# 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
10,340
1,850
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:
2. Conduction
3. Convection
Depending on their surroundings, some motors may survive and some will not.

 I think I'll revise the above statement.

Last edited: Dec 2, 2014
3. ### crutschow Expert

Mar 14, 2008
13,475
3,362
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
16,665
7,310
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.