Vehicle transmissions typically include one or more pumps to deliver pressurized hydraulic fluid for lubrication and actuation of transmission shift elements. These transmission pumps are typically driven by torque from an engine coupled to the transmission. In some of these transmissions, the pumps are required to operate to maintain oil pressure in the transmission even when the engine is idled or stopped, such as in electric drive mode in a hybrid electric vehicle. One such transmission may include a main pump and an auxiliary pump, which adds weight, cost, and complexity. Another such transmission may include two or more overrunning clutches to carry torque from the engine and an auxiliary electric motor to a single transmission pump. The latter transmission uses multiple overrunning clutches, and undesirable electric motor equipment and control techniques.
Auxiliary pumps are typically driven by a brushless direct current (BLDC) motor, which generally includes a controller, power electronics inverter, a stator with three phase windings, and a rotor with permanent magnets responsive to electricity flowing through the windings. The controller and power electronics inverter together control the motor by sequentially energizing the windings with electrical current to produce a rotating magnetic field in the motor. The magnetic field attracts the rotor magnets, which thus follow the rotation of the field and, therefore, cause the rotor to rotate. But proper motor phase winding commutation depends on rotational position of rotor magnets at any given time relative to the phase winding to be energized next.
Rotor position information for proper commutation of motor phase windings can be acquired by using a position sensor or encoder. However, such devices increase cost and reduce reliability of the motor and, thus, it has become increasingly desirable to control a brushless DC motor without using such devices. One typical sensorless control approach is to use back electromagnetic frequency (EMF) zero-crossing detections in an idle phase winding to estimate rotor position. This is possible because only two of the three motor phase windings are energized at any given time, and a third phase winding is idle and available for back EMF detection.
But prior technology for sensorless control of BLDC motors does not enable detection of back EMF zero-crossings while an inverter circuit is inactive. This is because when an inverter circuit is inactive, the power electronics switches are open and voltages in the phases are floating and isolated from ground reference. Also, it is impossible in this condition to reliably sense useful voltage signals and determine back EMF zero-crossings for estimating motor positions and, thus, motor speeds. Accordingly, commutation of present sensorless BLDC motors requires expensive sensors, costly low voltage control connections to the electric motor with hermetic seals, and/or complex motor position sensing techniques.