AN1354
®
APPLICATION NOTE
SINGLE-PHASE INDUCTION MOTOR DRIVE FOR
REFRIGERATOR COMPRESSOR APPLICATION
L.Gonthier
A.S.D
1.INTRODUCTION
Up to now, refrigerator compressors have been controlled by electromechanical switches (thermostat or
even electronically controlled relays). This choice was driven by the high inrush current that can appear
when the rotor is stalled. Furthermore, electromechanical relays are advantageous because they are less
sensitive to line voltage disturbances. Today, new semiconductor devices feature over-voltage protection
and high inrush current capability, allowing them to be used in cold appliances.
Electronic thermostats can so be implemented, allowing the appliance efficiency to be improved by more
than 20 W, for 150 W compressors, thanks to better temperature control and the removal of the PTC.
Hence, at a similar cost as Electromechanical thermostats, this technical breakthrough could allow refrigerators
or freezers to fulfill Class A consumption requirements, bringing the following advantages:
■
Better reliability:
- Higher switching robustness of static switches towards mechanic solutions
- Higher ACS and ACST overvoltage robustness towards Triacs, which allows Metal-Oxyde varistor
removal
■
■
■
■
Temperature regulation law flexibility (automatic defrost, Hysteresis threshold adaptation)
Reduction of the temperature ripple (better food preservation, appliance elements downsizing)
Possibility to add indication features for the end-user (inside temperature, open door)
Spark-free operation and EMI reduction (switches can be turned on at Zero Voltage and are turned off at
Zero Current)
■
Overcurrent protection of the motor winding.
This paper outlines the different topologies that can be used for electronic motor control, and lists the
electrical constraints that result from these different circuits. A comparison is also made between the
different performances of electromechanical or electronic thermostats.
All numerical examples are based on the specifications for a 1/5 Horse power compressor, which can be
used in 350 L freezers
2. SINGLE-PHASE INDUCTION MOTOR DRIVE TOPOLOGIES
2.1 One or Two Triac approach
Single-phase induction motors, used for compressor controls, embed an auxiliary winding. This winding
permits a higher torque at start-up to be applied. Two different ways can be implemented to control this
auxiliary winding. The different topologies are given in Figures 1 and 2.
The most popular method is to add a Positive Temperature Coefficient (PTC) resistor in series with this coil
and the thermostat (cf. 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 in-
creases from a few Ohms to several tens of kOhm. This results in reducing the Start winding current to a
few tens of mAmps. This winding can then be considered as open.
A second solution is to use a second triac to control the auxiliary winding and then replace the PTC duty
(cf. Figure 2). Then, at START Triac OFF state, no power is consumed contrary to the PTC, this results in
an improvement in the appliance efficiency (cf. 4.2.1 section).
April 2002
1/10
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