Electric Motors: Types, Performance, Standards and Selection: Locked-Rotor Current

Locked-Rotor Current

The steady-state current drawn from the line with the rotor locked and rated voltage and frequency applied. This is the maximum current the motor draws during the initial instant of startup.

NEMA Standard Pull-Up Torque Requirements

Pull-up torque is the hidden danger in motor selection. You can have adequate locked-rotor torque to break the load free, and adequate breakdown torque to handle the running load, but if pull-up torque dips below the load requirement during acceleration, the motor stalls at an intermediate speed.

NEMA Standard pull-up torques for single-speed, polyphase, squirrel-cage integral-horsepower motors, Designs A and B, with continuous ratings and rated voltage and frequency applied:

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

What this means in practice: A 50 HP Design B motor at 1800 RPM has a locked-rotor torque of 140% of full-load torque. Since 140% falls between 110% and 145%, the pull-up torque must be at least 100% of full-load torque. If your load demands more than 100% of full-load torque at any speed during acceleration, this motor will not reach operating speed.

Standard Direction of Motor Rotation

The standard direction of rotation for the following motor types is counterclockwise when facing the end of the motor opposite the drive (i.e., facing the back of the motor):

• All non-reversing direct-current motors
• All single-phase alternating-current motors
• All synchronous motors
• All universal motors

Important exception: This rule does not apply to two- and three-phase induction motors, as in most applications the phase sequence of the power lines is rarely known. Reversing any two of the three supply leads reverses rotation.

Types and Characteristics of Electric Motors

Direct-Current Motors

DC motors are grouped into three general classes based on their winding configuration:

Series-Wound Motors

In the series-wound motor, the field windings and armature windings are connected in series — all current passing through the armature also passes through the field.

Key characteristics:

• Speed decreases as load increases (field strengthens with more current)
• Speed increases as load decreases — at very light loads, speed may become excessive and dangerous
• Must be directly connected or geared to the load to prevent "runaway"
• Extremely high starting torque — up to 300–350% of full-load torque
• Speed can be adjusted with a variable series resistance, but speed cannot be held constant under varying load
• Speed regulation becomes worse as speed is reduced
• Limited to traction and lifting service where load is constant

Typical applications: Cranes, hoists, gates, bridges, car dumpers — anywhere very high starting torque is required and the load provides natural braking.

Shunt-Wound Motors

In the shunt-wound motor, both the armature and field are connected across the main power supply — armature and field currents are separate.

Key characteristics:

• Field strength is not affected appreciably by load changes
• Delivers fairly constant speed (about 10–12% drop from no load to full load)
• Becomes an adjustable-speed motor through either field control or armature control

Field control: A variable resistance in the field circuit controls speed. Speed increases as field resistance increases (field weakens). Torque decreases proportionately, giving nearly constant horsepower. Speed range of 6:1 is possible, but 4:1 is more common. Speed regulation ranges from 15–22%.

Armature control: A variable resistance in the armature circuit reduces voltage applied to the armature. Speed range of about 2:1. Since field current remains unchanged, torque remains constant. Speed regulation degrades as speed is reduced — about 100% for a 2:1 speed range.

Adjustable-voltage drive: Combines an AC-to-DC conversion unit (motor-generator set or electronic rectifier) with a separately excited shunt motor. Speed ranges of 20:1 below base speed and 4–5:1 above base speed are achievable — a total range of 100:1 or more. With special electronic controls, speed variation can be held to ½ to 1% from full load to no load.

Typical applications: Centrifugal pumps, fans, blowers, conveyors, elevators, wood- and metalworking machines. The adjustable-voltage variant excels on planers, milling machines, boring mills, and lathes.

Compound-Wound Motors

Both series and shunt field windings are provided. They may be connected for cumulative compounding (currents in same direction) or differential compounding (currents in opposite directions).

Key characteristics:

• Speed variation from load changes is much less than series-wound but greater than shunt-wound (up to 25% from full load to no load)
• Greater starting torque than shunt-wound (130–260% of full-load torque)
• Withstands heavier overloads than shunt-wound
• Narrower adjustable speed range than shunt-wound
• Standard motors use cumulative compounding; differential is limited to special applications

Typical applications: Reciprocating pumps, printing presses, punch presses, shears, bending rolls, conveyors, crushers — anywhere the starting load is very heavy or the load changes suddenly and violently.

