Electric Motors: Types, Performance, Standards and Selection: Torque and Current Definitions

Torque and Current Definitions: The Language Every Engineer Must Speak

Before you can select, specify, or troubleshoot any electric motor, you need absolute clarity on what the torque and current terms mean. These definitions are NEMA standards — not opinions, not approximations.

Locked-Rotor (Static) Torque

The minimum torque the motor will develop at rest, for all angular positions of the rotor, with rated voltage applied at rated frequency.

This is your startup muscle. If the locked-rotor torque is less than the breakaway torque of your load, the motor will not start. Period.

Breakdown Torque

The maximum torque the motor will develop with rated voltage applied at rated frequency, without an abrupt drop in speed.

Think of this as the motor's ceiling. Push past it and the motor stalls — speed collapses, current spikes, and windings overheat.

Pull-Up Torque

The minimum torque developed by an AC motor during acceleration from rest to the speed at which breakdown torque occurs. For motors without a definite breakdown torque, it is the minimum torque developed up to rated speed.

NEMA pull-up torque rules for Designs A and B:

For Design C motors, pull-up torque is not less than 70% of locked-rotor torque.

Locked-Rotor Current

The steady-state current taken from the line with the rotor locked and with rated voltage (and rated frequency for AC motors) applied.

the practitioner's Lesson: The motor that burned out on his conveyor had adequate horsepower but inadequate locked-rotor torque for a loaded start. If he had checked the torque curve instead of just the nameplate HP, three motors would still be running.

Standard Direction of Motor Rotation

The standard direction of rotation for all non-reversing DC motors, all single-phase AC motors, all synchronous motors, and all universal motors is counterclockwise when facing the end of the motor opposite the drive.

This rule does not apply to two- and three-phase induction motors, because in most applications the phase sequence of the power lines is rarely known in advance.

NEMA Reference Tables for Squirrel-Cage Induction Motors

Table 1: NEMA Standard Locked-Rotor Current — 3-Phase, 60 Hz, 230 V

Note: For motors designed for voltages other than 230 V, the locked-rotor current is inversely proportional to the voltage. For motors larger than 200 hp, refer to NEMA Standard MG 1-12.34.

Table 2: NEMA Standard Breakdown Torque — Wound-Rotor Motors (60 and 50 Hz)

All values are percent of full-load torque. These represent the upper limit of the range of application.

Motor Types in the supplied reference: Five Classifications

Before diving into specific motor types, you need to understand how motors behave under load from a speed perspective. NEMA classifies motors into five categories:

. Constant-Speed Motors

Normal operating speed is constant or practically constant regardless of load.

Examples: Synchronous motors, induction motors with small slip, DC shunt-wound motors.

. Varying-Speed Motors

Speed varies with load — ordinarily decreasing as load increases.

Examples: Series-wound motors, repulsion motors.

. Adjustable-Speed Motors

Speed can be varied gradually over a considerable range, but once adjusted, remains practically unaffected by load.

Examples: DC shunt-wound motor with field resistance control.

Key Definition: The base speed of an adjustable-speed motor is the lowest rated speed obtained at rated load and rated voltage at the temperature rise specified in the rating.

. Adjustable Varying-Speed Motors

Speed can be adjusted gradually, but once adjusted for a given load, it will vary considerably with changes in load.

Examples: DC compound-wound motor adjusted by field control, wound-rotor induction motor with rheostatic speed control.

. Multispeed Motors

Can be operated at any one of two or more definite speeds, each practically independent of load.

Examples: DC motor with two armature windings, induction motor with windings capable of various pole groupings. Exception: in multispeed permanent-split capacitor and shaded-pole motors, speeds are dependent on load.

Direct-Current Motors: Three Classes, Distinct Personalities

DC motors divide into three fundamental classes based on winding configuration: series-wound, shunt-wound, and compound-wound. Each has dramatically different torque-speed characteristics, and choosing the wrong one can be catastrophic.

Series-Wound DC Motors

How it works: The field windings (in the stator frame) and the armature windings (on the rotor) are connected in series — all current passing through the armature also passes through the field.

