DocID025296 Rev 1

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AN4363

Voltage rating selection

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Figure 6. Delta connection

according to Table 1 values, is very close or above 600 V. For such applications, AC

switches able to withstand at least 800 V are then required. For motor control, due to the

back electro-motive force (BEMF) that can be added to the voltage held by the AC switch at

OFF state, 1000 V or 1200 V devices are preferred.

2.2

Motor control applications

A BEMF can be applied by a motor when the series AC switch is OFF. A motor can still be

running due to inertia or if a mechanical torque is applied.

A BEMF will occur across a motor if it is turning and if an inductive field is applied. This is the

reason why, such a BEMF does not occur with universal motors (as the exciting field applied

by the rotor is switched off when the stator is turned off, as both windings are in series) and

asynchronous induction motors (as the rotor field is generated by the stator field). A low-

level BEMF could occur in case of permanent induction due to motor iron saturation but

such a BEMF can usually be neglected.

A BEMF will actually appear at stator winding turn-off usually when permanent magnets are

used. This is the case of permanent magnet synchronous motors (PMSM). Typically the

BEMF could be between 30 to 50% of the line voltage. So a 50% margin on the AC switch

voltage capability, versus the peak line voltage has to be taken into account.

There are also specific topologies where a capacitor can be used to control a motor. These

two topologies are:

•

Induction motors, with split-phase capacitors (see Figure 8): such motors are used for

drum motor control (mainly in vertical axis washing machines), rolling shutters, garage-

door openers, compressors, etc. The Triac voltage typically reaches 550-650 V at

device turn-off for 230-240 V applications (see STMicroelectronics Application note

AN2991).

•

Motors used in a diode-bridge in series with an AC capacitor (used as a ballast to step-

down the Line voltage, see Figure 7): such a topology is used to operate a low-voltage

electronic load like a BLDC motor, directly from the mains. At turn-off, the voltage

across the Triac reaches 2 times the line peak voltage due to the nearly perfect 180°

phase-shift between line and capacitor voltages. This means 746 V can be applied for

a 264 V line voltage (240 V + 10%).

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