STARTUP AT LOW VOLTAGE

Up to now, most common starters are described among those types complete with capacitors, contactors and connection modes. In fact they all are most expensive methods designed for the startup of a compressor. Nevertheless, some compressor manufacturers impose restrictions on the startup current and therefore it might be the case that even high power motors are designed so that they are started instantaneously in order to eliminate adverse effect of low voltage on some part of the compressors like arc generation, interferences to peripheral devices, etc. In some cases, electric suppliers permit switching to rated supply voltage after the voltage regulator is energized up to a portion thereof. This prevents sharp decreases in the supply voltage at the expense of other consumers fed via the same power supply circuitry. In some cases, the instantaneous switching is restricted by the power suppliers for specific rate and time. In some cases, the startup current is limited with respect to certain percent of rotor maximum stall current.

Isolating the compressor load may help in the reduction of the current drawn and thus the acceleration of the motor will be easier. Irrespective whether or not the compressor is loaded, the compressor will need full startup current even for the fraction of a second. The instantaneous current drawn under the stalled rotor is the main concern and therefore, isolating the compressor load may not provide the solution. In such cases, a special circuit arrangement for the startup will be needed to reduce the motor’s startup current.

In most widely used method is to incorporate low voltage starters, where the voltage of the supply line to the motor is reduced considerably, if not completely. Manuel startup is not useful for the cooling compressors and therefore, the magnetic starters will only be dealt with here.

There are five different types magnetic low voltage starters each of which are for different applications:

• Partial winding

• Star - triangle

• Automatic transformer

• Primary resistor

• Stepwise low voltage auxiliary starter

The startup torque will be reduced as the startup current is reduced and therefore, the starters might be selected to supply minimum compressor torque requirements. Commercially available starters complete with automatic transformers are usually designed to provide up to 64% of the maximum torque. The same is 45% and 33% for the partial winding and star – triangle connection, respectively. It seems that where star – triangle connection is used; the compressor should almost completely be isolated from the load.

The first law of the thermodynamics is an expression of the principle of conservation of energy and states that energy cannot be created or destroyed.

The reason of the low or under voltage is not only the requirements set by the electric power suppliers. In most cases, the cable connections between the compressor and condenser may not be appropriate. In this case, the line losses may increase beyond the normal limits and the voltage at the compressor’s terminal drop considerably. The situation might be even more complicated, where copper and aluminum conductors at different current capacities are used. Majority of the manufacturers provide list of recommended cable sizes and maximum cable lengths to keep the voltage loss between the main power source and compressor below 3%. The effect of the line losses on the compressor might be aggravated when minimum power supply voltage is taken into account. Here, it is important to provide correct data to the installer on the electric system (of course, if not the cooling system contract include the electrical installation).