A new motor drive offers to give fuel-cell-powered electric vehicles improved performance and efficiency throughout their operating range. It's called Phase Advance, and developers of the patented design claim to achieve both high starting torque and power output at high speeds without increasing the size and cost of the motor/controller package.
Both induction motors and permanent-magnet brushless motors are vying for the electric car market. Permanent magnet motors are capable of achieving higher efficiencies and lighter weight, but are handicapped by their characteristic of maintaining constant torque for constant current. Flux vector drives can achieve constant power output across the speed range, but engineers at Unique Mobility (Golden, CO) claim their Phase Advance method is simpler to implement and cheaper to produce.
In addition to the drive motor, the system consists of:
- An electronic switching amplifier capable of generating three phase ac waveform of varying frequency and amplitude.
- A low resolution sensor capable of measuring the relative position of the rotor to the motor stator to accomplish six-step commutation of the motor.
- A circuit which allows time adjustment of the commutation signal from the motor to the electronics for the purpose of implementing phase advance for constant power operation.
The systems split the motor's operation into three ranges. At low speed it operates in pulse width modulation mode only; in the second, it operates with pulse width modulation and phase advancement, and continues this operation until the pulse width modulation reaches 100% duty cycle; lastly, the torque reduces in hyperbolic fashion, maintaining constant power throughout the region (see graph).
The end result, say the inventors, is the ability to operate a vehicle with a brushless permanent-magnet motor as if it has an "electronic transmission," accelerating quickly off the line and achieving a high top speed.
Unique Mobility has incorporated the technology into several of its drive systems, ranging from a 5-hp motor for an electric scooter to a drivetrain for a bus rated at more than 130 hp.