electric motor 6

7/29/2019 Electric Motor 6

1/20

Note: The source of the technical material in this volume is the Professional

Engineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for Saudi

Aramco and is intended for the exclusive use of Saudi Aramcos

employees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,

or disclosed to third parties, or otherwise used in whole, or in part,

without the written permission of the Vice President, Engineering

Services, Saudi Aramco.

Chapter : Electrical For additional information on this subject, contact

File Reference: EEX21606 W.A. Roussel on 874-1320

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Motor Starter Components And Standards


2/20

Engineering Encyclopedia Electrical


Saudi Aramco DeskTop Standards

CONTENTS PAGE

MAJOR COMPONENTS OF AC MOTOR STARTERS......................................1

Overload Protection....................................................................................3

Controller....................................................................................................5

Fault Protection ..........................................................................................7

Disconnect..................................................................................................9

Enclosure..................................................................................................10

MOTOR STARTER STANDARDS....................................................................11

Industry Standards....................................................................................11

NEC Article 430 ...........................................................................11

ANSI/IEEE Standards...................................................................13

NEMA Standards..........................................................................14

Saudi Aramco Standards ..........................................................................14

SAES-P-113 Motors and Generators ............................................14

SAES-P-114, Chapter 6, Motor Protection...................................14

16-SAMSS-503 Low Voltage Motor Control Centers and

Switchracks (600 V and Below) ...................................................1416-SAMSS-506 Medium Voltage Motor Control Center .............14

Drawing 990-P-AB036766...........................................................15

GLOSSARY ........................................................................................................17

LIST OF FIGURES

Figure 1. Major Functional Components of a Motor Starter................................2

Figure 2. Typical Three-Pole Thermal Overload Relay.......................................4

Figure 3. Contact Arrangement for One Pole of a Typical

Controller..............................................................................................5

Figure 4. Typical Three-Pole Magnetic Contactor...............................................6

Figure 5. NEC Article 430 .................................................................................12

Figure 6. Commonly Used Symbols for One-Line Diagrams............................16


3/20



Saudi Aramco DeskTop Standards 1

MAJOR COMPONENTS OF AC MOTO R STARTERS

Alternating current (AC) motor starters have five major functional components as illustrated

in the diagram shown in Figure 1. These include:

components that provide motor overload protection.

components that perform the motor controller function.

components that protect against short-circuit and ground faults.

components that provide motor circuit disconnecting means.

an enclosure suitable to meet the requirements of the motor starter application.

These components and their functions are described in the following sections.


4/20




Figure 1. Maj or Functional Components of a Motor Star ter


5/20




Overload P r otection

One of the most important components common to all motor starters is the device used to

provide running overload protection for the motor, its control circuits, and the connecting

circuit conductors. Selection and sizing of this device is guided by specific requirementsgiven in the National Electric Code for motor running overcurrent (overload protection).

The definition of running overcurrent (overload) is the operating overcurrent up to and

including stalled-rotor (locked-rotor) current. Overcurrent, however, does not include fault

currents due to shorts or grounds. When overcurrent persists for a sufficient length of time, it

causes overheating and possible damage to the motor and its circuit components.

The function of the overload device is to monitor motor line current and to open the motor

circuit when predetermined safe levels are exceeded. In accordance with the National Electric

Code and based on specific circuit conditions, continuous duty motors rated more than one

horsepower may be protected for overloads using overload relays, integral thermal protectors,

fuses or circuit breakers. The most commonly used overload device is the overload relay.

Two types of overload relays that respond to the heating effect of the motor line current are

the thermal and the solid-state overload relays. The thermal overload relay uses the motor

line current to produce heat within itself at a designated rate that simulates load and conductor

heating. On the other hand, the solid-state overload relay monitors motor line current and

uses semiconductor circuits to determine the heating effects that the level of current will have

on the motor and conductors. The motor starter in Figure 1 shows an example of a thermal

overload relay being used for the overload protection. Figure 2 shows a typical three-pole

thermal overload relay.


