Electronic – How to avoid damaging relay used for controlling motors

motorrelay

I've got a circuit with two DPDT relays controlling a motor – the diagram below is borrowed from a related but different question. RLY1 turns everything on and off, RLY2 switches the direction of the motor.

I've found that after a brief amount of use the relays are damaged. Both relays have the same problem – on one set of contacts common/NO/NC are all connected, even when the relay coil is unpowered. (The other set of contacts is as expected – ie. common/NC are connected, NO is connected to neither of the other two – so only "half" of each relay is currently damaged).

Questions: what is likely to have caused NO/NC terminals to be connected, and how should I have avoided that problem?

Gory Details (possibly not needed):

  • motor runs on 30V
  • PSU that came with the motor is rated 30V/2A
  • relays are rated for 2A/30VDC (model Meishuo MCB-S-205-C-M)
  • relay is controlled from an ATMega MCU
  • relays both have an IN4001 protection diode across the relay coil – not shown in diagram
  • I did manage to "fix" one relay by tapping it on the table. Unknown if that will stay fixed or if permanent damage is done. The other relay doesn't respond to this treatment.

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Best Answer

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Snubbers on reversible motor.

Provided your relay contacts are break-before-make you shouldn't be causing a direct short. The problem is likely to be the inductive kick causing arcing of the contacts when they open.

One solution is to put diodes on the motor to shunt the current to the PSU. This may look a little strange but is really just a rearrangement of the standard H-bridge protection as shown in Figure 2.

schematic

simulate this circuit

Figure 2. The same circuit in H-bridge configuration.

schematic

simulate this circuit

Figure 3. Redrawn to use relay configuration on OP's schematic.