Question

An induction motor is driven by a PWM driver. Can the value of modulation frequency of PWM waveforms affect the performance of the motor? Give your explanation.

Answer 1

Note :All figures are below the matter

F1 shows the proposed block diagram of the DC
motor speed controller system consists of DC motor,
PWM control unit, DC- Cbuck converter and 12V DC
power supply. The driver circuitis used to trigger the
power MOSFET in the buck converter. The IRF540
MOSFET has the gate–source voltage of ± 20 V and
the maximum drain-source voltage of 100 V. The MOSFET exhibits the minimum gate–source
threshold voltage of 2V. The output pin of the driver
circuit is connected to the gate terminal of the
MOSFET. The PWM control unit can provide PWM
signal with the voltage ranging from 0 to 15 V.
Figure 2 shows the DC to DC converter
circuit which consists of MOSFET, 150n Hinductor,
47 pF capacitor, diode and a load at the output
terminals. In this project the buck converter is
operating in continuous conduction mode at different
frequency. The buck converter regulates the speed by
controlling the average DC voltage which is applied to
the armature windings of the motor through the
switching element MOSFET.
The output voltage from the converter is fed
to the armature windings of the motor and the armature
voltage is varied with the use of pulse width
modulation technique where by the duty cycle of the
pulse is varied and supplied to the converter switch
(MOSFET) through a motor driver. The input voltage
to a DC-DC converter is an unregulated DC voltage Vs
which is equal to 12 V. The converter supplies a
variable DC output voltage V0to the DC motor. The
DC motor has the rating of 12 V and the maximum
speed of 5000RPM. The speed of the DC motor is
measured by a tachometer which gives the reading in
RPM.
The buck converter has the inductor value of
150nH on the output side act as a filter to provide a
smooth continuous output current to the load. The
voltage across the inductor is given by
V=L di/dt (1)
where L is the value of inductor in Henry (H), and di/dt
is the rate of rise of inductor current.
The value of inductor is calculated based on
the switching frequency, load voltage and load current.
The capacitor is connected in parallel to remove the
ripple current from the inductor to a stable output
voltage. The diode, D conducts current when the
MOSFET is OFF and provide a path for the inductor
current to flow to the load. The diode has the feature
of providing low leakage current to the circuit. The
power dissipated by the diode is given by
PD= IFx VF (2)
whereIF is the average forward current of the rectifier
diode and VF is the forward voltage of the rectifier
diode.
Figure 3 shows the implementation of PIC
microcontroller to control the motor speed. The
microcontroller is connected to the driver circuit. A
microcontroller is regarded as the PWM controller
since it is the one which generates and control the
PWM signal which is being applied to the gate terminal of the MOSFET (Arvind et al., 2014). As the
amount of time that the voltage is on increases
compared with the amount of time that it is off, the
average speed of the motor increases and vice versa.
The PWM frequency is fixed at 1 kHz.PIC16F877A is
a 40/44-pin device which can operate at up to 20 MHz
clock speed. The microcontroller supplies the PWM
pulse about 5 volt DC to the driver circuit. The driver
circuit will provide the necessary voltage to turn on the
MOSFET. This enables the speed of DC motor to be
controlled through duty/PWM cycle supplied by
microcontroller.
The PWM signals are the one responsible for
turning ON/OFF the power MOSFET in the buck
converter hence the armature windings of the motor
receives average DC voltage that will determine the
motor speed. The time that it takes a motor to speed up
and slow down under switching conditions is depends
on the inertia of the rotor (basically how heavy it is),
friction and load torque (reference). By controlling the
TON and TOFF (duty cycle) the average of output DC
voltage can be varied. Two push buttons are applied in
the microcontroller circuit to increase and decrease the
duty cycle. LCD display is connected to
microcontroller to display the duty cycle. Figure 4 shows the pulses at different duty cycles. The
pulse with higher duty cycle turns ‘ON’ at longer time
than that of lower duty cycle. The duty cycle, d is
governed by equation
d= t on / T
where T is the duration of one period and ton is the
‘ON’ time. The ratio of ON to OFF time is called as
duty cycle which determines the speed of the motor.
The desired speed can be obtained by changing the
duty cycle. The PWM pulse is used to control duty
cycle of DC motor drive. Power is supplied to the
motor in square waveof constant voltage but varying
pulse-width or duty cycle. Duty cycle refers to the
percentage of one cycle during which duty cycle of a
continuous train of pulses. Since the frequency is held
constant while the on-off time is varied, the duty cycle
of PWM is determined by the pulse width. Thus the
power increases duty cycle in PWM.(Srivastava). The
PWM ON period at 60 % of duty cycle is higher than
at 40 % duty cycle. This contributes to higher motor
speed at 60 % duty cycle compared to 40 % duty cycle.
Figure 5 shows the pulses at switching
frequency of 500 Hz and 1500 Hz. The frequency of
operation, f is defined as
f= 1/(f on + f off)
Where ton is the ON time of the PWM pulse, toff is the
‘OFF’ time in which the value of PWM pulse is at zero
level and T is the total time period of one duty cycle.
Higher switching frequency increases the output voltage.

The motor speed controller system using PWM
technique was developed. The relationship between
the duty cycle and the converter output voltage has
been investigated. It is found that the wider the pulse
width, the more average voltage applied to the motorterminals. This leads to the stronger the magnetic flux
inside the armature windings. Hence, the faster the
motor will rotate. The microcontroller provides
flexibility in controlling the speed by changing the
duty cycle of the PWM pulse. The effect of the PWM
pulse width on the motor voltage and speed has been
studied.