6/20




Figure 2. Typical Thr ee-Pole Therm al Overload Relay

All thermal overload relays have an operating memory. Operating memory means that the

relays respond as if they remember that the load they are protecting has been operating at

some rate, and that it may not be cold when returned to service after an overload. Some types

of solid-state overload relays have an equivalent operating memory to keep track of the

accumulated overload duty. This equivalent operating function can be provided in various

ways, such as by charging a capacitor or by altering the count in a register of a

microprocessor.

As illustrated in Figure 1, overload relays are connected on the loadside of the magnetic

contactor. Each relay has two major parts. There is the thermal sensing element, often

referred to as the heater, and there are the overload relay contacts. The thermal sensing

element is directly acted on by the line current drawn by the motor. If the motor load current

exceeds the rated value of the thermal element for a specified length of time, the relay reacts

to open the overload contacts, which in turn breaks the control circuit to the starter coil, thus

shutting down the motor. The time-current response characteristics of the overload relay

ensure that the motor is automatically shut down before an overload can persist to the point

where the motor becomes overheated and damaged.

Specific types of overload relays (e.g. bi-metallic, solder-pot and solid-state) and the methods

used to select them are described in Module EEX 216.04.


7/20




Controller

A major component common to all AC motor starters is the controller. The controller consists

of the contactor that carries the motor line current and the control circuit for the contactor.

When the overload relays (described in the preceding section) are added to the controller, theassembly is referred to as a basic magnetic motor starter.

The contactor portion of the controller is essentially an on-off device operated by

electromagnetic means. When the controller coil is energized through a control circuit, the

resulting magnetic field mechanically forces the main contacts to close, thus starting the

motor. The coil is then continuously energized to hold the contacts closed and keep the motor

running. When the coil is deenergized, the main controller contacts are forced open by either

spring pressure or gravity, thus stopping the motor. Figure 3 illustrates the arrangement of the

current carrying contacts, coil, and moving armature for one pole of a contactor. Contactors

are typically designed in two-, three-, four-, and five-pole configurations. Note in Figure 3

that the flow of power for the contact arrangement is in a straight line, into the line side of theassembly and out of the load side. This feature, common in most controllers, is referred to as

straight-through wiring. Figure 1 illustrates a controller that uses a three-pole contactor to

make up the full-voltage, non-reversing AC motor starter. A typical three-pole magnetic

contactor is depicted in Figure 4.

Figur e 3. Contact Ar rangement for One Pole of a Typical Contr oller


8/20




Figur e 4. Typical Thr ee-Pole Magnetic Conta ctor

The contactor portion of the controller must be able to continuously carry the full-load current

and to safely interrupt the locked-rotor current of the motor being controlled. If the motor

stalls, or if it does not start properly (it is jammed and cannot begin to turn), the current drawn

by the motor is referred to as the locked-rotor current. This current is typically 400-600% of

the full-load current of the motor (the same as the starting current), and the contactor must be

capable of safely opening the circuit under this condition.

Because both the full-load and locked-rotor currents are a function of the horsepower rating at

a specified voltage, motor controllers are rated for the maximum horsepower they can safely

handle at these voltages. Horsepower ratings for low voltage controllers (NEMA sizes 00

through 9) and medium voltage controllers (NEMA sizes H2 through H6) are listed in NEMA

Standard ICS-2. Saudi Aramco Standard SAES-P-114 requires that motor controllers be

applied in accordance with their horsepower rating.

Horsepower ratings and NEMA standard sizes for controllers are described in Module EEX

216.04.


9/20




Fault Pr otection

Another of the major components that is common to all AC motor starters is the device used

to provide protection against short circuit and ground faults.

The devices used for this protection are typically an instantaneous trip circuit breaker, an

inverse time circuit breaker, nontime delay fuses, or dual element (time-delay) fuses. The

example motor starter illustrated in Figure 1 shows that either nontime delay fuses or an

inverse circuit breaker can be used for the short-circuit and ground fault protection.

Saudi Aramco standards require that only circuit breakers be used for the fault protection

function. The one exception to this requirement is for cases where combination Class E2

starters with current limiting fuses, may be used for induction motors 4000 volts or greater

and 1500 horsepower or less. When circuit breakers are used, they serve as two components

for the motor starter. First, they serve as the fault protection device, and second, they serve as

the disconnecting device.

