Induction Motor Rating and Efficiency

Study Notes

Motor power is rated in terms of mechanical output power.
Electrical power input is determined by: √3 x V₁ x I₁ x λ x P.f, using line voltage (V₁), line current (I₁), power factor (λ).
Mechanical power output is determined by: 2πnT / 60 using rotor speed (n) in rpm and motor torque (T) in Nm.
Motor efficiency is the ratio of output power to input power: η% = (Pout / Pin) x 100.
Output power is always less than input power due to losses in energy conversion.

Rotor current increases with increased load, leading to greater torque demand.
Increased rotor current has a corresponding opposite change in stator current.
Stator current has three components: magnetising current, iron-loss current, and current to balance the rotor mmf.

No-load: Energy is used to establish the rotating magnetic field and overcome motor losses.
With load: Energy is used to establish the rotating magnetic field, overcome motor losses, and drive the load.
Stator current on load is the phasor sum of magnetising (Iμ), iron-loss (In), and load components.

A load test determines motor performance at different loads.
Constant supply voltage and frequency are crucial for accurate results.
Load can be applied using a direct-coupled generator or a brake.
Quantities measured include: speed, rotor current (slip-ring motors only), stator current, power factor, power input, efficiency, and power output.
Stator and rotor currents measured are line values.

Speed: Falls fairly uniformly as power output increases.
Rotor current: Rises almost uniformly with increased load, increasing the torque component.
Stator current: Increases proportionally to rotor current, as it neutralizes the rotor mmf.
Power factor: Improves with increased load, as the stator current becomes more in phase with the applied voltage.
Power input: Increases with load, reflecting increased energy conversion.
Efficiency: Increases to a maximum at full-load, then decreases under overload conditions.

Reduced voltage significantly impacts motor performance:
- Reduced rotating magnetic field strength.
- Reduced induced voltages and currents in the rotor.
- Reduced torque output, proportional to the square of the applied voltage.
Reduced voltage increases the supply current, potentially stalls the motor, and prolongs the acceleration period.

This table provides efficiency and power factor values for various synchronous speeds, pole numbers, and motor sizes.

The motor's drooping speed characteristic is a result of the speed adjusting to the required torque.
The power factor improvement with load is due to the increased load component of stator current, reducing the lag.
The efficiency curve reaches a maximum at full-load, where variable copper losses roughly equal constant losses.
Reduced voltage significantly reduces torque due to the squared relationship between voltage and torque.