- Induction Motor Definition: An induction motor is a type of electric motor that uses electromagnetic induction to generate mechanical power.
- Low Power Factor Definition: Low power factor in induction motors means the motor operates inefficiently at light or no load, typically with power factors between 0.2 and 0.4.
- Causes of Low Power Factor: The causes of low power factor in induction motors include the presence of magnetizing current, which is highly inductive and does not contribute to work output.
- Impacts of Low Power Factor: Low power factor operation increases the burden on generators, conductor sizes, transmission costs, and reduces efficiency and voltage regulation.
- Power Factor Correction: Power factor correction, using capacitors or synchronous phase modifiers, helps manage reactive power demand and improve transmission efficiency.
Low power factor operation of an induction motor is the usual light-load and no-load condition. Induction Motors need magnetic fields to work, so they draw magnetizing current from the source. That current sets up flux in the air gap, often about 20% to 60% of full-load current and more on small machines. Magnetizing current does little real work. It only provides the field for power exchange between stator and rotor. Typical power factor is about 0.2 to 0.4 at light or no load, and about 0.8 to 0.9 at full load. The low power factor on the power system at low load is that lagging magnetizing current.
A low power factor raises kVA demand on generators. For the same power at constant voltage, current rises, so conductors must be larger and transmission lines cost more. The extra current also raises copper loss and voltage drop, so regulation is poorer. Motors sized close to the real load avoid sitting at that light-load power factor.
To cut those losses on transformers and distribution equipment, power factor correction is used. A capacitor bank or a synchronous phase modifier can supply the reactive demand of induction motors. Utilities often aim near 0.9 to 0.95 lagging rather than exact unity, to avoid a leading power factor.
Power factor correction can be a capacitor in parallel with the motor, at the distribution board or at the source. On star-delta motors the capacitor is often on the main or delta contactor, so it is not left across the winding in star. Care is needed with a static capacitor because motor reactive demand changes with load. Overcompensation of a disconnected motor can leave it self-excited as an induction generator. Some APFC panels use a controller to switch capacitor steps at light load.
