HVAC compressor current and motor speed control



The information on this page is illustrative. It is not to be used for training purposes or as guidance or instruction. It is also incomplete. A full version of this content can be found in our PicoScope 7 Automotive software, which is downloadable from here.



The purpose of the test is to check the current drawn by a High Voltage (HV) Heating Ventilation and Air Conditioning (HVAC) compressor and its motor speed control circuit.

Electric Vehicle High Voltage Heating Ventilation and Air Conditioning compressor current and speed measurements.

Electric Vehicle High Voltage Heating Ventilation and Air Conditioning compressor current and speed measurements.

Further information

Conventional Internal Combustion Engine (ICE) vehicles use belt and pulley systems to take power from the engine crankshaft and drive their HVAC system compressors to help cool the cabin. The heat generated by their hydrocarbon burning engines passes into coolant which is then circulated through heater matrices to warm the cabin.

In hybrid vehicles, the ICE doesn't run continuously and in electric vehicles there is no ICE at all. Instead, these vehicles use their high voltage power source to drive the HVAC system and control cabin temperature.

The use of a HV power source means that the compressor motor current can be kept low (relative to a lower voltage system requiring the same motor power output) and its thermal losses will be reduced. This makes HV driven HVAC compressors an efficient way of controlling cabin temperature.

Some vehicle manufacturers use the HVAC compressor as a fully functioning heat pump capable of both cooling and warming the cabin. These systems warm the cabin by switching the flow of the hot, compressed, refrigerant gas to a condenser within the vehicle cabin, rather than at the front of the vehicle, where it can give up its heat.

Other systems use Positive Temperature Coefficient (PTC) resistive heaters to heat the cabin. However, these systems consume significant power from the vehicle HV system and can significantly affect the HV battery's range.

In some vehicles, the HVAC system plays an essential function in helping to cool the coolant system that regulates HV battery and motor temperature.

In the example above, compressor motor control is achieved through LIN bus communications with the HVAC control unit. The arrangement allows a command signal to be sent to the compressor and a motor speed (or position) feedback signal to be returned to the HVAC control unit on the same line. Other vehicle manufacturers may use other means to signal the desired compressor speed and receive feedback as to its actual speed.

Although some indication of the desired or actual motor speeds isn't directly available from the waveform, PicoScope's Serial decoding feature can be used to check the communication between the HV control unit and the compressor.

Extreme care is required when carrying out routine maintenance or repairs on vehicles with HV compressors: The manufacturer specific, non-conductive, oil must be used in these HVAC systems, as can come into contact with high voltage components. The incorrect oil could cause damage to the vehicle, through insulation breakdown (between the HV system and the vehicle chassis ground) or, potentially, death. Always refer to the vehicle manufacturer's technical information to find the correct oil for the vehicle.


This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

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Guided test: HVAC compressor current and motor speed control