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Start and Run Capacitors Are Critical for Single-Phase AC Motors

Published Date
Author Bill Schweber

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Most engineers are familiar with the basic brushed, brushless, and stepper DC motors used in many products. However, another class of motors has been in use since the earliest days of electricity and magnetics—predating electronics—and is still widely used: the single-phase, standard AC-line motor.

This type of induction motor, available with power ranging from a fractional horsepower rating to a few horsepower, is relatively inexpensive and very reliable, due to its lack of brushes. As a result, single-phase AC motors are often used in machinery such as fans, appliances, and air conditioners. Once the motor's power exceeds a few horsepower, technical constraints usually demand a three-phase AC line and matching motor for smooth and efficient operation.

The basic AC motor was developed in the 1890s as a competitor to the brushed DC motor. This led to a fierce rivalry between DC proponents—led by Thomas Edison—and AC advocates—notably Nikola Tesla and George Westinghouse—for power generation and transmission. AC won this battle around 1910.[1]

In addition to bearings and wiring connections, a basic AC induction motor has two parts, a stator and a rotor (Figure 1):

  • The stator is a stationary component of the induction motor. In a single-phase induction motor, the stator has top and bottom windings and is powered by a single-phase AC source, such as a standard AC line from a wall outlet.
  • The rotor is a rotating component of the induction motor and transmits mechanical motion via the shaft. In most designs, the rotor is designed as a "squirrel cage" with conductive bars embedded in slots around the rotor's periphery. These bars are short-circuited at both ends by end rings, which form a loop.

Figure 1: The basic inductive AC motor uses rotating magnetic fields that drive the interaction between the stator and the rotor. (Source: Mouser Electronics)

The two oppositely-rotating magnetic fields induce currents in the rotor. This generates a magnetic field in the rotor that interacts with the stator's field and produces rotation; however, there is no electrical connection between the stator and the rotor. Applying an AC current to the stator generates a single north/south magnetic field, rotating at the line frequency. At full speed, the rotor will spin in accordance with these fields.

This all seems simple enough, and it is. However, there is one significant problem: This motor will run but not start on its own, nor will it start in a predictable direction. This is because the single-phase induction motor lacks a naturally rotating magnetic field.

Unlike three-phase motors, which generate a rotating magnetic field with three alternating currents, single-phase motors rely on a single alternating current, which produces a static, pulsating magnetic field. This field cannot initiate continuous rotation in the motor's rotor due to its non-uniform nature and inability to provide sufficient starting torque.

In the motor's starting condition, the forward and backward components of alternating flux from the AC line are equal in magnitude but opposite in direction. As such, they cancel each other out, which results in zero net torque on the rotor. This zero-torque condition is why single-phase induction motors are not self-starting.

The Start Capacitor Solution

A simple solution for starting single-phase induction motors is to start the motor manually, but this is undesirable in practice and impractical for larger motors. Instead, early power engineers analyzed the electromagnetic field issues and devised a better solution by incorporating a start capacitor. While other starting methods for single-phase motors exist, such as using additional windings, the start capacitor method is the most popular.

One way to solve the single-phase problem is to build a two-phase motor, deriving two-phase power from a single-phase AC line. This requires a motor with two windings electrically spaced 90° apart, fed with two phases of current displaced 90° in time. A start capacitor creates the needed phase shift between the currents in the two coil windings.  

With the appropriate capacitance value, the current in the main winding—characterized by its inductive reactance and internal resistance—is phase-shifted to lag the current in the auxiliary winding by 90°. The resulting stator currents are equal in amplitude and orthogonal in phase, thereby producing a rotating magnetic field like that of a balanced two-phase induction motor. This type of motor, called a permanent split-capacitor motor, achieves improved starting torque and smoother operation.

