[Editor's Note] We spoke to Bill Schweber of Mouser Electronics about the importance of starting and running capacitors in single-phase AC motors.


■ Single-phase AC motor brushless, low cost and highly reliable

Most engineers are familiar with the basic brushed, brushless, and stepper DC motors used in a variety of products.

However, there is another motor that has been around since the early days of electricity and magnetism, predating electronic technology, and is still widely used today. It is a single-phase standard AC line motor.

These induction motors offer a power range from less than a horsepower to several horsepower, and because they are brushless, they are relatively inexpensive and reliable.

For this reason, single-phase AC motors are often used in mechanical equipment such as fans, home appliances, and air conditioners.

When motor output exceeds several horsepower, technical constraints generally require a three-phase AC line and a matching motor for smooth and efficient operation.

The basic AC motor was developed in the 1890s to compete with brushed DC motors.

During this process, a fierce competition over power generation and transmission took place between the direct current camp centered around Thomas Edison and the alternating current camp centered around Nikola Tesla and George Westinghouse.

Around 1910, the winner was the exchange.

A basic AC induction motor consists of two main parts: a stator and a rotor, in addition to bearings and wiring connections (Figure 1).

o The stator is the fixed part of an induction motor. In a single-phase induction motor, the stator has upper and lower windings and is powered by standard single-phase AC power supplied from a wall outlet.

The rotor is the rotating component of an induction motor, transmitting mechanical motion through the shaft. In most designs, the rotor is constructed in the form of a "squirrel cage" with conductive bars inserted into slots around its periphery. This bar is shorted at both ends with end rings to form a loop.


Two magnetic fields rotating in opposite directions induce currents in the rotor. This creates a magnetic field inside the rotor, which interacts with the magnetic field of the stator to produce rotation.

However, there is no electrical connection between the stator and rotor. Applying an alternating current to the stator generates a single north-south magnetic field that rotates at the line frequency. When the full speed is reached, the rotor rotates in accordance with this magnetic field.

It seems simple on the surface, and it is indeed. But there's one important problem.

This motor does not start on its own and does not start in a certain direction. This is because single-phase induction motors do not inherently have a rotating magnetic field.

A three-phase motor generates a rotating magnetic field using three alternating currents, but a single-phase motor relies on a single alternating current, so only a stationary, pulsating magnetic field is generated.

This magnetic field is non-uniform and does not provide sufficient starting torque to initiate sustained rotation of the rotor.

At the moment the motor starts, the forward and backward components of the alternating magnetic flux generated by the AC power source are equal in magnitude and opposite in direction, so they cancel each other out.

As a result, the net torque acting on the rotor becomes zero, which is why single-phase induction motors cannot start on their own.

■ Startup Capacitor Solution

The simplest way to start a single-phase induction motor is to turn it manually, but this is not practical and is impractical for large motors.

Early power engineers analyzed the electromagnetic field problem and came up with a better solution: the introduction of a starting capacitor. There are several methods for starting single-phase motors, including using additional windings, but the starting capacitor method is the most widely used.

One way to solve the single-phase problem is to derive two-phase power from a single-phase AC line to construct a two-phase motor. This requires a motor with two windings electrically separated by 90°, and to supply two currents that are 90° out of phase with each other. A start capacitor creates the necessary phase shift between the two winding currents.

With the appropriate capacitance value, the current in the primary winding is phase shifted 90° behind the secondary winding current due to the influence of inductive reactance and internal resistance. As a result, the stator currents have equal amplitudes and orthogonal phases, forming a rotating magnetic field similar to that of a balanced two-phase induction motor.

This type of motor is called a permanent split-capacitor motor and provides improved starting torque and smooth operation.

But this raises a new problem.

Although the capacitor and auxiliary winding ensure motor starting and the desired direction, leaving them connected once the motor starts spinning is not only unnecessary but also detrimental to performance. Therefore, when the rotor reaches a sufficiently high speed, i.e. about 75 to 90% of the rated speed, a simple centrifugal switch is activated to disconnect the starting capacitor and auxiliary winding.

■ Is it a motor failure or a capacitor failure?