DC Motor Characteristics Summary

Polyphase Alternating-Current Motors

Squirrel-Cage Induction Motors

The most widely used industrial motor. Consists of a wound stator connected to the AC power supply and a laminated steel core rotor with heavy aluminum or copper conductors set around its periphery, connected at each end by a heavy ring — forming the characteristic "squirrel cage."

General characteristics:

• Simple design and construction
• Rugged and reliable service
• Essentially constant-speed — speed changes very little with load
• Speed is not subject to adjustment (except in multi-speed designs)
• Classified by NEMA into Designs A, B, C, and D (covered in detail above)

Wound-Rotor Induction Motors

This type adds a series of coils set into the rotor, connected through slip-rings to external variable resistors. By varying the resistance, the rotor current and hence the motor speed can be controlled.

Key characteristics:

• Extremely low starting current with high starting torque (up to 300%)
• Speed can be reduced to 50% of synchronous speed by rotor resistance
• Speed regulation: 5–10% at maximum speed, 18–30% at low speed
• Speed varies inversely with load when resistance is inserted
• Breakdown torque: 200% when slip rings are short-circuited

Typical applications: Blowers, conveyors, compressors, fans, pumps, steel plate-forming rolls, printing presses, cranes, lathes, milling machines. Also used for reversing service in cranes, gates, hoists, and elevators.

Multiple-Speed Induction Motors

Feature multiple stator windings arranged to change the number of effective poles and hence the speed. Available in constant horsepower or constant torque designs.

Typical speed combinations:

• 600, 900, 1200, and 1800 RPM
• 450, 600, 900, and 1200 RPM
• 600, 720, 900, and 1200 RPM

Where gradual speed change is also required, a wound rotor may be provided in addition to the multiple stator windings.

High-Frequency Induction Motors

Used with frequency changers for very high speeds — grinders, drills, routers, portable tools, and woodworking machinery. These motors operate at relatively constant speed over the entire load range, an advantage over series-wound high-speed motors.

Frequency converters can deliver three-phase power at frequencies from 360 to 2,160 cycles.

Synchronous Motors

The rotor is connected to a DC supply providing a field that rotates in step with the AC stator field. After reaching synchronous speed, the motor operates at this exact constant speed throughout its entire load range.

Key characteristics:

• Speed is governed by frequency of power supply and number of poles
• Inherently high power factor — often used to correct low power factor of other motors on the same system
• Starting torque: 40% for slow speed to 160% for medium speed designs
• Pull-out torque: 170% for unity power factor motors, 225% for 80% power factor motors (special designs up to 300%)
• Available from 25 HP to several thousand HP

Typical applications: Electric timing devices, machines requiring synchronous operation, compressors, rolling mills, crushers, paper mill screens, shredders, vacuum pumps, and motor-generator sets.

Polyphase AC Motor Characteristics Summary

Single-Phase Alternating-Current Motors

Used in sizes up to about 7½ HP for applications where polyphase power is not available. Most are basically induction motors distinguished by different starting arrangements.

Capacitor-Start Motor

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

• Normal starting torque designs: Centrifugal pumps, fans, blowers
• High starting torque designs: Reciprocating compressors, loaded conveyors

Capacitor-Start, Capacitor-Run Motor

The auxiliary circuit provides high effective capacity for starting and remains connected with reduced capacity during running. Exceptionally quiet in operation when loaded to at least 50% of capacity.

Capacitor Split-Phase Motor

A small continuously-rated capacitor is permanently connected in one stator winding. Requires the least maintenance of all single-phase motors but has very low starting torque. High maximum torque makes it useful for floor sanders and grinders where momentary overloads occur.

Repulsion-Start, Induction-Run Motor

Has higher starting torque than capacitor motors. Disadvantages include electrical and mechanical noise and extra maintenance requirements. Used for compressors, conveyors, and stokers starting under full load.

Repulsion-Induction Motor

Has relatively high starting torque, low starting current, and a smooth speed-torque curve. Particularly suitable for severe starting and accelerating duty and high-inertia loads such as laundry extractors. Brush noise is continuous.

Split-Phase and Universal Motors

Limited to about ⅓ HP ratings. Used chiefly for small appliance and office machine applications. The universal (series-wound single-phase) motor can also operate on DC.

Motor Types Classified in the supplied reference this classification helps you match the motor to the load characteristic — not just the horsepower requirement.

Base speed of an adjustable-speed motor is the lowest rated speed obtained at rated load, rated voltage, and the temperature rise specified in the rating.

