17-11-2012, 06:02 PM
Brushless DC Motors
brushles motor.pdf (Size: 322.86 KB / Downloads: 218)
Introduction
Conventional dc motors are highly efficient and their characteristics make them suitable
for use as servomotors. However, their only drawback is that they need a commutator
and brushes which are subject to wear and require maintenance. When the functions of
commutator and brushes were implemented by solid-state switches, maintenance-free
motors were realised. These motors are now known as brushless dc motors.
In this chapter, the basic structures, drive circuits, fundamental principles, steady state
characteristics, and applications of brushless dc motors will be discussed.
Structures and Drive Circuits
Basic structures
The construction of modern brushless motors is very similar to the ac motor, known as
the permanent magnet synchronous motor. Fig.1 illustrates the structure of a typical
three-phase brushless dc motor. The stator windings are similar to those in a polyphase
ac motor, and the rotor is composed of one or more permanent magnets. Brushless dc
motors are different from ac synchronous motors in that the former incorporates some
means to detect the rotor position (or magnetic poles) to produce signals to control the
electronic switches as shown in Fig.2. The most common position/pole sensor is the
Hall element, but some motors use optical sensors.
Comparison of conventional and brushless dc motors
Although it is said that brushless dc motors and conventional
dc motors are similar in their static characteristics, they
actually have remarkable differences in some aspects. When
we compare both motors in terms of present-day technology,
a discussion of their differences rather than their similarities
can be more helpful in understanding their proper
applications. Table 1 compares the advantages and
disadvantages of these two types of motors. When we
discuss the functions of electrical motors, we should not
forget the significance of windings and commutation.
Drive circuits
Unipolar drive
Fig.4 illustrates a simple three-phase unipolar-operated motor that uses optical sensors
(phototransistors) as position detectors. Three phototransistors PT1, PT2, and PT3 are
placed on the end-plate at 120o intervals, and are exposed to light in sequence through a
revolving shutter coupled to the motor shaft.
As shown in Fig.4, the north pole of the rotor now faces the salient pole P2 of the stator,
and the phototransistor PT1 detects the light and turns transistor Tr1 on. In this state, the
south pole which is created at the salient pole P1 by the electrical current flowing
through the winding W1 is attracting the north pole of the rotor to move it in the
direction of the arrow. When the north pole comes to the position to face the salient pole
P1, the shutter, which is coupled to the shaft, will shade PT1, and PT2 will be exposed
to the light and a current will flow through the transistor Tr2. When a current flows
through the winding W2, and creates a south pole on salient pole P2, then the north pole
in the rotor will revolve in the direction of the arrow and face the salient pole P2. At this
moment, the shutter shades PT2, and the phototransistor PT3 is exposed to the light.
These actions steer the current from the winding W2 to W3. Thus salient pole P2 is deenergized,
while the salient pole P3 is energized and creates the south pole. Hence the
north pole on the rotor further travels from P2 to P3 without stopping. By repeating such
a switching action in sequence given in Fig.5, the permanent magnet rotor revolves
continuously.
Bipolar drive
When a three-phase (brushless) motor is driven by a three-phase bridge circuit, the
efficiency, which is the ratio of the mechanical output power to the electrical input
power, is the highest, since in this drive an alternating current flows through each
winding as an ac motor. This drive is often referred to as 'bipolar drive'. Here, 'bipolar'
means that a winding is alternatively energised in the south and north poles.
We shall now survey the principle of the three-phase bridge circuit of Fig.6. Here too,
we use the optical method for detecting the rotor position; six phototransistors are
placed on the end-plate at equal intervals. Since a shutter is coupled to the shaft, these
photo elements are exposed in sequence to the light emitted from a lamp placed in the
left of the figure. Now the problem is the relation between the ON/OFF state of the
transistors and the light detecting phototransistors. The simplest relation is set when the
logic sequencer is arranged in such a way that when a phototransistor marked with a
certain number is exposed to light, the transistor of the same number turns ON. Fig.6
shows that electrical currents flow through Tr1, Tr4, and Tr5, and terminals U and W
have the battery voltage, while terminal V has zero potential. In this state, a current will
flow from terminal U to V, and another current from W to V as illustrated in Fig.7. We
may assume that the solid arrows in this figure indicate the directions of the magnetic
fields generated by the currents in each phase. The fat arrow in the centre is the resultant
magnetic field in the stator.