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
CONTENTS PAGE
PHYSICAL ARRANGEMENTS OF MOTOR STARTER ENCLOSURES ...1
Introduction... ...1
Common Enclosure Components...1
Disconnecting Means ...1
Lock and Tag Features ...1
Interlocks and Latches ...2
Open Panel Type Enclosures...3
Description...3
Saudi Aramco Applications...4
Enclosed Type Motor Starter Enclosures ...5
Single (Wall-Mount)...5
Group (Wall-mount) ...6
Motor Control Centers (MCC’s)...8
Saudi Aramco Applications...11
NEMA ENCLOSURE CLASSIFICATION SYSTEM...13
NEMA 1 - General Purpose Enclosures...17
NEMA 12 - Dust-Tight Industrial Enclosures...19
NEMA 3R - Rain-Resistant Enclosures ...20
NEMA 4/4X - Water, Dust-Tight and Corrosion-Resistant Enclosures...21
NEMA 7 - Hazardous Location Enclosures ...22
Operating Principles ...29
Solder-Pot O/L Relays...30
Components...30
Operating Principles ...31
Solid-State O/L Relays...32
Components...32 Operating Principles ...34 Classes...36 Class 10 ...37 Class 20 ...37 Class 30 ...37 Types...37 Type A ...37 Type B ...38
Temperature Compensation Criteria ...39
Environmental Conditions ...39 Ambient ...40 Non-Ambient ...40 Pole Arrangements ...41 Single-Pole ...41 Three-Pole ...41 Other Considerations...42 Single-Phasing...42
NEMA Motor Contactor Sizing Criteria ...48
Horsepower...48
Motor Voltage...50
Continuous Current...50
Special Criteria ...51
Motor Contactor Auxiliary Devices ...53
Contacts ...53
Interlocks ...54
Motor Contactor Coil Voltage Ratings...55
SELECTING A LOW VOLTAGE MOTOR DISCONNECT/FAULT PROTECTIVE DEVICE...57
Types...57
Disconnect Switch With Fuses ...57
Molded Case Circuit Breakers (MCCBs) ...58
Low Voltage Power Circuit Breakers (LVPCBs) ...60
Ratings...61
Disconnect Switch and Fuses ...61
Molded Case Circuit Breakers (MCCBs) ...64
Low Voltage Power Circuit Breakers (LVPCBs) ...68
Combination Motor Starters ...70
Fuse T/C Characteristics...72
Log-Log T/C Paper...72
Motor Nameplate Data ...88
Full-Load Amperes...88
kVA Code/Locked-Rotor Amperes ...88
Voltage and Horsepower ...88
Fault/Starting Currents ...89
Symmetrical Current...89
Asymmetrical Current...89
NEC Maximum Settings ...90
Inverse-Time MCCBs...90
Magnetic-Only MCCBs and MCPs ...90
LVPCBs...90
WORK AID 1: RESOURCES USED TO SELECT A LOW VOLTAGE MOTOR O/L RELAY...91
Work Aid 1A: NEC Article 430...91
Work Aid 1B: 16-SAMSS-503 ...91
Work Aid 1C: Vendor’s Literature, Westinghouse Catalog 25-000 ...91
Work Aid 1D: Applicable Selection Procedures...91
WORK AID 2: RESOURCES USED TO SELECT A LOW VOLTAGE MOTOR CONTACTOR ...96
Work Aid 2A: NEC Article 430...96
Work Aid 2B: 16-SAMSS-503, Chapter 4...96
Work Aid 2C: Vendor’s Literature, Westinghouse Catalog 25-000 ...96
Work Aid 3E: Vendor’s Literature, Westinghouse SA-11647, Low Voltage Metal Enclosed Switchgear - Type DS...98 Work Aid 3F: Applicable Selection Procedures ...98 GLOSSARY... ...101
LIST OF FIGURES
Figure 1. Common Motor Starter Enclosure Components...2
Figure 2. NEMA General Purpose Contactor Rating...4
Figure 3. Single Wall-Mount Enclosure ...5
Figure 4. Group Wall-Mount Enclosure ...7
Figure 5. Typical Low-Voltage Motor Control Center ...8
Figure 6. MCC Drawout Unit ...9
Figure 7. Handle Mechanism Locked-Out With Padlock ...10
Figure 8. NEMA Wiring Classes for Motor Control Centers...12
Figure 9. Comparison of Specific Applications of Enclosures for Indoor Nonhazardous Locations ...14
Figure 10. Comparison of Specific Applications of Enclosures for Outdoor Nonhazardous Locations ...15
Figure 11. Comparison of Specific Applications of Enclosures for Indoor Hazardous Locations ...16
Figure 12. Conversion of NEMA Type Numbers to IEC Classification Designation...18
Figure 13. Example of Full-Load Ampere Range for Various Sizes of Overload Relays ...25
Figure 14. Maximum Overload Relay Trip Rating Based on Motor Service Factor (S.F.)...26
Figure 15. Bimetallic Type Overload Relay ...27
Figure 16. Solder-Pot Type Overload Relay...30 Figure 17. Current Sensing (Heater) Plug-In Module for Solid-State Overload
Figure 18. Solid-State Overload Relay Time-Current Curve ...35
Figure 19. Typical Time-Current Characteristics for Class 20 and Class 30 Overload Relays ...36
Figure 20. Typical Air-Magnetic Contactor with O/L Relay ...45
Figure 21. Typical Vacuum Contactor...47
Figure 22. Horsepower Ratings for Three-Phase Single Speed Full-Voltage Magnetic Contactors (Controllers) for Nonplugging and Nonjogging Duty ...49
Figure 23. Typical Auxiliary Contact ...53
Figure 24. Typical Auxiliary Interlocks...54
Figure 25. Example of AC Coil Voltage Ratings for NEMA Size 3 and 4 Low Voltage Contactors ...55
Figure 26. Disconnect Switch ...57
Figure 27. Dual-Element Cartridge Fuse ...58
Figure 28. Molded Case Circuit Breaker (MCCB) ...58
Figure 29. Switch Nameplate...61
Figure 30. Fuse Label ...61
Figure 31. Disconnect Switch Ratings ...62
Figure 32. Low Voltage Fuse Ratings...63
Figure 33. MCCB Asymmetrical Factors...64
Figure 34. Typical MCCB Ratings ...66
Figure 35. Typical MCP Ratings and Settings...67
