AN302

Application examples

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1.2

Example 2: motor control (1)

Figure 4.

Control of a small motor by TRIAC

The designer wishes to control a small high-impedance motor (2500

Ω, for example) by a

TRIAC. He obtains the parts and an operating manual and carries out some tests. The

circuit, based on that in Figure 4, operates as expected at first. However, after one year of

production, the manufacturer complains of low torque in the motor and blames the TRIAC.

What’s happened?

was 50 mA. But the TRIACs used for the tests were not worst-case, they were more

results.

After a year of delivery, the component manufacturer continues to deliver parts which are in

With these different values the conduction time decreases, the asymmetry is greater as

shown in Figure 5, and the resulting DC component of current causes the motor to gradually

lose torque.

To prevent this kind of difficulty, one must, when designing the circuit, take into account not

the typical value of the sample used but the worst-case value specified by the component

manufacturer.

Figure 5.

Voltage across the TRIAC and current for the motor control

M

Induction motor

I

IH +

V

t

IH –