Factors Governing Motor Selection

Selecting the right motor is a multi-variable problem. Horsepower alone tells you nothing about starting capability, speed range, or environmental suitability.

Speed, Horsepower, Torque, and Inertia Requirements

Speed Range in the supplied reference

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

Horsepower Determination

For loads that follow a definite cycle, a horsepower-time curve enables determination of both the peak horsepower and the root-mean-square (RMS) average horsepower, which indicates the proper motor rating from a heating standpoint.

Inertia (WR²) Considerations

The inertia of the rotating parts of a driven machine will, if large, appreciably affect accelerating time and hence the amount of heating in the motor. For synchronous motors, the inertia of both the motor rotor and the rotating parts of the machine must be known, since pull-in torque varies approximately as the square root of the total inertia.

The accelerating time can be calculated:

$$t = \frac{N \times WR^2}{T_a \times 308}$$

Where:

• $t$ = Accelerating time (seconds)
• $N$ = Speed (RPM)
• $WR^2$ = Total inertia of motor and load (lb·ft²)
• $T_a$ = Average accelerating torque (lb·ft)
• $308$ = Constant

And full-load torque in foot-pounds:

$$T_r = \frac{5252 \times HP}{N}$$

Space Limitations

When space is at a premium, a partial motor may be specified:

• Complete motor: Stator + rotor + shaft + two end shields with bearings
• Partial motor: Missing one or more of these elements

Common partial motor configurations:

• Without drive-end shield and bearing — directly connected to the machine (e.g., headstock of a lathe)
• Shaftless type — supplied without shaft, end shields, or bearings — for built-in application in multiple drilling machines, precision grinders, deep well pumps, compressors, and hoists where the rotor becomes part of the driven machine

Critical design note: When using a partial motor, proper ventilation, mounting, alignment, and bearings must be arranged by the designer of the machine to which it is applied.

Frame size reduction option: A motor with a smaller frame wound with Class B insulation can be subjected to a higher temperature rise than a larger-frame Class A insulated motor of the same horsepower rating — potentially saving critical space.

Temperature Ratings and Insulation Classes

The applicability of a motor is limited not only by its torque capability but also by the temperature it reaches under load.

Insulation Classes

Temperature Rise Ratings

Normal ambient temperature is taken to be 40°C (104°F).

Motors Exposed to Injurious Conditions

When motors must operate in hostile environments, the manufacturer should be consulted for any of the following conditions:

• Chemical fumes or acid/alkali fumes
• Damp locations or steam exposure
• Combustible or explosive dust (Class II hazardous locations)
• Gritty or conducting dust
• Lint exposure (textile mills, cotton processing)
• Inflammable or explosive gases (Class I hazardous locations)
• Temperatures below 10°C (50°F)
• Oil vapor or salt air exposure
• Abnormal shock or vibration from external sources
• Excessive departure from rated voltage
• Unbalanced AC supply voltage
• Operation in pits or where entirely enclosed in boxes
• Poorly ventilated rooms
• Speeds in excess of specified overspeed

Enclosure Selection Guide

Motors with Built-In Speed Reducers

When the driven machine requires a speed lower than standard motor speed, a built-in speed reducer eliminates separate gearboxes and coupling assemblies.

Types of Speed Reducers

Electric Motor Maintenance

A motor that runs perfectly on day one can fail catastrophically on day 365 without proper maintenance. The following inspection schedule — designed for average conditions of duty and dirt — protects your investment.

Weekly Inspection

Surroundings:

• Check for dripping water, acid or alcoholic fumes near windings
• Inspect for unusual amounts of dust, chips, or lint on or about the motor
• Verify that no boards, covers, or canvas interfere with ventilation or could jam moving parts

Lubrication (sleeve-bearing motors):

• Check oil level (if a gauge is used) and fill to the specified line
• For journals under 2 inches diameter, stop the motor before checking
• For special systems (wool-packed, forced lubrication, flood and disk), follow the instruction book
• Add oil only when the motor is at rest
• Check for oil creeping along the shaft toward windings — oil damages insulation

Mechanical condition:

• Listen for unusual noise indicating metal-to-metal contact
• Check for odor of scorching insulation varnish

Ball or roller bearings:

• Feel bearing housings for vibration
• Listen for unusual noise
• Inspect for grease creepage inside the motor

Commutators and brushes (DC motors):

• Check for sparking — observe through several cycles if the motor is on cyclic duty
• Note color and surface condition of the commutator
• A stable copper oxide-carbon film on the commutator (as distinguished from a pure copper surface) is essential for good commutation

the practitioner's Transformation: From Specification Failure to Standards Mastery

Six months after the conveyor disaster, the practitioner sat at his desk with the NEMA standards manual open beside his laptop. He was specifying motors for a new material handling system — this time at a cement plant with even higher stakes.

