AN863 Datasheet, PDF (6/22 Pages) STMicroelectronics – Improved sensorless control with the ST62 MCU for universal motor

IMPROVED SENSORLESS CONTROL WITH THE ST62 MCU FOR UNIVERSAL MOTOR
fying the triac firing delay. Moving from Figure 2 to Figure 3, we can see two effects
of the inductive term. First effect: at triac turn-on, the motor current does not exhibit a
discontinuity, but changes gradually from 0 to a finite value. Second effect: at mains
voltage zero crossing, the motor current does not reach zero immediately, but keeps
trailing for a few milliseconds. This trailing gives us more freedom to chose the meas-
urement time t0. We can even chose the zero crossing instant, which is the easiest
time for the micro-controller (no need to measure time t0 with the timer, we can use
the mains zero-crossing as an interrupt to the micro, which is necessary anyway for
other matters). This is illustrated in Figure 3, where the arrow shows that the current
is constant at the zero-crossing time while the motor load changes, provided that the
speed is kept constant by adjustment of the firing delay td. For a detailed mathemat-
ical discussion, see annex 1.
To summarize, if we want to regulate the speed at a pre-defined set speed, for ex-
ample 1000 rpm, we must measure the instantaneous motor current on every mains
period at the same time t0 (for example, on every mains voltage zero crossing fol-
lowing the positive half-cycle), and maintain this measured current at a fixed set cur-
rent, 1 ampere for example. For a different set speed, 2000 rpm, we need to regulate
the current around a different value, 0.5 ampere for example (note that a lower cur-
rent corresponds to a higher speed).
The regulation is performed by adjustment of the triac firing delay td.
The above described method gives very good results at medium and high speeds.
At very low speeds, we have a second order effect coming into the picture: Figure 4
is identical to Figure 3, except for the presence of a fourth current curve, corre-
sponding to a very large firing delay td (this corresponds to a very low speed, com-
bined with a low load). We can see on this fourth curve that the current at time t0 is
smaller than i0, current of the other 3 curves at this time. This effect is described in
detail in annex 1. We have two solutions to cope with it. The simplest one is to limit
the maximum firing delay (below values in the range of 7 mS, depending on the motor
type). This means limiting the minimum speed which can be regulated. If the applica-
tion cannot tolerate this reduction in its speed dynamic range, a second and more
complex solution is to compensate this current measurement distortion, by means of
look-up tables memorized in the micro-controller memory. When the current i(t0) is
measured, if td is larger than a limit value (7 mS for example), i(t0) is increased by a
compensation value extracted from a memory table before it is used to calculate the
current error. This value is a function of the triac firing delay td, and is determined ex-
perimentally by characterizing the motor. The regulation system is not very sensitive
to this value, so it is enough to characterize one motor of a given type, it is not neces-
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