Behavior under load:

• Increasing load → more current through armature and field → field strengthens → speed decreases
• Decreasing load → less current → field weakens → speed increases
• Very light loads → speed may become dangerously excessive

Critical safety rule: Series-wound DC motors must be directly connected or geared to the load to prevent runaway. A belt-driven series motor that throws its belt can accelerate to destruction.

A variant called the series-shunt wound motor includes a light shunt field winding specifically to prevent dangerously high speeds at light loads.

Characteristics (1–300 hp):

Shunt-Wound DC Motors

How it works: Both armature and field windings are connected across the main power supply — armature and field currents are separate.

Three configurations exist:

Shunt-Wound, Constant Speed:

Shunt-Wound, Adjustable Speed:

Shunt-Wound, Adjustable Voltage Control:

The Adjustable Voltage Drive Advantage:

When controlled by an adjustable voltage supply, the shunt motor operates at constant torque up to base speed and constant horsepower above base speed. Speed ranges of at least 20:1 below base speed and 4–5:1 above base speed yield a total range of 100:1 or more.

With special electronic controls, speed maintenance of ½ to 1% variation from full load to no load is achievable — even over a line voltage variation of ±10% and normal variations in motor and ambient temperature.

Compound-Wound DC Motors

How it works: Both series and shunt field windings are provided. These can be connected in two ways:

• Cumulative compounding — currents in both windings flow in the same direction (standard configuration)
• Differential compounding — currents flow in opposite directions (limited to special applications)

Characteristics:

Design Insight: The compound-wound motor gives you greater starting torque than a shunt-wound motor and better overload capacity, but with a narrower adjustable speed range. It is the compromise between the brute starting force of the series motor and the speed stability of the shunt motor.

Polyphase Alternating-Current Motors: The Industrial Workhorses

The vast majority of industrial drives in the world are powered by polyphase AC motors. Understanding the distinctions between types is not optional — it is the difference between a motor that runs for decades and one that burns out in weeks.

The Squirrel-Cage Induction Motor

Construction: A wound stator connected to external AC power, and a laminated steel core rotor with heavy aluminum or copper conductor bars set into the core around its periphery and parallel to its axis. These bars are connected at each end by a heavy ring, forming closed paths for induced currents. The bars and rings form the characteristic "squirrel cage."

Operating Principle: The motor operates by transfer action — analogous to a transformer with a short-circuited secondary that is free to rotate. The rotor is not connected to the power supply.

General characteristics: Simple design, rugged construction, essentially constant-speed operation (speed changes very little with load and is not subject to adjustment).

NEMA Design Letters: A, B, C, and D

This is where the practitioner's conveyor story becomes a permanent lesson. The design letter on a squirrel-cage motor tells you exactly what torque and current characteristics it has:

Design A

• Locked-rotor torque: As shown in NEMA Table 2
• Breakdown torque: As shown in NEMA Table 3
• Locked-rotor current: Higher than Table 1 values
• Slip at rated load: Less than 5%
• Status: Not commonly used — Design B offers similar characteristics with lower starting current

Design B — The General-Purpose Workhorse

• Locked-rotor torque: As shown in NEMA Table 2
• Breakdown torque: As shown in NEMA Table 3
• Locked-rotor current: Not exceeding Table 1 values
• Slip at rated load: Less than 5%
• Starting torque at 1800 RPM: 250–275% of full-load for 3 hp and below; 185–150% for 5–75 hp
• Starting current: Usually no more than 5–6× full-load current
• Starting method: Full voltage

Applications: Fans, blowers, rotary compressors, centrifugal pumps, woodworking machines, machine tools, line shafts, drill presses, grinders, hammer mills, lathes, planers, polishers, saws, screw machines, shakers, stokers.

Design C — The High-Starting-Torque Specialist

• Locked-rotor torque: Up to values in NEMA Table 2 (high-torque applications)
• Breakdown torque: Up to values in NEMA Table 3
• Locked-rotor current: Not exceeding Table 1 values
• Slip at rated load: Less than 5%
• Starting torque: Up to 250% of full-load torque with low starting current

Applications: Compressors requiring loaded start, heavy conveyors, reciprocating pumps, crushers, pulverizers, agitators — anywhere high starting torque is needed at infrequent intervals.