The primary crusher discharge conveyor required a loaded start. He immediately eliminated Design B from consideration. The bucket elevator needed to handle shock loads from irregular feed. Design D went on the short list. The kiln feed system required absolute speed precision for proper material flow. Synchronous motor, no question.

For each motor, the practitioner documented not just the horsepower but the design letter, locked-rotor torque requirement, breakdown torque margin, pull-up torque verification, starting current impact on the electrical system, insulation class for the ambient temperature, and enclosure type for the operating environment.

The commissioning went flawlessly. Every motor started on the first attempt. Every conveyor moved under full load. Every elevator handled surge conditions without tripping.

an illustrative engineering practitioner's firm three weeks later — not with a complaint, but with a referral.

That's what standards mastery looks like in practice.

Your Motor Selection Decision Framework

Use this systematic checklist every time you specify a motor:

Step 1 — Define the load:

• What is the required horsepower?
• What is the starting condition — unloaded, partially loaded, or fully loaded?
• Does the load involve impact, shock, or cyclic peaks?

Step 2 — Select the design letter:

• Unloaded or light starts → Design B
• Loaded starts with high breakaway torque → Design C
• Impact loads, flywheel applications → Design D
• General purpose with emphasis on low starting current → Design B

Step 3 — Verify the torque chain:

• Locked-rotor torque ≥ breakaway torque of the load?
• Pull-up torque ≥ load torque at all speeds during acceleration?
• Breakdown torque ≥ maximum expected running torque (with safety margin)?

Step 4 — Check the electrical impact:

• Locked-rotor current × starting method factor ≤ available supply capacity?
• Voltage drop during starting within acceptable limits for other equipment?

Step 5 — Address the environment:

• Ambient temperature → Insulation class (A or B)?
• Atmospheric contaminants → Enclosure type (open, TEFC, TENV, explosion-proof)?
• Moisture, chemical exposure → Special treatment or enclosure?

Step 6 — Confirm the physical fit:

• Frame size compatible with mounting arrangement?
• Shaft size and extension adequate for coupling?
• Conduit entry accessible?

Step 7 — Plan for maintenance:

• Bearing type and lubrication requirements documented?
• Spare parts identified?
• Inspection schedule established?

Engineering takeaway

Every table in this guide, every percentage, every design letter exists because someone — probably many someones — learned the hard way what happens when motors are specified by horsepower alone.

The NEMA standards transform motor selection from guesswork into engineering. They give you the data to calculate, the framework to decide, and the vocabulary to communicate precisely with manufacturers, contractors, and maintenance teams.

the practitioner's 200,000-unit mistake came down to a single oversight: he treated the motor as a commodity defined by one number (horsepower) when it's actually a precision machine defined by dozens of interdependent specifications.

You now have every specification, table, and decision framework you need to avoid that mistake — permanently.

What's Your Next Step?

If you're specifying a motor for a new installation, pull up the NEMA design letter comparison table and verify that your selected design letter matches the starting condition of your load — not just the running condition.

If you're troubleshooting a motor that trips on startup, compare the locked-rotor torque of your installed motor against the actual breakaway torque of the driven equipment. The answer is almost always in the design letter.

What's the most critical motor in your facility — and when was the last time someone verified that its NEMA design letter actually matches the load it's driving?

This guide is designed as a permanent reference. Bookmark it, print it, share it with your engineering team. The standards don't change with trends — they change with physics. And physics is forever.

The Complete Engineering Guide to Every Motor Type, Selection Factor, and Maintenance Schedule You Will Ever Need

The Conveyor That Ate a Quarter-Million Dollars

the practitioner had been a plant engineer for eleven years. He knew conveyors, he knew throughput targets, and he knew how to keep a production line moving. What he did not know — not until three ruined motors and 14 days of unplanned downtime proved it — was that choosing the wrong electric motor is one of the most expensive mistakes in industrial manufacturing.

