Relay contact protection against 24V 2-pole brushed DC motor inrush current

Question

I'm driving a 24V DC motor applied to a small gearbox. Motor control is via (2) 8A rated PCB mount relays that provide motor power as well as direction control. Motor inrush is rated at 8A for 10-20ms and varies depending on the status of the geartrain backlash loading in conjunction with the DC motor startup current. Running current is 1.6A at full load. If the motor command cycle is excessive, the relay contacts are burning and eventually fail. I'm looking for a simple soft-start solution (i.e. NTC thermistor or limiting resistor on the 24vdc feed to the motor) but with minimal impact on the running characteristics, and without adding a micro for control. A power resistor seems to be a simpler solution, but takes space, whereas an NTC takes less space but might run hotter. Somethign like a "zero-crossing" switch but for DC applications. Any ideas on a solution?

Answer 1

The best way and most reliable is to purchase relays that meet or exceed your motor's requirements. You could use an H-Bridge such as the BTS7960 which will require additional electronics. PWM (Pulse Width Modulation) will give you the soft start etc. A micro such as an Arduino (another forum on stackexchange) would do the job but it can also be accomplished with discrete electronics.

Answer 2

Here's the schematic, featuring flyback diodes and braking.

One relay is to be energised for clockwise direction and the other for counter clockwise.

Direction reversal is to be effected only after the motor has come to a halt.

With these precautions, relay contact burning would be effectively prevented.