Experimental implementation using ST7FMC

AN2372

8/13

Electrical speed

ω = 2π /(T

Or,

ω = 2π /(6. T

If only one Hall sensor is present, the captured period corresponds to 180°, and hence

ω = π / T

180

In the PLL method,

ω = k.Σ(θ

2.2

Current control

A conventional speed control loop sets current reference for an inner current loop. Though it

is very relevant for the overall control structure, it poses a few challenges. Since the phase

currents are sinusoidal, extracting a DC equivalent torque current requires at least two

phase currents and Park Clark transformations in the control loop. This needs two current

sensors and a high MIPS computing engine. Attempting to extract all three phase current-

related information from the DC link current requires additional timer hardware/logics and

CPU MIPS. These requirements are obviously complicated and expensive.

Figure 6.

Voltage foldback current limit control implementation using ST7FMC

Hence a cost effective voltage foldback current limit control is implemented that fits very well

with the ST7FMC architecture. The control block diagram is shown in Figure 6. ST7FMC

has an on-chip opamp and a comparator. This opamp promotes the use of a low value shunt

resistor whose weak output can be amplified, thereby minimizing its power dissipation. The

current signal from the opamp can be connected to the comparator whose output is tied to

an interrupt generator. If the applied motor voltage is high leading to heavy currents in the

DC link, such that the comparator detects over current condition, it generates an interrupt

where, in its interrupt subroutine, the applied voltage is decremented marginally by dV. If this

condition is continuously identified during succeeding PWM cycles, the applied voltage is

constantly decremented in each interrupt, until the peak current flowing through the DC link

is below the reference value. At this point, the motor operates at a voltage and speed

corresponding to current limit. Since the voltage magnitude is reduced instead of clipping

the on times of inverter switches, the motor currents continue to be sinusoidal. From a

control standpoint, current loop implementation is highly simplified and the PMSM motor is

controlled like a DC motor.

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Digital

translator

0

dV