What Is a Single Phase Motor

Single phase motors generally refer to low-power single phase asynchronous motors powered by a single phase AC power supply. It is also called a single phase induction motor. This kind of motor usually has two phase windings on the stator, and the rotor is of ordinary squirrel cage type. The distribution of the two phase windings on the stator and the different power supply conditions can produce different starting characteristics and operating characteristics.

However, without other provisions, when a single-phase motor is energized to start from a completely stopped condition, the motor would remain stalled, or if it does run it might do so forward or backward depending on its initial position. To overcome this problem, AC motors commonly include a secondary starting coil wired in series with a capacitor. Upon startup, this circuit causes a magnetic field phase shift, which starts the rotor turning in a predictable direction. Once the rotor is spinning sufficiently fast, a centrifugal switch opens to disconnect this starting circuit. The starting coil is designed to define the desired rotation direction.

The stator is the stationary part of the motor and contains the primary winding and an auxiliary winding. The main winding is designed to produce a magnetic field when an AC voltage is applied. The auxiliary winding, found in motors, provides a phase shift to create a rotating magnetic field

Capacitor Start vs Capacitor Run Motors

When working with single-phase electric motors, it is important to understand the difference between motor-start capacitors and motor-run capacitors. They are both capacitors, but they have fundamental differences in purpose, design, and operational characteristics, making them non-interchangeable. At its most basic level, a capacitor is a device used to store energy in an electrical charge. So, what sets motor-start capacitors apart from motor-run capacitors, and how do these differences impact their roles in a motor’s operation?

These have electrostatic capacitors permanently connected in series with auxiliary winding. They provide the running torque all the time, maintaining the phase difference between windings. The loss factor is low, being mostly MPP film capacitors from 1.25 μF tom36 μF for different applications. (Till 1990s, paper capacitors were common before the advent of film capacitors). The capacitor is called motor run capacitor. Capacitor value is a compromise between good staring torque and full speed load torque.

This is commonly used in single phase FHP motors to give high starting torque. This is usually an aluminium electrolytic capacitor placed in series with auxiliary winding.

Though the capacitor-run motor has a simple construction and can be operated easily, the capacitance rating must be varied depending on the supply frequency, i.e., 60Hz or 50Hz, to have the ratio 1:1.4. Furthermore, since a motor with the same specifications will have different rotational speeds, having the ratio 1.2:1, in areas supplied by 60Hz and 50Hz mains, different designs were required, particularly for tape-recorders. The brushless motor was developed to avoid this inconvenience, although it requires a complex drive method

These are designed for short-term use to provide a high burst of energy required to start a motor. They typically have a higher capacitance value compared to motor-run caps and are not built to handle continuous current flow. Motor-start caps normally have a range of rated capacitance compared to a specific rating that run-caps have. Using a motor-start capacitor in place of a motor-run capacitor would result in overheating and potential failure since it cannot withstand prolonged operation.

A capacitor start capacitor run motor is a type of single-phase induction motor that incorporates both a start capacitor and a run capacitor. These capacitors are used to create a phase shift in the motor's windings, improving its starting torque and running efficiency.

Capacitor start / Capacitor run motors are a type of split phase (two phase) induction motor designed to operate from single phase ac power.  Unlike Resistive Start / induction motors, they do not have a start winding that is switched out of the circuit once the motor is running.  Instead, they have two windings with a capacitor in series with one of the windings.

The capacitor-run motor is a representative example of the rotating-field single-phase AC induction motor. Its main application lies in fans. It has a two-phase winding configuration consisting of the main and auxiliary windings. A capacitance of about 3μF is connected to the auxiliary winding, as shown in the figure. Its function is to advance the phase of the current of the auxiliary winding by about 90° relative to the current of the main winding to create a rotating field. Switching of the rotation direction, for applications that require it, is done by reversing the terminals of either winding. In the figure, the main winding is reversed.