# Force calculation for lock-rotor motor

1. Oct 26, 2007

### Tagline

I have 1700 HP induction motors that I need to do lock-rotor test on. Force applied to the bolts on the lock-rotor flange should be mass X acceleration I guess but what I'm stuck on is what would the acceleration be of a motor that's not turning ... is that a dumb question or what! Eventhough the motor won't be moving, it's still applying force. Anyone know how to figure that one out? Thanks!
~Larry

2. Oct 26, 2007

### stewartcs

If the rotor is locked it's not accelerating.

I imagine that the amount of force on the bolts is related to the locked-rotor torque. The manufacturer of the motor should have that value. The locked-rotor torque may vary somewhat with different standstill positions of the rotor with respect to the stator.

If you don't have the manufacturers data you can calculate the locked-rotor torque with this equation:

T = (21.12*Rr*Ebr^2)/(s*ns*[(Rr/s)^2 + Xbr^2])

where,

T = developed torque in ft-lbs
Rr = actual resistance per phase of the rotor windings (ohms)
Ebr = blocked rotor voltage
s = slip
ns = synchronous speed
Xbr = blocked rotor reactance

Just set s (the slip) equal to 1 since you are trying to find the locked-rotor condition.

3. Oct 27, 2007

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Thank you Sir! I'll see if I can't puzzle that out.

I always say: Well ... it seems we don't have time to do it right, we only have time to do it over.
~Larry