Users often think that the motor is faulty, but in reality, the start capacitor is damaged or completely failed.

If the start capacitor fails, the motor will continue to draw much higher current than normal, causing it to overheat and trip the circuit breaker.

Conversely, when the start capacitor is functioning properly, it provides the necessary phase shift to allow the rotor to begin rotating.

This causes a short current spike as the motor picks up speed, after which it stabilizes at the motor's normal operating current level.

However, if the start capacitor becomes weak or completely damaged, the motor will have difficulty reaching normal speed.

If the capacitor fails, the motor will just make a humming or grinding noise, won't spin, and will only draw high current.

This continuous current draw causes the motor to overheat and trigger the circuit breaker as a safety device.

If the capacitor performance is weakened, the motor will eventually start, but it will consume more current than usual, resulting in excessive heat generation and reduced efficiency.

Replacing the start capacitor in these cases often restores motor performance, making it one of the first things to check when diagnosing motor problems. Additionally, ensuring that the voltage and capacitance of the capacitors are properly rated is important to prevent repetitive failures and maintain motor efficiency.

■ Selecting the capacity of the starting capacitor

The capacitance value of a starting capacitor is determined by the motor's size (output horsepower or power). Typical starting capacitors range from a few hundred microfarads (?F) to about 500?F. Important factors to consider when selecting a capacitor include:

o Voltage rating: This refers to the maximum voltage that the capacitor must withstand during operation.
o AC power frequency: either 60Hz or 50Hz, many capacitors are compatible with both frequencies.
o Operating Temperature: Capacitors are often used in high temperature environments and are often located next to operating motors, so they may be rated for 65°C, 85°C, or even 100°C.
o Physical size: The capacitor must be able to be installed inside or next to the motor housing.
In most applications, start capacitors are used only intermittently with low duty cycles. This characteristic simplifies the design, manufacturing, and cost of capacitors.

■ Driving capacitor

Driving capacitors are used in single-phase motors to maintain constant torque in the auxiliary winding even under load. However, not all single-phase motors have a drive capacitor.

In a motor circuit with both a starting and a running capacitor, the running capacitor is always connected to the circuit, and the starting capacitor is disconnected by a centrifugal switch when the motor approaches synchronous speed.

This configuration is called a 'start-run capacitor motor', where the start capacitor is used only for starting and the run capacitor is used for continuous operation.

Unlike start capacitors, which are used only intermittently, run capacitors must be designed to withstand continuous loads, as they must operate continuously while the motor is powered.

The capacitance values are generally much smaller than those for starting, ranging from a few microfarads to about 20 microfarads, but they must have a more robust and heat-resistant structure.

■ Overall performance improvement by using driving capacitors

Single-phase AC induction motors have been widely used and have a successful track record for over 100 years.

However, from the beginning, the limitation that this motor could not start rotating on its own was well known.

The solution to this is to use a starting capacitor and an auxiliary winding, which are disconnected when the rotor reaches sufficient speed.

In addition, using smaller capacity driving capacitors can further improve overall performance.


※ About the author

Bill Bill Schweber is an electronics engineer and contributing writer for Mouser Electronics. He has authored three textbooks on electronic communication systems and hundreds of technical articles, opinion columns, and product features. Previously, he ran several topical technology websites at EE Times and served as a senior editor and analog editor at EDN. At Analog Devices, a leading company in analog and mixed-signal ICs, he worked in marketing communications (PR) and on both sides of the technology PR spectrum. He not only introduced the company's products and messages to the press, but also served as a media outlet, receiving the company's stories. Previously, he was an associate editor for Analog Devices' technical journals and held positions in product marketing and applications engineering. Earlier in his career, he worked directly with Instron, designing analog and power circuits and integrating systems for materials testing machine control. He holds a Master of Science in Electrical Engineering (MSEE) from the University of Massachusetts and a Bachelor of Science in Electrical Engineering (BSEE) from Columbia University. He is a Certified Professional Engineer (PE) and holds an Advanced Level Amateur Radio license. He has also planned, written, and taught online courses on a variety of engineering topics, including MOSFET fundamentals, ADC selection, and LED driving.