I have a question about the demand of Reactive power at motor
stalling. It is always mention that "reactive power demands of load
increase with motor stalling". However, should it be the other way
around? When it is stalling, the voltage drops and it actually takes
less current through the inductance.
Typically, line voltage does *not* drop very much when the motor is stalled.
The frequency induced into the rotor is proportional to slip, so at stall
(100% slip), the frequency in the rotor is equal to line. The inductive
reactance of the rotor is higher then the resistance at stall, so the line
current power factor is very low (0.1 is not unheard of).
So, stalled motor, high current, low power factor => Hi reactive demand.
On 5/15/07 11:59 AM, in article
I have some trouble understanding the details of the question.
Nevertheless, an induction motor is pretty much like a transformer. At
stall, slip is one and the motor truly is a transformer. The primary is the
main motor winding that provides the rotating field. The secondary is the
rotor that can be either a wound rotor or the more likely squirrel cage.
This secondary is a short. Current is limited by the leakage reactance and
secondary resistance which is usually kept low. Thus, except for the
residual resistance, primary current is set by this reactance. The current
will be mostly reactive.
-- Fermez le Bush--about two years to go.
| I have a question about the demand of Reactive power at motor
| stalling. It is always mention that "reactive power demands of load
| increase with motor stalling". However, should it be the other way
| around? When it is stalling, the voltage drops and it actually takes
| less current through the inductance.
The voltage across the load will still be mostly there. The current
will be high. Less work is being done (the "work" that is being done
is heating the windings and other wires). With less work there is less
power used. Less power used relative to volt*amps means low power factor
which means more reactive.
The inrush current in a motor is often about six times the normal full
load current. This current is often called the 'blocked rotor'
current because it is the current that flows when the rotor is at zero
speed -- as occurs at startup, or a with a blocked rotor, or when
The resistance of the rotor reflected to the stator circuit is "R2'/s"
where s is slip ... so with the motor running at rated speed with slip
of say s=0.04, the rotor resistance as seen by the stator is much
higher than when the motor is stalled, with slip s=1. I believe this
is what gives rise to the higher current at lower speed.