Page 398 - Power Electronics Handbook

Electrical machine control 387

14.3.3.3 Electronic switches
The switches used to change the direction of stator current, at a signal from
the rotor-position sensors, need to satisfy several requirements. First, they
must be adequately rated to handle the full winding power, which can vary
widely with the size of machine used. It is also important to establish
whether the current through the switches will be unidirectional or
bi-directional. A mechanical commutator can readily pass current in either
direction whereas electronic switches are usually restricted in this aspect,
therefore special provisions must be made if bi-directional operation is
required. This often arises when the same machine is to act as a motor and
a generator and also in some closed stator winding arrangements.
A mechanical switch can only be off or on, but an electronic device can
often have a controlled turn-on, as in a transistor. This means that, for
instance, in Figure 14.38 instead of controlling the applied voltage (V,) to
the motor to change its speed, the value of this voltage can be kept fixed
but the voltage across the switch (V,) varied. Therefore an electronic
switch can act in a switching mode, as for a mechanical commutator, or in
an amplified mode, where the extent to which it is on or off is controlled,
this mode having several advantages.

Figure 1438 Distribution of voltage across the
electronic commutator switch and the motor

For instance, Figure 14.39 shows a four-segment commutator operated
from Hail effect sensors. The currents through the sensors are constant,
but if the rotor flux is assumed to be sinusoidally distributed, then the Hall
voltage, being proportional to this flux, would cause the transistor to
operate in an amplification mode. The stator current distribution in the
four windings would then be such as to maintain the stator flux constant,
irrespective of rotor position. Since motor torque is proportional to the
product of stator and rotor flux, and these are now constant, such a motor
is capable of extremely smooth operation. The amplification mode has one
obvious disadvantage; the dissipation across theaswitches is much greater
than if they were always either off or saturated, so its use is limited to
relatively small motors.