Motor capacitor

Lore & Background

Start capacitors lag the voltage to the rotor windings, creating a phase shift between field windings and rotor windings. Without the start capacitor, the north and south magnetic fields line up and the motor hums, only starting when physically turned. A start capacitor stays in the circuit long enough to rapidly bring the motor up to about 75% of full speed, then is taken out, often by a centrifugal switch. Start capacitors above 20 μF are always non-polarized aluminium electrolytic capacitors with non-solid electrolyte, suitable only for short motor starting times.

Reader's Guide

Run capacitors are designed for continuous duty while the motor is powered, so electrolytic capacitors are avoided and low-loss polymer capacitors are used, mostly polypropylene film capacitors (historically metallised paper). They are energized the entire time the motor is running. A wrong capacitance value causes an uneven magnetic field around the rotor, leading to hesitation, noise, increased energy consumption, performance drop, and overheating. Dual run capacitors support two motors (fan and compressor) in air conditioner units, saving space by combining two capacitors into one case with three terminals labeled C (common), FAN, and HERM. A 440-volt capacitor can replace a 370-volt, but not vice versa. Capacitance must remain within 5% of original value. Round cylinder-shaped dual run capacitors are common for air conditioning; an oval dual run capacitor could be used if the mounting strap fits. Older capacitors may be labeled with obsolete terms 'mfd' or 'MFD' for microfarads.

Did You Know?

• A faulty run capacitor often becomes swollen, with sides or ends bowed out, and may leak electrolyte.
• Motor capacitors manufactured before 1978 likely contain polychlorinated biphenyls (PCBs), which are extremely toxic.
• A start capacitor stays in the circuit until the motor reaches about 75% of full speed, then is removed by a centrifugal switch.
• A dual run capacitor has three terminals labeled C (common), FAN, and HERM (for hermetically-sealed compressor).

Fundamental Architecture & Magnetic Interaction

AC motors consist of two core components: an outer stator with coils fed by alternating current that generates a rotating magnetic field, and an inner rotor mounted on the output shaft that creates a second rotating field. The rotor's field can originate from permanent magnets, reluctance saliency, or electrical windings powered by either DC or AC. The interplay between these two rotating fields is what converts electrical energy into mechanical rotation. While the vast majority of AC motors are designed for rotary motion, a less common variant—the AC linear motor—applies the same underlying principles but arranges the stationary and moving elements in a straight line, yielding translational motion rather than circular rotation. This architectural duality highlights how the same electromagnetic logic can be reconfigured to serve fundamentally different mechanical needs, from spinning a shaft to driving a platform along a track.

The Slip Phenomenon & Speed Determination

Induction motors, also called asynchronous motors, depend on a small speed differential between the stator's rotating field and the rotor shaft—this gap is termed slip. Slip is what induces current in the rotor's AC winding, and without it, no torque can be generated near synchronous speed. In a squirrel-cage design, if the rotor matched the field speed exactly, the flux at any point on the rotor would remain constant, producing zero induced current. Consequently, these motors always operate slightly below synchronous speed, with standard units showing 2–3% slip under load and specialized designs reaching up to 7%. Even unloaded, internal mechanical losses keep slip from reaching zero. The synchronous speed itself is governed by a straightforward relationship: it equals 120 multiplied by the supply frequency in hertz, divided by the number of poles per phase winding. The constant 120 emerges from converting 60 seconds per minute and accounting for the two poles each phase requires.

A Century of Competitive Invention

The path to the modern AC motor was forged by a remarkable succession of inventors across Europe and America. The theoretical groundwork traces to Faraday and Henry's 1830–31 discovery that a changing magnetic field induces current, with Faraday typically receiving credit for publishing first. Hippolyte Pixii followed in 1832 with the first alternator—a revolving horseshoe magnet sweeping past wound coils. In 1879, Walter Baily demonstrated a battery-operated polyphase motor with a commutator to London's Physical Society, conceiving the rotating magnetic field. Marcel Deprez published a similar two-phase concept in 1880, though his design was never practically realized. Galileo Ferraris and Nikola Tesla independently developed commutatorless induction motors around 1885–1888, with Tesla receiving a U.S. patent that year. Mikhail Dolivo-Dobrovolsky then delivered the first three-phase induction motor in 1890, which became the standard prototype across Europe and the United States.

Synchronous Motors & the Broader Motor Family

While induction motors dominate many applications, synchronous motors represent the other principal category of AC machines. Unlike their asynchronous counterparts, synchronous motors do not depend on slip to induce rotor current. Instead, they employ permanent magnets, salient projecting poles, or an independently excited rotor winding to produce rated torque at precisely synchronous speed. A notable variant, the brushless wound-rotor doubly fed synchronous motor, features an independently excited winding that bypasses slip-induction entirely and can operate at the supply frequency or at sub- or super-multiples thereof. Beyond these two families, the AC motor landscape also includes eddy current motors and mechanically commutated machines—both AC and DC—whose speed varies with applied voltage and winding configuration. This diversity of topologies ensures that designers can match electromagnetic architecture to specific torque, speed, and control requirements across a wide range of applications.