Figure 39. Typical Log-Log Paper...73
Figure 40. Non-Time Delay Fuse Characteristics ...74
Figure 41. Time Delay Fuse T/C Characteristics ...75
Figure 42. Thermal Magnetic MCCB Fault Protection...77
Figure 43. Magnetic-Only MCCB Fault Protection...78
Figure 44. MCP Fault Protection ...79
Figure 45. Ground Fault Protection With Shunt Trip ...80
Figure 46. Long Time Pickup (LTPU) T/C Characteristics ...81
Figure 47. Long Time Delay (LTD) T/C Characteristics ...82
Figure 48. Short Time Pickup (STPU) T/C Characteristics ...82
Figure 49. Short Delay Time (SDT) With I2t T/C Characteristics...83
Figure 50. Instantaneous Trip (IT) T/C Characteristics...84
Figure 51. GFP With Window-Type CT...84
Figure 52. Sample GFPU Code Letters and Settings ...85
Figure 53. Ground Fault Pickup (GFPU) T/C Characteristics ...85
Figure 54. Ground Fault Time (GFT) With I2t T/C Characteristics ...86
Figure 55. LVPCB Motor Protection ...87
Figure 58. NEC Table 430-32...92
Figure 59. Problem 1 Motor Nameplate Data ...106
Figure 60. Problem 1 One-Line Diagram...107
Figure 61. Problem 2 Motor Nameplate Data ...111
PHYSICAL ARRANGEMENTS OF MOTOR STARTER ENCLOSURES Introduction
A major component of a motor starter is the enclosure. To properly select low voltage motor starters, it is necessary to understand the physical arrangements of motor starter enclosures. This Information Sheet explains the physical arrangements of enclosures by describing components that are common to all enclosures and by describing various types of enclosures.
Common Enclosure Components
Motor starter enclosures have several components that are common to all types of enclosures. These components include a disconnecting means, lock and tag features, and enclosure interlocks. Descriptions of these common components are given in the following paragraphs.
Disconnecting Means
A common component that is included on all types of enclosures is a means of externally operating the disconnect device that is mounted inside of the enclosure. This component is typically a flange mounted handle located on the outside of the enclosure as shown in Figure 1. The handle is mechanically fastened to an operating mechanism that is located inside of the enclosure and that attaches to the disconnecting device (disconnect switch or breaker). The handle provides for external operation of the disconnecting device, and it gives positive visual indication of its status (open or closed).
Lock and Tag Features
A common component of enclosures that is very important for safety is the provision to padlock the operating handle. This provision allows one or more padlocks to be inserted through a hole in the operating handle to lock it in the “Off” position. The purpose of this feature is to allow the motor starter to be locked in the de-energized position and tagged with a “Warning” tag to provide for safe inspection and maintenance of the motor. The location of this locking provision is identified for the enclosure shown in Figure 1. In addition to the capability of padlocking the operating handle, enclosures also allow padlocking of the cover to prevent access by unauthorized personnel.
Interlocks and Latches
Another set of components that are common to all types of enclosures is the cover safety interlock. The typical enclosure has two interlocks. These interlocks are illustrated in Figure 1. One is connected between the external operating handle and the enclosure cover to prevent opening of the cover while the handle is in the “On” position. In order to open the cover, the handle must be moved to the “Off” position. However, to allow access by trained and authorized personnel for purposes of special maintenance, an interlock bypass is provided. The second interlock is designed to function when the cover is open. This interlock prevents the breaker or disconnect switch from being operated in the “On” position while the cover remains open. The one exception to the operation of this interlock is that trained and authorized personnel are provided the option of activating the interlock bypass.
Open Panel Type Enclosures Description
Motor starters are typically mounted in NEMA-type enclosures. However, for some applications, motor starters are mounted on flat, open panels. In accordance with NEC Article 430-132, motor starters operated at 50 volts or more between terminals must be guarded against accidental contact by mounting in an enclosure or by locating in a controlled room, controlled balcony, or at an elevation of 8 feet or more.
In older manufacturing facilities, open panel mounting was normally accomplished by mounting the motor starters on pole-supported slate or micarta panels. These panels, which are sometimes called “electric switchboards”, were also used to mount other electrical controls needed for the facility. The switchboards, which are usually supplied by open-type uninsulated bus, were typically located in a dedicated room where access was allowed only to qualified electricians and to authorized managers.
