Page 580 - The Mechatronics Handbook

0066_Frame_C20 Page 50 Thursday, January 10, 2002 10:37 AM

Input
Sensor B
Sensor A
φ A Rotor
Power
Controllers
Amplifiers

φ
B

FIGURE 20.80 Two-phase brushless motor.

In a similar manner, the torque output of phase B is

T B = I B K T cos ( pθ/2) (20.30)

If the motor currents are arranged to be supplied in the following relationships

I A = Isin ( pθ/2) and I B = Icos ( pθ/2)

then the total torque for a two-pole motor becomes

[
T = T A + T B = IK T sin 2 ( pθ/2) + cos 2 ( pθ/2)] = IK T (20.31)

Equation (20.31) shows that if all of the above conditions are satisfied then the brushless dc motor
operates in a similar manner to the conventional dc motor, i.e., the torque is directly proportional to the
armature current. Note that the armature current in this context refers to the stator windings. Excitation
of the phases may be implemented with sinusoidal, or square wave inputs. The sine wave drive is the
most efficient but the output transistors in the drive electronics must be capable of dissipating more
power than that dissipated in square wave operation. Square wave drive offers the added advantage that
the drive electronics can be digitally based. The brushless dc motor will duplicate the performance
characteristics of a conventional dc motor only if it is properly commutated. Proper commutation involves
exciting the stator windings in a sequence that keeps the magnetic field produced by the stator approx-
imately 90 electrical degrees ahead of the rotor field. The brushless dc motor therefore relies heavily on
the position feedback system for effective commutation. It might also be apparent that the brushless
motor as described is not strictly a dc machine, but a form of ac machine with position feedback.

Motor Selection

For the mechatronics engineer the main concerns regarding electric motors will be those of selection for
purpose. At the very least the motor must be capable of matching the power requirements of the driven
load. In all cases, therefore, the motor power available should be enough to cope with the anticipated
demands of the load. Other requirements are the need for the motor to have enough torque available
on start-up to overcome the static friction, accelerate the load up to the working speed, and be able to
handle the maximum overload. Too much excess motor torque on start-up might result in a violent
initial acceleration. Some systems therefore require a “soft start” whereby the motor torque is gradually
increased to allow the load to accelerate gently.

©2002 CRC Press LLC