0
\$\begingroup\$

i am using two Relays to create an H-Bridge. This H-Bridge drives a 24V Motor ~3A in both directions. Now i often got the problem, when i turn on a motor and turn it off again, the relay contacts are getting burned (because the motor gets shorted between the two GND pins and its kinetic force will get stopped and it creates an electric arc)

Do you think i have attached the snubber (R = 27Ohm | C = 470nF) in the right way? Would it be better to create a second snubber on the GND Pin of the Relay, so that both: Motor ON and Motor OFF gets secured by snubber?

Unfortunately we need to short the motors when they are turned off, so it is harder to rotate the rotor of the motor.

The Relay contacts when the motor is getting turned on are always looking very nice, it is mostly the GND contacts on the relay when the motor gets shortet after it is turned off.

Any Ideas? :)

Schematic -> See in the Pictures.

H-Bridge

I am also using Free-Wheeling Diodes

Freewheeling Diodes

\$\endgroup\$

1 Answer 1

1
\$\begingroup\$

The arc is formed when the moving relay contact moves away from the 24v terminal and draws the arc to the 0v terminal of the relay. So the arc forms from 24V to ground, and can destroy the relay as the current becomes uncontrolled.

There are specific relays designed for use in the H-Bridge Mode. For example the G8NDL from OMRON are specifically designed to deal with the arc. It is only suitable for 12V systems however.

24V makes things much more difficult as the arc is much worse. There are a few choices:

  1. You can use two separate dual relays, opening and closing each contact with a dwell time of 100ms or so. 4 coils, 4 contacts.
  2. Insert a MOSFET from the Grounds of the relay to 0V, and use the mosfet to interrupt the voltage, while switching the relay polarity.

Option 2. is often more cost effective.

Update - There are also specific 24V relays with wider contact gaps, intended for truck applications that could help. For example the Fujitsu FBR572 / 582.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.