For modern applications, open panel mounting of motor starters is typically accomplished by fastening the starters to a flat, painted, steel panel. The panel is then mounted in a large steel cabinet or in a separate control room. When the starters are mounted in this manner, their continuous current rating is increased in accordance with the NEMA ICS2 contactor ratings shown in Figure 2. With reference to this figure, it is noted that the ratings for open panel mounting are 110% of the ratings for enclosed mounting.
Size of Contactor Enclosed Mounting Continuous Current (amperes, rms) Open Mounting Continuous Current (amperes, rms) 00 9 10 0 18 20 1 27 30 2 45 50 3 90 100 4 135 150 5 270 300 6 540 600
(Reference NEMA ICS2-210) Figure 2. NEMA General Purpose Contactor Rating
Saudi Aramco Applications
Saudi Aramco standards do not permit the use of open panel-type enclosures.
In accordance with SAES-P-114, a motor starter for a low voltage motor must be either a combination motor starter or a circuit breaker depending on the horsepower rating of the motor. With reference to a combination motor controller, it is defined by NEMA ICS2-321 as an externally operable circuit-disconnecting means and a magnetic controller mounted in a single enclosure. On the other hand, circuit breakers are by design enclosed in their own case or housing. As a result, both types of low voltage motor starters allowed by Saudi Aramco
Enclosed Type Motor Starter Enclosures Single (Wall-Mount)
The single wall-mount enclosure is the most commonly used type of enclosure. A typical single enclosure, similar to the one illustrated in Figure 3, offers the advantage of placing individual starters at their most convenient location while still providing all of the common component features described above (i.e. disconnecting means, lock and tag features, and enclosure interlocks).
Single enclosures are also designated by a NEMA-type number that indicates the environmental conditions for which they are suitable. NEMA enclosure types and classifications are described in the following Information Sheet (NEMA Enclosure Classification System).
Single wall-mount enclosures are available from manufacturers in a number of sizes. The required size for an enclosure is recommended by the manufacturer and is determined by the type and size of combination controller to be housed. When needed, extra space can be requested by the user to accomodate field-mounted control components.
Group (Wall-mount)
A group wall-mount enclosure is essentially several single enclosures designed and manufactured as one unit but with individual internal compartments. The group-type enclosure is designed to save time, space, and expense when installing multiple control devices.
Figure 4 shows an example of a group enclosure with four compartments for mounting four combination controllers. Group enclosures are typically partitioned into either four or six compartments. Each compartment is designed to hold a combination starter, incoming or feeder circuit breakers, fusible switches, or other auxiliary devices. The barriers between compartments can be removed to provide oversize spaces allowing for installation of a lesser number of larger size controllers.
In addition to the barrier compartments, the group enclosure normally contains internal wiring troughs. Typically, one trough is located at the top and is fitted with power terminal straps for extension to adjoining compartments. Another wiring trough is located along the bottom for interconnecting wiring and outgoing cables.
The compartments have hinged doors that are interlocked to prevent opening them when the breaker switch is in the “On” position. In addition, the disconnect operating mechanism can be padlocked in either the “On” or “Off” positions.
Motor Control Centers (MCC’s)
A motor control center (MCC) is a group of combination starters assembled into a single metal enclosure with individual compartments for each starter. Control centers are arranged in straight-line, L-shaped, U-shaped, or back-to-back configurations. Figure 5 shows a typical arrangement of a motor control center in a straight-line configuration.
The metal enclosure of the motor control center is built with a single steel-channel frame that has compartment-like spaces for insertion of individual combination starters. The individual compartments of the enclosure share common bus systems and wireways. With regard to the bus systems, a main horizontal bus is installed across the top of the unit to provide three-phase power distribution from the incoming line or primary disconnect device to each vertical structure. A vertical bus is mounted in each vertical unit to provide distribution of the main bus power to each of the individual vertical compartments. Completing the arrangement of bus systems is a neutral bus mounted on stand-off insulators across the bottom of each vertical unit and a ground bus mounted across the top of each unit. With regard to the wireways, the enclosure has both vertical and horizontal wireways to provide for convenient servicing and controller change-outs. All wireways are provided with hinged panel covers for easy access and as a barrier to fire.
For this type of enclosure, a steel compartment shell, referred to as a drawout case, is provided for each compartment. Figure 6 shows the construction of a typical drawout case. The drawout case, comprised of three sides and a base, serves as a housing for mounting of each starter. Four mounting points on the drawout case allow it to engage guide rails, located near the top of the compartment space, for easy insertion and withdrawal. A quarter turn latch located at the top of the case securely holds it in the compartment after insertion.
Figure 7 shows the arrangement of a typical handle mechanism that is located on the front of a drawout case. The handle mechanism is designed to operate the controller disconnecting device located inside of the drawout case. Similar to other types of enclosures, the handle mechanism for this enclosure provides common safety features. These features include an interlock that prevents the compartment door from being opened when the handle is in the “On” position. When the compartment door is open and the handle is in the “On” position, an interlock prevents the drawout case from being removed from the compartment. In addition, the handle mechanism can be padlocked in the “Off” position to insure that individual starters are not energized accidentally or by unauthorized personnel during maintenance procedures.
