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08Mar

AC Motors – IP Rating and Thermistors

AC motors run on alternating current delivery, exactly as domestic electrical power is delivered to the home. Both the voltage and frequency are different across the world, and appliances designed for sale in a particular country must be able to cope with the power supply in that area, as using the wrong type of appliance with the wrong power supply could cause a fire or electrocution. It might be annoying having to take travel adaptors to different countries (especially so if you forget it with you!) but there is a very good reason for that!

Thermistors in AC motors serve a vital function preventing the unit from overheating by limiting the input current and initiating shut off systems when overheating is detected. Thermistors are thermally sensitive resistors which act when temperatures change, either decreasing or increasing resistance. A PTC (Positive Temperature Coefficient) thermistor increases resistance as temperature goes up and are used to protect against surges in current. An NTC (Negative Temperature Coefficient) Thermistor decreases resistance as temperature rises and is used in temperature sensors, such as toasters, freezers and commercial food processing equipment.

PTC thermistors in AC hydraulic motors can be located just after the power input in order to regulate current if the motor starts getting too hot while still maintaining operation. The feedback from the system can affect a surge in cooling activities or alert the operator to shut down the machine in the event of serious overheating. In environments where temperature increases could lead to a potentially explosive atmosphere an AC motor with PTC thermistor regulation is ideal, as the AC motor produces no spark that could ignite any material and the PTC system can ensure the system is shut down before dangerous temperatures are reached. An NTC thermistor could be used to control the action of a cooling system to ensure that the correct temperature is achieved and maintained during motor operation.

Another thing which makes AC motors ideal for use in potentially hazardous environments is the ease with which they can be isolated and contained, generally rated IP55 as standard. DC motors are much harder to house in a sealed protective casing, so an AC motor is the obvious choice. IP (Ingress Protection) ratings denote the ability of any piece of equipment to withstand material and water entering it. The first number given relates to the ability to withstand solids from hands (1) to total enclosure from even minute dust particles (6). The second number relates to the waterproof factor of the item, from dripping water (1) to immersion beyond 1 metre deep (8). The highest level of protection is IP68, and the lowest would be IP00, although it is hard to think of any item of equipment that could realistically achieve that rating! AC motors are therefore ideal in dusty, dry environments where contamination and heat are present.

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