what does blocked rotor mean???

Discussion in 'Homework Help' started by Rizu, Dec 2, 2014.

  1. Rizu

    Thread Starter New Member

    Nov 1, 2012
    11
    0
    In any motor, when we do Blocked Rotor Test, what actually happens? Is it that the full power is consumed in core loss? Is the current that flows through the armature is rated current? In fact what sort of physical phenomena occurs when the rotor is blocked ?

    E(a)=kΦω .....E(a) being the induced voltage
    right? so when the rotor is blocked, ω=0 i.e. E(a)=0......so the current flow will be maximum in the armature. Is this maximum current called the rated current in a motor ? then the induced torque will be maximum as

    ζ (ind)=k*Φ*I(a)

    see, I cant make anything out of it?
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    When a rotor is blocked, current flows through the windings limited only by the DC resistance of the wire. There is no back EMF from the rotation to reduce the current in the windings. As a result thing inside the motor will get warm, then hotter and hotter. There are three ways to get rid of heat:
    1. Radiation
    2. Conduction
    3. Convection
    Depending on their surroundings, some motors may survive and some will not.

    [Edit] I think I'll revise the above statement.
     
    Last edited: Dec 2, 2014
  3. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,221
    Often a blocked rotor test is done at reduced voltage to reduce the dissipated power. Motors can't generally stand a block rotor test at full voltage for more than a few tens of seconds. Also you are likely to blow a breaker using full voltage for any significant length of time.

    The blocked rotor full voltage current would be the maximum startup current and the maximum starting torque of the motor.
     
  4. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    Papabravo stated the basic principle in his first sentence.
    I do locked rotor tests often. The label on the machine says, "LRA". Like, LRA = 76 amps
    I guess it depends on what country you are in to call it locked rotor or blocked rotor.
    Anyway, a clamp meter will settle on a reading in about 1 second, and I can read the meter in about 1 second. That's quick enough to almost always avoid popping a circuit breaker.
    This test also shows up defects in the power supply wiring.
    A peak holding amp meter would be even quicker.
    I can't read your math, but I can do the test in practical terms.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,503
    2,367
    For us in UK it was known as Locked Rotor. Same in Canada!;)
    Max.
     
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