In addition to the handle mechanism, a control panel is mounted on the front of the drawout case. The control panel allows mounting of pushbuttons, indicator lights and related control devices. The arrangement of mounting both the handle mechanism and the control panel on the front of the drawout case helps to make inspections and maintenance easier.
A final feature of this type of enclosure is the compartment door. Each compartment of the motor control center has a separate hinged door that allows the handle mechanism and control panel to protrude through the door when it is closed. The doors are typically secured in the closed position using two quarter turn indicating type fasteners. As described above, an interlock prevents the door from being opened when the handle is in the “On” position.
Saudi Aramco Applications
With regard to Saudi Aramco application of enclosures, SAES-P-114 requires that a motor controller be either a combination motor starter or a circuit breaker. When the controller is a combination motor starter, the enclosure for the controller is provided by the manufacturer as an integral part of the starter. The provided enclosure is designed and assembled in accordance with NEMA Standards 250 and ICS-6 to meet specific application environmental conditions (NEMA enclosure types and classifications are described in the following Information Sheet). The enclosure provided by the manufacturer also includes the common enclosure components described above (a disconnecting means, lock and tag features, and enclosure interlocks).
When the controller is a circuit breaker, the enclosure is provided by one of two means. Either the circuit breaker is designed and constructed with a self-encasing enclosure, or the breaker is designed for mounting inside of a metal-enclosed switchgear compartment.
With regard to enclosures applied for low voltage motor control centers (MCC), 16-SAMSS-503.4.2 requires that MCC’s be rigid, free-standing, metal-enclosed structures, consisting of vertical sections assembled into a group having a common bus and forming an enclosure to which additional sections may readily be added. The enclosures must be suitable for back-to-wall or back-to-back mounting. Back-to-back constructions having a common horizontal bus are not acceptable. The MCC cubicle design must be NEMA Class I, Type B, with all ventilation openings suitably filtered or screened with a specified corrosion-resistant material arranged to prevent entrance of rodents and other foreign matter.
NEMA Standard ICS2-322.08 describes Class I motor control centers as consisting of mechanical groupings of independent combination motor control units, feeder tap units, other units, and electrical devices arranged in a convenient assembly. The “Type” designation indicates whether wiring between motor control units is allowed and whether unit and/or master terminal blocks are required. Figure 8 shows in summary form the NEMA wiring classes for motor control centers. With reference to this figure, it is seen that Class I does not allow wiring between independent motor control units and that Type B requires that terminal blocks be provided for field wiring to the units.
Class I
(No interwiring between units.)
Class II
(Interwiring between units.)
A. No Terminal Blocks Type A
---B. Unit Terminal Blocks Type B Type B
C. Unit and Master
Terminal Blocks Type C Type C
NEMA ENCLOSURE CLASSIFICATION SYSTEM
NEMA Standard 250 provides a classification system for enclosures of electrical equipment. The primary purpose of the classification system is to permit potential users to determine:
• The type of enclosure appropriate for the application. • The features that the enclosure is expected to have.
• The tests applied to the enclosure to demonstrate its conformance to the description.
The system provides for enclosures to be designated by a “Type” number that indicates the environmental conditions for which the enclosure is suitable. Applicable type numbers for nonhazardous application include Types 1, 2, 3, 3R, 3S, 4, 4X, 5, 6, 6P, 7, 8, 9, 10, 11, 12, and 13. Type numbers applied to enclosures for hazardous location use include Types 7, 8, 9, and 10. Enclosures covered by this classification system are nonventilated, except that Types 1, 2, and 3R enclosures may be either nonventilated or ventilated.
Figures 9, 10, and 11 give a brief overview of the types of enclosures included in the NEMA classification system and the environmental conditions that they protect against. Figure 9 shows an overview comparison of enclosures used for indoor nonhazardous locations, Figure 10 shows a comparison of enclosures used for outdoor nonhazardous locations, and Figure 11 compares enclosures applied to indoor hazardous locations.
Detailed descriptions for selected enclosure types are provided in the sections that follow these figures.
Types of Enclosures Provides a Degree of
Protection Against the Following Environmental Conditions
1* 2* 4 4X 5 6 6P 12 12K 13
Incidental contact with enclosed
equipment X X X X X X X X X X
Falling dirt X X X X X X X X X X
Falling liquids and light
splashing --- X X X X X X X X X
Circulating dust, lint, fibers and
flyings --- --- X X --- X X X X X
Settling airborne dust, lint
fibers, and flyings --- --- X X X X X X X X
Hosedown and splashing water --- --- X X --- X X --- --- ---Oil and coolant seepage --- --- --- --- --- --- --- X X X Oil or coolant spraying and
splashing --- --- --- --- --- --- --- --- --- X
Corrosive agents --- --- --- X --- --- X --- --- ---Occasional temporary
submersion --- --- --- --- --- X X --- ---
Types of Enclosures Provides a Degree of Protection
Against the Following Environmental Conditions
3 3R* 3S 4 4X 6 6P
Incidental contact with the enclosed
equipment X X X X X X X
Rain, snow, and sleet X X X X X X X
Sleet --- --- X --- --- ---
---Windblown dust X --- X X X X X
Hosedown --- --- --- X X X X
Corrosive agents --- --- --- --- X --- X
Occasional temporary submersion --- --- --- --- --- X X Occasional prolonged submersion --- --- --- --- --- --- X
* Note: These enclosures may be ventilated.
