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Glentek Inc., 208 Standard Street, El Segundo, California 90245, U.S.A. (310) 322-3026
SMA8105, SMA8205, and SMA8305 MANUAL
Brushless Motors/Amplifiers
Brushed Motors/Amplifiers
Advantages
Disadvantages
No scheduled maintenance and no brush dust
is generated.
Motor brushes must be checked periodically for
wear and excess brush dust.
Higher RPM limits.
Approximately 3000RPM maximum.
Lower inertia/torque ratio.
Higher inertia to torque ratio.
Dissipates heat more efficiently due to windings
being located in stator.
Not as efficient at dissipating heat. Heat is
trapped at rotor and shortens bearing life.
Safer for explosive atmospheres. Quieter and
less electrical noise generated.
Brushes spark and generate electrical and
audible noise.
Disadvantages
Advantages
Amplifiers are complicated and expensive.
Amplifiers are simpler and less expensive.
Higher torque ripple.
Lower torque ripple.
No Industry standard packaging.
Industry standard packaging.
2.5 Sinusoidal vs Trapezoidal:
Figure 2.4 shows the two most common waveforms used to drive a brushless motor.
Note that in each case, there are actually three different waveforms. Each waveform drives
a motor winding and is 120
o
out-of-phase with the other two. Again, the waveform may be
generated from a DC source by linear or PWM techniques.
The first waveform is known as trapezoidal or six-step since the voltage is literally
stepped from winding to winding (like the Christmas-light analogy). This is the simplest and
least expensive method of driving a brushless motor. Its principal disadvantage is that the
large current steps produce high torque ripple. (Torque ripple is a repetitive fluctuation in
torque as the motor turns and is independent of load.) The SMA8105 trapezoidal mode
amplifier produces a trapezoidal output.
The second waveform is known as sinusoidal. To minimize torque ripple, the motor
current needs to be constantly varied according to the orientation of the magnets and
windings. As it happens, this is a sine function. In fact, a sine wave is defined as a rotating
radius (like a motor shaft) revolving through time (see figure 2.4). A sine wave is the most
natural way to drive a motor and produces the minimum torque ripple. The SMA8205
sine/resolver mode amplifier produces a sinusoidal output.
Figure 2.4
Trapezoidal and sinusoidal waveform used to drive brushless motor.
