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I am having a control panel to operate a green house fan and pad lab. in this lab i am having,

  • 2 fans
  • 1 temperature sensor(PT-100)
  • 1 humidity sensor
  • 1 motor for water flow(2 HP)
  • 1 motor for fogger (1.5 HP)

In the control panel are the following components:

  • MCB 6A TP- 5 Quantity
  • Temp. controller- 2
  • Humidity controller- 2
  • contactor 9A- 5
  • overload relay 1.6-2.4A- 3
  • overload relay 2.5-4A 2
  • PLA relay- 2
  • E push button stop- 1
  • Selector switch- 5

The issue is that when the motor starts the overload relay is getting trip after each 5 minits. The motor needs to run continuously. So my question: Is there any problem with the rating of the overload relay? If so then what should be the rating of the overload relay?

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Could you post a schematic and add component values for the motor and the relays? – suha Jun 27 '12 at 19:23
You rate the fan in horsepower and the overload relay in amps. How are we supposed to relate the two without knowing what voltage the fan is running at? Once that is know, it's simple to compare the fan amps with the overload limit amps and see if the specs make sense or not. – Olin Lathrop Jul 27 '12 at 23:36

1 Answer

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I think you have indeed underrated the circuit breaker. As you are not having problems with the start-up current of the motor, I guess the average current drawn by all the machines together is slightly more that what the circuit breaker can handle. The bimetallic plate gets heated up very slowly and after 5 minutes turns off the power. This seems a particularly messy situation, as on hot days it will turn off faster, for example.

No guessing what would be a correct value for an overload relay, tho. Seems you have to either calculate a worst case scenario, based on the datasheets of the machines you have, or measure the average/worst-case current.

Note: I am not an electrical engineer and this post is probably rubbish.

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