Environmental Engineering Reference
In-Depth Information
The features of brushless dc machines are high start-up torque and high effi-
ciency. Voltages are nominally up to 60 V
dc
, or 100-240 V
ac
, at power levels from
5 W (for computer disk drives) to 2.5 kW for CVT transmission oil pump drive.
Speed ranges up to 30 krpm have been attained (e.g. as a sub-atmospheric refrig-
erant pump). High rotor speeds and high power are problematic because of the
concern with rotor surface magnet retention.
5.2.2 Brushless ac
When the stator back-emf is sinusoidal, the inverter is controlled in 180
conduc-
tion mode. The switch commands listed in Figure 5.12 have a dwell of 180 elec-
trical degrees. For this conduction interval, the commands to switches in the
A-phase leg, for instance, are auto-complementary. When switch A
รพ
is ON, then
switch A
must be OFF because bus shorting is not permitted. During the switch
commutation interval, a built-in dead time of 3-5
m
s is used as a guard band to
prevent switch shoot through conduction.
The commutation logic for a brushless ac motor differs in several key re-
spects from its brushless dc cousin. Because of 180
conduction, the actual phase
current must be monitored rather than bus current to regulate the sinusoidal
waveshape. Also, due to the need for precise rotor position information, some form
of mechanical rotor sensing such as an absolute encoder or resolver is required.
Figure 5.11 is a schematic for the brushless ac motor having the features noted
above. When the SPM motor back-emf voltage is lower than the dc link, the power
inverter operates in pulse width modulation (PWM) mode to synthesize sinusoidal
current waveforms in each of the motor phases with an amplitude set by the current
regulator. Current regulator command magnitudes can be in response to torque if used
in torque control mode, which hybrid propulsion motors are generally designed for, or
to motor speed when used in speed control mode if used to drive electrified ancillaries
such as pumps, fans or compressors. For example, the hybrid air conditioning used at
times in hybrid electric vehicles uses a brushless motor rated 1.5-2.0 kW to maintain
cabin climate control during engine-off periods. Brushless ac motors are also used in
electric assist steering, a necessary feature when idle stop mode is used while the
vehicle is still in motion. Other applications of brushless ac motors are active sus-
pension actuators in a fully active, wide bandwidth, wheel position controller.
Brushless ac motors are used in this application because of its smoother operation and
softer detent torques than the BDCM.
Application voltage ranges of the brushless ac motor controller range from 60
to 600 V (IGBT power inverter). An example of a pre-packaged motor controller is
the International Rectifier Plug-N-Drive module [2] IRAMS10UP60A. This mod-
ule is rated 10 A at 600 V and is capable of switching up to 20 kHz PWM. The
power electronics relies on non-punch-through (NPT) (motor drive) IGBTs in a
single in-line package (SIP). The module is rated for direct control of 750 W
brushless motors. Compared to Figure 5.13, it contains all components except the
motor, current sensors, power supply and the part of the microcontroller handling
communications and outer loop control. Internal to the module is an IR21365C