(Reference NEMA Standard Publication No. 250) Figure 10. Comparison of Specific Applications of Enclosures for Outdoor
Type of Enclosure 7 & 8, Class I Groups
Type of Enclosure 9, Class II Groups Provides a Degree of Protection
Against Atmospheres Typically
Containing Class A B C D E F G 10
Acetylene I X --- --- --- --- --- ---
---Hydrogen, manufactured gas I --- X --- --- --- --- --- ---Diethel ether, ethylene,
cyclopropane I --- --- X --- --- --- ---
---Gasoline, hexane, butane, naphtha,
propane, acetone, toluene, isoprene I --- --- --- X --- --- ---
---Metal dust II --- --- --- --- X --- ---
---Carbon black, coal dust, coke dust II --- --- --- --- --- X --- ---Flour, starch, grain dust II --- --- --- --- --- --- X ---Fibers, flyings III --- --- --- --- --- --- X ---Methane with or without coal dust MSHA --- --- --- --- --- --- --- X
(Reference NEMA Standard Publication No. 250) Figure 11. Comparison of Specific Applications of Enclosures for
NEMA 1 - General Purpose Enclosures
All enclosure types included in the NEMA classification system are intended to provide a degree of protection to personnel against incidental contact with the enclosed equipment. In addition to this common protection provided by all enclosures, each enclosure is identified by a Type number that indicates the degree of protection provided to the enclosed equipment against environmental conditions.
A NEMA Type 1 enclosure is intended for general purpose indoor applications. It is used primarily to provide a degree of protection against falling dirt in locations where unusual service conditions do not exist.
When properly installed, Type 1 enclosures:
• Prevent the insertion of the end portion of a straight rod of specified diameter into the equipment cavity of the enclosure.
• Provide a degree of protection against limited amounts of falling dirt. • Provide suitable rust-resistance protection.
Type 1 enclosures are evaluated to demonstrate their conformance to environmental protection requirements by the following NEMA specified tests:
• Rod entry test (reference NEMA 250.6.2) • Rust-resistance test (reference NEMA 250.6.8)
A similar -- but different -- classification system for enclosures is provided by the International Electrotechnical Commission (IEC) in standard IEC-529. Figure 12 shows a comparison of the two enclosure classifications, and it provides for conversion from NEMA-type numbers to IEC classification designations. However, Figure 12 cannot be used to convert IEC classification designations to NEMA-type numbers. The reason Figure 12 cannot be used to convert from IEC designations to NEMA-type numbers is because the tests and evaluations between the two systems are not identical.
NEMA Enclosure
Type Number IEC EnclosureClassification Designation 1 IP10 2 IP11 3 IP54 3R IP14 3S IP54 4 and 4X IP56 5 IP52 6 and 6P IP67 12 and 12K IP52 13 IP54
NEMA 12 - Dust-Tight Industrial Enclosures
A NEMA Type 12 enclosure is intended for indoor applications. It is used primarily to provide a degree of protection against circulating dust, falling dirt, and dripping noncorrosive liquids. This type of enclosure is not intended to provide protection against such conditions as internal condensation.
When completely and properly installed, Type 12 enclosures:
• Prevent the entrance of water under test conditions intended to simulate an environment of light splashes, seepage, and dripping of noncorrosive liquids. • Exclude dust under test conditions that are intended to simulate an indoor
industrial environment of circulating dust, lint, nonignitable fibers, and noncombustible flyings.
• Have no knockouts or unused openings.
• Have doors with provisions for locking or the requirement that a tool be used to gain entry. All closing hardware is captive.
• When intended for wall mounting, have mounting means external to the equipment cavity. When intended for floor mounting, have closed bottoms. • Have gaskets, if provided, that are oil-resistant.
• Have suitable rust-resistance protection.
Type 12 enclosures are evaluated to demonstrate their conformance to environmental protection requirements by the following NEMA specified tests:
• Drip test (reference NEMA 250.6.3)
• Circulating dust test (reference NEMA 250.6.5.1.2) • Rust-resistance test (reference NEMA 250.6.8)
NEMA 3R - Rain-Resistant Enclosures
A NEMA Type 3R enclosure is intended for outdoor applications. It is used primarily to provide a degree of protection against rain and sleet and to be undamaged by the formation of ice on the enclosure. This type of enclosure is not intended to provide protection against such conditions as internal condensation or internal icing.
When completely and properly installed, Type 3R enclosures:
• Prevent water from contacting live parts, insulation, and wiring under test conditions that are intended to simulate rain.
• Are undamaged after being encased in ice under test conditions.
• Prevent the insertion of the end portion of a straight rod of specified diameter into the equipment cavity of the enclosure.
• Require the use of a tool to gain access to the equipment cavity or have provisions for locking.
• Are permitted to have a conduit hub or equivalent provision to exclude water at the conduit entrance if the entrance is above the lowest live part.
• Have provisions for drainage.
• Have suitable rust-resistance protection.
Type 3R enclosures are evaluated to demonstrate their conformance to environmental protection requirements by the following NEMA-specified tests:
• Rod entry test (reference NEMA 250.6.2) • Rain test (reference NEMA 250.6.4)
• External icing test (reference NEMA 250.6.6)
NEMA 4/4X - Water, Dust-Tight and Corrosion-Resistant Enclosures
NEMA Type 4 and 4X enclosures are intended for indoor or outdoor applications. Both types are used primarily to provide a degree of protection against windblown dust and rain, splashing water, and hose-directed water. In addition, the Type 4X enclosure is also intended to provide a degree of protection against corrosion. These types of enclosures are not intended to provide protection against such conditions as internal condensation or internal icing.
