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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 .sin

ωt0 / (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)

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-

Vmains

Imotor

1

2

3