Fpwm

FREQUENCY CONTROLLED AC
MOTOR DRIVE

where Frequency is described in cycles per second (Hz),
and Speed and Slip are expressed in RPM.

1.0 Features of Standard AC Motors
The squirrel cage induction motor is the electrical motor
motor type most widely used in industry. This leading
position results mainly from certain excellent features of
the squirrel cage motor such as:
•Uncomplicated, rugged construction. For the user
this means low initial cost and high reliability.
• Good efficiency coupled with low maintenance costs
resulting in low overall operating costs.
Squirrel cage motors are asynchronous induction
machines whose speed depends upon applied frequency, pole pair number, and load torque. At a constant
supply voltage and frequency, if the effect of temperaturevariations is disregarded, the motor torque will depend
upon slip - Shown in Figure as An.
At a positive slip, the squirrel cage machine will act as a
motor - at a negative slip, as a generator. To reverse the
machine's direction of rotation, the phase sequence to
the motor must be changed.
Assuming similar conditions, the phase current drawn by
a squirrel cage motor will depend only on theslip. A
motor running at synchronous speed will only draw
minimum current. As shown in Figure 1, as the slip is
increased in either direction from zero, the current begins
to increase rapidly.
The peak torque of standard squirrel cage motors is two
to three times the permissible continuous torque, I e. the
motor's rated torque. As shown in Figure 1, the positive
and negative peak torquesare respectively produced at
slips of ±∆ nh, determined by the electrical characteristics
of the motor.
For efficient operation, the slip of a squirrel cage motor
must be kept small, i.e. within a narrow range of -∆h…+∆
nh. Because of this, speed control of squirrel cage motors
can best be implemented by means of infinitely variable
(stepless) frequency control. The nominal slip in mostcases is 3% of the synchronous speed, and depends
mainly on the motor size.
1.1 Motor Speed - In order to use the polyphase AC
motor as an adjustable speed device, it is necessary to
control and adjust the frequency of the 3 φ power applied
to its terminals. The operating speed of the AC motor is
determined by the following relationship:

Figure 1 -Typical Squirrel Cage Motor Torque T andCurrent I at Constant Supply Voltage and Frequency

When a synchronous or synchronous reluctance motor is
used, slip in the above equation is zero, while the motor's
shaft speed is said to be synchronous, and completely
controlled by applied power frequency.
Operation of an induction (NEMA Type B) machine,
however, will result in a slip whose degree is a function
of motor design, and a lowerthan synchronous operation
shaft speed.
1.2 Frequency Controlled Squirrel Cage Motors
In frequency converter drives, squirrel cage motors are
usually run within the range between the peak torques.
The single-phase equivalent circuit of the motor, shown
in Figure 2, can be used to obtain equations for the
torque T, the peak torque Th and the slip ∆nn corresponding to the peak torque. Theequations are based on
the assumption that the magnitude of the phase current
I1, does not affect the voltage Ui

1.3 Motor Torque - AC motor torque capability is
proportional to magnetic flux density in the motor's air
gap. In order to maintain constant torque capability at
the output shaft of an AC motor used as an adjustable
speed device, the flux density in the air gap must bemaintained constant over the operating frequency
range. Since the motor presents a highly inductive
load to the power supply, its impedance increases
directly with applied frequency. It is necessary,
therefore, to increase the applied voltage in direct
proportion to the increase in frequency or increase in
speed. Therefore the ratio of volts to frequency must
be kept constant, as shown in the...