HEAVY MACHINERY AND VEHICLE CAB AIRCONDITIONING SYSTEM. FIELD OF THE INVENTION This invention relates to battery operated air conditioning systems. In particular, it relates to an efficient and power conserving air conditioning system for the operational cabins, without limitation, typically of industrial vehicles such as trucks, haulage vehicles, mobile cranes, mining and earth moving machinery. BACKGROUND OF THE INVENTION The adaptation of prior art automobile air conditioning systems to cool the operator cabins of trucks, cranes, mining and earthmoving machinery etc. is known. Problems arise when the equipment has to be parked for a period of time, and the driver has to remain in it. Standard prior art compressors running the air conditioning in most machinery is driven by the machine's engine, so to maintain a safe working temperature for an operator, the engine needs to be running continuously. This results in wear and tear on the engine and the air conditioning system and consumes fuel even when the machine is not actually engaged in productive work. This is a common situation, especially with large earthmoving machinery. The problem is even more acute in mining where drivers of large haulage vehicles, draglines and dozers have to be kept cool for example, in the Pilbara, often in temperatures up to 50 degrees C. If the machines are to be parked up or shut down, battery life becomes critical when having to maintain cool cab temperatures in very high ambient heat for at least two hours, such as during lunch breaks or while operators are waiting for truck turnarounds. Prior art attempts to develop solutions and systems with current condenser and compressor technology have not been very successful. Systems using standard compressors coupled to an electric motor run at full speed and then cut out much like a clutch in a car. The stop and start mode of conventionally installed systems results in a current draw that is always at maximum when in operation and difficult to control without over condensing.
There is therefore a need for a system wherein compressor function can be controlled to reduce current draw as the temperature drops and only ramped up as the temperature begins to rise such that the compressor does not sit and cycle but can be continuously modulated to maintain a desired temperature in the interest of prolonging battery life, It is therefore a general object of the present invention to seek to eliminate or ameliorate the disadvantages and limitations of the prior art. It is however a specific objective to provide an efficient and inventive air conditioning system for the operator's cabs of industrial vehicles and heavy machinery which significantly reduces battery power consumption. SUMMARY OF THE INVENTION In one aspect, the invention resides in a battery conserving air conditioning system for operator cabins, typically of industrial vehicles and heavy machinery, including: a variable speed compressor; the compressor associated with a controller specially adapted to increase or decrease power supplied to the compressor's motor; a temperature sensor adapted to send signals to be processed by the speed controller, wherein, power supplied to and hence, speed of the compressor is automatically and continuously varied ( and not of a stop/start mode ) according to temperature detected by the temperature sensor in the interest of a reduction in battery power consumption and a smoother more seamless temperature controllability . The compressor has a brushless direct current (DC) motor controlled by the controller which automatically and continuously varies input voltage to the motor via pulse width modulation in accordance with the signals received from the temperature sensor. 2 Preferably, the temperature sensor detects the return air temperature inside the cab. In the alternative, the temperature sensor may detect ambient temperatures outside the cab or in the air conditioning circuit itself. Preferably, in cooperation with the temperature sensor the controller includes a function that enables air temperature inside the cabin to be adjusted or set to a predetermined temperature or range. Suitably, the speed of the compressor motor is increased when the temperature detected approaches a higher set temperature or a higher end of the set temperature range and is decreased when the temperature detected approaches the lower set temperature or a lower end of the set temperature range to conserve battery power and facilitate smooth and seamless temperature controllability. The power is preferably a 24 volt battery supply, but can be adapted to work with other machinery battery capacities and systems. BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention be understood, reference is made to the accompanying drawing wherein: FIG. 1 shows a flow diagram of the invention in operation. DETAILED DESCRIPTION OF THE EMBODIMENT. As shown in the flow diagram of FIG. 1, when the air conditioning 10 is turned on at the controller 12, power from the electrical system which includes the battery 14 ignition switch 16 and alternator 18 is sent to the relays which operate the condenser fans 20 and the evaporator fans 22. The controller 12 receives signals from the return air temperature sensor 24 located inside the cabin. The controller processes the signals to control the compressor 26 via 3 pulse width modulation wherein altering the width of the pulse determines the speed of the compressor motor. In a preferred embodiment there is included a temperature selector 28 to enable the operator to adjust and set the pre-determined or required cabin temperature or temperature range. As an example, if the operator in the cabin has selected 22 C. degrees as a required temperature and the temperature inside the cabin is actually 35 C. degrees, the controller 12 will provide the compressor 26 with full battery power to maximally cool the cab. As the temperature is reduced in the cabin, less cooling is required. The return air sensor 24 continues to send signals back to the controller 12 wherein the controller will recognize and process the variation due to the temperature drop. Utilizing pulse width modulation, the controller will slow down the compressor motor, which in turn reduces the cooling capacity of the compressor 26. As a consequence, the compressor does not draw as much power thereby preserving battery storage to enable significantly longer operational time when compared to existing prior art systems. It must be emphasized that the principal advantage of the invention is the overriding benefit of being able to prolong battery life when the engine is switched off by continuous control of compressor motor function. This is achieved by reducing the current draw as the temperature drops and only increasing it as the temperature rises. In this way, the compressor does not sit and cycle but is continuously modulated to maintain the desired cabin temperature solely on battery power for a longer period without compromising the machinery's other required electrical functions. It will of course be realized that while the foregoing has been given by way of illustrative example of this invention, all such and other modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of this invention as is herein set forth. In the specification the terms "comprising" and "containing" shall be understood to have a broad meaning similar to the term "including" and will be understood to imply 4 the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. This definition also applies to variations on the terms "comprising" and "containing" such as "comprise", "comprises", "contain" and "contains". 5