Design D — The Heavy-Duty, High-Slip Powerhouse

• Locked-rotor torque: As indicated in NEMA Table 2
• Locked-rotor current: Not greater than Table 1 values
• Slip at rated load: 5% or more (speed drops appreciably as load increases)
• Starting torque: Up to 275% of full-load torque
• Noise: Quiet operation
• Starting current: Relatively low

Applications: Impact and shock loads, high-peak loads, flywheel drives — trains, elevators, hoists, punch and drawing presses, shears, die stamping, bulldozers, balers.

The the practitioner Rule: Design D is what his conveyor needed from Day 1. Its high slip characteristic allows flywheel energy storage, and its 275% starting torque handles loaded starts without flinching.

Design F (Historical)

No longer standard. These had low starting torque (about 125% of full-load) and low starting current. They were used for machines requiring infrequent starting at no load or very light load.

Quick-Reference: Polyphase AC Motor Comparison

Multiple-Speed Induction Motors

These motors contain multiple stator windings arranged and connected so the number of effective poles — and therefore the speed — can be changed.

Available configurations:

• Constant horsepower at all rated speeds
• Constant torque at all rated speeds

Typical speed combinations (60 Hz):

Where gradual speed change is needed between the fixed steps, a wound rotor may be provided in addition to the multiple stator windings.

Wound-Rotor Induction Motors

Construction: In addition to a squirrel cage, the rotor contains a series of coils connected through slip rings to external variable resistors. By varying rotor circuit resistance, current flow and motor speed are controlled.

Designed for:

• Extremely low starting current with high starting torque — blowers, conveyors, compressors, fans, pumps
• Adjustable varying-speed service down to 50% of synchronous speed — steel plate-forming rolls, printing presses, cranes, blowers, stokers, lathes, milling machines
• Reversing service — cranes, gates, hoists, elevators

Speed regulation:

High-Frequency Induction Motors

Used in conjunction with frequency changers when very high speeds are required.

Applications: Grinders, drills, routers, portable tools, woodworking machinery.

Key advantage over series-wound/universal high-speed motors: Operates at a relatively constant speed over the entire load range.

Frequency supply options:

• Motor-generator set
• Two-unit frequency converter
• Single-unit inductor frequency converter (self-driven, self-excited from general polyphase power supply)

Available frequency range: 360 to 2,160 cycles per second from single-unit converters.

Synchronous Motors

Operating principle: Unlike induction motors, the synchronous motor's rotor is connected to a DC supply that provides a field rotating in step with the AC stator field. After reaching synchronous speed, the motor operates at this constant speed throughout its entire load range.

Synchronous speed is governed by:

$$N_{sync} = \frac{120 \times f}{P}$$

Where:

• $N_{sync}$ = Synchronous speed (RPM)
• $f$ = Supply frequency (Hz)
• $P$ = Number of poles

Applications:

• Electric timing devices
• Machines that must operate in synchronism
• Compressors, rolling mills, crushers (started without load)
• Paper mill screens, shredders, vacuum pumps
• Motor-generator sets
• Power factor correction — synchronous motors have an inherently high power factor and are often used to correct the low power factor of other motors on the same system

Torque characteristics:

Single-Phase Alternating-Current Motors: When Polyphase Is Not Available

Most single-phase AC motors are fundamentally induction motors distinguished by their starting method. A critical fact: a single-phase induction motor with only a squirrel-cage rotor has no starting torque. Every single-phase motor type below solves this starting problem differently.

Capacitor-Start Motor

How it works: An auxiliary stator winding is connected in series with a capacitor and a centrifugal switch. During starting and accelerating, the motor operates as a two-phase induction motor. At approximately two-thirds full-load speed, the centrifugal switch disconnects the auxiliary circuit, and the motor runs as a single-phase induction motor.

