1. The calculation of the rated current of the motor
(1) Speed calculation
Motor rated current (A): "Motor power doubled," that is, "one kW two amps." Usually refers to the commonly used 380V, three-phase asynchronous motor with a power factor of about 0.8, "double the kilowatt number" is the motor's rated current.
(2) Empirical formula
Motor rated current (A) = motor capacity (kW) × 2
The above-mentioned calculation results of the quick calculation and the empirical formula are the same. The calculated rated current is very close to the actual current value on the motor nameplate, which meets the practical requirements, such as a Y132S1-2, 10kW motor, with quick calculation or experience. The formula calculates its rated current: 10 × 2 = 20A.
2, the choice of the motor with circuit breaker
Low-voltage circuit breakers are generally divided into plastic shell type (also known as device type) and frame type (also known as universal type). In addition, molded case circuit breakers are often used for motors of 380V 245kW and below. According to the application, the circuit breaker can be divided into protection distribution lines, protection motors, protection lighting lines, and leakage protection.
2.1 Principles for selection of circuit breakers for motor protection
(1) The long delay current setting value is equal to the rated motor current.
(2) Instantaneous setting current: For a circuit breaker protecting a cage motor, the instantaneous setting current is equal to (8 to 15) times the rated current of the motor, depending on the type, capacity, and starting conditions of the caged motor to be protected. For a circuit breaker protecting a wound rotor motor, the instantaneous setting current is equal to (3 to 6) times the rated current of the motor, depending on the type, capacity, and starting conditions of the wound rotor motor.
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