could realistically be drawing in excess of 1000A if the cable run is short enough not to contribute much resistance itself. We measured peak current of approx. 1400A into an 11kW 110V DC machine which had some fairly major wiring errors in the timestarter cubicle. At this level of current the battery voltage will sag significantly and may affect other loads sharing the battery. We experienced a sag of about 45V on a 1430AH battery.






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If we learn from our mistakes,
I'm getting a great education!

 

[powergage]

ScottyUK,

Since the original posting, I have since found that the motors are started with resistor time-starter controllers, exactly as you guessed.

The 140 HP was a type-o. The nameplate FLA is 140, and the rated HP is 20. We measured the inrush at about 510A, about 360% of FLA.

Sounds like you learned some of these lessons the hard way! I'm getting shivers just thinking about getting a phone call saying, "Hey, we're seeing a 45V drop in our DC system!!" I guess now I'll have a good idea where to start looking!!

Thanks!!!