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I want to remove the need for contactors in a motor application. Currently we are using a 1.5HP (1.1KW) motor and actuating it via a contactor.

I have a PCB I am designing and want to use the following relay to actuate this motor: https://www.digikey.com/en/products/detail/american-zettler/AZ2150-1C-24DEF/12171625

Aside from all of the practicalities of tracing 220VAC on a PCB such as air gaps, I noticed that the datasheet only rates this relay for 6,000 cycles on the motor load I am looking for.

only 6000 cycles for 2 HP motor

I am curious how this cycling is calculated, as the motor is only drawing a max of 6 amps or so and the relay is rated for 40. If anyone has any general insight into the use of large relays for motor loads like this that would be very helpful.

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    \$\begingroup\$ Are you essentially asking "Do the number of cycles increase if I draw less than the rated current through this relay?" \$\endgroup\$
    – JYelton
    Commented Apr 29, 2022 at 16:52
  • \$\begingroup\$ No J, I am asking why the number of cycles for a motor load is so much less than a general purpose load for a given current. \$\endgroup\$
    – radhlw
    Commented Apr 29, 2022 at 20:36

1 Answer 1

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A motor is an inductive load. As the contacts open the inductor attempts to maintain current. Because of this the voltage momentarily increases. This effect causes arcing of the contacts. Arcing greatly shortens the life span of relay contacts.

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    \$\begingroup\$ That makes a lot of sense. Is this something a flyback diode is able to help with, preventing the arc? \$\endgroup\$
    – radhlw
    Commented Apr 29, 2022 at 20:37
  • \$\begingroup\$ @radhlw With a DC motor, yes. I don't know if it would apply to your situation but you might want to look into TRIACs; these are like solid state switches that can be switched at the zero-crossing of the AC waveform. I don't know if they're the right solution but I thought I'd mention as something to look into. \$\endgroup\$
    – JYelton
    Commented Apr 29, 2022 at 21:29
  • \$\begingroup\$ Thank you for the TRIAC tip. I will look into it for our application. For an AC motor, how do I look for a relay that will provide sufficiently high cycle life that can be mounted on a PCB? \$\endgroup\$
    – radhlw
    Commented Apr 29, 2022 at 21:41
  • \$\begingroup\$ @radhlw A relay may not be available in the form factor that you want. Look into snubberes. While i'm not 100% sure they will increase the contact life of a relay, they are required for triac use to properly shut off triacs with inductive loads. \$\endgroup\$
    – vini_i
    Commented May 3, 2022 at 11:13

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