The fault protection device must provide protection for the complete motor branch circuit,

which includes the circuit conductors, the control apparatus and the motor.

To provide for safe and effective sizing of the fault protection device, the National Electric

Code defines requirements and maximum ratings to be used. The first requirement for the

fault protection device is that it be capable of carrying the starting current of the motor

without opening the circuit. The next requirement is that the rating or setting of the device

not exceed maximum values given in the National Electric Code. Sizes, ratings and functions

of fault protection devices are described in detail in Module EEX 216.04.

As mentioned above, the device used for fault protection may be a type of fuse or circuitbreaker, depending on the application and circuit requirements. The following paragraphs

briefly describe the characteristics for some of these devices. Low voltage fuses (600 volts or

less) come in two basic shapes: the plug fuse and the cartridge fuse. They may be current

limiting or non-current limiting. Cartridge fuses are either renewable or nonrenewable.

Nonrenewable cartridge fuses are assembled at the factory, and are replaced after they open in

service. Renewable cartridge fuses can be disassembled, and the fusible element can be

replaced. A special type of low voltage fuse that is sometimes used is the dual-element or

time-delay fuse. The dual-element fuse has one element that is fast-acting and responds to

overcurrents in the short-circuit range, while its other element permits short-duration

overloads, but melts if the overload is sustained.

Medium voltage fuses (over 600 volts) are called power fuses. Power fuses are either current

limiting or non-current limiting. A non-current limiting power fuse is more commonly called

an expulsion fuse. Current limiting fuses are categorized as either general purpose (E-rated)

or R-rated. R-rated current limiting fuses are specifically used for fault protection, in Class

E2 controllers, for medium voltage motor short circuit protection.


10/20




Molded-case circuit breakers are a class of breaker rated at 600 volts and below. These

breakers consist of a switching device and an automatic protective device assembled in anintegral housing of insulated material. These breakers are capable of clearing a fault more

rapidly than a low voltage power breaker. Solid-state trip units incorporated into some styles

of molded-case breakers provide for their coordination with power breakers. Molded-case

breakers are generally sealed to prevent tampering, which in turn precludes any inspection of

the contacts. They are generally not designed to be maintained in the field, and manufacturers

recommend total replacement if a defect appears.

Molded-case circuit breakers are available in several different types. The thermal magnetic

type, the most widely used, employs thermal tripping for overloads and magnetic tripping for

short-circuits. The magnetic type employs only instantaneous magnetic tripping for cases

where only short-circuit interruption is required. The integrally fused type combines regularthermal magnetic protection together with current limiting fuses to respond to applications

where higher short-circuit currents are available. In addition, the current limiting type offers

high interrupting capacity protection while at the same time limiting the let-through current to

a significantly lower value than is usual for conventional molded-case breakers.

The motor circuit protector (MCP) is a specific class of molded-case circuit breaker having a

special adjustable instantaneous trip circuit designed primarily for motor short-circuit

protection. Major features of the MCP are that it can respond to the occurrence of low level

faults in motor windings and typically clear them faster (less than one cycle) than a fusible

device. SAES-P-114 permits MCP use for low voltage motor protection greater than 1 to 100

horsepower.

Low-voltage power circuit breakers (LVPCB), like molded-case breakers, are rated 600 volts

and below. They differ, however, because they are typically open-construction assemblies on

metal frames with all parts designed for accessible maintenance, repair, and ease of

replacement. They are intended for service in switchgear compartments or in other enclosures

of dead front construction. Tripping units are field adjustable, and include electromagnetic,

direct acting, and solid-state types. They can be used with integral current-limiting fuses to

meet interrupting requirements up to 200 kA RMS symmetrical. SAES-P-114 requires the

use of LVPCBs for low voltage motor fault protection greater than 100 horsepower.

