PWM controlled MOSFET based DC motor driver, stuck with reverseing direction

Question

See the circuit diagram below and help me to find the problem:

I can drive the motor in one direction as expected and to reverse the direction I added a relay circuit. I used two SPDT relays instead of DPDT shown in diagram.

But some times one relay get turned off. Spark see inside the relays too while changing direction. Is there any way to remove the high voltage spike that produce as a result of reversing the direction of motor?

Answer 1

The reason you see the sparks is because you are open-circuiting a charged inductor (ie the stator of the motor)

Thou shall not open-circuit a charged inductor, thou shall not short-circuit a charged capacitor.

The stator of the motor has been "charged"... you have at one point applied a voltage to it to permit current to flow. For whatever reason (intent or oversight...) the relay has opened the stator terminals. The inductor MUST keep the current flowing & due to: \$V = L\frac{dI}{dt}\$, a sudden (attempted) change in current can only result in an increase in voltage which leads to sparks, avalanche or insulation breakdown.

To remove such sparks you could consider using a H-Bridge arrangement but these are more involved. As a "poor mans" replacement, some sort of snubber across the motor would be the next best thing. An R-C-Z would be one option

Answer 2

^{simulate this circuit – Schematic created using CircuitLab}

*Figure 1. (a) Two back-to-back Zener diodes or (b) a bridge rectifier arrangement.

• (a) requires two Zeners with a breakdown voltage greater than the normal motor current. For your 12 V circuit 18 V Zeners might be a good choice. Their current rating should be >= motor current for safety.
• (b) uses four diodes or bridge rectifier to shunt the voltage back to the supply. Note that the supply has to be able to absorb this.