When completely and properly installed, Type 4 and 4X enclosures:
• Exclude water under test conditions that are intended to simulate a hosedown condition.
• Are undamaged after being encased in ice under test conditions.
• Are permitted to have a conduit hub or an equivalent provision to exclude water at the conduit entrance.
• Have mounting means, if provided, that are external to the equipment cavity. In addition to the above features, Type 4 enclosures have suitable corrosion protection, and Type 4X enclosures, in order to provide a degree of protection against specific corrosion agents, are made of American Iron and Steel Institute Type 304 Stainless steel, polymerics, or materials with equivalent corrosion resistance.
Type 4 and 4X enclosures are evaluated to demonstrate their conformance to environmental protection requirements by the following NEMA specified tests:
• External icing test (reference NEMA 250.6.6) • Hosedown test (reference NEMA 250.6.7)
• Corrosion protection test (reference NEMA 250.6.9.1 for Type 4 and NEMA 250.6.9.2 for Type 4X)
NEMA 7 - Hazardous Location Enclosures
NEMA Type 7 enclosures are intended for indoor use in hazardous locations classified as Class 1, Group A, B, C, or D, as defined in the National Electric Code. When properly installed and maintained, this type of enclosure is designed to contain an internal explosion without causing an external hazard.
Type 7 enclosures are designed to be capable of withstanding the pressures resulting from an internal explosion of specified gases and to sufficiently contain the explosion to the extent that an explosive gas-air mixture existing in the atmosphere surrounding the enclosure will not be ignited. Additionally, Type 7 enclosures are designed such that heat generating devices contained within the enclosure will not cause external enclosure surfaces to reach a temperature capable of igniting explosive gas-air mixtures in the surrounding atmosphere. When completely and properly installed, Type 7 enclosures:
• Provide a degree of protection to a hazardous gas environment from an internal explosion or from operation of internal equipment.
• Do not develop, when equipment is operated at rated load, surface temperatures that exceed prescribed limits for the specific gas corresponding to the atmospheres for which the enclosures are intended.
• Withstand a series of internal explosion design tests that determine: a. The maximum pressure effects of the gas mixture.
b. Propagation effects of the gas mixture.
• Withstand, without rupture or permanent distortion, an internal hydrostatic design test based on the maximum internal pressure obtained during explosion tests and the specified safety factor.
• Are marked with the appropriate Class and Group(s) for which they have been qualified.
Type 7 enclosures are tested and evaluated in accordance with the applicable portions of: • ANSI/UL 698, Industrial Control Equipment for Use in Hazardous Locations. • ANSI/UL 877, Circuit Breakers and Circuit Breaker Enclosures for Use in
Hazardous Locations, Class 1, Groups A, B, C, and D, and Class II, Groups E, F, and G.
• ANSI/UL 886, Outlet Boxes and Fittings for Use in Hazardous Locations, Class 1, Groups A, B, C, and D, and Class II, Groups E, F, and G.
• ANSI/UL 894, Switches for Use in Hazardous Locations, Class 1, Groups A, B, C, and D, and Class II, Groups E, F, and G.
SELECTING A LOW VOLTAGE MOTOR O/L RELAY Introduction
Overload relays are protective devices that guard low voltage AC motors against a variety of abnormal conditions that can overheat motor windings. The overload relays are designed to accomplish this protection by reflecting the heating characteristics of the motors that they protect. The two main components of an overload relay are the relay itself and the heater element.
When selecting an overload relay and its heater elements for application, several factors must be considered. These factors include the motor full-load current and service factor and the relay style, class, type, temperature compensation, and pole arrangement. This Information Sheet describes these overload relay selection factors. Note: Work Aid 1 has been developed to help the Participant select an overload relay.
Motor Data
Full-Load Amperes
An important factor used in the selection of the overload relay is the motor nameplate full-load amperes. The amperes marked on the motor nameplate represents the amount of amperes that the motor will draw continuously when delivering its nameplate-rated horsepower at nameplate-rated voltage and frequency. When an overload relay is applied to a motor circuit, it senses the motor line currents either directly or indirectly. For the case where the overload relay senses the current directly, the motor amperes flow directly through the relay and its heater elements. For the case where the overload relay senses the current indirectly, the motor amperes flow through the primary winding of a current transformer (CT) and allow the relay to sense the current via the secondary winding of the CT.
Because overload relays sense the line currents of a motor, they are sized according to the amount of amperes that they are capable of handling. Each size of relay is rated with a range of amperes that it can safely and continuously carry. Figure 13 shows an example of the ampere rating range for a few sizes of one particular manufacturer’s overload relay. When selecting an overload relay, the selected size must have a current range that covers the full-load nameplate amperes of the motor to which it is applied.
Motor Full-Load Amperes Overload Relay
0.25 - 26.2 AA13P
26.3 - 45 AA23P
19 - 90 AA33P
19 - 135 AA43P
Figure 13. Example of Full-Load Ampere Range for Various Sizes of Overload Relays
In addition to selecting the overload relay, the motor nameplate full-load amperes are also used to select the heater elements that are mounted in the relay block. The heater elements are in series with the power conductors of the relay, and they use the full-load amperes to generate and provide the heat that operates the bi-metallic contact in the relay. Similar to the overload relay, heater elements are sized and selected according to a range of full-load amperes for which they are designed.
