Is it advisable to exceed the rated currect of a relay for a brief moment, when using it to drive an inductive load (a motor)?

Question

I am building a relay circuit for pump control with a microcontroller. It is a 1.5 HP single-phase water pump that draws around 13A during normal running and has a starting current that briefly (< 1 second) reaches 25A.

My relay is rated for 20A. The datasheet rates it for over 50,000 operations for steady state currents at about 13A, which is my nominal power draw. Can anyone with experience in this kind of thing provide some insight on the degradation of the relay over time? I expect the relay to accumulate less than 15,000 cycles over 20 years.

Answer 1

Motors are inductive and take start currents that are much higher than their run currents .This is hard on relays .Reputable relay manufacturers show a curve of current Vs number of operations .You can read down on the curve and estimate product life .Finding a suitable relay is easy when the number of operations per day is low and difficult when the number of daily operations is high .Some larger relays that are sometimes referred to as contacters give a rating in terms of HP so selecting a relay for your motor is easy .

Answer 2

You should use a relay that has a motor rating, obviously a rating that is high enough to include the motor you intend to operate.

Here is a relay with such a rating.

If you use an underrated relay it's possible to have a failure that is not as benign as failing to close after a fair number of cycles. For example, the contacts may well weld closed. And maybe after 10 cycles or 20.