Calculating A/C Current draw for large DC motor

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derekk

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I will be installing a new panel to feed a piece of equipment with the following motor specs:

885HP, 230V DC, 3055 AMPS

The electrical service is 277/480V AC. A custom controller is being built for this motor. My question is how much current @ 480V 3-phase AC this motor will be likely to draw and how to size the disconnect. I realize current drawn may partially be a function of how the DC is generated, but can anyone help me with an estimate?

Thanks in advance.
 
Re: Calculating A/C Current draw for large DC motor

Your DC KVA draw would be 230 * 3055 = 703 KVA

Divide 703/gen efficiency/motor efficiency

Assuming efficiencies 85% - 703/. 85/. 85 = 973 KVA

Round up the motor to 1000 KVA

@ 480/3 phase - FLC = 1000*1000/1.71/480 = 1203 AMPS

Round up to 1250 or 1500 AMPS

The final results would be subject to the actual equipment selection.
 
Re: Calculating A/C Current draw for large DC motor

If you mush all this into one formula you have:

I = Vdc x Idc/(Vline x eff x PF x sqrt(3))

where eff and PF are the efficiency and PF of the controller. The motor efficiency is wrapped up in the full load current and it has no PF. However the efficiency figures out to be 94% which sounds a bit high.

= 230 x 3055/(480 x 0.85 x 0.85 x 1.71)

= 1170A at full load.

Fudging that up to 1500A sounds quite reasonable. I would think that the controller designers could predict the efficiency and PF from their preliminary design so you can plan a little better.

[ February 16, 2005, 09:47 AM: Message edited by: rattus ]
 
Re: Calculating A/C Current draw for large DC motor

I would only divide by .85 once. The current given (3055 amps) should already include the effiency factor for the motor. In other words, the 3055 amps is the input current to the motor.

I see another possible error. Does the motor only have two wires? I'm not that familiar with DC motors. The calcualtion given is assuming the input to the controller is three phase (a good asumption, but you may be SOL if some moron designs the controller for a single phase input) and the input to the motor is one phase.

Why not a 480V motor or even 600 V or higher? That would cut the current (and the size of the wires) in half. How do you get 3000 Amps into a motor anyway?? I assume they wouldn't wind a motor with 8 sets of 500's! I think someone behind these specs is off their rocker.

I'm also pretty skeptical about the "custom" controller. There should be an off the shelf solution for most motor controls. The fact that they need a "custom" controller may indicate that the design has gone off the beaten path.

Steve
 
Re: Calculating A/C Current draw for large DC motor

This wouldn't be an exam question, would it? ;)


That's quite a motor to be operating at 230 volts.

Ed
 
Re: Calculating A/C Current draw for large DC motor

Steve, the second 0.85 constant is is a guess at the PF of the controller which I presume will be a rectifier and switching regulator.

I must agree that running such a large motor from 230V is unusual, and perhaps that is the reason they need a custom controller.

DC motors are of two basic types: Series wound like the starter in your car, or shunt wound like the generator in older cars which would function as a motor as well.

I wonder too if they do the commutation electronically and feed the armature through slip rings?

[ February 16, 2005, 10:09 AM: Message edited by: rattus ]
 
Re: Calculating A/C Current draw for large DC motor

How big is the motor junction box?...it will take a lot of wire for 3000A. I have hooked up a number of 1500hp DC motors, but they were 700V. We had 16 535kcmil DLO/RHW (8 plus, 8 negitive) from the drive to the motor and had to add a large extension to the motor junction box to make the terminations. The DC FLA ws 1775 amps.
Don
 
Re: Calculating A/C Current draw for large DC motor

Thank you for the answers. No, this is not an exam question. This motor is part of a large industrial mixer and was originally built for use in US Navy submarines which is probably the reason for the unusual combination of low voltage and high power (I'm guessing close it was meant to be installed in close proximity to the generation point).

The motor had last been in operation about 8 years ago. The original motor controller burned out and the company shuttered the equipment instead of having a new controller built. Our client recently purchased the machinery and is installing it. They have contracted with a local firm to design and build a controller to support this machinery. I had figured a current draw of about 1400A and had sized a new distribution panel disconnect at 2000A. I just wanted to have someone else verify that this number seems reasonable.

Here is some additional information on the motor if anyone is interested: Form A, shunt wound commutator 75C, insulated windings armature 60C, Bare copper windings 70C, shunt field 60C
 
Re: Calculating A/C Current draw for large DC motor

Does the controller for this unit fall under the new Article 409??

If so would the input kVA not be on the control unit??


Just curious

Charlie
 
Re: Calculating A/C Current draw for large DC motor

That explains a lot. I had mistakenly assumed it would be a new motor.

Rattus:

Adding a power factor of .85 does sound reasonable. I was actually refering to the first reply. You posted while I was pecking away on the keyboard :)

Steve
 
Re: Calculating A/C Current draw for large DC motor

Sounds like this is a propulsion motor from a Diesel submarine which ran from batteries. I am sure the price was right for the motor.
 
Re: Calculating A/C Current draw for large DC motor

Derekk

The DC motor control can be a simple thing. Just control voltage of the shunt winding of the generator and connect generator to the motor without any interrupting devices. The start is smooth and it is real pleasure to see this giant machinery to start getting into life without screaming.

This kind of system (Two 5000 HP DC motors @ 2000 VDC working on the same shaft - 110 rpm) I use to operate on ships. There were 4 DC diesel generators working in various combinations with the propulsion motors. The only thing big was the "selector switch".

Have fun
 
Re: Calculating A/C Current draw for large DC motor

Oliver, if I remember correctly, the speed of the motor is inversely proportional to the field current, right?

The shunt wound motor does not produce the highest starting torque, but that should not be a problem when turning screws in water, right?

Streetcar motors, yes I am that old, used a compound motor, part series, part shunt, to provide high starting torque, then at higher speeds, the shunt windings came into play. This was fed from a 600V overhead line. The return was through the rails, except in SF where they drag a monstrous cable under the street, right Sam?
 
Re: Calculating A/C Current draw for large DC motor

Rattus you are correct. The other way to control the speed is with a constant voltage in the shunt windings and a variation voltage at the rotor. In this regime the speed-current curve is a straight line.
 
Re: Calculating A/C Current draw for large DC motor

Thank you for the posts. I am also looking forward to seeing this motor turn for the first time. This is being used to power a large internal mixer used for processing rubber chemicals. The entire machine will be just over 40 feet tall when fully assembled and weigh at 275,000 pounds. Variable speed control is quite important in this application. This motor was used to power this same system 8 years ago and spares are available (a huge plus).

The motor was built in 1942. It's nice to see something that some might term a "wartime relic" used again so many years later in such a totaly different application.
 
Re: Calculating A/C Current draw for large DC motor

I wonder what the motor was originally used for? Probably to drive a propeller? Or maybe even to turn one of the big guns on a battleship?

Steve
 
Re: Calculating A/C Current draw for large DC motor

This takes me back to my Navy days aboard a DC ship. The amount of 3-phase, 480 volt power required would be Volts x amperes of the DC load. The ampere rating of 3055 Amps includes the losses in the motor. Therefore, 3055A x 230V = 702,650 VA. if the rectifier is a 3-phase configuration, I 3-phase = 702,650va divided by 480 x 1.73 which results in 847amps at 480 volts. Based on Art 430, this would have to be increased by 25 % which results in 1,058 amps.
 
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