Actuators - Introduction to Biomechatronics - page 115

FIGURE 3-26
Torque-speed
characteristics for a
brushless DC motor.

achieved using dead time control, in which extra time is allowed for one driver to turn
off before the next one is activated. In this configuration there is a floating phase period
between high and low drive periods so that the problem will never occur.
The torque-speed characteristics of BLDC motors are different from those of DC
motors. There are two torque parameters: (1) the stall torque,

τ s ; and (2) the rated torque,
τ r . During continuous operation the motor can be loaded up to the rated torque, which
remains constant for a speed range up to the rated speed, as shown in Figure 3-26. The
motor can be run faster than this, up to about 150% of the rated speed, but the torque
drops. In addition, the motor can generate torques higher than the rated value, but these
must be limited in duration; otherwise, the motor can overheat.
It is possible to control a BLDC without Hall sensor feedback. If the back EMF of each
of the windings is monitored, it will transition through zero at about the same time that the
Hall sensor state would change. This transition can be used to control the commutation as
already discussed.
Though BLDC motors are much more complicated than normal brushed types, they are
often used in demanding applications because of their improved reliability and
compactness. Table 3-3 provides a comparison between the main features of BLDC and
brushed DC motors.

3.2.3.1 Selecting a BLDC Motor
The parameters that govern motor selection for a particular application are as follows
(Yedamale 2003):

• Peak torque required for the application
• Root mean square (RMS) torque required
• The range of operating speeds required
Peak torque: This can be thought of as the same as the stall torque and can be determined
by summing the required load torque, τ l , the inertial torque, τ j , and the torque required to
overcome friction,

τ f . Other factors also contribute to the peak torque requirements. They
include windage loss, which is contributed by the resistance of the air in the gap. These
additional factors are difficult to calculate, so it is easier to add a safety factor of 0.2 when
doing the calculation.

τ s = 1 . 2 (τ l + τ j + τ f ) (3.38)