Variable speed control for an inductive load.

Thread Starter

Max Kreeger

Joined Oct 1, 2013
94
Hey guys,

I have a brushed DC motor which I have hooked up to a PWM speed controller. However I noticed in its specs sheet it said "For inductive loads such as DC motors it is recommended to place a 400V 3A diode in parallel with the load"

Why is that?

Thank you in advance,

Max
 

crutschow

Joined Mar 14, 2008
23,770
As Dd said.
An inductive load tries to keep the current moving through it when the controller shuts off (rather like the inertia of a mechanical load).
If this current has no place to go, it will generate a large negative voltage spike at the controller output, which can damage the controller.
The diode provides a low impedance path for this current, thus preventing the damage.
It's common practice to add a diode across any switched inductive load, such as motors and relay coils, to prevent inductive spikes.
 

MikeML

Joined Oct 2, 2009
5,444
Does the motor connect between controller and the most negative voltage on the power supply, or between controller and the most positive voltage on the power supply?
 

MikeML

Joined Oct 2, 2009
5,444
Is the motor reversible, or does it always turn one direction?

If it always runs only one direction, put the cathode of the diode to the + (red) lead, and the anode to - (blk) lead.
 

Thread Starter

Max Kreeger

Joined Oct 1, 2013
94
So should I still add the diode in like it said in its manual? If so, would adding it directly in parallel with the motor contacts be ok?
 

Thread Starter

Max Kreeger

Joined Oct 1, 2013
94
No, the motor is reversible.

EDIT: So what would happen if I added a diode in?
By reversible I mean if I change the polarity.
 
Last edited:

Glenn Holland

Joined Dec 26, 2014
682
If the motor is reversible (it looks like a plain vanilla permanent magnet motor), the polarity reversing contacts (an "H" Bridge configuration) would be at the input connections to the motor and the diode would be at the output of the PM controller.
 

Glenn Holland

Joined Dec 26, 2014
682
The diode is in parallel with the inductive load (the motor) so it provides a path for the induced voltage (and current) when current to the load suddenly decreases or shuts off completely.

If the diode is next to the PWM controller, it is in parallel with the motor and it will always have the same polarity regardless of the state of the motor reversing contacts.
 

DickCappels

Joined Aug 21, 2008
6,005
Yes, put it in according to the instructions that came with the speed control. If you put it in backwards it will look a lot like a short circuit to the speed control and if it is not protected against a short circuit, something could be damaged.
 
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