However, this creates a new problem. While this capacitor and winding ensure that the motor starts and does so in the desired direction, it is undesirable to have them connected once the motor is running, as it is unnecessary and detrimental to performance. Therefore, once the rotor is spinning at a high enough speed—typically 75 to 90 percent of full speed—a simple centrifugal switch opens to disconnect the start capacitor and auxiliary winding.

Motor or Capacitor Failure?

Often, a user may assume a motor has gone bad when it is really the start capacitor that is failing or has failed completely. A failed start capacitor results in a much higher continuous motor-current draw than normal, thus overheating the motor and tripping the circuit breaker. When the start capacitor is functioning correctly, it provides the necessary phase shift to get the rotor spinning. This is why there is a brief spike in current during startup as the motor ramps up to speed, after which the current draw levels out at the motor's operating current.

However, if the start capacitor is weak or completely dead, the motor will struggle to reach its operating speed. When the capacitor fails, the motor may only hum or produce a grinding sound, drawing higher current without successfully turning. This continuous current draw not only overheats the motor but can also cause circuit breakers to trip as a safety measure. For weak capacitors, the motor might eventually start but will consume more current than normal during operation, leading to excessive heat and reduced efficiency.

Replacing the start capacitor often restores the motor's performance, making it one of the first things to check when diagnosing motor issues. Ensuring the capacitor is properly rated for voltage and capacitance is crucial to avoid recurrent failures and maintain motor efficiency.

Sizing the Start Capacitor

The value of the start capacitor is a function of the motor size (i.e., horsepower or watts). Typical start capacitors range from a few hundred microfarads (µF) to about 500µF. Other critical parameters to consider when selecting a start capacitor include the following:

  • Voltage rating: This denotes the peak voltage (V) the capacitor will experience during operation.
  • AC mains frequency: This value is either 60Hz or 50Hz, and many capacitors are specified for both.
  • Operating temperature: These types of capacitors are often used in hot operating environments and are also located next to a running (i.e., hot) motor. They may be rated for 65°C, 85°C, or even 100°C.
  • Physical size: The capacitor must fit within the motor housing or next to it.

Note that the start capacitor is used only intermittently at low duty cycles in most applications. This greatly simplifies their design, construction, and cost issues.

The Run Capacitor

A run capacitor is used in single-phase motors to maintain a running torque on an auxiliary coil while the motor is loaded. However, not all single-phase motors have run capacitors.

In a motor circuit with both run and start capacitors, the run capacitor is permanently connected to the circuit. The start capacitor is disconnected by a centrifugal switch as the motor approaches its synchronous speed. This arrangement is called a "capacitor-start capacitor-run" motor, as the start capacitor is used only at the start, while the run capacitor is used for continuous operation.

Unlike the start capacitor, which is used only intermittently, the run capacitors must be designed to handle the stress of continuous duty while the motor is powered. Its typical capacitance value is much smaller than that of the start capacitor, ranging from a few microfarads up to about 20µF, but it must have a more rugged and temperature-tolerant design.

Conclusion

Single-phase AC induction motors are widely used and have a successful track record of over one hundred years. Since the earliest days of these motors, engineers have recognized their inability to self-start rotation. One solution is to use a start capacitor and auxiliary winding, which are disconnected once the rotor reaches a sufficiently high speed. In addition, a smaller run capacitor is often used to improve overall performance.

 

[1]https://www.energy.gov/articles/war-currents-ac-vs-dc-power

About the Author

Bill Schweber is a contributing writer for Mouser Electronics and an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical web-site manager for multiple topic-specific sites for EE Times, as well as both the Executive Editor and Analog Editor at EDN. At Analog Devices, Inc. (a leading vendor of analog and mixed-signal ICs), Bill was in marketing communications (public relations); as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these. Prior to the MarCom role at Analog, Bill was associate editor of their respected technical journal, and also worked in their product marketing and applications engineering groups. Before those roles, Bill was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls. He has an MSEE (Univ. of Mass) and BSEE (Columbia Univ.), is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. Bill has also planned, written, and presented on-line courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.

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