Medium-voltage power circuit breakers (MVPCB) are primarily circuit opening and closingdevices for use on circuits with distribution voltages up to 15 kV. Unlike low-voltage

breakers, they do not incorporate built-in automatic trip units. Separate protective relays are

used to monitor the circuit conditions and then to send an electrical signal to trip the breaker.

MVPCBs are available in a wide variety of designs, primarily identified by their interrupting

medium: air, air blast, vacuum, or gas (SF6). MVPCBs are basically an enlargement of the

low-voltage power circuit breaker with larger insulators and interrupting assembles to handle

the higher voltages and larger interrupting currents. SAES-P-114 requires the use of

MVPCBs for medium-voltage motor fault protection greater than 1500 horsepower.


11/20




Disconnect

Another major component that is common to all AC motor starters is the disconnecting

device. In accordance with the National Electric Code, motors and controllers must be

provided with a means of safely disconnecting them from their source of supply so thatmaintenance of the controller, motor, and its driven equipment can be accomplished with no

hazard to personnel.

The devices permitted by the NEC to be used for disconnecting include circuit breakers,

motor-circuit switches, and molded case switches (nonautomatic circuit interrupters). For

purposes of definitions, a motor-circuit switch is a horsepower-rated switch capable of

interrupting the maximum overload current of a motor. A molded case switch (nonautomatic

circuit interrupter) is a circuit breaker-like device without the overcurrent element and

automatic trip mechanism. It is rated in amperes and is suitable for use as a motor circuit

disconnect based on its ampere rating.

The disconnect, additionally, must open all the ungrounded supply conductors and be gang-

operated so that the one operating mechanism opens all poles simultaneously. The device

must give a clear indication of its status, open or closed, be rated in horsepower, and be

capable of interrupting the locked-rotor current of a motor of the same horsepower rating.

The horsepower rating is required in the event that a motor stalls, and the motor controller

fails to properly open the circuit. When the disconnect switch is operated, it must interrupt

the locked-rotor current of the motor, which is typically 400-600% of the motor full-load

current.

With regard to location, the disconnecting means must be located in sight from the controller,

the motor location, and the driven machinery location. For motors over 600 volts, thecontroller disconnecting means may be out of sight of the controller, provided the controller

has a warning label indicating the location and identification of the disconnecting means. In

addition, the disconnect must be capable of being locked in the open position.

The NEC requires that a switch, circuit breaker, or other device serve as a disconnecting

means for both the controller and the motor, thereby providing safety during maintenance and

inspection shutdown periods. The disconnecting means also disconnects the controller;

therefore, it cannot be a part of the controller. However, separate disconnects and controllers

may be mounted on the same panel or contained in the same enclosure. This type of

arrangement is referred to as a combination starter, and it is the most common arrangement

used in motor control centers.


12/20




Enclosure

A final component common to AC motor starters is the enclosure. An enclosure is a

surrounding case constructed to provide a degree of protection to personnel against incidental

contact with the enclosed equipment, and to provide a degree of protection to the enclosedequipment against environmental conditions.

Although other types of materials (e.g. polymeric) are sometimes used, enclosures are

typically sheet metal or cast metal construction. However, in all cases, only material that will

not support combustion in air is selected for enclosures.

In many cases, the controller and the overload relay are contained in the same enclosure. For

some motor starters, the disconnecting means and/or the branch-circuit fault protective device

are also included in the enclosure with the controller and overload relays. When all of the

elements (devices) are in the same enclosure, the assembly is referred to as a combination

starter.

Enclosures are designated by a type number which indicates the environmental conditions for

which they are suitable. Applicable type numbers include Types 1, 2, 3, 3R, 3S, 4, 4X, 5, 6,

6P, 7, 8, 9, 10, 11, 12, and 13 as identified in NEMA Standard Publication No. 250. Types 7,

8, 9 and 10 enclosures are for use in hazardous (classified) locations. (Definitions for these

classifications are given in the National Electric Code, Article 500, and ANSI/NFPA 497M.)

NEMA enclosure types and classifications are described in Module EEX 216.04.


13/20




MOTOR STARTER STANDARDS

An engineer should be familiar with the following Saudi Aramco and industry standards in

order to select low and medium voltage motor starters for AC induction motors.

