Electric Motors convert electrical energy to mechanical energy which can be used for various applications like fans, compressors, refrigerators, vacuum cleaner, elevators etc. They are useful in home as well as industrial applications.
Pulse Wave Modulation (PWM) waveforms of different pulse width and duty cycle can be generated from PWM generators. These waveforms are configurable using different parameters using which it can control motor of most types: AC induction motors, Permanent Magnet AC motors, both brushless and brush DC motors, switched and variable reluctance motors, stepper motors etc.
Such a kind of control is easy to implement and offers cost effective. PWM controlled motors provide smooth torque at low speed and also provide complete speed control from zero up to the nominal rated speed of the motor with only small additional motor losses.This paper talks about the basic operation of motor, types of electric motors, how PWM generators can be
used to control them and the typical architecture of a PWM generator.
II. Motor Basics
When a wire carrying electricity wire is kept perpendicular to the magnetic field, it experiences force in opposite
direction. The force then creates a turning influence which is called a torque which rotates the wire. This is shown in Figure 1, below.
The direction of current in the wire is then reversed so that the polarity of the electromagnet changes due to
which it experiences a torque again, forcing the wire to rotate (Figure 2). This is the basic principle of a motor.
In a three-phase AC motor the direction of current in the wire changes by itself because of the nature of AC
supply. DC motors depend on mechanical or electronic commutation to reverse the direction of current and a single-phase AC induction motor depends on some extra electrical components to produce this reversal.
The basic components of a motor are:
1. Stator - The stationary part of a motor is called as stator. It can be a permanent magnet or an electromagnet.
2. Rotor - The non stationary part of a motor is called as rotor. It rotates because the wires and magnetic
field of the motor are arranged so that a torque is developed about the rotor's axis. If stator is a permanent magnet then rotor is an electromagnet and vice versa.
3. Commutator - It’s a switch which rotates the direction of current in rotor.
4. Brushes - It carries current between wires and the rotor.
III. Types of Electric Motors
The different types of motors, shown (below) in Figure 3, are described below.
AC Motors: An AC motor is an electric motor that runs on alternating current (AC). It can be of two types: Asynchronous and Synchronous motors.
1. Synchronous Motor: A synchronous motor has two electromagnets, one on the rotor and one on the stator. The rotor is supplied with DC current which produces a circular magnetic field around the rotor. The stator is fed with a 3 phase AC supply which creates a rotating magnetic field. The two rotating fields are locked to each other by magnetic attraction and both are in perfect synchronism i.e. the rotor rotates at the same frequency as the revolving magnetic field of stator. Synchronous motors are of two types:
a. Non Excited Motor: Manufactured in reluctance and hysteresis designs, these motors employ a self starting circuit and require no external excitation supply. This means that the rotor does not require any external supply to generate the magnetic field.
b. Direct Current Excited Motor: These motors require direct current supplied through slip rings for excitation. The direct current can be supplied from a separate source or from a dc generator directly connected to the motor shaft.
2. Asynchronous Motor: Asynchronous motors have one electromagnet as rotor and one permanent magnet as stator. The stator winding is energized with an AC supply which creates a rotating magnetic field. This changing magnetic field induces current in the rotor conductors. This current interacts with the rotating magnetic field created by the stator and causes a rotational motion on the rotor. The mechanical speed of the rotor is generally different from the speed of the revolving magnetic field.
a. Induction Motor: In an induction motor the current is induced in the rotor without contacts by the magnetic
field of the stator, through electromagnetic induction. It does not have any permanent magnets on the rotor; instead, a current is induced in the rotor.
b. Commutator Motor: In commutator motor, both rotor and stator are electromagnets with same current flowing through them. Since same poles are formed on stator and rotor, they repel each other causing rotation. The commutator changes the direction of current causing rotation again.
DC Motors: A DC motor is an electric motor that runs on direct current (DC). It requires a controller to
reverse the direction of current. It is of the following types:
1. Brushed Motor: The brushed DC motor is mechanically commutated. The stator of a brushed DC motor generates a stationary magnetic field that surrounds the rotor. The rotor contains one or more windings which when energized produces a magnetic field. The windings are constantly being energized in a different sequence so as to produce a rotating motion in the rotor for alignment. This energizing in a different sequence is done by a commutator. As the rotor reaches alignment, the brushes move to the next commutator contacts, and energize the winding again. This result in misalignment between the rotor and the stator. The rotor will rotate until it
is almost aligned with the stator's field magnets and thus producing a rotating motion again.
