Motors 102 — How to Size an AC Motor

Servo and stepper sizing starts from torque and speed. AC induction motors are sized primarily by horsepower (HP)—the mechanical power required to do work. The goal is to match motor HP, speed, and torque capacity to the driven load’s demand.

    Key idea: In AC motor sizing, you start with mechanical power and load type rather than motion profile. 
    Once HP is known, you confirm speed (RPM), duty cycle, and starting torque.
  

Step 1 — Define the Load

1. Load Type

Constant torque: conveyors, mixers, positive displacement pumps.

Variable torque: centrifugal fans, blowers, pumps (torque ∝ speed²).

Constant power: machine tools, winders, grinders (torque decreases with speed).

2. Load Inertia & Start-Stop Requirements

AC motors don’t like rapid reversals or high-inertia starts. Identify load inertia and whether you’ll use soft-start or VFD ramp control.

3. Duty Cycle

Continuous → select nameplate HP for continuous load.
Intermittent → higher HP may not be required (motor can cool between runs).
Frequent start/stop → consider NEMA design letter and service factor.

Step 2 — Determine Required Horsepower

The fundamental sizing relationship:

Horsepower (HP) = (Torque × Speed) / 5252
Torque (lb·ft) = (HP × 5252) / RPM

Example: A conveyor requiring 45 lb·ft at 1750 RPM → (45 × 1750)/5252 ≈ 15 HP.

Step 3 — Select Motor Speed (Poles & Synchronous RPM)

Poles	Synchronous Speed (60 Hz)	Typical Full-Load Speed	Notes
2-pole	3600 RPM	3400 – 3500 RPM	High speed, low torque
4-pole	1800 RPM	1725 – 1750 RPM	Common industrial speed
6-pole	1200 RPM	1150 – 1175 RPM	Higher torque, quieter
8-pole	900 RPM	850 – 875 RPM	Low speed, high torque

Select motor poles to match the required shaft speed—use gearing, pulleys, or VFD if exact speed control is needed.

Step 4 — Decide Control Method

Across-the-Line Start

Simple, full-voltage connection. High inrush current (~6–8× rated). Use only if fixed speed and utility limits allow.

Soft-Start

Reduces inrush current; gentle acceleration. Common in pumps and compressors.

Variable Frequency Drive (VFD)

Varies frequency and voltage to control speed/torque. Enables energy savings, smooth acceleration, and closed-loop vector control if paired with encoder.

Step 5 — Consider Environment & Mounting

Enclosure Type

ODP (open drip-proof), TEFC (totally enclosed fan cooled), TENV (non-vented), Explosion-Proof, Washdown, etc.

Mounting

Foot-mount (B3), C-face (B14), D-flange (B5), or custom OEM. Confirm shaft height and frame (NEMA 56–449 or IEC 63–315).

Step 6 — Check Service Factor & NEMA Design

Service Factor (SF): extra capacity built in—e.g., 1.15 SF means the motor can handle 15% overload without overheating.

NEMA Design	Starting Torque	Slip	Applications
A	Low	Low	Fans, pumps (low start torque)
B	Normal	Normal	General-purpose industrial (most common)
C	High	Moderate	Compressors, conveyors (high inertia)
D	Very High	High	Punch presses, crushers (high peak torque)

Teaching Summary

    Servo/Stepper: size from required torque & speed (dynamic motion).
AC Induction: size from required power (HP) & continuous torque.
Drive type: across-the-line → simple; VFD → variable speed/energy savings.
Key checks: HP, RPM, load type, duty cycle, enclosure, service factor.

  

Rule of thumb: for constant torque applications, choose the smallest HP motor that delivers required torque at full load with 1.15 service factor. Validate current draw and thermal rise under your duty cycle.