Note: Work Aid 1 describes the procedures for using the motor full-load amperes to select both the overload relay and its heater elements.
Service Factor
Another factor that is used in the selection of the overload relay is the motor service factor (S.F.).
In accordance with NEMA MG-1, the service factor of an AC motor is a multiplier, which when applied to the rated horsepower, indicates a permissible continuous horsepower loading for the motor. When the voltage and frequency of a motor are maintained at nameplate values, the motor may be loaded up to the horsepower obtained by multiplying the rated horsepower by the service factor.
As a result of the maximum continuous horsepower load and, thus, maximum continuous amperes for a motor being affected by the service factor for the motor, the service factor is used in determining the maximum trip rating for the overload relay. In accordance with NEC Article 430, the overload relay must be selected to trip, or it must be rated at no more than the percent of motor nameplate full-load amperes shown in Figure 14.
Motor Parameter Percent of Motor Nameplate Full-Load Amperes (FLA)
Motors with S.F. > 1.15 125%
Motors with temperature rise < 40oC
125%
All other motors 115%
(Reference NEC Article 430-32) Figure 14. Maximum Overload Relay Trip Rating Based on
Motor Service Factor (S.F.)
Bi-Metallic O/L Relays Components
As schematically shown in Figure 15, a bi-metallic overload relay has two basic components: the relay itself, which contains the bi-metallic actuated contact, and the heater elements. The relay is available as either a single-pole relay or a three-pole (block) relay. The heater elements are constructed of resistance wire or similar material, and they are mounted inside of the relay body. Following is a description of each of these basic overload relay components.
Block-type relays are three-pole bimetallic, thermally actuated relays.
The physical construction of the block-type relay includes three sets of motor current-carrying connection terminals mounted on an insulated housing and used for connection to a three-phase motor circuit. Contained within the insulated housing (body) of the relay are provisions for inserting and connecting interchangeable heater elements. The relay provides a circuit that allows motor current to flow into the relay connection terminals, through the heater elements, and back out to the motor circuit.
Also contained within the insulated housing (body) of the relay is a bimetallic strip that is used to detect the heat generated by the interchangeable thermal elements. The bimetallic strip is mechanically connected to and operates a single-pole, single-throw, snap action switch. The snap-action switch is used to open the control circuit of the starter.
The block-type relay is rated in accordance with the range of full-load current that it is capable of carrying, the NEMA size of contactor it connects to, and the interchangeable heater elements designed for use with it.
Heater elements are constructed of resistance wire or similar material. They are designed to be inserted into and connected to the overload relay. Each block-type relay is constructed with three individual compartments to accept three individual heating elements. The heaters are connected to the relay in an arrangement that allows the motor current or CT secondary current to flow directly through them.
Individual heating elements are marked with their heater type numbers. Each manufacturer has its own form of designating the heater ranges and ratings. The precise current that a heater element is rated at depends on many factors, such as the number of heaters included in the overload relay and the type of enclosure used for the starter. However, in all cases, heaters are rated based on a range of motor amperes at which they will generate sufficient heat to cause the overload relay to operate. Typically, the heater(s) selected will provide for the overload relay to operate at 115% to 125% of heater rating at an ambient of 40oC.
Operating Principles
With reference to Figure 15, the operation of the bimetallic type of overload relay can be described by noting that the bimetallic strip is in a straight or unflexed state when it is relatively cool (e.g. when current through the heater is below the rating of the heater). In this position, the normally closed (NC) contact mechanically connected to the bimetallic strip is in its normal (closed) state. With the terminals of the heater connected to the motor circuit, motor current flows through the heater. As current flows, the power consumed by the heater (I2R) is converted to heat that acts directly on the bimetallic strip. In accordance with the inverse time versus current curve for the relay, when the motor current becomes excessive for a sustained period of time, the heat from the heater element will cause the bimetallic strip to deflect and operate the NC contact. Opening the contact, in turn, opens the coil circuit to the starter.
Solder-Pot O/L Relays Components
Solder-pot overload relays are thermally responsive relays that contain two basic component: a ratchet mechanism that operates a NC contact and a heater element as schematically shown in Figure 16. Following is a description of these basic components.
Ratchet Mechanism - With reference to Figure 16, it is noted that the ratchet mechanism is comprised of several parts. One part is a small cylinder that contains an alloy (e.g. solder) that will melt due to heat produced by excessive current flow. Within this cylinder is a portion of a shaft that is prevented from turning by the holding action of the alloy. The other end of the shaft is connected to a toothed ratchet wheel that interlocks with a pawl and holds a spring loaded actuator in the loaded position. At the end of the actuator travel path is an NC contact that is operated when the actuator is released and allowed to reach the end of its travel path.
Heater - The heater element for this relay is designed in the form of a resistance wire coil that mounts around the cylinder containing the alloy. Similar to the heater elements used for the bi-metallic type relay, the heater elements for the solder-pot relay are designed to produce a precise amount of heat in direct proportion to the motor current that flows through them. The heater elements are rated in accordance with a range of motor current that will cause the overload relay to operate when excessive motor current flows for a specified period of time. The characteristics of the heater cause the overload relay to operate with an inverse time-current characteristic.