2. Brushless Motor: A brushed DC motor has constant friction which results in wear and tear of the motor. Hence a brushless motor came into existence. This motor is electronically commutated. Unlike Brushed DC motor, a brushless DC motor has permanent magnets on the rotor and electromagnets to the stator. Then an electronic commutator is used to charge up the electromagnets as the rotor turns. It has more reliability and longer life.
IV. Controlling Motors using PWM
Speed of the motor is controlled by controlling the voltage across the motor. Torque is controlled by controlling the current and direction is determined by the direction of the current across the motor.
A typical control mechanism of a motor is shown in Figure 4, below. In controlling a three-phase motor, three
perfectly sinusoidal current waveforms with relative phase displacements of 120° are desired. This is achieved by applying PWM waveforms to the switching devices and which then produce the effect of a continuously varying analog signal. This PWM conversion generally has very high electrical efficiency.
There are 3 half H-Bridge circuit which is driven by 6 PWM signals. These PWM signals control the voltage
waveform on the motor. This is shown below, in Figure 5. The PWMs fed to the transistors have 50 % duty cycle. The top and bottom transistors are fed with complementary PWM waveforms. The square wave of the H2 Half H-Bridge is phase shifted compared to the H1 half H-Bridge.
Similarly the square wave of the H3 Half H-Bridge is phase shifted compared to the H2 half H-Bridge. As a result, the motor sees a waveform across its terminals which in shown in Figure 6, below. The amount of phase shift of the square waves control the RMS value of waveform across the motor terminals. This technique is ideally suited for transformer loads as no DC component appears in the load voltage as long as the duty cycle of each square wave remains at 50%.
There is an ADC which measures current of the sinusoidal PWM waveform. Fault input to PWM generator can also be seen. These faults when asserted disable the PWM output.
V. PWM Generator
There are various PWM generators available in market which can generate PWM waveform of all types:
Centre Aligned, Edge Aligned, Phase Shifted, Double Switching, Synchronous Switching etc.
The block diagram of a typical PWM generator is shown in Figure 7, below. The building blocks are:
Clock Selection Unit: This unit selects the main counter clock. In a PWM generator, an option of selecting the clock is provided so that PWM waveforms of different frequency range can be generated. The various clock options are: IP clock, External clock, PLL clock etc. After the clock is selected it can also be divided using a prescaler.
PWM Counter: PWM generator has a PWM counter which is the main controlling block of the generator. Its initial value can be programmed by writing into an initialization register. The maximum value up to which the
counter counts is also programmable. It gets its clock from the Clock Selection Unit.
Counter Initialization Unit: This unit selects the signal which when asserted initializes the PWM counter. The PWM counter can be initialized either by software selection or by assertion of some local signals or external
PWM Generation Unit: It consists of two comparators and two registers. The register contains the value whose match turns on/off the edges. One comparator and register is used to control the turn on edge while a second comparator and register control the turn off edge.
Force out Logic Unit: The force out logic selects the output that should go out on the PWM channels. It chooses from one of four signals that can be supplied to the outputs: the PWM signal, the inverted PWM signal, a binary level specified by software or the external control signals.
Deadtime Logic Unit: The deadtime generators automatically insert software-selectable delays into the pair of PWM outputs. Deadtime forces PWM outputs to the inactive state.
Output Logic Unit: The PWM signal generated in the above logic is passed to the outside world only when all faults are disabled and output generation is enabled in the Output Logic Unit. This unit also has the ability of Polarity control giving user the ability to convert positive true signal to negative true signal and vice versa.
Electric motors find their way at home, office, industry and many more places. There are various types of
motors available in market and different motors are used for different applications.
AC motors are smaller, lighter, more commonly available, and less expensive than DC motors. It also does not
require much maintenance and are better suited for high speed operation.
But controlling the speed and torque of AC motor requires greater understanding of the design and motor characteristics as compared to DC motor.
There are various ways by which motors can be controlled namely, direct torque control, indirect torque control, PWM control etc.
PWM control uses switching devices like power MOSFETs to produce the affect of continuously varying
signal. This conversion has very high electrical efficiency. If a linear amplifier would have been used then the max efficiency achieved would have been 64%. But with PWM waveform with fast electronic switching devices the
efficiency can be greater than 95%. PWM control is widely used as it’s cost effective and easy to implement solution.
Aijaz Fatima is a senior design engineer at Freescale Semiconductor India and can be reached at B21192@freescale.com