The story started simply enough. A new aggregate conveyor needed a drive motor. The purchasing department found a "great deal" on a batch of general-purpose squirrel-cage induction motors — Design B, 50 horsepower, 1800 RPM. On paper, the numbers checked out. The horsepower matched, the voltage matched, the frame size fit the mounting plate.

But the conveyor was a drag-chain unit handling crushed limestone. It started under full load, every single cycle. The peak torque demand during startup hit 275% of full-load torque — well beyond what a Design B motor could deliver without stalling. Within three months, the first motor burned out. The replacement lasted six weeks. The third died in eleven days.

By the time the practitioner traced the root cause, the plant had lost over a quarter-million dollars in motor replacements, emergency rewinding, production downtime, and expedited shipping for the correct motors — Design D squirrel-cage units rated for exactly the high-slip, high-starting-torque service the conveyor demanded.

the practitioner's mistake was not ignorance. It was incomplete knowledge — the kind that comes from treating electric motor selection as a horsepower-matching exercise instead of a systems-engineering decision involving torque curves, speed regulation, insulation class, enclosure type, environmental exposure, and long-term maintenance strategy.

This guide exists so you never make that mistake.

What You Will Master in This Guide

By the time you reach the final section, you will understand:

• NEMA standards — what they govern, how they classify motors, and why they matter
• Every major motor type — DC series, shunt, and compound-wound; AC squirrel-cage Designs A through D; wound-rotor; multiple-speed; high-frequency; synchronous; and every single-phase variant
• Torque definitions — locked-rotor, breakdown, full-load, pull-up, pull-in, and pull-out, with the NEMA reference tables that define them
• Speed classification — constant, varying, adjustable, adjustable-varying, and multispeed
• Motor selection engineering — speed ranges, horsepower sizing, torque matching, inertia calculations, space constraints, temperature limits, and enclosure requirements
• Complete application tables — DC motors (1–300 hp) and polyphase AC motors cross-referenced by type, rating, torque, speed regulation, and industrial use
• Single-phase AC motors — capacitor-start, capacitor-run, split-phase, repulsion-start, repulsion-induction, and universal types
• Built-in speed reducers — worm, parallel-shaft, and planetary configurations
• Explosion-proof and hazardous-location motors — classifications and requirements
• A full maintenance program — weekly, monthly, and annual inspection schedules for both AC and DC motors

This is not a summary. This is the reference you keep on your workstation.

NEMA Standards: The Foundation of Every Motor Decision

Classes of NEMA Standards

The National Electrical Manufacturers Association (NEMA) publishes two classes of standards that govern electric motor design and application:

• NEMA Standard — Relates to a commercially standardized product subject to repetitive manufacture. Approval requires at least 90% agreement among eligible voting members of the relevant Subdivision.
• Suggested Standard for Future Design — May not yet be applied to a commercial product, but represents a sound engineering approach to future development. Requires approval by at least two-thirds of eligible voting members.

A third category, Authorized Engineering Information, provides explanatory data and engineering guidance that does not fall into either standard classification.

Why this matters to you: When a motor datasheet references "NEMA MG 1" compliance, it is telling you that the motor's performance characteristics — locked-rotor current, torque curves, frame dimensions, insulation class — have been tested and certified against a standard that the entire North American motor industry has agreed upon. If you are specifying motors for international applications, be aware that IEC standards serve a parallel function, but the NEMA design letter classifications (A, B, C, D) are unique to the North American framework.

Mounting Dimensions and Frame Sizes

NEMA standardizes every critical physical dimension of foot-mounted electric motors:

• Bolt-hole spacing in the motor feet
• Shaft centerline height from the bottom of the feet
• Conduit box size and location
• Shaft length and diameter
• Face-mounted and flange-mounted dimensions via standard motor frame numbers
• Belt-tightening base and rail-mounting dimensions

The standards also prescribe:

• Lettering conventions for dimension drawings
• Terminal housing locations and dimensions
• Symbols and terminal connections
• Grounding provisions for field wiring
• Knock-out and clearance hole dimensions
• Tolerances on shaft extension diameters and keyseats
• Methods for measuring shaft run-out, eccentricity, and face runout of mounting surfaces

Engineering Note: When replacing a motor in an existing installation, the NEMA frame number is your fastest path to a drop-in replacement. A motor with the same frame designation will match bolt patterns, shaft height, and shaft dimensions — eliminating the need for adapter plates, coupling changes, or foundation rework.