An AC motor is an electromechanical device that converts alternating current electrical energy into mechanical rotation using a rotating magnetic field. In a real circuit or installation, an AC motor changes electrical power (watts) into mechanical torque (lb-ft or Nm), but it also introduces inductive reactance, lowers the system power factor, and demands a massive inrush current upon startup that completely dictates your breaker and wire sizing.

While the basic physics of electromagnetic induction governs all of them, the physical construction of the rotor and stator varies wildly depending on the job. Choosing the wrong type means dealing with burnt-out windings, tripped breakers, or incompatible speed controls. Below, we break down the primary AC motors and types you will encounter on the jobsite or bench, how to size circuits for them, and where they actually end up.

The Core AC Motors and Types Compared

Not all alternating current motors are created equal. The NEMA MG-1 standard categorizes them based on their rotor construction and how they achieve synchronous speed. Here is the data-dense breakdown of the four main categories you need to know.

Motor Type Speed Regulation Starting Torque Typical Efficiency Primary Use Case
Squirrel Cage Induction (3-Phase) Excellent (Constant speed under load) High (150-200% of full load) 90% - 95.4% (NEMA Premium) Industrial compressors, CNC mills, large HVAC blowers
Split-Phase / PSC Induction (1-Phase) Good (Slight slip under heavy load) Low to Medium (75-150%) 75% - 85% Residential HVAC fans, garage door openers, drill presses
Synchronous (Reluctance / PM) Perfect (Locked to line frequency) Low (Requires soft-start or VFD) 92% - 97% Robotics, precise conveyors, large utility power factor correction
Universal (Series-Wound) Poor (Speed varies wildly with load) Very High (300%+) 60% - 75% Handheld power tools, vacuum cleaners, kitchen blenders
The 'Slip' Concept: In an induction motor, the stator's magnetic field rotates like a magnet dragged in a circle over a copper plate; the plate (rotor) chases the magnet but always lags slightly behind. That lag is called 'slip'. If the rotor ever caught up to the magnetic field (zero slip), induction would stop, and torque would drop to zero. Synchronous motors, by contrast, use permanent magnets or DC-excited rotors to lock perfectly to the AC frequency with zero slip.

Circuit Impact: Sizing and Inrush Calculation

Because AC motors are highly inductive loads, you cannot use standard Ohm's Law to size their circuits. The U.S. Department of Energy's Motor Systems guide and the National Electrical Code (NEC) Article 430 require specific multipliers to handle the massive magnetic field collapse and startup surges.

Let's run a worked numeric example for wiring a standard 5 HP, 230V, 3-phase squirrel cage induction motor (like a Baldor-Reliance EM3615T) in a workshop.

Step 1: Determine Full Load Current (FLC)

Do not use the nameplate amps for wire sizing; the NEC requires you to use the standardized tables to prevent undersizing if a motor is swapped later. According to NEC Table 430.250, the FLC for a 5 HP, 230V, 3-phase motor is 15.2 Amps.

Step 2: Size the Conductors

Motor branch circuit conductors must be sized at 125% of the FLC.

  • 15.2A × 1.25 = 19 Amps.
  • Looking at NEC Table 310.16 (75°C column for standard terminations), 12 AWG copper THHN is rated for 25A. However, for mechanical durability and to mitigate voltage drop on runs over 50 feet, most electricians will step up to 10 AWG copper THHN (rated 35A at 75°C).

Step 3: Size the Overload and Short-Circuit Protection

This is where people get confused and trip breakers constantly.

  • Overload Protection (Thermal): Sized at 115% to 125% of the nameplate amps (let's assume nameplate is 14.8A). 14.8 × 1.25 = 18.5A thermal overload setting.
  • Short-Circuit Breaker: To allow the motor to start without tripping the breaker during inrush, NEC 430.52 allows an inverse-time breaker sized up to 250% of the FLC. 15.2A × 2.5 = 38A. The next standard breaker size up is 40 Amps.
Locked Rotor Amps (LRA): When that 5 HP motor starts, it will momentarily draw roughly 6 times its FLC (approx. 91 Amps) for a fraction of a second. The 40A breaker's magnetic trip curve is designed to tolerate this brief 91A spike without opening, whereas a standard 20A lighting breaker would trip instantly.

Where You Meet AC Motors in Practice

Theory is great, but you will encounter these specific AC motor types in distinct real-world environments. Knowing what is inside the housing dictates how you troubleshoot and replace them.

HVAC and Air Handling

Older residential furnaces and AC condensers use Permanent Split Capacitor (PSC) single-phase induction motors. They are cheap, reliable, but inefficient at part-load. Modern high-SEER systems have largely replaced them with ECM (Electronically Commutated Motors). While ECMs are technically brushless DC (BLDC) motors internally, they are fed by AC mains and contain an internal inverter. If an ECM fails, you cannot just swap in a standard PSC motor; the control board logic is entirely different.

The Machine Shop

Lathes, knee mills, and heavy grinders rely almost exclusively on 3-phase squirrel cage induction motors. They are the workhorses of industry because they have no brushes to wear out and can run continuously at high loads. If your shop only has single-phase power, you will either need a bulky rotary phase converter or a Variable Frequency Drive (VFD) with a single-phase input and 3-phase output to run these machines.

Kitchen and Handheld Tools

Open a vacuum cleaner, a router, or a blender, and you will find a Universal motor. These are series-wound motors with carbon brushes and a commutator. They are technically capable of running on both AC and DC. They are chosen here because they can spin at 20,000+ RPM (induction motors are physically limited by line frequency to roughly 3600 RPM max) and deliver massive starting torque in a very lightweight package. The trade-off is that the brushes wear out, and they generate significant electrical noise (EMI).

Common Confusions and Selection Mistakes

When spec'ing or replacing AC motors, DIYers and junior technicians frequently fall into a few specific traps.

Confusion 1: VFDs on Single-Phase Motors

The Mistake: Buying a cheap Variable Frequency Drive (VFD) and wiring it to a standard 120V/240V single-phase split-phase or PSC motor to get speed control.
The Reality: Standard VFDs output a 3-phase PWM waveform. If you connect this to a single-phase motor with a centrifugal start switch or a run capacitor, you will likely fry the capacitor, weld the start switch shut, or destroy the VFD's IGBTs. VFDs are meant for 3-phase induction motors. For single-phase speed control, you need a dedicated triac-based fan speed controller or an ECM swap.

Confusion 2: Output Power vs. Input Power

The Mistake: Assuming a '1 HP' motor draws exactly 746 Watts from the wall.
The Reality: 1 Horsepower equals 746 Watts of mechanical output. Because no motor is 100% efficient, a standard 1 HP single-phase motor might draw 1,100 to 1,300 Watts of electrical input at full load. Always size your generator or inverter based on the input VA (Volt-Amps), factoring in the power factor, not just the mechanical HP rating.

Confusion 3: Universal vs. Induction Brushless

The Mistake: Confusing the high-RPM whine of a universal motor with a high-frequency induction setup.
The Reality: If the motor has carbon brushes, a commutator, and sparks slightly under heavy load, it is universal (or DC). If it is completely sealed, has no brushes, and hums at a lower pitch, it is induction. This distinction matters for maintenance: induction motors usually only require bearing replacements every 10 years, while universal motors require brush and commutator servicing much more frequently.