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IMPROVED SENSORLESS CONTROL WITH THE ST62 MCU FOR UNIVERSAL MOTOR

1 UNIVERSAL MOTOR PRINCIPLES

The universal motor can be driven in AC or DC mode. Figure 1 shows the two most

popular variable speed drive principles. The goal is to adjust the voltage seen by the

motor in order to adjust its speed. In AC mode, the motor voltage is adjusted by var-

ying the firing delay of a triac. This is done with a diac, resistor and capacitor when

lowest cost is desired, and with an 8 bit micro-controller when higher performance

and added features are desired, such as user interface or monitoring [1]. The switch

is usually a triac, the cheapest power switch.

In DC mode, the motor is supplied by a high frequency pulse width modulated (PWM)

DC voltage. The voltage seen by the motor is proportional to the PWM duty cycle,

which can be adjusted to modify the speed. The power switch used to chop the DC

voltage at high frequency is a power MOS or an IGBT. The DC mode has several ad-

vantages versus the AC mode (less acoustic noise, higher efficiency, lower har-

monics content, all due to lower current ripple, as is shown in Figure 1). However, in

low cost appliances, the AC mode still dominates due to its lower cost (no rectifier

bridge, no fast diode, triac cheaper than IGBT or MOS). This is the mode we will focus

on in the next pages.

The universal motor is a serial excitation motor. Therefore, the motoring torque is pro-

portional to the square of the motor current:

Tm = k. I²

The mechanical power is the product of the torque by the speed:

Ω

If we assume 100% efficiency, the mechanical power equals the electrical power V.I

input to the motor. It follows that the speed is proportional to

V/I:

consequence is that the motor speed is proportional to the average motor voltage.

At constant speed, the resistive torque Tr is equal to the motoring torque Tm. It

comes from the previous equations that the speed is given by:

.

This equation shows that when the resistive load torque increases at a given voltage,

the motor speed will decrease, hence the need for speed regulation. This regulation

can be performed by adjusting the average voltage V.

Tr