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The relay enjoys a wide range of applications with greatly differing ambient conditions and technical requirements. The relay used on different occasions in the same field requires different conditions.
Due to the constraint of space, this chapter deals mainly with the major areas of the applications of the relay. A few examples are cited here to illustrate its usage in brief. It is essential to use the information in Section I to choose the right relay to cater to the specific needs of the customer.
▶ The Automobile Sector
The relay is enjoying an increasingly wide area of application in the automobile sector. Frequently used relays include motor starting relays, horn relays, circuit breaking relays for motors or electric generators, charging voltage and current regulating relays, flashing relays for signal transformation, lamp brightness control relays, air-conditioner control relays, on/off control relays for sliding doors, and lift control relays for glass windows.
The automobile generally uses a 12V power source with a designed coil voltage of 12V. As the power is supplied by the battery cell, the voltage is not stable; additionally, the ambient conditions are poor, the operating voltage V￡60%VH (rated operating voltage); the maximum coil overvoltage is 1.5 VH; the coil power consumption is relatively high, usually in the range of 1.6 - 2 W, with a high temperature rise. The ambient conditions are very tough: the ambient temperature is in the range of -40-100°C; the relay used in the motor case must withstand the erosion of gritty dust, water, salt and oil; it is subject to serious vibration and impact, with an impact strength of 100g and an impact stability of 10g; the vibration is in the range of 10-40Hz, double-amplitude 1.27mm, 40-70Hz, 0.5g; 70-100Hz, 0.5mm (double-amplitude); 100-500Hz, 10g and several other rates
▶ Household Appliances
Relays are primarily used to control the compressor motor, the fan motor and the cooling pump motor to execute relevant commands. The power of compressor motors in household appliances are generally in the range of 1-3 horse-powers, while that of fan motors and cooling pump motors in household appliances are usually in the range of 1/4-2 horse-powers. At the moment of loaded starting, a heavy surge current usually occurs, which is 6 times the current of a full load operation. It takes some time for the compressor motor to reach its full speed, which constitutes a significant threat to the contacts of the relay. Therefore, a sufficient margin is required of the load capacity of the relay; the rebound of the relay when put into operation should be minimized, and the release of the relay should be completed in the shortest possible time to minimize the rebound of the contacts. The relay must meet a high safety standard; its safety must be certified by the relevant safety certification organization.
Product Ambient Conditions: ambient temperature -40-55°C; relative humidity 40°C, 95%; rainwater-proof and even resistant to salt mist in coastal areas. Because weight and size are not essential factors in this case, the relay must be sturdy and impact-proof.
2. Household Appliances
Relays are mainly used in washing machines, microwave ovens and electric heaters among the household appliances. Relay contact load: from heavy load in 220V and 5000W heater (or 1-horse power motor) to light load in drive solenoids, other relay coils and indication lamps.
The relay must have a service life expectancy of 5-10 years; in other words, the electric life of the relay must be 100,000 times to 200,000 times. Ambient temperature: -40-55°C (85°C for microwave ovens and electric heaters); relative humidity 20%-95%. The relay must meet a high safety standard; its safety must be certified by the relevant safety certification organization.
3. Machine Tool Control
Major control functions on machine tools are performed by a general-purpose AC relay, which usually is a push-button or limit-switch drive relay. The contacts of the relay can control electromagnetic valves, medium-sized actuating motors and indication lamps. The voltage is usually 24VDC, 220VAC. Digital control has greatly expanded the area of application of the relay.
Kellering and coordinate boxing can be operated through digital programming which transmits signals to the control unit, memory element and other logical elements of the machine tool to control 2-5 axials of the coordinate servo-actuator. This digital control method is very convenient in controlling drilling machines, turret lathes, plain lathes and automatic forming verifiers.
The digital control system requires the relay be able to suit low level signals, medium sensitivity and quick moves and have a high switching reliability. The ambient conditions for the installation of the machine tool must also be taken into consideration. Running machine tools and equipment in the vicinity are liable to transmit certain impact and vibration to the control cabinet and the control cabinet be contaminated with splashed cutting coolant. Usually the equipment manufacturer has taken some precautions in this regard, however, it is always advisable to take these unfavorable factors into consideration when selecting and designing relays. Relays used in this field must meet a high safety standard regarding electric insulation, voltage with standing and fire retarding capabilities.