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4. INTRODUCTION TO V/F DRIVES
4.1 AC motor
The first electric motor built in the early 1800's was a dc
motor. It was simple to control speed by controlling the armature
and field voltage and provides very fast torque response. Many
application requirements were met by changing the speed of the dc
motor. But they had many disadvantages of higher cost, higher rotor
inertia, and maintenance problems with commutators and brushes.
The first ac motor was designed around the turn of the century.
Ac motors are simpler and more robust than dc motors. The fixed
speed-torque characteristics of ac motors are not suitable for all
applications. Ac motors convert electric energy into mechanical
energy by electromagnetic induction. This principle behaves in a
manner where a voltage will be induced if a conductor is moving
through a magnetic field. If the conductor is a closed circuit, a
current will flow in the conductor (rotor). That current creates
another field, which interacts with the stator field.
The induction motor and the synchronous motors are two types of
ac motors. In principle the stator functions the same way in both
motor types. The difference is in rotor construction and how rotors
operate in the magnetic field created by current flowing in the
stator windings. With synchronous motors, the rotor and the
magnetic field are running at the same speeds. With induction
motors, the rotor and the magnetic field are running at different
speeds. The difference is defined as slip speed or slip. When the
rotating speed of the motor is greater than the speed of the
rotating magnetic field, the motor will act as a generator,
transferring power from the motor back to the electrical source.
When the motor rotates at the same or lower speed than the magnetic
field, power is transferred from the electrical source to the
motor.
Among all types of ac machines, the induction machine,
particularly the cage type, is most commonly used in industry.
These machines are very economical, rugged and reliable,
practically no maintenance and are available in the ranges of
fractional horsepower (FHP) to multi-Megawatt capacity. FHP
machines are available in single-phase, but poly-phase
(three-phase) machines are often used in variable speed drives.
An ac Squirrel Cage Induction Motor (SCIM) has two basic parts:
The stationary part called the stator and the rotating part called
the rotor as shown in fig 4.1. The stator is a series of wound
coils, while the rotor is a cage of aluminium or copper bars
connected by end rings. The phase windings and the stator core must
produce the magnetic field in a number of pole pairs.
It is the number of pole pairs, which determines the speed of
the rotating magnetic field. Synchronous speed (Ns) of the motor is
equal to 120/number of pole pair times the applied frequency. The
continuous torque that a motor will produce is 1.5 times the number
of pole pairs times the motor horsepower. The speed is dependent on
the number of poles pairs of the motor and the frequency of the
supply voltage. The frequency applied to the motor sets the upper
speed motor limit.
The power applied to the stator creates a magnetic field that
rotates. As these lines of flux cut across the rotor bars, rotor
current is induced. This induced current creates its own lines of
flux. If the rotor conductors are initially stationary, its
conductors will be subjected to a sweeping magnetic field, inducing
current in the short-circuited rotor (as shown in fig 4.2) at the
same frequency. The interaction of air gap flux and rotor mmf
produces torque.
At synchronous speed of the machine, i.e., when the rotor
catches up with the stator, no lines of stator flux would be
cutting across the rotor, so no rotor current, no rotor magnetism,
no attraction, no rotation, and the rotor begins to slow down. At
any other speed Nr, the speed differential Ns- Nr called slip
speed, induces rotor current and torque is developed.
Speed/Torque/Current curves
In an ac motor, torque varies by:
Hence V/F ratio is altered to obtain higher starting
torques.
Where: E/F is proportional to motor flux
I is current drawn by the motor
The applied voltage controls the torque that the motor produces.
Torque is proportional to the voltage squared. Torque is also
dependent on the slip S. The operating zones of the motor basing on
slip can be defined as plugging (1.0
