UNIT 4
STATOR SIDE CONTROL OF 3 PHASE INDUCTION MOTOR DRIVE
Control of Induction motor by AC voltage Controller:
•By controlling the firing angle of the thyristors connected in each
phase, the rms value of stator voltage can be varied.
•As a result of this, the motor torque and the speed of the motor
are varied.
•In star connected controller, all the thyristors carry line currents.
But in delta connection all the thyristors carry phase current
only.
•Hence low rating thyristors may be employed in delta controller.
•But delta controller produces circulating currents due to third
harmonic voltages. This may increase power loss across each
device.
By reducing Stator Voltage Control of Induction Motor, speed of a high-slip
induction motor can be reduced by an amount which is sufficient for the speed
control of some fan and pump drives .
While torque is proportional to voltage squared , current is proportional to
voltage .
Therefore, as voltage is reduced to reduce speed, for the same current motor
develops lower torque.
Consequently, method is suitable for applications where torque demand reduces
with speed, which points towards its suitability for fan and pump drives.
The variation of torque with speed for Fan / Blower load is given as T = k ω2
where k is some constant and w is speed.
Thus the starting torque requirement is zero for such load and their torque
requirement increases as speed increases.
If stator copper loss, core loss, and friction and windage loss are ignored, then from
eqns , motor efficiency η is given by
The equation shows that the efficiency falls with decrease in
speed.
The speed control is essentially obtained by dissipating a
portion of rotor input power in rotor resistance.
Thus, not only the efficiency is low, the power dissipation
occurs in the rotor itself, which may overheat the rotor.
Because of these reasons, this drive is employed in fan and
pump drives of low power rating and for narrow speed range.
Variable voltage for speed control is obtained using ac voltage
controllers.
Control by AC Voltage Controllers and Soft Start:
Domestic fan motors, which are always single-phase, are controlled by a single-phase triac
voltage controller (Fig. 6.32(a)). Speed control is obtained by varying firing angle of the
triac. These controllers, commonly known as solid state fan regulators, are now preferred
over conventional variable resistance regulators because of higher efficiency. Industrial fans
and pumps are usually driven by three-phase motors.
Fig. 6.32(b) shows a commonly used thyristor voltage controller for speed control of 3-
phase motors. Motor may be connected in star or delta. In delta connection, third harmonic
voltage produced by motor back emf causes circulating current through the windings which
increases losses and thermal loading of motor. Speed control is obtained by varying
conduction period of thyristors. For low power ratings, anti-paralleled thyristor pair in each
phase can be replaced by a triac
Since Stator Voltage Control of Induction Motor, both single- and three-phase, allow a
stepless control of voltage from its zero value, they are also used for soft start of
motors.
The power factor of an ac regulator is defined by eqn. . With increase in firing angle,
both distortion factor and displacement factor reduce, giving a low power factor.
In fan and pump drives, the fluid flow has to be maintained constant against
variations in pressure head and nature of pumped fluid.
Therefore, it is always operated with closed-loop speed control. For closed-loop
control, scheme of Fig., consisting of inner current loop and outer speed loop is used.
Braking is not used because fluid pressure provides adequate braking torque.
Variable voltage variable frequency control:
•Varying the voltage alone or frequency alone has some
disadvantages with regards to the operation of induction motor.
•The maximum torque in an induction motor is given by
•Here K is a constant and Ls & Lr’ are the stator and stator referred
rotor inductances.
•At high frequencies, the value of (Rs / f) will be very much less
than 2π (Ls+ Lr’). So (Rs / f) can be neglected and hence the torque
equation becomes
•From the above equation it is clear that it is clear that if the ratio
(V / f) is kept constant, the motor can produce a constant
maximum torque.
• At low frequencies(when speed is reduced), the term (Rs / f)
will be high and it cannot be neglected.
• Hence if maximum torque needs to be maintained constant at
low speeds, then (V / f) ratio must be increased.
•Near to base speed (or rated speed), the supply voltage will be
maximum and it cannot be increased further.
•Therefore, above base speed, the frequency is changed by
keeping supply voltage constant.
From V-F Relation graph
•(V/f) ratio is increased at low frequency to keep maximum torque
constant.
•(V/f) ratio is kept constant at high frequencies up to base
frequency
•V is kept constant and frequency is varied above base frequency
From speed-Torque characteristics
• It is clear that maximum torque is same at all different speeds.
• volts / Hertz control offers speed control from standstill up to
rated speed of IM.
Variable voltage variable frequency of induction motor by VSI:
• In voltage source inverters, the input voltage is kept constant.
• The magnitude of output voltage of VSI is independent of the
load.
• But the magnitude of output current depends on the type of
load.
• A VSI converts the input dc voltage into an ac voltage with
variable frequency at its output terminals
•When VSI is operated as a six step inverter, the transistors are
turned ON in the sequence of their numbers with a time interval
of T/6 seconds if T is the total time period of one output cycle
• Frequency of the inverter output is varied by varying the time
period (T) of one cycle.
• VSI using normal transistors is shown above but any other self
commutated device can be used in place of transistor
•MOSFET is used in low voltage and low power inverters.
•IGBTs and power transistors are used up to medium power levels.
•GTO and IGCT are used for high power levels.
•VSI may be a six step inverter or a PWM inverter.
PWM CONTROL:
• If a PWM inverter is used as VSI as shown in below figure, then
the input voltage may be a constant dc which is obtained from a
simple diode rectifier.
• The output of a PWM inverter is a variable voltage and variable
frequency.
• In a PWM inverter, it is possible to control the output voltage and
frequency as well as the harmonic content can be minimized
Closed loop v/f control of Induction Motor Drives:
•Closed loop system employs an inner slip speed loop and an outer
speed loop as shown below.
•The slip speed loop acts as inner current control loop. It also
ensures the motor to operate between synchronous speed and
the speed at which maximum torque occurs for all frequencies.
• Thus a high torque will be produced for a small current drawn
from supply.
• The speed error is processed through a speed controller, usually
a PI controller, and a slip regulator.
Increasing Speed:
•If the motor speed is to be increased, then the reference speed
ωm*will be set to the required speed.
• Now the comparator compares the actual speed and reference
speed and produces a positive error.
• This will set the slip speed command ωsl* at its maximum value.
• Hence the motor starts accelerating (i.e., speed increases) at the
maximum inverter current and hence the speed error decreases.
•When the actual motor speed reaches the reference value, the
drive finally settles at that speed. At this speed, the motor torque
equals the load torque.
Decreasing Speed:
•If the motor speed is to be decreased, then the reference speed
ωm*will be set to the required speed.
•Now the comparator compares the actual speed and reference
speed and produces a negative error.
•This will set the slip speed command ωsl* at its maximum
negative value.
•Hence the motor starts decelerating (i.e., speed decreases) at
the maximum inverter current and hence the speed error
decreases. Here regenerative braking is applied.
•When the actual motor speed reaches the reference value, the
drive finally settles at that speed. At this speed, the motor torque
equals the load torque.