Available in:

• Normal starting torque — for centrifugal pumps, fans, blowers
• High starting torque — for reciprocating compressors, pumps, loaded conveyors or belts

Capacitor-Start, Capacitor-Run Motor

The auxiliary circuit provides high effective capacity for high starting torque and remains connected during running with reduced capacity.

Key advantage: Exceptionally quiet operation when loaded to at least 50% of capacity.

Available in:

• Low-torque designs for fans and centrifugal pumps
• High-torque designs for applications similar to capacitor-start motors

Single-Value Capacitor (Capacitor Split-Phase) Motor

A relatively small, continuously-rated capacitor is permanently connected in one of the two stator windings. The motor both starts and runs as a two-phase motor.

Advantages:

• Least maintenance of all single-phase motors
• High maximum torque (useful for floor sanders, grinders with momentary overloads)

Disadvantage: Very low starting torque.

Applications: Slow-speed direct-connected fans, grinders, floor sanders.

Repulsion-Start, Induction-Run Motor

A drum-wound rotor circuit connects to a commutator with short-circuited brushes set at an angle to the stator field. At about two-thirds full-load speed, brushes lift, commutator short-circuits, and the motor runs as a single-phase squirrel-cage motor.

Advantages: Higher starting torque than capacitor motors.

Disadvantages: Electrical and mechanical noise; extra maintenance sometimes required.

Applications: Compressors, conveyors, stokers starting under full load.

Repulsion-Induction Motor

Has an outer repulsion winding and an inner squirrel-cage winding on the rotor. As speed increases, induced current partially shifts from the repulsion winding to the squirrel-cage winding.

Advantages:

• Relatively high starting torque with low starting current
• Smooth speed-torque curve with no break
• Greater ability to withstand long accelerating periods than capacitor types

Disadvantage: Continuous brush noise.

Applications: Severe starting and accelerating duty, high-inertia loads such as laundry extractors.

Repulsion Motor

Has no limiting synchronous speed — speed changes with load. At certain loads, slight load changes cause wide speed changes. A brush-shifting arrangement provides adjustable speed with a range of up to 4:1 at full rated constant torque.

Reversing: Accomplished by shifting brushes beyond the neutral point.

Applications: Machines requiring constant torque and adjustable speed.

Split-Phase Motor

An auxiliary stator winding is used for starting, with either a resistance in series with the auxiliary winding (resistance-start) or a reactor in series with the main winding (reactor-start).

Universal Motor (Series-Wound Single-Phase)

Has a rotor winding in series with the stator winding, identical in principle to a series-wound DC motor. Because it operates on both AC and DC, it is called a "universal" motor.

Rating limit: Split-phase and universal motors are limited to approximately ⅓ hp and are used chiefly for small appliance and office machine applications.

Single-Phase Motor Summary

Motors with Built-In Speed Reducers

When compact packaging matters, electric motors with integrated speed-changing units eliminate the need for external gearboxes.

Worm Gearing Type

• Quiet operation
• Ideal where the slow-speed shaft must be at right angles to the motor shaft
• Best for high speed ratios
• Double reduction worm gearing available for very low speeds (both slow-speed and armature shafts become parallel)

Parallel-Shaft Type

• Slow-speed shaft is parallel with the armature shaft
• A pinion on the armature shaft meshes with a larger gear on the slow-speed shaft
• Available with constant-mesh change gears for varying the speed ratio

Planetary Gearing Type

• Permits large speed reduction with few parts
• Economy and compactness
• Slow-speed shaft is in line with the armature shaft

Factors Governing Motor Selection: The Engineering Decision Framework

Selecting a motor is not a catalog exercise — it is a systems engineering decision. Here are the variables you must evaluate, in order.

. Speed Requirements

Speed regulation (variation from no load to full load) is greatest with series field winding motors and entirely absent with synchronous motors.

. Horsepower Sizing

When the load is not constant but follows a defined cycle:

• Plot a horsepower-time curve to determine peak horsepower
• Calculate the root-mean-square average horsepower — this indicates the proper motor rating from a heating standpoint
• For constant loads sustained 15 minutes to 2 hours (depending on motor size), the HP rating will usually not be less than the constant load value

Critical rule for induction motors: They operate at maximum efficiency when loaded to full capacity. Oversizing wastes energy; undersizing causes overheating.

