Motors and Drives Fundamentals



Capacitor-Start/Capacitor-Run Motors


The most complicated and expensive single-phase motor is the Capacitor-Start/Capacitor-Run Motor. These motors contain both a starting and running capacitor. Smaller sizes will generally have two capacitor enclosures on the top of the motor, while larger sizes may have both capacitors in a large housing on the side of the motor. Capacitor-start/capacitor-run motors are used over a wide range of single-phase applications primarily for starting hard loads. They are available in sizes ranging from ½ to 25 horsepower; however, common sizes are between 3-and-10 horsepower. Maximum sizes allowed are generally dictated by the electrical power supplier. The speed of this motor varies about 10% from no load to fully loaded conditions. Some capacitor-start/capacitor-run motors are available as multi-speed motors with two-or-three fixed operating speeds when appropriate control equipment is used.

Torque-Speed Characteristics

Capacitor-start/capacitor-run motors have moderate-to-high starting torque compared to other types of single-phase motors. Starting torque generally ranges from 200%-350% of normal full-load torque. Their main advantage over the capacitor-start motor is their lower starting-torque that results in lower starting current. Because of the lower starting current, they are generally used for most single-phase applications between 3-and-10 horsepower due to their lower starting current. Common applications include: larger single-phase compressors, pumps, grinders, conveyors, and larger single-phase air-conditioning compressors.

Relative Cost

Capacitor-start/capacitor-run motors require less running current and are therefore more energy efficient than capacitor-start motors; however, they cost more to purchase than many other motors. Capacitor-start/capacitor-run motors typically cost from 100%-to-115% of the cost of a comparable 3-Phase Induction Motor.


These motors have two capacitors in series with the main stator winding. The starting capacitor is connected in series with the centrifugal switch, while the running capacitor is NOT. The starting capacitor optimizes starting-torque during the starting period, while the running capacitor optimizes the motor's current flow leading to better energy efficiency when operating at running speed.

When the motor initially starts with the centrifugal switch in the closed position, both capacitors are functional. Once the motor gets to about 70%-to-80% of normal operating speed, the switch disconnects the starting capacitor. Since the run capacitor is NOT wired in series with the switch, the run capacitor remains functional to optimize the motor's performance. This design provides optimum levels of both starting-torque and efficient running characteristics. The motor will have starting problems if the centrifugal switch sticks open or closed which can be relatively common in dirty environments. If the switch sticks closed, it will cause the starting winding to burn out of the motor. If the switch sticks in the open position, the motor will NOT start the next time, while sitting there making a humming sound.