Answer: The output horsepower between the 3KW single-phase motor and the 3KW three-phase motor is equal. Because horsepower is also a unit of power, it is just an old concept of an industrial revolution. The concept of horsepower was named by James Watt, the improver of the steam engine, to indicate the power of his steam engine relative to the pulling force of the horse. At that time, Watt What is defined is imperial horsepower. After conversion, it is 1 imperial horsepower = 745.7 watts, and the unit of horsepower is "HP"; then there is a metric horsepower unit, which is defined as: 1 horsepower = 735 watts, that is, 1 horsepower is equal to Complete the 75 kg weight up to 1 meter in 1 second. The three-phase motor has three sets of running windings, and the symmetrical phase angle among them is 120⁰ AC. After the power is turned on, it can rotate by itself according to the phase sequence of the power supply, and does not need other auxiliary starting components. Therefore, the current of the winding of the single-phase motor is greater than the current of the winding of the three-phase motor, so the size of the single-phase motor must be much larger, and the cross-sectional area of the enameled wire of the winding of the single-phase motor is larger than that of the three-phase motor. In the formula, P2 is the rated output power, η is the motor efficiency, n is the motor speed, M is the motor torque, and 9550 is a constant. The formula for calculating the torque of a single-phase motor is the same. Now the question is, why is the same output power, but why are three-phase motors commonly used in industry instead of unidirectional motors? In fact, this is because the power supply system in our country is all three-phase four-wire system or three-phase five-wire system (their line voltage is 3×380V), and the L1, L2, L3 live wires in the three-phase winding coil Point O leads out a wire as the neutral line N of the three-phase four-wire system power supply, so it can output three lines with a voltage of 220V. So when we choose a motor, if the load requires a large torque, then choose a motor with a large number of poles. Because the more poles the motor has, the lower the speed and the greater the torque. It's like a car that downshifts when going uphill. Because in the case that the engine output power cannot be increased, reducing the speed can be exchanged for greater torque, thereby obtaining stronger traction. Due to the power supply, the starting torque of the three-phase motor is much larger than that of the single-phase motor; because the single-phase motor itself has no starting torque, it can only run through a special structure auxiliary start (for example: capacitor start). Compared with single-phase motors, three-phase motors have the advantages of small size, large starting torque, high power factor, high efficiency, uniform torque, and small current. So the daily industry uses three-phase motors.
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