AN2372

Experimental implementation using ST7FMC

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Experimental implementation using ST7FMC

The power of ST7MC to control BLDC motors with trapezoidal flux distribution, in 6 step

mode, is well known. In addition to this, ST7MC is also capable of delivering three phase

sinusoidal complementary PWMs with programmable dead time insertion to control a two-

level three-phase inverter that can drive any three-phase loads. It has a speed feedback

block that can either count the number of encoder pulses in a given time frame (in encoder

mode) or identify the time lapsed between two consecutive tacho edges (in tacho mode). In

the experiment performed using ST7MC on a three-phase PMSM, three-phase PWM

generation and speed feedback in tacho mode are used.

The control block diagram is shown in Figure 3. A speed command from the user is passed

through a ramper that sets the acceleration and deceleration rates and generates a speed

command for closed loop control. This is compared with a speed feedback estimate and the

error is passed through a PI controller that generates the magnitude reference for a 3 phase

sine voltage to be applied on the motor. Usually speed loops set the current reference for an

inner current loop for current controlled ramp up and ramp down. But this is handled in a

simplified manner and is described in Section 2.2. Speed feedback is estimated as

described in Section 2.1.

Figure 3.

Implementation block diagram

θ represents the estimated angular position of phase back emf A at any given instant. δ

represents the angle enforced between the back emf and applied stator voltages. By

controlling this value, the motor can be made to operate in unity power factor. In this

experiment, the load is assumed to be a friction load. This means that the load torque

increases linearly with speed. To obtain close to unity power factor at all speeds, the load

angle is varied linearly with speed. Provision is given on this test setup to exclude load angle

compensation to study the difference in performance. The effect of load angle compensation

is predominantly visible at higher loads and speeds. With load angle compensation, the

phase currents and DC link currents are appreciably lower than without it under same load

conditions and the waveforms are shown in Figure 4.

ω

set

ω*

+

-

ω

ρ

δ

+

+

θ

Position

Hall

Ramper

PI

Controller

Current

limiter

3 Phase

PWM

Generator

and Inverter

PMSM

Speed and

Absolute position

estimator

F(

ω)