National Electric Code (NEC)

American National Standards Institute/Institute of Electrical and Electronics

Engineers (ANSI/IEEE)

National Electrical Manufacturers Association (NEMA)

Saudi Aramco Engineering Standards (SAES)

Saudi Aramco Materials System Specification (SAMSS)

Industry Standards

NEC Art icle 430

The NEC is published by the National Fire Protection Association (NFPA). The intent of the

NEC is the practical safeguarding of persons and property from the hazards that can arise

from the use of electricity. The NEC is updated every three years through proposals that are

submitted by the public. The proposals must be reviewed and approved by a series of

committees and councils before the public proposal can become part of the standard. The

latest edition of the NEC is the 1993 edition (NFPA 70-1993). The NEC Article covers the

topics itemized in Figure 5.

Scope - NEC Article 430 covers motors, motor branch and feeder conductor circuits and theirprotection, motor overload protection, motor control circuits, motor controllers, and motor

control centers (MCCs).

Low Voltage Motor Articles - All of the Sections contained in Article 430, except for

Section J, pertain to low voltage motors.

Medium Voltage Motor Art icles - Section J Articles 430-121 through 430-127 recognize the

additional hazards associated with higher (medium) voltages.

Tables -Tables 430-150, 430-151 and 430-152 will be used throughout the entire course and

will be discussed in detail later.

NOTE: Although Article 430 covers motors, motor starters, etcetera, other articles in the

NEC, pertaining to motor circuits, cannot be ignored. For example, the number of

conductors in a raceway, and ampacity ratings of conductors are covered in other articles.

The fact that the conductors are for a motor circuit is irrelevant.


14/20




Figure 5. NEC Art icle 430


15/20




ANSI/IEEE Standards

ANSI/IEEE Standards provide information on how to design, test, measure, and specify

electrical systems and the preferred ratings of different pieces of electrical equipment. The

information in the ANSI/IEEE Standards represents the consensus opinion of a group ofsubject matter experts. The requirements and procedures that are given in ANSI/IEEE

Standards are useful in the design and application of motor starters.

C37.06-1987 Standard for Switchgear - AC High Voltage Circuit Breakers Rated on a

Symmetrical Current Basis - Preferred Ratings and Related Required Capabilities -

This standard lists the preferred ratings for medium voltage circuit breakers that are used in

Saudi Aramco installations for all motors above 5 kV as well as larger horsepower .600 to 5.0

kV motors.

C37.2-1991 Standard Electrical Power System Device Function Numbers - This standard

lists all of the device numbers and their functions. For example, ANSI Device 49 is a thermal

relay used to provide overload protection for a motor. Saudi Aramco requires all drawings,

diagrams, etc. to use the standard device numbers as listed in C37.2-1991.

C37.13-1990 Standard for Low Voltage AC Power Circuit Breakers Used in Enclosures

- This standard lists the ratings for low voltage power circuit breakers used in Saudi Aramco

installations for low voltage motors (under 600 V) and above 100 horsepower.

C37.46-1981 Standard Specifications for Power Fuses and Fuse Disconnecting Switches

- This standard lists the specifications for power fuses (NEMA Type R) used in medium

voltage 5 kV Class E2 motor starters, which are used in many of Saudi Aramcos

installations. Note: This standard was reaffirmed in 1987.

260-1978-Standard Letter Symbols for Units of Measurement - This standard lists the

letter symbols for all units of measurement; for example, mm for millimeter or f(x) for

function of x. Note: This standard was reaffirmed in 1991.

315-1975-Standard Graphic Symbols for Electrical and Electronics Diagrams - This

standard lists all of the graphic symbols for electrical and electronics diagrams. For the most

part, Saudi Aramco Drawing 990-P-AB036766 complies with 315-1975. Note: This

standard was reaffirmed in 1988.


16/20




NEMA Standard s

ICS 1-General Standards for Industrial Control and Systems - This standard covers the

requirements for industrial control apparatus rated 750 volts or less DC, and 7.2 kV volts or

less AC. Of particular note is that this standard lists all definitions and graphic symbols forcontrol systems.

