AN863 Datasheet, PDF (5/22 Pages) STMicroelectronics – Improved sensorless control with the ST62 MCU for universal motor

IMPROVED SENSORLESS CONTROL WITH THE ST62 MCU FOR UNIVERSAL MOTOR
At first order, we will neglect the inductive term j.L.ω. This yields: i(t) = k.Ω + r.
The universal motor behaves as a speed-dependent resistor of value k.Ω + r.
As Ω and r do not change very quickly, and v is known, measuring i at a specific time
within the mains period (example: at time t0 in Figure 2) gives an image of the motor
speed: Ω = (V0 / k. i(t0)) sinωt0 - r / k.
Therefore if we want to keep the speed at a fixed value Ω0, we need to keep i(t0) at
a corresponding fixed value i0:
i0 = V0 .sinωt0 / (k.Ω0 + r )
In practice, each mains period, the micro-controller measures the current at time t0
with its internal analog to digital converter. Then, the current error i(t0)-i0 is calcu-
lated, and the micro executes the speed regulation algorithm, which results in a new
firing delay td. This delay is used to fire the triac on the next mains period. It is
counted by the micro controller internal timer. The delay is measured starting from the
mains voltage zero-crossing.
Figure 3. Measured Current in a Real Motor (resistive and inductive): Constant
Speed, Variable Load (1: lowest load, 3: highest load)
Vmains
3
2
1
t0
Imotor
If we now consider the real universal motor, including the inductive term j.L.ω, we
can apply the same method. Figure 3 shows the current and mains voltage for a real
motor. On this figure, while the load is modified, the speed is kept constant by modi-
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