Electric Motors: Types, Performance, Standards and Selection: Three-Phase Motors

Failure trigger and engineering context

The two most common types of electric motors used in engineering are:

• Three-phase squirrel cage motors — the workhorses of industrial applications
• Single-phase squirrel cage motors — used where three-phase supply is unavailable (domestic, commercial, light industrial)

Most engineering designs require a three-phase motor. But single-phase motor data has been included in this guide because these motors are frequently required for domestic and commercial applications where three-phase supply is not readily available.

Three-Phase Motors: The Industrial Standard

The standard industrial three-phase motor is the Totally Enclosed Fan Cooled (TEFC) type with:

• Protection designation: IP55 or higher (dust-tight and protected against water jets from any direction)
• Insulation class: F (rated for 155°C maximum winding temperature)
• Power ratings: 0.18 kW to 315 kW (and beyond for special orders)
• Frame sizes: 63 to 355 (and larger)

What is "frame size"? The frame size of a motor is the distance in millimetres between the base of the motor feet to the centreline of the rotor shaft. This is a universal designation used by all electric motor manufacturers worldwide. As the frame size increases, the motor power increases.

The range of three-phase motor types includes:

• Totally enclosed fan cooled (standard off-the-shelf)
• Dust ignition proof
• Non sparking
• Flameproof
• Two-speed
• Brake motors
• Geared motors
• Slip ring motors

For standard applications, the totally enclosed fan cooled type with IP55 protection is the standard off-the-shelf offering. Higher protection ratings can be made to order.

The Four Synchronous Speeds

The speed of an AC motor depends on the number of magnetic poles in the stator winding and the supply frequency. At 50 Hz supply, four synchronous speeds are available:

The formula:

$$N_s = \frac{120 \times f}{p}$$

Where:

• N_s = synchronous speed in rev/min
• f = supply frequency in Hz (50 Hz or 60 Hz depending on region)
• p = number of poles

Critical note for global readers: If your supply frequency is 60 Hz (as in the Americas, parts of Asia, and others), the synchronous speeds become 3600, 1800, 1200, and 900 rev/min respectively. Always verify your local supply frequency before selecting a motor.

Understanding Speed Under Load

Under no-load conditions, the actual motor speed is approximately equal to the synchronous speed. At full load, the speed drops slightly below synchronous speed—this difference is called slip.

The speed-load relationship is very close to linear, which means you can use simple linear interpolation for intermediate loads and speeds:

$$N_{design} = N_{sync} - \frac{P_{design}}{P_{full}} \times (N_{sync} - N_{full\ load})$$

Where:

• N_design = speed at your design load (rev/min)
• N_sync = synchronous speed (rev/min)
• P_design = your required design power (kW)
• P_full = motor full load (maximum) power (kW)
• N_full load = speed at full load from data tables (rev/min)

Selection Method — Three-Phase Motors (7-Step Process)

Step 1: Determine the mechanical data

Establish the required torque, power, and speed for your application.

Step 2: Choose a motor from the performance data tables

For the appropriate synchronous speed, select a motor that has a torque/power output at least equal to that required.

Important distinction: The manufacturer's catalogue refers to "full load" which is actually the maximum continuous load of the motor. The full load requirement of the driven machine will often be less than the maximum motor load. To avoid confusion, the full load requirement of the driven machine is called the "design load" (design power).

Step 3: Determine motor speed at design load

If the design load is less than full load, use the performance data tables to find the speed at full load. Then use linear interpolation:

$$N_{design} = N_{sync} - \frac{P_{design}}{P_{full}} \times (N_{sync} - N_{full\ load})$$

Step 4: Check the overhung (radial) load

If there is a gear, pulley, chain-wheel, flywheel, or other mechanism directly attached to the motor shaft, calculate the overhung load to ensure it does not exceed the allowable value.

The overhung load formula:

$$F = \frac{2fT}{d} = \frac{60fP}{\pi dN}$$

Where:

• F = overhung load in N
• T = motor torque in Nm (design, not maximum or full load torque)
• P = motor power in W (design, not maximum or full load power)
• d = PCD of the pulley, sprocket, or gear in m
• N = speed at design load in rev/min
• f = drive application factor:

Why does the drive type matter? Different drive mechanisms create different radial load patterns. A vee belt wraps tightly around the pulley, creating tension on both sides that adds up to a higher radial force. A chain drive transmits force more directly. The application factor accounts for these differences.

Step 5: Check the axial (thrust) load

If there is a mechanism directly attached to the motor shaft that causes a thrust (axial) load only, calculate it to ensure it does not exceed the allowable value from Table 1 (see below). If the allowable thrust load is exceeded, the bearing life will be reduced. To avoid this, choose a larger motor.

Alternative approach: Use an intermediate shaft (layshaft) with its own bearings coupled to the motor with a flexible coupling. This prevents transmission of the overhung load to the motor and eliminates the need to increase pulley or motor sizes.

