Shaded pole motors are the original type of AC single-phase induction motor. Also called a single phase induction motor, simply connecting to a single voltage line and an external capacitor is required to make this motor turn. The different types of single phase induction motors vary depending on the method used to started them. The four basic types are split phase, capacitor start, permanent split capacitor, and capacitor start/capacitor run.
A split phase motor uses a switching device to disconnect the start winding once the motor gets to 75% of its rated speed. Although this type has a simple design which makes it less expensive for commercial use, it also has low starting torques and high starting currents.
The capacitor start motor is a split phase capacitor motor with a capacitor in series with the starting winding to create more starting torque. This motor is more expensive due to the switching and capacitor components that are required.
A permanent split capacitor motor does not have any starting switch. For this type, a capacitor is permanently connected to the starter winding. Since this requires a capacitor for continuous use, it does not provide starting power, therefore starting torques are typically small. These motors will not perform well for heavy starting load applications. However, they do have low starting currents, quieter operation, and higher life/reliability, thereby making them a good choice for high cycle rates. They are also the most reliable capacitor motor on account of not having a starting switch. Different designs offer higher efficiencies and power factor at rated loads.
The capacitor start/capacitor run motor has both a start and runs capacitor in the circuit. Once achieving full start-up, the start capacitor is disengaged. This type of motor has higher starting, lower loaded currents, and higher efficiency. The drawback is the expense required for two capacitors and a switching device. Reliability also plays a factor on account of the switching mechanism.
To compare, this resistance split induction motor types supply only a low to medium amount of starting torque, and this restricts them to the low power applications they are best suited for. These motors use a single auxiliary winding smaller than typical, creating a lower inducting rate and much higher resistance than other types. Simple models like these can only be used when the load is low, and there is little starting drive needed.
Some applications, like small fans, grinders, and heaters, do not need the higher starting torques but in most cases, the more torque at starting the motor, the more of a load can be put on the machine. A Single phase motor that comes with high starting torques often is more expensive than the simpler split induction types. However, the difference in power can be worth it for different industrial needs. In a single phase motor that has a high starting torque you can expect a different level of performance, this can save time and energy.
The alternating currents that flow in a single phase motor reach their peak values at the same time; this makes one single phase. In a three phase systems, peak current values are achieved in a sequential fashion, making three separate stages. To compare to three phase systems, these motors do not have the same high-efficiency levels but can last indefinitely with little maintenance.
Induction electrical motors have different classifications according to the source of electric power and type of construction. Induction type motors, also called asynchronous motors, operate using an alternating current (AC) produced by electromagnetic induction, as opposed to the commutators commonly used in other AC motor types. Induction motors have been used across the industry as well as in standard appliances such as refrigerators, washing machines, dishwashers and clothes dryers.
Induction type electric motors were the original AC motor to be created; Nikola Tesla came up with the prototype in 1883. These induction motors use a very simple design and operation when compared to modern AC motor designs but, they are still very rugged, quiet and durable. Induction motors are distinctive because they use an induced current in the rotor to produce rotational motion.
Induction motors consist of two simple parts, the copper winding stator, and the armature or rotor assembly. The stator windings are held in slots around the stator, with a balance between the number of north and south poles. The rotor assembly comes manufactured in a few variations: squirrel cage rotors, slip ring rotors, and solid core rotors.
These motors are best for low power needs and applications where it would be inefficient to use more high-powered mechanisms. Many single-phase motors are ideal for applications with low inertia while others are engineered to meet high starting torque requirements.