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I'm fairly new to electronics and working on a project that uses a 25A 30VDC DPDT relay to reverse the polarity of a brushed DC motor (powered by a 24V battery). The relay has been failing by arcing and sometimes fusing in one position when the motor is switched back and forth too quickly.

I'm trying to figure out if the cause is high inrush current when the motor changes direction, back EMF from the motor, or possibly both. During direction changes, current spikes up to ~30A (normal draw is around 10A), and I'm assuming voltage spikes are occurring too, though I haven’t been able to capture them accurately.

I've read that relay arcing can be mitigated using flyback diodes, capacitors, or NTC thermistors, but I’m not sure which approach is most effective in this kind of application.

What are the best practices or recommended protection methods for this setup? I'd really appreciate advice from anyone with experience handling this kind of issue.

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  • \$\begingroup\$ May be related/helpful: electronics.stackexchange.com/q/322120/2028, electronics.stackexchange.com/q/376237/2028 \$\endgroup\$ Commented Apr 16 at 16:23
  • \$\begingroup\$ Can you provide a schematic? It sounds like you using relays instead of an H-bridge to switch motor polarity. \$\endgroup\$ Commented Apr 16 at 16:26
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    \$\begingroup\$ Your basic operating principle is flawed. You shouldn't reverse a DC motor like that and expect to find any components that will survive reliably within any reasonable cost budget. \$\endgroup\$ Commented Apr 16 at 18:00

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The issue is not just arcing, but also due to high back-EMF when trying to reverse a spinning DC (or universal wound) motor.

If the motor is spinning, and current abruptly cut off, the motor generates a voltage about equal to that driving it before, until the speed drops -- 24 VDC, in your application. If that motor is then fed 24 VDC in the opposite direction, effectively 48 VDC is flowing through the motor, doubling the current and quadrupling the power! No wonder the switch is being destroyed, and the effects on the motor brushes and armature are also damaging. It would be like driving the family car forward at speed, and forcing the gears into reverse.

Notice that switches on reversible drills usually have a lockout, preventing instantly reversing direction. One must stop drilling, then switch to reverse.

Either implement a similar lockout, or use a three position (SPTT) switch, with the center connected to a power resistor, braking the motor to a stop before reversing. That might better be done through a relay with a short time delay at center, to prevent the user from skipping through the brake position too quickly.

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