Yes, a 3KW single-phase motor and a 3KW three-phase motor, they both have equal rated output power, and therefore they have equal rated output horsepower. Because horsepower is a unit of power, the concept originated during the Industrial Revolution and was named by James Watt, an improver of the steam engine, to denote the power of his steam engine relative to the pull of a horse. Watt defined horsepower in imperial terms, with 1 imperial horsepower ≈ 745.7 watts (W) in "HP". Metric horsepower (PS) is defined as 1 metric horsepower = 735.5 watts, which is equivalent to the work done to lift a 75 kg weight 1 meter in 1 second.
Three-phase asynchronous motor with worm gear reducer
A three-phase motor has three sets of windings that are 120⁰ apart from each other in space. When a three-phase alternating current with a phase difference of 120⁰ is passed through the motor, a rotating magnetic field can be generated, causing the rotor to rotate on its own without the need for any other auxiliary starting elements.
In contrast, a single-phase motor can only generate a pulsating magnetic field under a single-phase power supply and cannot start on its own, requiring additional starting devices (e.g., capacitors, resistors to split the phase, or shaded poles) to generate the initial torque. To achieve the same rated output power, single-phase motors usually require higher currents (because their power factor is usually lower than that of three-phase motors), so their stator windings tend to be thicker (larger cross-sectional area), and the motor's size and weight are usually larger than that of a three-phase motor of the same power.
Where 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 output torque of a single-phase motor and a three-phase motor (M ≈ 9550 * P2 / n) is formally the same.
Now the question arises, why is it the same output power, but why are three-phase motors commonly used in industry instead of single-phase motors?
This is mainly because our country's power supply system is generally used three-phase four-wire system or three-phase five-wire system (line voltage of 380V).The zero line (N) is drawn from the neutral point of the star-shaped secondary winding of the three-phase power transformer, thus providing a single-phase voltage of 220V between any of the phase lines (L1, L2, L3) and the zero line (N).
Therefore, when there is ready three-phase power available, the use of three-phase motors has significant advantages:
1. Good starting performance: Three-phase motors are self-starting and usually have a much higher starting torque than single-phase motors of the same power (single-phase motor starting torque is dependent on the starting device).
2. High efficiency: in the same power level, three-phase motor efficiency is usually higher than single-phase motor.
3. High power factor: When operating near the rated load, the power factor of a three-phase motor is usually better than that of a single-phase motor.
4. Good torque characteristics: three-phase motors run with smoother torque and less pulsation.
5. Small size and light weight: For the same output power, three-phase motors are usually more compact and lighter than single-phase motors.
6. Smaller running current: Due to high efficiency and high power factor, three-phase motors draw less line current from the grid than single-phase motors when outputting the same power.
7. High reliability: the structure is relatively simple (no start switch or capacitor), maintenance may be easier.
In addition, when we select a motor, if we need a large starting torque or running torque, we can choose a motor with a large number of poles (low speed). This is because according to the torque formula (M ≈ 9550 * P2 / n), the lower the rotational speed n is, the higher the output torque M will be for a certain output power P2.This is like downshifting a car going uphill: where the engine output cannot be increased instantaneously, a lower wheel speed (n decreases) through the gearbox can be exchanged for a larger drive wheel torque (M increases), resulting in more traction.
So despite the same rated power (horsepower), three-phase motors are generally superior to single-phase motors in terms of starting torque, operating efficiency, power factor, size and weight, current draw, torque smoothness, and operating reliability. In industrial environments, where three-phase power is commonly available, three-phase motors are the natural mainstream choice. Single-phase motors are mainly used in applications where only single-phase power is available (e.g., household appliances, small tools, agricultural machinery, etc.).