# PWM through relay power contacts

Hello, I have done research but failed to find. The question is: " if the current passing through the power contacts of a relay is PWM to say 50% does this alter the effective current capacity of the relay?"
If for example the relay switches a DC motor which draws 14 amps and PWM is applied to the current flowing through the relay contacts does this relieve the relay contacts so that a relay rated at less than 14 amps could be used safely? I understand that the motor will run slower . Thanks R.

Hello.

Relays commonly have separate ratings for switching and continuous current, but it sounds like you are asking about a case where the relay is closed continuously and there is some other switch in the circuit that is performing the PWM.

If you simplify this problem to a case where the limiting factor for the relay is its heat dissipation, assume the relay has a fixed resistance (let’s say 10 milliohms), and that the current through the relay is a square wave, then the average current the relay would be able to handle would be lower as is illustrated by the following example.

In most real systems, there are things that stray from those assumptions. Inductance and capacitance between the relay and the load will change the slope of the current and voltage changes and higher PWM frequencies reduce the time those variables have to change.

-Nathan

Thanks Nathan, Yes the situation is that I have ordered a DC powered gear pump which I know I will have to run slower [ to reduce PSI output from the pump] and I already have 2X DPDT relays set up as a reversing method for the pump which is controlled by an inclinometer with relay outputs to stabilise a platform. Its a long story, but I know that I will be applying PWM to the pump motor to achieve the desired output pressure from the gear pump that is attached to it. The relays are rated at 10 amps/ 12v and the pump is rated at 14 amps 12v. so on the face of it the relays need to be upgraded but… I wondered if the current being PWM this would allow the lower rated relays to survive.
I guess I will need to observe what happens. The relays are not continuously closed as they respond to the inclinometer to balance the platform so they are on for say 1 second off for another and then the other relay might come on as they balance the platform. via the gear pump fluid flow direction to an actuator.
Thanks for your input I will see what happens. Regards R

If the load has an inductance, which is the case for DC motors, the current does not follow the PWM cycle, but stays almost constant instead.

Ok got it now, thanks R.