# Series inductor to limit motor current?

Short question: Would it be possible to use a series inductor to limit the inrush current when switching on (or briefly loading) a DC motor?

Longer question: I have a battery-powered motorized toy which for convenience I'd like to run from a power socket instead (to avoid having to recharge and replace batteries). Currently it uses 4 1.5v batteries and if I measure it in steady state it draws about 0.8A at about 6V. The way that the toy works is that the motor is usually running continuously with very low load, and occasionally it briefly gets some load on the motor. I don't know exactly what kind of motor is used, but assume it's cheap and simple.

I bought a (cheap) 6V power supply which is allegedly capable of delivering 1.2A, but its current protection cuts in when switching on the motor. As I understand it (please forgive my very basic knowledge here!) the motor presents an extremely low resistance when it's starting up, and so tries to draw a lot more than its usual 0.8A. When running from batteries, I guess they have a maximum current that they're able to provide, so everything gets automatically limited. When the motor is up to speed, its "effective" resistance rises due to its motion, and it reaches its steady state of drawing 0.8A. I expect that when the motor experiences some mechanical load, then the drawn current also temporarily rises, but again the batteries can only provide what they can provide.

When it's connected to the mains power supply, the motor tries to draw much more than 0.8A (I'm not sure exactly how much more), trips the power supply's protection and the power supply cuts out. The motor slows down again, the power supply recovers and switches back on, but then the motor tries to draw too much again and it cuts out again.

I've read about "motor inrush current" and "current limiting" but for this simple toy I'm hoping it won't be necessary to build a custom driving circuit with transistors or mosfets and I can't believe that a temperature-based thermistor solution is ideal (partly because of wasted heat, partly because my intermittent load is not temperature-related). From what (very) little I know about inductors, it seems that one of those in series would "fight against" the sudden change in current and develop a voltage across itself at startup. So I'm hoping this would temporarily limit the current to say 1A. In the steady state this voltage across the inductor would drop to zero and it would then act as a pure low resistance, which is exactly what I think I want.

It doesn't need to be super-precise or to have a perfectly-flat 1A current limit, it would be fine to curve around 0.9A or 1.1A, but I don't know of an easy way to measure what's going on with a simple DC amp-meter.

Could this be the right way to go, or is it too simplistic? And how would I calculate what kind of inductor would be suitable?

• an inductor weighing less than 10Kg will likely not limit the current sufficiently. Commented Aug 15, 2018 at 9:43
• What @Jasen is trying to say is: Yes theoretical this is possible but the inductor needed to do what you want will be huge. Al tough you don't want to, an electronic solution is the easiest one. Commented Aug 15, 2018 at 9:57
• The other thing would be to simulate the internal resistance of the batteries with a series resistor before the motor. Its wasteful, but might be the easiest solution. Commented Aug 15, 2018 at 10:24
• Very interesting, thank you @Jasen, Oldfart. I expected it to be (very very much) smaller as there are only a few volts involved. Good to know! Commented Aug 15, 2018 at 11:27

I'd go with an off-the-shelf DC-DC converter with built-in current-limit and feed that from a plug-pack with a higher voltage

eg:

https://www.aliexpress.com/store/product/DC-DC-CC-CV/1326062_32803647489.html

The three brass screws control the output parameters, one sets voltage, one sets output current, and the other sets input current, set the voltage one for 6v then set the input current one so that the powersupply's over-current current limit does not activate.

You'll have to feed this thing with more than 6V though, the output is always less than the input, 9V or 12V etc would work fine.

you're going to need a voltmeter so you can set the voltage.

The time is too long for a usefully small inductor.

A big electrolytic capacitor across the power supply can supply the start surge. Try 4700uF or 10000uF.

You can also make a soft-starter using a power fet. Adjust R and C to get right rate.

simulate this circuit – Schematic created using CircuitLab

• If I put a big capacitor across the power supply, wouldn't that draw even more current at startup time to charge up the capacitor? And the "soft starter" circuit wouldn't handle increased load, right? Commented Aug 15, 2018 at 10:32
• It is connected to the power supply before you turn the supply on. Most psu are happy with this (some cheapies aren't). The soft starter makes heat, but not for very long. Commented Aug 15, 2018 at 10:42

You could fit a simple inrush current limiter (ICL) in series with the power supply. These are basically negative temperature coefficient resistors - when you power the device, a t=0 the ICL has a high resistance which limits the current. As the small amount of current slowly heats up the ICL, it's resistance decreases. This eventually leads to the resistance being near zero when the ICL stabilises at a certain temperature.

The effect is it squashes inrush currents and stretches the pulse out, then once stable has almost no effect on the circuit.

Added bonuses, its one small cheap 2 pin device.

Although its possible to model the behaviour of ICL devices, and select an appropriate through simulation alone - its incredibly difficult. There are a lot of factors in play (heatsinking of ICL to board, ambient temperature, shape of current pulse, repetition etc). In my experience its best to just buy a selection of values and give them a try. In the lab I test them with a scope and current probe, but just works/not works testing is also fine.

• That sounds like the thermistor-based solution I mentioned in the question - my concern is that the intermittent load on the motor will happen when the ICL is already warm and the high-current shutoff will still be triggered. Commented Aug 15, 2018 at 10:13
• Apparently I only skim read the question then. You need the power supply to shift into a constant current mode really, which is probably not going to keep your motor happy. Either you store some energy to give in bursts (big caps), or use a better power supply. Commented Aug 15, 2018 at 10:18