Step 6: Check combined radial AND axial loads

If there is a mechanism directly attached to the motor shaft that causes both radial and thrust loads simultaneously, the allowable thrust load must be reduced below the value in Table 1. Use Figure 1 (the combined radial and axial load capacity graph) to determine the reduction.

How to use the combined load graph:

For example, if a frame 71 motor has a radial load acting on it equal to the maximum value allowed, then the allowable thrust load is the Table 1 value multiplied by 0.68. If the radial load is 50% of the maximum allowed, then the allowable thrust load is the Table 1 value multiplied by 0.84.

Step 7: Obtain performance data and dimensions

From the relevant performance data tables and dimension tables, extract all the values you need for your specification.

The Data Tables: Your Motor Selection Bible

Performance Notes Before You Read the Tables

• Precise motor performance varies with each individual motor and can only be ascertained by performance testing
• A squirrel cage motor self-adjusts to the power and torque required by the load—as load increases, current draw increases. Do not overload the motor as the extra current draw can cause overheating and eventual failure
• Do not significantly oversize the motor either—an oversized motor will cost more, take up more space, and operate at lower efficiency
• Efficiency is listed at three load points: full load (1.00 FL), 75% full load (0.75 FL), and 50% full load (0.5 FL). Linear interpolation is sufficiently accurate for intermediate values

Three-Phase Motor Performance Data — 2 Pole, 3000 RPM Synchronous Speed (415 V, 50 Hz)

Three-Phase Motor Performance Data — 4 Pole, 1500 RPM Synchronous Speed (415 V, 50 Hz)

Three-Phase Motor Performance Data — 6 Pole, 1000 RPM Synchronous Speed (415 V, 50 Hz)

Three-Phase Motor Performance Data — 8 Pole, 750 RPM Synchronous Speed (415 V, 50 Hz)

Table 1: Maximum Shaft Loads (Based on 30,000 Hours Bearing Life)

This table is critical for checking whether your overhung (radial) load and/or thrust (axial) load will exceed the motor bearing capacity.

Key note: Thrust loads are based on the assumption that the thrust is acting toward the motor. No data was available for thrust in the opposite direction (away from the motor).

The Problem

A three-phase electric motor is to provide a design power of 12 kW at approximately 1450 rev/min. The motor shaft will have a wedge belt pulley of pitch circle diameter 100 mm directly attached to it. The maximum bore of the pulley is 42 mm.

Select a suitable foot-mounted motor and complete the specification table.

Step-by-Step Solution

Step 1: Design power = 12 kW at approximately 1450 rev/min

Step 2: Synchronous speed = 1500 rev/min → therefore a 4-pole motor is needed

From the 4-pole performance data table, choose a motor with a maximum power output (full load) of at least 12 kW.

Selected motor: F160LO4 (Frame size 160, 4 pole)

• Maximum power output (full load): 15 kW
• Full load speed: 1455 rev/min

Why not choose a motor rated at exactly 12 kW? Because the "full load" is the motor's absolute maximum continuous rating. Your design load should always be less than or equal to the motor's full load. A 15 kW motor running at 12 kW (80% of full load) gives you a safety margin and better bearing life.

Step 3: Calculate speed at design load

No load speed = 1500 rev/min (synchronous) Full load (15 kW) speed = 1455 rev/min

By linear interpolation, speed at design load (12 kW):

$$N_{design} = 1500 - \frac{12}{15} \times (1500 - 1455) = 1500 - 0.8 \times 45 = 1464 \text{ rev/min}$$

Step 4: Check the overhung load

Using the overhung load formula:

$$F = \frac{60 \times f \times P}{\pi \times d \times N}$$

Where:

• f = 1.5 (wedge belt pulley application factor)
• P = 12,000 W (design power)
• d = 0.1 m (100 mm pulley PCD)
• N = 1464 rev/min

$$F = \frac{60 \times 1.5 \times 12000}{\pi \times 0.1 \times 1464} = \frac{1,080,000}{460.1} = 2348 \text{ N}$$

From Table 1, the maximum radial load for a frame 160, 4-pole motor is 2800 N.

2348 N < 2800 N → The overhung load is OK ✅

Step 5: No axial load in this application ✅

Step 6: Calculate torque at design power

Using the power-torque relationship:

$$P = T \omega = T \times \frac{\pi \times N}{30}$$

$$12000 = T \times \frac{\pi \times 1464}{30}$$

$$T = \frac{12000 \times 30}{\pi \times 1464} = 78.3 \text{ Nm}$$

Quick approximation: T ≈ (P_design / P_full) × 9550 × (P_full / N_full) = (12/15) × (15 × 1000 / 1455) × 9.55 ≈ 78.4 Nm. Close enough for estimation purposes.

Step 7: Determine efficiency at design load

The design load is 80% (12/15) of full load power. From the performance table:

• Efficiency at full load = 88%
• Efficiency at 75% load = 87%

By linear interpolation, efficiency at 80% load ≈ 87.2%