Soft start a D.C. Motor

[anishkgt]

Hi all,

Forgive me if this sounds lame

I have a motor rated 12-24v which I plan to use to drill several pcbs. I wish to soft start the motor upon powering it.

I have read somewhere that this is possible with a resistor and a capacitor yea adding a 25v 1000uf capacitor in parallel would help but I wish to have the startup to be a little more slower by adding a resistor in series with the cap but not sure of the values.

Would be grateful if somebody could help.


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[Zero999]

What current does it draw?

A capacitor would make the current inrush worse, not better.

Here's an example using a MOSFET. The maximum gate-source voltage of most MOSFETs is around 20V, so for 24V operation, you'll need to add another 1Meg resistor between the gate and 0V.

https://www.eevblog.com/forum/beginners/powering-a-dc-motor-with-plug-in-transformer/

[anishkgt]

Just clearing a doubt. Wouldn't adding resistor actually slow down charging of the cap and there by slowly start the motor ?


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[Zero999]

Just clearing a doubt. Wouldn't adding resistor actually slow down charging of the cap and there by slowly start the motor ?

No, it wouldn't, because the capacitor and resistor are in parallel with the motor, the full voltage will be applied to the motor, as soon as the power is applied and it will start at full speed. The extra current, required to charge the capacitor, will be added to the motor's inrush current.

You could theoretically connect a large inductor in series with the motor, to soft start it, but the inductor would need to be significantly larger and heavier than the motor.

Anyway, why do you want to do this? The usual reason for soft starting a motor is because the current surge is causing the power supply's over current protection to kick in.

[anishkgt]

Reason to do it was prevent the sudden twisting of motor and as well learn to overcome it.


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[oldway]

Very easy: as the DC motor start with no load, simply connect a power resistor in serie with the motor so inrush start current is limited to 1 or 2x nominal current and short circuit the resistor by a relay after 2 or 3 seconds.

[Zero999]

Reason to do it was prevent the sudden twisting of motor

Again why? In an application such as a drill, I believe it's better if the motor starts as quickly as possible. If your power supply can start the motor, there's no need for a soft start.

[anishkgt]

Hmmm yea. But it was good if it was possible. I could dot it with an arduino but I don't intent to use it for this application.


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[james_s]

Just use the circuit posted above with the mosfet, or the resistor with a bypass relay, I've used the latter before and something similar to the former except I used a NPN power transistor. All you need is something that will limit the current initially then bypass it. Not really necessary for small stuff but it does reduce the mechanical load on larger machinery. You could also replace the switch with a simple PWM speed control and just turn it on at low speed the way the trigger on a variable speed hand drill works.

[eKretz]

Of course there are needs for soft starting motors other than current overload. That's kind of a silly statement. The are many applications for soft start that are beneficial/necessary. For drills it can help prevent issues with the chuck loosening or tightening, as well as decrease wear on the drivetrain (belts, pulleys, gears etc.). I have a DeWalt brushless hand drill that brakes so hard when the trigger is released that the chuck ends up dropping the drill if I let it brake 4 or 5 times in a row. I've learned to slowly release the trigger or else. The same can happen in reverse (on motor startup) and make the chuck very difficult to loosen. There are many many applications in other areas as well, obviously.