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ATTENTION ENGINEERS: Inductor to limit inrush when starting an electric motor? Answered

The situation: I am using a contactor (like a big switch) to start a brushed permanent magnet DC motor. I am not using a motor controller. The motor is a Mars 0909 which draws 4.8 KW continuous and 15KW for 30 seconds

The problem: This current can cause arcing at the contactor which can erode the contactor's electrical contact points and perhaps stress the batteries or blow a fuse needlessly.

The goal: Reduce inrush while the motor starts up.

The question: My teacher suggested I use a large inductor in series with the contactor. Theoretically the inductor would resist the initial inrush of current and then its resistance would go way down. We looked around a bit and didn't find any motors in series with an inductor in series. Is there a reason why inductors an not typically in series with motors?

Is this something like a "Peterson coil" or an "arc suppression coil"?

Any idea what the formula would be to calculate the size of such a coil?

3 Replies

steveastrouk (author)2011-06-08

Run the math, and size the inductor needed.

You should be using a motor starting contactor rated for the load, probably with magnetic arc breakers.

Unless you have active control big DC motor= big starting surge.


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Also, big motors like that can ramp to speed VERY VERY fast. Make sure the motor is bolted to a strong suport, or it WILL bounce down the workshop


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iceng (author)steveastrouk2011-06-08

Your very correct Steve, we called them magnetic starters some automatic
some manual.  They would limit motor inrush current in several steps and
were designed to Quench DC arcs.

I still have a Repulsion Start, Induction Run power planer motor.   Alex

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