Operation principle

UM0575

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1

Operation principle

1.1

Compressor starter application

Single-phase induction motors, used for compressor control, use an auxiliary winding. This

winding permits a higher torque to be applied at startup. The most popular method to control

the start winding is to add a positive temperature coefficient (PTC) resistor in series with this

winding and the thermostat (Figure 2). 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 W to several tens of kW. This

results in reducing the start winding current to a few tens of mA. This winding can then be

considered as open. The PTC then behaves like a switch in OFF state, but with a high

leakage current, resulting in high power losses (approx. 2.5 W).

Figure 2 gives the typical schematics of this application where a run or a start capacitor can

be connected in parallel (point 1) or in series (point 2) respectively with the PTC.

1.2

Standard PTC behavior

The transition between PTC ON and OFF states brings a voltage increase across this

variable resistor. Figure 3 and Figure 4 show two oscillograms of the same PTC in two

different operating conditions, for a 230 V compressor which can use both a start and run

capacitor. We see that, at the end of the PTC conduction, the voltage across it reaches

approximately 250 V (refer to "VPTC_OFF" indication). This voltage level is similar whatever

the operating conditions are (min or max RMS line voltage, run capacitor or not, etc.). The

PTC could be turned off as soon as this level has been reached. Section 1.3 explains how to

implement an electronic solution to achieve this function.

Figure 2.

Compressor starter application