Need to simulate an induction motor load

Discussion in 'General Electronics Chat' started by jitinl, Oct 15, 2013.

  1. jitinl

    Thread Starter New Member

    Oct 15, 2013
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    Hi,

    I have to demonstrate the functioning of relays to someone. Now these relays would be used to switch the supply to an induction motor. I want to check/show that the relays can be turned on-off repeatedly under such a load without getting damaged. However its not possible to use an actual induction motor for the purpose. I was wondering if I could instead use inductors and resistances to simulate the load of a motor in a single phase circuit, and maybe blink an led instead of turning on the motor. The purpose is to roughly estimate the load of a real motor and not to get an accurate judgement.

    Can anyone help me with any references on deciding the values of resistances and inductances for testing purpose - maybe there is a typical range for induction motors' equivalent resistances and inductances - I tried finding one but with no luck.
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    I don't have those values. But depending on the relay contact rating I would tend to believe that they can be damaged over time if you repeatedly open / close them with current flowing through them. There are also different contact materials...
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    What is the purpose of this test and how do you intend to "prove" if a relay is suitable or not?
    Normally you would just choose a relay/contactor with a contact design rated to switch inductive loads..

    Have you looked into solid state relays/snubbers
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    All mechanical relays will suffer contact damage in time. Mcgyvr has the answer.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You could use an inductance such as a large transformer with a rectifier and large capacitors and a high lighting load on the secondary to simulate an induction motor.
    An induction motor at switch on actually represents a transformer with a shorted turn secondary.
    For applications that switch motors or similar loads very frequently such as HVAC etc, a specially made relay is used call a 'Special Purpose Contactor' type, this posses heavy duty contacts and arc Chutes etc.
    Max.
     
    Last edited: Oct 15, 2013
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    If you want to continue.. Its easiest to just use a real induction motor..
     
  7. crutschow

    Expert

    Mar 14, 2008
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    Because it's difficult to simulate the large starting current of an induction motor I agree with mcgyvr, use a real motor.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I find the whole exercise somewhat a little pointless, all relays and are typically chosen or selected to suit their respective loads, if you undersize a relay contact rating, especially for an inductive load, the life of the relay is going to be shortened, whatever the application?
    No different for motor loads, this is why as I previously pointed out the use of Special Purpose for repeated inductive motor load switching.
    It may be more enlightening to use undersized relays on a high inductive DC load and show the results that can occur when the selection is wrong?
    Once those being demonstrated to observe a plasma arc that continues on a miss-selected relay to the point of melting the contact. The observer usually does not forget it.
    Also an opportunity to get into Magnetic arc-blowout.
    Much more enlightening!
    Max.
     
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