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This query is from my project on automatic motor switching of water tank. My question is that what should be the rating of switching device(relay or contactor) for single phase induction motor (2hp 8A class E) ?

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  • \$\begingroup\$ You didn't specify the voltage. At 120VAC, 2HP is about 24 amps. At 240, it is 12 amps. Use a contactor rated for 30A for a 120V motor, and 16A for a 240V motor, and switch both motor legs. Either leave the third pole open, or wire it in series with the one of the other poles. I'd also add a Motor Starter Protector in case of an overload. \$\endgroup\$ – R Drast Feb 26 '17 at 11:53
  • \$\begingroup\$ I'm sorry to miss important details of the question (its first time online query), but its happy and informative to see the data for both voltages. Thank you so much. \$\endgroup\$ – Rohith K Feb 28 '17 at 10:17
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You should use a contactor or motor starter that is rated in horsepower. Devices rated in horsepower are specifically designed for use with motors considering the brief surge of high current that occurs when the motor is switched on and accelerated to full speed. The design also considers the need to interrupt the high current that would occur in the event that the motor becomes stalled. Even some relatively small relays are sold with motor horsepower ratings. Purchase a contactor or relay based on motor horsepower and voltage ratings, not motor current.

A motor starter is a motor contactor with an overload relay sold as a single device. A combination starter also includes a fused disconnect switch or circuit breaker to provide ground-fault and short-circuit protection.

There is not much difference between a motor contactor and a relay that has a motor power rating. For a given power rating, the contactor can be expected to have a longer service life and a better tolerance for frequent on/off operation. Relays rated for motor use are likely to be available only for small motors. A contactor may be the only option for a 2 Hp motor. For automatic switching of a water tank pump motor the on/off switching could be quite frequent. It is probably not the best idea to buy the least expensive switching device that you can find. Unless the motor has built-in overload or over-temperature protection, external overload current protection should be provided.

8 amps is quite a low current for a 2 Hp, 230 volt, single-phase induction motor. For 8 amps, the efficiency multiplied by the power factor be 0.81. I believe that "class E" may indicate a high efficiency design. If that is the case, it is probably a better than average quality motor.

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  • \$\begingroup\$ I feel privileged from your reply Sir. My plan was to use electromechanical relay because contactor seems to be costly. Could you please let me know the practical advantages of contactor over relay, i mean like its stability in fluctuations of supply parameters and initial inrush. \$\endgroup\$ – Rohith K Feb 28 '17 at 10:35
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Contactor and relay are interchangeable terms, there is no official definition that means anything different. But in general, what Mr. Cowie was referring to is that whatever you call the device, it must be rated for the induction motor you intend to switch, you cannot rely solely on the current rating. That's because not only is there a high inrush current when you first turn the motor on, but when you want to turn it off, the inductive coils that make up the motor will sustain the arc that forms as the contacts separate, and that arc, the same temperature as the surface of the sun, will destroy the contacts. So what is usually sold as a contactor will be designed to quench the arc quickly, and effectively dissipate the heat it creates, typically with a design life of at least 1,000,000 operations; not so with cheap relays.

So what might happen with a cheap relay is that it will work for a while, then some day the contacts will melt and weld together, making it so that your pump cannot stop until someone is there to open the disconnect switch or a protective device clears to prevent a fire. but in the process, it usually ruins the motor, costing you a LOT more than having just done it right the first time.

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