Specifications

122 Control and Automation Solutions Guide
Controllers for SRMs
SRMs are similar to stepper motors
because they need power switched to the
proper windings at the appropriate times.
The most common conguration is similar
to an H-bridge, but diers somewhat. The
driver is called an N+1 switch and diode
asymmetric bridge converter (Figure 7). It
allows each phase of a 3-phase motor
to be energized by the top FET and the
appropriate bottom FET, which are both
turned on simultaneously (Figure8). The
current is allowed to ramp up to a limit,
at which point the top FET is turned o.
This is the freewheeling mode, where the
winding inductance keeps the current
relatively constant, ramping down only
very slowly with the bottom diode
closing the loop around the winding.
Then to discharge the phase quickly in
preparation for the next step, the bottom
FET is also turned o. The voltage across
the winding is now clamped to the
opposite polarity by the top and bottom
diodes. This causes the current to ramp
down at about the same rate that it
ramped up, except for the eect of two
additional diode drops making it ramp
down slightly faster. This conguration
allows each phase to be switched on
and o quickly, especially with a high-
voltage supply, allowing for high-speed
motor operation at high torque. Figure8
shows only a single current-sense amp
sensing the current on the high-side FET.
This is only adequate for simple control
systems. Complete control also requires
current sensing on each low-side FET.
DISPLAY
DRIVER
SWITCH
DEBOUNCER
KEYBOARD
SCANNER
KEYBOARD
SWITCHES
DISPLAY
FRONT PANEL
LOW-SIDE
GATE DRIVERS
HIGH-SIDE
GATE DRIVER
325V DC OR 650V DC
ISOLATIONTRANSCEIVER
DC-DC
CONVERTER
FIELDBUS
RESET
µP
I/0
CURRENT-SENSE AMP
I/0
TEMPERATURE
SENSOR
SWITCHED
RELUCTANCE
MOTOR
VR OR
HALL-EFFECT
SENSOR
SENSOR
INTERFACE
VOLTAGE
SUPERVISOR
AC-DC
CONVERTER
AC MAINS (3-PHASE)
230V AC OR 460V AC
UART
ADC
Figure 8. Controller for a switched reluctance motor.
1 2 3
V
0
1
1
2
2
3
3
Figure 7. “N+1 switch and diode” asymmetric bridge for driving SRMs. The control circuitry needed for the IGBTs shown is
shown in Figure 8.