What relay to use for controlling a 120V 1/3HP ac motor

Discussion in 'General Electronics Chat' started by vijaybala85, Apr 7, 2010.

  1. vijaybala85

    Thread Starter Member

    Jan 7, 2010
    92
    0
    Hi Guys!

    what type of relay (specs and manufacturer requested) should I use to control a 120V 1/3 HP ac motor? This is how the control flow works.

    I have a data acquisition card that can output 5V. I am using an opamp with 24V and GND on the rails , whose input will be the 5V to get an output of 24V to power a 24V dc relay. Now can I use that output from the relay to power another relay (one that I need specs on) to turn on the ac motor?

    Hope my question is clear?

    Thanks!

    V
     
  2. vijaybala85

    Thread Starter Member

    Jan 7, 2010
    92
    0
    Oh and from the details I received about the motor, it draws about 7-8A from the ac power supply.

    V
     
  3. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    For an AC motor rated at 1/3hp, I would highly suggest using an AC Motor Contactor/ Starter..... These are normally switched with AC power but, using the 5 volt from the DAQ card, you could use a Solid State Relay to switch the motor contactor on or off... although motor contactors are very high priced....

    [​IMG]

    B. Morse
     
  4. vijaybala85

    Thread Starter Member

    Jan 7, 2010
    92
    0
    Thanks BMorse!! And for the picture too.. But is there any cheaper than contacters?

    V
     
  5. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234

    You could try a heavy duty relay, but every time you turn on the motor you will see the contacts arc and could fuse them together which would be bad..... so I would really suggest a contactor, they maybe expensive, but will last longer and could be more reliable in the long run.....

    P.S.
    If you do go with a motor contactor, make sure you get an MOV or a surge suppressor for the contactor (small device in picture above to the right of the contactor) this will prevent back EMF from causing any problems when the contactor is turned off....

    B. Morse
     
  6. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    Just to let you know, there are some relays out there that can handle motor loads, I had found some at www.Mcmaster.com , this particular one was only $25.00 and can handle up to a 1 1/2 HP motor.... I am using it with a Motor contactor to reverse a single phase 3/4 HP AC motor....

    AC Motor Relay.jpg

    B. Morse
     
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