Single-phase induction motor drive topologies

AN2991

2/14

Doc ID 15772 Rev 2

1

Single-phase induction motor drive topologies

1.1

One or two Triac approach

Single-phase induction motors, used for compressor controls, have an auxiliary winding.

This winding permits a higher torque to be applied at start-up. Two different ways can be

implemented to control this auxiliary winding. The different topologies are given in Figure 1

and Figure 2.

The most popular method is to add a positive temperature coefficient (PTC) resistor in

series with this coil and the thermostat (see Figure 1). Then, each time the thermostat is

closed, the current flows through the start winding and begins to heat the PTC. After a few

hundreds of milliseconds, the PTC value rapidly increases from a few ohms to several tens

of thousands of ohms. This results in reducing the start winding current to a few tens of mA.

This winding can then be considered as open.

A second solution is to use a second Triac to control the auxiliary winding and replace the

PTC function (see Figure 2). Then, at off state no power is consumed to keep the PTC hot,

this results in improving the appliance efficiency (see Section 3.2.1: PTC losses).

A start capacitor is sometimes connected in series with this winding in location

➀ (see

Figure 1 and Figure 2). It is important to note, even for the same motor, that this capacitor

can be placed or removed, without disturbing the motor operation.

When the capacitor is placed in location

➁ (split-phase capacitor), it always sinks a current,

even when the PTC is hot or when the start Triac is off. This allows power factor

improvement and power consumption reduction. The capacitor C will be added if the

refrigerator or freezer does not reach the required efficiency level without it.

In the following study, we assume that C is always placed at location

➁, if present.

Figure 1. One Triac topology

C

VAC

1

PTC

2

T(°C) Klixon

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