Direct current compressors

Friday, 4 June, 2010

Nowadays convenience is increasingly important in many areas, including refrigeration systems. This is especially true for mobile refrigeration applications, ranging from mobile cool boxes with compressor refrigeration to cab air conditioning for lorries. Accordingly, in this instalment we focus on compressors for DC operation and mobile use.

Chapter 1. Direct current compressors

Nowadays convenience is increasingly important in many areas, including refrigeration systems. This is especially true for mobile refrigeration applications, ranging from mobile cool boxes with compressor refrigeration to cab air conditioning for lorries. Accordingly, in this instalment we focus on compressors for DC operation and mobile use.

Compressors with EC motors 

There are two points that are especially important for compressors used in mobile applications. 

Firstly, they must be mechanically suitable for use in mobile applications, such as motor homes, lorries, boats and cool boxes. 

The mechanical stresses on compressors in mobile operation, such as in a lorry, are considerably higher than with stationary operation, so the compressors must be specifically designed for this type of use ("battery driven" (BD) type). For example, the internal suspension and various internal components of BD compressors are mechanically reinforced so they can withstand alternating centrifugal forces, rough road surfaces, and so on. 

Secondly, using a DC power source has a major impact. In most mobile applications, the vehicle electrical system is a DC system. In normal passenger cars - including luxury limousines with a BD compressor in the champagne cooler - this is usually a 12-volt DC system, while 24-volt DC systems are standard in lorries. 


Due to this situation, the electronics modules of BD compressors are substantially different from the start-up devices for stationary compressors designed for AC operation. 

With compressors for DC operation, the current from the supply voltage remains constant and always flows in the same direction, unlike the situation with an AC power source. This means that a rotating field must be generated electronically. In other words, a BD compressor fundamentally requires an electronic module in order to operate, and this module can easily be equipped with extra features, including the possibility of running the compressor at a different speed. The standard speed of a BD compressor is 2,000 rpm, compared to approximately 2,900 rpm for an AC compressor with one pole pair operating at 50 Hz. 

However, with most BD compressors the speed can easily be increased to a maximum of 3,500 rpm, or even 4,500 rpm with some types. This can be done by connecting a resistor between the C and T terminals of the electronics module. The desired speed can be obtained by using a suitable resistor. This can simply be a standard resistor with a value close to the figure listed in the resistor value table of the BD data sheet. In practice, it does not matter if the compressor runs at a slightly different speed, such as 2,511 rpm instead of 2,500 rpm. 

However, the cooling capacity increases at higher compressor speed, since the compressor pumps more vapour or the volumetric flow is higher. 

This means that it is sometimes possible to use a smaller BD compressor operating at a somewhat higher speed as a service substitute for a slightly larger compressor. For example, a 2 cm3 BD 35F compressor running at slightly more than 2,500 rpm can be used as a substitute for a BD 50F with a rated capacity of 2.5 cm3 at 2,000 rpm. 

Resistor (8) Ohms Motor speed rpm Contr.circ.current mA
0 2,000 5
277 2,500 4
692 3,000 3
1523 3,500 2

Under voltage protection is not common in stationary refrigeration systems operating from the AC mains, and in most cases it would not be worthwhile. The situation with DC compressors is completely different. It would be extremely irritating if you couldn't start your luxury car just because the BD compressor for the back-seat cooling compartment had sucked all the power out of the battery. To prevent this, standard BD electronic modules are equipped with a battery protection cut-out function. This is factory set to a cut-out voltage of 9.6 V and a restart voltage of 10.6 V for a 12-V electrical system. As these values are already rather low, they can be altered using resistors in a similar manner to speed control.

The protection cut-out resistor is connected between the C (common) and P (protection) terminals. This allows the cut-off voltage to be set to a higher value, which reduces the allowable battery discharge. In the case of special electronics modules, such as those used in solar power systems, this function is usually omitted because the output voltage of a solar panel is variable and may drop into the under voltage range without creating any cause for concern.


Resistor (R2) (kOhms) 12V cut-out (V) 12V cut-in (V) 12V max. Voltage (V) 24V cut-out (V) 24V cut-in (V) 24V max. Voltage (V)
0 9.6 10.9 17.0 21.3 22.7 31.5
1.6 9.7 11.0 17.0 21.5 22.9 31.5
2.4 9.9 11.1 17.0 21.8 23.2 31.5
3.6 10.0 11.3 17.0 22.0 23.4 31.5
4.7 10.1 11.4 17.0 22.3 23.7 31.5
6.2 10.2 11.5 17.0 22.5 23.9 31.5
8.2 10.4 11.7 17.0 22.8 24.2 31.5
11 10.5 11.8 17.0 23.0 24.5 31.5
14 10.6 11.9 17.0 23.3 24.7 31.5
18 10.8 12.0 17.0 23.6 25.0 31.5
24 10.9 12.2 17.0 23.8 25.2 31.5
33 11.0 12.3 17.0 24.1 25.5 31.5
47 11.1 12.4 17.0 24.3 25.7 31.5
82 11.3 12.5 17.0 24.6 26.0 31.5
220 9.6 10.9       31.5

The cooling control signal can be connected directly to the electronics module of a DC compressor. For this purpose, a floating contact should be connected between the C and T (thermostat) terminals. A simple thermostat (refrigerator thermostat or KP thermostat) should be provided for this purpose.

