When you choose a motor for an application, whether for a household appliance or a large industrial project, there is a range of factors to consider. One of the main questions you’ll be confronted with is choosing a single-phase or a three-phase motor. The following article will go over the key differences between single-phase and three-phase power, the motors designed for each, and the comparative advantages and disadvantages of single-phase and three-phase motors.
Single-Phase and Three-Phase Power
Single-phase power is what is available in most households and residential buildings at the wall socket. It is supplied through a single active wire and a ground wire. Single-phase power comes in an alternating current that follows a periodic sine wave, with the current changing direction at every half-cycle.
Three-phase power basically consists of three single-phase lines, each one-third of a cycle out of phase from the previous one, producing a more consistent power level. This is how most power is generated, transmitted and distributed along the grid before being converted to single-phase wall socket power. Three-phase power is supplied through three active wires and a single ground wire. It has double the single-phase power voltage, which means that the wires do not need to be as thick for the same power requirements.
How a Motor Works
The basic components of a motor are a rotor and a stator. The rotor is the part in the middle of a motor that rotates, while the stator is attached to the motor's body and does not move. The rotor and stator contain coils that generate a magnetic field when current is applied, attracting and repelling each other depending on the direction of the current, making the rotor turn and delivering mechanical power.
Single Phase Motors
Single-phase motors are motors designed to run on single-phase power. The most basic design of a single-phase motor contains a single-coil each for the rotor and stator. As the alternating current passes through these coils, changing direction every half-cycle of the sine wave, the rotor and stator, in turn, attract and repel each other, producing a continuous revolution of the rotor. Single-phase motors are not self-starting, requiring additional capacitors or start-up winding mechanisms at the beginning of each operation.
Single-phase motors are typically used for applications where the motor power requirement is less than 5HP, such as for appliances in households, shops and small factories. They are usually cheaper than three-phase motors due to being simpler in design and are suitable for connection to the standard wall-socket power supply. However, they are less efficient than three-phase motors and generally have a lower life expectancy due to higher vibration.
Three Phase Motors
Three-phase motors are designed to run on three-phase power. Instead of having a single corresponding pair of coils for the rotor and stator, they have three pairs of coils, spaced 120 degrees apart around the rotor shaft, each connected to one of the active wires of the three-phase power. As the rotor completes one 360-degree revolution, each coil is attracted and repelled three times, producing a higher-torque, higher-efficiency operation compared to a single-phase motor.
Because of this design, three-phase motors have significantly higher efficiency at converting electrical to mechanical energy, and therefore lower operating costs. They are inherently self-starting, and because of their higher torque, especially during starting, they are ideal for heavy industrial applications. Their longer service lifetime also makes them ideal for applications where reliability is important. However, they are generally more costly than single-phase motors to purchase and maintain. Additionally, three-phase power can incur higher installation costs as well as higher service fees. For this reason, three-phase power and three-phase motors are well suited to large industrial applications where the high torque and operating efficiency offsets the costs of the motor and power supply.
Three-phase motors can run on single-phase power, but when doing so, they are not self-starting. If necessary, a phase converter can convert single-phase power to three-phase power to run three-phase motors at maximum efficiency.
Now that you have a grasp of the key differences between single-phase and three-phase motors, you will have a better idea of which type of motor to choose for your application. eMotors Direct has a wide variety of single-phase and three-phase motors available to suit a wide range of applications, from household appliances to large industrial projects.