AN1506 - APPLICATION NOTE

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5. THE LOW VOLTAGE AC MOTOR DRIVE APPLICATION.

In the frame of a funded program aiming to develop an innovative traction system for wheelchair

application, a sinusoidal brushless motor drive has been developed. The system is powered by two lead-

acid batteries, which are connected in series, and the rated voltage in the dc bus is 24V, while the rated

capacity is 45Ah, thus the gross available energy to the traction system is about 1kWh. The

innovativeness of such low voltage applications is represented by traction systems based on chopper

converters feeding dc motors. Generally speaking, in the past such converters required paralleled

operations of many MOSFETs, since the on-state resistance of a single device was unacceptably high. In

the case study an injected current value of 80A in the motor will cause about 0.5 V-0.7V voltage drop in

the used MOSFET, which is quite acceptable.

The mechanical actuators of the drives are two permanent magnet brushless motors specifically

designed [4], rated speed 110 rpm, rated torque 20Nm, rated power 250W, rated current 15 A, number of

rotor poles 16, and equipped with coaxial position transducers (absolute encoders). Two-separated

three-phase current-regulated pulse-width modulated (CRPWM) inverters feed the motors. The two

motors have separated torque references, which are simultaneously given by means of a joystick

command; thus the steer action is automatically performed by control of the manipulator position. In

particular cases the motors can be required to develop a peak torque, and consequently the current fed

by the inverter should increase up to 70 A-80A for several tenth of seconds in order to obtain a torque

five times greater than the rated one.

The inverter current is controlled by a tolerance band technique [1, 2], which allows supplying sinusoidal-

shaped currents which amplitudes can be changed according to the load requirements. The devices in

width of the hysteresis band (between 2-4%). Figure 6 reports the block diagram of the application, figure

7 the schematic of the 3-phase full bridge inverter.

Figure 6. Block diagram of the traction system for wheelchair applications

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CURRENT

REFERENCE

DC-SIN

CONVERSION

CIRCUIT

Ref. A

Ref. B

Ref. C

SIN WAVE

GENERATOR

Current error amplifier

Current error amplifier

Current error amplifier

Driver

Driver

Driver

Driver

Driver

Driver

ROTOR POSITION

DETECTOR

3-PHASE

FULL

BRIDGE

INVERTER

ABSOLUTE

ENCODER

+

+

+

-

-

-

-

-

-

-