ICS 2-Industrial Control Devices, Controllers, and Assemblies - This standard covers

general purpose mechanical, electromechanical, and/or solid state (static) devices and controls

that are principally used in industrial applications for the control of motors.

MG-1-Motors and Generators - This is key industry standard governing the performance

and construction of motors and generators.

250-1991-Enclosures for Electrical Equipment (1000 Volts Maximum) - This standard

covers the classification and description of enclosures for electrical equipment. It is

considered the best source for determining specific applications of the numerous types of

enclosures.

Saudi Ara mco Standar ds

There are two types of Saudi Aramco standards pertaining to the selection of motor starters;

SAESs and SAMSSs.

SAES-P-113 Motors and Generator s

This SAES contains the minimum mandatory requirements for the procurement and

installation of induction motors. Any deviations from this standard must be approved by theSaudi Aramco Chief Engineer (Dhahran). This standard, in particular, lists the nominal

ratings (phases, voltages, hp/kW) for Saudi Aramco-procured motors.

SAES-P-114, Chapter 6, Motor Protection

This SAES (Chapter 6) specifies the minimum protection requirements for three-phase

motors. The chapter is categorized by motor type, voltage, and horsepower/kilowatt rating.

16-SAMSS-503 Low Voltage Motor Contr ol Center s and Switchr acks (600 V and

Below)

This SAMSS defines the minimum technical requirements for three-phase, 600 volts and

below, indoor motor control centers (MCC).

16-SAMSS-506 Medium Voltage Motor Contr ol Center

This SAMSS defines the minimum technical requirements for medium voltage (5 kV class)

metal-enclosed indoor motor control centers. This standard also applies to NEMA Class E2

current limiting fused controllers.


17/20




Drawing 990-P-AB036766

This drawing describes the standard electrical symbols used for power one-line diagrams in

Saudi Aramco. Figure 6 lists several of the more common symbols that will be used in this

course. Again, as in most of the Saudi Aramco standards, the symbols are in accordance with

the more nationally recognized ANSI/IEEE Standard 315-1975.


18/20




Figur e 6. Commonly Used Symbols for One-Line Diagrams


19/20




GLOSSARY

ambient temper atur e The temperature of the medium such as air, water, or earth

into which the heat of the equipment is dissipated.

combinat ion star ter A complete motor starter consisting of a disconnect device, a

magnetic contactor, and protective devices for short circuit

and overload. All devices are assembled in a single

enclosure.

contactor A magnetic device that has sufficient capability to start and

stop a motor under normal and overload conditions.

continuous ra ting The maximum constant load that can be carried continuously

without exceeding established temperature-rise limitations

under prescribed conditions of test and within the limitationsof established standards.

disconnect switch A switch intended for isolating an electric circuit from the

source of power.

fault curr ent A current that results from the loss of insulation between

conductors or between a conductor and ground.

fault cur r ent, low-level A fault current that is equal to or less than the

(as applied to a motor maximum operating overload.

branch circuit)

interrupting capability The maximum value of current that a contact assembly is

required to successfully interrupt at a specified voltage for a

limited number of operations under specified conditions.

inverse time A qualifying term applied to a relay indicating that its time of

operation decreases as the magnitude of the operating

quantity increases.

motor circuit pr otector A magnetic-only molded case circuit breaker used

(MCP) in low voltage combination starters. This device has onlyinstantaneous functions to protect the motor, starter, and

branch circuit from short circuit and ground fault currents.

operat ing over load The overcurrent to which an electric apparatus is subjected in

the course of the normal operating conditions that it may

encounter.


20/20



overload r elay A device that is used to sense an overload on a motor circuit.

The most common type uses a heater that heats a bi-metallic

strip that operates a set of contacts.

overload protection The effect of a device operative on excessive current, but not

necessarily on short circuit, to cause and maintain the

interruption of current flow to the governed device.

symmetr ical (curr ent) A currrent waveshape where the envelopes of the peaks are

symmetrical about the zero axis.

electric motor 6

Documents