6.2 Electrical Components: Capacitors, Inductors & Resistors

Capacitors: Storing Energy in an Electric Field

A capacitor is a passive component built from two conductive plates separated by an insulating material called a dielectric — air, ceramic, paper, plastic film, or an oxide layer, depending on the type. When a voltage is applied across the plates, opposite electric charges accumulate on each plate, and energy is stored in the electric field that forms in the dielectric between them. Unlike a resistor, a capacitor does not dissipate this energy as heat — it stores the energy and can release it back into the circuit later. The amount of charge a capacitor can store per volt applied is its capacitance, measured in farads (F). A farad is a large unit for practical purposes, so most capacitors encountered in the field are rated in microfarads (µF), one-millionth of a farad.

Motor Start and Run Capacitors

The capacitor a Registered Master Electrician meets most often in the field is on a single-phase induction motor. A single-phase motor has no rotating magnetic field on its own the way a three-phase motor does, so it needs an auxiliary winding whose current is shifted out of phase with the main winding to produce starting torque. A capacitor wired in series with this auxiliary (start) winding creates that phase shift:

• A start capacitor is a high-capacitance, electrolytic-type capacitor used only for the brief instant of starting. It is switched out of the circuit — typically by a centrifugal switch on the motor shaft or a current-sensing relay — once the motor reaches roughly 75% of full speed. Because it only operates for a second or two at a time, it can be built with a construction that would overheat under continuous duty.
• A run capacitor is a lower-capacitance, oil-filled or metallized-film capacitor left permanently in the circuit while the motor runs. It improves running torque, running efficiency, and power factor. A capacitor-start, capacitor-run motor uses both: a start capacitor for high starting torque that is switched out after start-up, and a separate run capacitor that stays in the circuit continuously.

Confusing these two capacitor types in the field is a common and costly mistake. Wiring a start capacitor into continuous duty typically causes it to overheat and fail, sometimes explosively on electrolytic types, because it was never designed for sustained current.

Power-Factor Correction and Filtering

Capacitors are also the standard tool for power-factor correction: connected in parallel with an inductive load such as a motor or a bank of fluorescent ballasts, a capacitor supplies leading reactive power that cancels part of the load's lagging reactive power, raising the overall power factor. This application, including how to size a correction bank, is covered in depth in Chapter 1. A third common application is filtering: in a direct current (DC) power supply, a capacitor connected across the output smooths out voltage ripple left over after rectification (covered in section 6.3), holding the voltage up between the peaks of the rectified waveform.

Inductors and Reactors: Storing Energy in a Magnetic Field

An inductor — often called a reactor in heavy electrical equipment — is, at its simplest, a coil of wire, sometimes wound around an iron or ferrite core to concentrate the magnetic field. When current flows through the coil, it establishes a magnetic field, and the energy of that current is stored in the field rather than dissipated as heat. Because a magnetic field cannot change instantaneously without an infinite voltage, an inductor opposes any change in the current flowing through it — the same inductive-reactance behavior introduced with alternating current (AC) impedance in Chapter 1, but now viewed as a physical, deliberately built component rather than a circuit-analysis quantity.

Two applications come up regularly in RME-level trade work:

• Current-limiting reactors — an inductor deliberately inserted in series with a motor or a feeder to add impedance and limit inrush or fault current. This serves a similar overall goal to the reduced-voltage starters in Chapter 5, but by a different mechanism: a series reactor adds impedance directly in the current path rather than reconfiguring the motor's winding connections.
• Motor windings and chokes — every motor stator winding is itself an inductor, which is why induction motors draw a lagging (inductive) current and require the power-factor correction discussed above. A choke is simply an inductor used to block or smooth high-frequency current, commonly seen on the input or output of variable-frequency drives to reduce harmonic distortion — a topic revisited in section 6.3.

Resistors: Opposing Current Flow

A resistor opposes the flow of current and, unlike a capacitor or inductor, dissipates the resulting energy as heat rather than storing and returning it to the circuit. Resistors relevant to trade practice fall into a few categories:

• Fixed resistors — a set resistance value that does not change, used for current-limiting, voltage-dividing, or bleeder functions in control and electronic circuits.
• Variable resistors (rheostats) — an adjustable resistor, historically used for functions like dimming or motor field-current adjustment; a rheostat is a two-terminal variable resistor, distinct from a three-terminal potentiometer used for signal-level adjustment.
• Resistive heating elements — a resistor purpose-built to dissipate as much heat as possible, using a high-resistance alloy such as nichrome wire, found in electric water heaters, space heaters, and industrial process heat.

Power (Wattage) Rating

Every resistor and heating element carries a power (wattage) rating — the maximum rate at which it can safely dissipate heat, governed by P = I² × R (or equivalently P = V²/R). Exceeding that rating, whether from an oversized current, a wiring error that increases current beyond design, or simply choosing an undersized resistor for the application, causes the component to run hotter than it was designed for. The result is accelerated insulation breakdown, discoloration, and eventual open-circuit failure — or, in a sealed or poorly ventilated enclosure, a fire hazard. Sizing a resistor or heating element by its wattage rating, not just its resistance value, is essential trade practice.

Comparing the Three Component Types

Key Takeaways

• A capacitor stores energy in an electric field and opposes a change in voltage, measured in farads (F)
• Start capacitors operate only during starting and are switched out; run capacitors stay in the circuit continuously
• An inductor/reactor stores energy in a magnetic field and opposes a change in current — every motor winding is itself an inductor
• A resistor opposes current flow and dissipates energy as heat; exceeding its power (wattage) rating causes overheating and failure
• Capacitors, inductors, and resistors are distinguished by what they store or oppose — voltage, current, and heat dissipation, respectively