Induction Motor Calculations
Motor Efficiency and Power Factor
First, it is important to identify which KW you are referring to. There is mechanical output power and electrical input power.
Electric circuits are rated in electrical input power and mechanical systems in mechanical shaft power.
For the purposes of the description, let us identify these as KWe and KWm Where KWe is the electrical input power and KWm is the mechanical output power.
The power rating of an induction motor is usually the rated shaft power of that motor. (KWm)
The efficiency of the induction motor is the ratio between
the mechanical output power and the electrical input power.
The power factor of an electrical circuit is the ratio between the electrical KW input to the KVA input, so pf = KWe/KVA
The mechanical shaft power is therefore KWm =
KVA x pf x eff
So KVA = KWm / pf / eff = KWm / (pf x eff) = KWe / pf
Be careful to ensure that you know whether you are referring
to mechanical output power (usual with motors) or electrical input power.
Motor Synchronous Speed
The synchronous speed is a function of the frequency of
the supply and the number of poles in the stator winding.
F = Supply Frequency (Hz)
The torque produced by an induction motor is a function of the shaft power and the shaft speed where the torque reduces with speed for constant power.
T = 9.5493 x P / RPM
T = Torque (Nm)
T = 5252 x P / RPM
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