Page 349 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

One example in Hong Kong using VVVF technology can be found at Po On Market and WSD Staff Quarters at

Kwai Fong. It has been shown that this low noise level drive enables over 40% of energy saving with improved
riding comfort.
When compared with other modes of drives, one can see that the VVVF drive offers low starting current
(approximately 1.8x of the rated current), high power factor (i.e. better efficiency in power supply) with good
ride quality and floor leveling. It can reduce motor starting currents by as much as 50 to 80% compared with
conventional motor drives. Further, wear and tear of the equipment can also be less during start/stop of the motor
by using VVVF motor drive.
One example in Hong Kong using VVVF technology can be found at Po On Market and WSD Staff Quarters at
Kwai Fong. It has been shown that this low noise level drive enables over 40% of energy saving with improved
riding comfort.

3-Phase Induction Motor

A 3-phase induction motor is an electromechanical energy conversion device which converts 3-phase input
electrical power into output mechanical power.

A 3-phase induction motor consists of a stator and a rotor. The stator carries a 3-phase stator winding while the
rotor carries a short-circuited winding called rotor winding. The stator winding is supplied from a 3-phase supply.
The rotor winding drives its voltage and power from the stator winding through electromagnetic induction and
hence the name.
Working Principle of a 3-Phase Induction Motor
The working principle of a 3-phase induction motor can be explained by considering a portion of it as follows:
When the 3-phase stator winding is fed from a balanced 3-phase supply, a rotating magnetic field (RMF) is
produced in the motor. This RMF rotates around the stator at synchronous speed which is given by,
SynchronousSpeed,NS=120fPSynchronousSpeed =120
The RMF passes through the air gap and cuts the rotor conductors, which as yet are stationary. Due to the relative
motion between the RMF and the stationary rotor conductors, EMFs are induced in the rotor conductors. As the
rotor circuit is closed with short-circuit so currents start flowing in the rotor conductors.

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CITS : E & H - Electronics Mechanic - Lesson 223 - 236