When operating at several speeds, evaluate the horsepower requirement for each speed.

. Torque Matching

Starting torque requirements vary from 10% to 250% of full-load torque depending on the driven machine.

Variables affecting starting torque:

• Frequency of starts
• Temperature
• Type and amount of lubricant

The motor's torque curve must exceed the driven machine's torque demand at every point from zero to full speed. The greater the excess torque, the faster the acceleration.

Acceleration Time Formula

$$Time = \frac{N \times WR^2}{T_a \times 308} \quad \text{(seconds)}$$

Where:

• $N$ = Full-load speed (RPM)
• $WR^2$ = Inertia of rotating parts (lb·ft²) — where $W$ = weight and $R$ = radius of gyration
• $T_a$ = Average torque available for acceleration (lb·ft)
• $308$ = Combined constant converting minutes to seconds, weight to mass, and radius to circumference

Warning: If acceleration time exceeds 20 seconds, special motors or starters may be required to prevent overheating.

Running Torque Formula

$$T_r = \frac{5250 \times HP}{N} \quad \text{(lb·ft)}$$

Where:

• $HP$ = Horsepower supplied to the driven machine
• $N$ = Running speed (RPM)
• $5250$ = Combined constant converting horsepower to foot-pounds per minute and work per revolution into torque

The peak horsepower determines the maximum torque required — the motor must have a maximum running torque in excess of this value.

. Inertia Considerations

The inertia (flywheel effect) of rotating parts in the driven machine will, if large, significantly affect accelerating time and therefore motor heating.

Special consideration for synchronous motors: The $WR^2$ of both the motor rotor and the rotating parts of the machine must be known. The pull-in torque varies approximately as the square root of the total inertia of motor and load.

. Space Limitations

When the motor must become an integral part of the driven machine:

Complete Motor: Stator + rotor + shaft + two end shields with bearings.

Partial Motor Options:

Important: When using a partial motor, proper ventilation, mounting, alignment, and bearings must be provided by the machine designer.

Frame Size Optimization: Sometimes a motor with a smaller frame size wound with Class B insulation (higher permissible temperature rise) can replace a larger-frame Class A insulated motor at the same horsepower rating.

. Temperature Limits

Motors are temperature-rated based on insulation class and frame type:

Insulation Materials

Class A: Cotton, silk, paper, and similar organic materials (impregnated or immersed in liquid dielectric); molded and laminated materials with cellulose filler and phenolic resins; cellulose acetate films and sheets; varnish (enamel) as applied to conductors.

Class B: Mica, glass fiber, asbestos, etc., with suitable bonding substances. Other materials demonstrated capable of Class B temperature operation.

Temperature Rise Ratings (Normal Ambient: 40°C / 104°F)

. Environmental and Enclosure Selection

When operating conditions are unusual, consult the motor manufacturer for any of the following exposures:

• Chemical fumes
• Damp locations
• Speeds exceeding specified overspeed
• Combustible or explosive dust
• Gritty or conducting dust
• Lint
• Steam
• Poorly ventilated rooms
• Pits or fully enclosed boxes
• Inflammable or explosive gases
• Temperatures below 10°C (50°F)
• Oil vapor
• Salt air
• Abnormal shock or vibration
• Excessive departure from rated voltage
• Unbalanced AC supply voltage

Enclosure Decision Guide

NEC Class I, Group D covers: gasoline, naphtha, alcohols, acetone, lacquer-solvent vapors, and natural gas.

Electric Motor Maintenance: The Schedule That Prevents the practitioner's Mistake

Frequency and thoroughness of inspection depend on four factors:

1. Importance of the motor in the production scheme
2. Percentage of operating days
3. Nature of service
4. Winding conditions

The following schedules cover both AC and DC motors and are based on average duty and environmental conditions.

Weekly Inspection

Monthly or Bimonthly Inspection