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Usually to control our water pump motor, we have to turn the switch on, wait for 15-20 minutes and then turn the switch off. Often times someone forgets to turn the motor off. I want the motor to turn on for 15 minutes and turn off automatically. I am comfortable programming with Arduino and first considered using a 5 V relay module to control the motor. After looking at the datasheet of my relay module, I found out that the 10 A 250 VAC rating of the relay is only for resistive loads. For inductive loads the rating drops to 6 A for the relay module I have. Which I am not comfortable with, but the problem is the starting current of motor which is around 7-8 times the rated current. So around 42-48 A of starting current, which could damage the relay module. I did some research and found out that the magnetic contactor can be used to control the motor. I could use an Arduino and the relay module to control the magnetic contactor which would control the motor. I also heard about time relays. Can time relays handle the starting current of the motor or should I use a magnetic contactor instead?

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  • \$\begingroup\$ Do you have a datasheet for the relay? \$\endgroup\$
    – jonathanjo
    Sep 16, 2022 at 10:30
  • \$\begingroup\$ I do have the datasheet of SRD-05VDC-SL-C relay module. But I don't know how to read a time relay datasheet to look for how much transient current it can handle. \$\endgroup\$ Sep 16, 2022 at 10:54

3 Answers 3

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It's standard practice to use a motor contactor, along with a thermal overload relay, to protect the motor.

The timer contact could be used to energise the contactor coil.

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Can time relays handle the starting current of the motor or should I use a magnetic contactor instead?

You can use both. Use a timer relay to power the operating coil of the contactor. Check in the contactor datasheet that the coil operating current is compatible with your timer relay rating. Also check that the operating coil of the contactor is compatible with your supply voltage. (Operating coils can come with different voltage ratings so make sure you get one to suit your application.)

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You can use a spring-wound timer to control the coil of a contactor. These are simple mechanical timers with no electronics inside, and are very reliable. They come in many different time scales from about 10 minutes to 12 hours. (Photo credit: Amazon.in)

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