The thermostat contact closes when the compressor needs to be switched on. When the desired temperature is reached (such as inside a cool box), the thermostat contact opens and the compressor stops.

The thermostat contacts do not need to meet any special load requirements, since they aren't connected to a power circuit. However, currents in cars and lorries are generally rather high because the electrical system operates at a much lower voltage than with stationary systems, so the current must be correspondingly greater in order to obtain the same power consumption.


A diagnostic function is integrated in the compressor electronics module to make commissioning and servicing easier. This function can detect five different fault conditions: battery protection cut-out, fan overcurrent, start-up against back pressure, minimum speed, and electronics overtemperature.

"Battery protection cut-out" means that the battery voltage has dropped below the cut-out threshold or has not yet reached the restart threshold of the battery protection circuit. This means that the compressor may not be started, in order to avoid further discharging the battery.

"Fan overcurrent" means that the fan connected to the + and F terminals is drawing too much current (more than 1 A). A 12-V fan must always be used, regardless of whether the BD compressor is operated from a 12-V or 24-V electrical system.

"Start-up against back pressure" means that compressor start-up is blocked because the back pressure in the refrigeration system (the pressure difference between the compressor inlet and outlet) is more than 5 bar. Ideally the pressure in the system should be fully equalised for start-up with a DC compressor, but the compressor can cope with a small pressure difference during start-up. In the worst case, this error code may mean that there is a mechanical defect in the compressor that prevents it from starting (such as scored crankshaft bearings). This will cause this error code even if there is no measurable pressure difference between the compressor inlet and outlet.

Another error code is "Underspeed", which is issued when an excessive load on the compressor (such as excessive outlet pressure) causes the speed to drop below 1,850 rpm.

The final fault diagnosis option is "Electronics overtemperature". Excessive temperatures are especially likely to occur with mobile units operating in very small compartments or in engine compartments in the summer. Cut-out may be necessary to protect the unit against high thermal stress.

These five diagnostic codes are issued as pulse signals. If a LED is connected to the BD electronics module, the error code can be read directly from the blink rate. Error code 3 (start-up against back pressure) is indicated by a triple blink followed by a pause, another triple blink, and so on. If no LED is connected, the pulses can be measured using a multimeter connected to the + and D (diode) terminals (set the meter to a range higher than 24 V DC). In this case a sequence of two short pulses followed by a pause, two short pulses again and so on corresponds to error code 2, "fan overcurrent".


Number of flashes Error type
5 Thermal cut-out of electronic unit

(if the refrigeration system has been too heavily loaded, or if the ambient temperature is high, the electronic unit will run too hot).
4 Minimum motor speed error

(if the refrigeration system is too heavily loaded, the motor cannot maintain minimum speed at approximately 1,850 rpm).
3 Motor start error

(the rotor is blocked or the differential pressure in the refrigeration system is too high >5bar).
2 Fan over-current cut-out

(the fan loads the electronic unit with more than 1A)
1 Battery protection cut-out

(the voltage is outside the cut-out setting)

BD compressors are basically single-cylinder machines with vertical crankshafts. They are normally delivered prefilled with an adequate oil charge, so additional oil is usually not necessary. Due to their relatively high heat dissipation, these compressors also have oil slinger cooling.

Static cooling by the atmospheric air is usually adequate with compressors at the lower end of the capacity scale, but fan cooling is necessary with larger models or high evaporating temperatures. For this reason, these compressors should not be fitted with acoustic covers. Check the compressor data sheet to see whether static cooling ("S") is adequate or fan cooling ("F") is necessary.

The solder connections of these compressors are closed with "Capsolute" caps to prevent the entry of undesirable foreign objects or moisture into the compressor. A special cap remover is available for removing these caps, and it should be used to remove all of the caps. This means that even if some process stubs are not needed, you must always remove their caps and solder them closed.

The compressor is fitted on rubber mounts. You should always use these rubber mounts because they work together with the suspension springs inside the compressor block to absorb compressor starting and stopping forces, centrifugal forces and vibrations.

Technology comparison

There are two main technologies that are widely used as alternatives to compressor refrigeration systems for mobile DC operation: Peltier coolers and absorption refrigerators.

Peltier coolers are attractive in terms of price, but they have very low cooling capacity compared to BD compressor systems. They are often installed in simple, relatively small cool boxes for the consumer market. Their range of use is also limited with regard to the maximum temperature difference. For instance, an evaporating temperature below freezing with an external temperature of 30 °C is not possible with a Peltier cooler, but it is certainly possible with BD technology. From an energy standpoint, DC compressor technology is significantly more efficient, in the most part due to EC motor technology. As a rule of thumb, you can assume that electricity consumption with a DC compressor is only around one-third that of a comparable Peltier cooler.

The energy efficiency of absorption refrigerators is similar to that of Peltier coolers. They also consume three times as much electricity as compressor refrigeration. Absorption refrigerators are used as minibars in hotels and - as a mobile application - in many motor homes. Absorption refrigerators in campers are generally powered from a heat source, which is usually a propane burner.

Compressor Cooling and Installation