Universal motor drive

AN392

2/11

Doc ID 1863 Rev 2

1

Universal motor drive

1.1

Power control

The power device is a TRIAC because it is the most economical online switch. In a TRIAC-

based controller the output power, and, for example, the motor speed, are controlled by the

phase delay of the TRIAC drive. This delay is referred to the zero crossing of the line voltage

which is detected by means of a connection to the mains neutral (Figure 1). Changing

operation from 60 Hz to 50 Hz can be achieved by making simple modifications to the MCU

EPROM/ROM table defining the TRIAC conduction angle versus power level. Automatic

selection of the 50 Hz/ 60 Hz tables can be implemented.

The TRIAC can be directly driven by the MCU. A very short gate current pulse (~ 100 µs)

could be enough to trigger the TRIAC for rms load currents above 2 A. Such pulse control

allows the low voltage MCU power supply consumption to be reduced. The snubberless

TRIAC is driven in quadrants QII and QIII with a 60 mA gate current provided by three I/O

bits of the ST6210 in parallel. This pulse is sufficiently long to ensure the TRIAC is latched at

the end of the pulse. Pulse length can be modified if another TRIAC or motor is used.

Figure 1.

Mains synchronization

1.2

User interfaces

Different user interfaces can be implemented - a touch control, a push button or a

potentiometer. The circuit diagram in Figure 2 show that the three modes are implemented

on the board to let the system designer choose the preferred user interface.

Control action is obtained when the sensor or the button is touched for more than 330 ms. If

the touch duration is between 50 ms and 330 ms, the circuit is switched on or off. A contact

of less than 50 ms causes no action.