what does blocked rotor mean???

Thread Starter

Rizu

Joined Nov 1, 2012
11
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?
 

Papabravo

Joined Feb 24, 2006
21,158
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:

crutschow

Joined Mar 14, 2008
34,280
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.
 

#12

Joined Nov 30, 2010
18,224
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.
 
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