Operating Principles
With reference to Figure 16, the operation of the solder-pot relay can be described by first noting, when the overload relay is connected for operation, that its heater terminals are connected to the motor circuit to allow motor current to flow through the heater. Prior to an excessive flow of current, the alloy in the cylinder is in a solid state allowing the ratchet to hold the actuator in place. When an excessive amount of current flows through the heater for a specific amount of time, the heat generated by the heater element acts directly on the alloy film, melting it at a precise temperature. Once the alloy is converted to a liquid state, the shaft within the cylinder is released allowing it to turn and rotate the ratchet wheel. Rotation of the wheel releases the pawl, which in turn releases the spring-loaded actuator. The released actuator then travels to the NC contact, and operates it to open the coil circuit of the starter.
Solid-State O/L Relays
Solid-state overload relays monitor motor line current and use semiconductor circuits to determine the heating effects that the level of current will have on the motor and conductors.
Components
The basic components that make up a solid-state relay are the main body (or block) and a selection of current sensing and special function plug-in modules. Following is a description of these components.
Block - The main body (or block) of the solid-state overload relay is physically constructed to hold three sets of motor current-carrying connection terminals mounted on an insulated housing. When placed in operation, the terminals are connected to the motor circuit to allow motor current to flow through the relay.
Contained within the relay body are built-in current transformers that are used to monitor the motor line currents and to translate them into logic level signals. Also contained within the body of the relay is a semiconductor circuit that represents a thermal model of the motor. The thermal model is typically calibrated to have an exponential function with NEMA overload relay Class 10 characteristics.
The main body of the relay provides for mounting of selected plug-in modules to build in the amount and type of protection desired. The selection of plug-in modules include current sensing modules and special function modules.
The main body of the relay also houses an electromechanical relay contact that is used for opening the coil circuit of the starter. This contact is normally provided as a single-pole single-throw (SPST) NC contact that is closed when the relay is energized and that opens when the relay trips or when control power is removed.
In addition to the above features, the solid-state overload relay is ambient-compensated, has both manual and automatic reset capabilities, and indicates overload trip operations through use of light emitting diodes (LEDs).
Figure 17. Current Sensing (Heater) Plug-In Module for Solid-State Overload Relay
Although the current sensing plug-in module receives logic level signals but does not receive actual motor amperes, it is still rated in units of motor line amperes. Nominal ratings for the current sensing plug-in module range from 0.54 amperes to 150 amperes. When a current sensing module for the solid-state relay is selected, the selection is made in accordance with the percent of full-load current desired to trip the overload relay. Similar to thermal type relays, the solid-state overload relay normally provides for trip operation at 115% to 125% of motor full-load amperes at 40oC.
In addition to the current sensing plug-in module that is required for operation of the solid-state overload relays, several special plug-in modules are available for optional selection to provide additional types of protection for the motor. These modules are physically
plugged-Operating Principles
Operation of the solid-state overload relay with a properly sized plug-in current sensing module follows the inverse time-current curve shown in Figure 18. Based on this curve, the relay will trip after 7 seconds at 600% full-load amperes for “cold” starts, after 4 seconds at 600% full-load current for “hot” starts, and ultimately at 115% of full-load current for long periods of time.
A principle advantage of the state relay over the thermally actuated type is that the solid-state relay operates with a one percent accuracy. The thermal type relay is not as accurate because small variations in tolerances in the mechanical elements of a thermal relay result in large variations in performance. On the other hand, solid-state overload relays are more expensive than thermal types, which make them less popular for smaller, less critical motors and loads.
Operation of the solid-state relay is accomplished with the CTs monitoring all three phases of the motor current. The current signals from the CTs are transposed, via solid-state circuits, to a logic level signal and then transmitted to the current sensing plug-in module. The plug-in module, which also contains solid-state circuitry, receives the logic signals and, using the thermal model circuit built into the relay, it determines the corresponding heating effects on the motor. When the current sensing module determines that the flow of current is excessive for a specified period of time (in accordance with Figure 18), it sends a trip signal to the NC electromechanical relay contact in the main relay, operating the contact and thus opening the external coil circuit of the starter.
Classes
Inverse-time overload relays are described by time-current characteristics, and, in accordance with NEMA ICS-2, they are designated with a class number indicating the maximum time in seconds at which they will operate (trip) when carrying a current equal to 600% of their current rating. The class number applies to the relay under the condition that overcurrents are balanced in all three phases. NEMA overload relay classes include Classes 10, 20, and 30. Figure 19 shows typical time-current characteristics for Class 20 and Class 30 overload relays. A description of each class follows.
Class 10
A NEMA Class 10 overload relay operates (trips) in 10 seconds or less when carrying a balanced overload current of 600% of its current rating.
Class 20
A NEMA Class 20 overload relay operates (trips) in 20 seconds or less when carrying a balanced overload current of 600% of its current rating.
Class 30
A NEMA Class 30 overload relay operates (trips) in 30 seconds or less when carrying a balanced overload current of 600% of its current rating.
Types
Thermally actuated bi-metallic overlay relays are available as one of two types, either Type A or Type B. Following is a description of each type.
Type A
The Type A overload relay is designed to protect industrial motors against overload conditions. Using a block-type, bi-metallic design, this relay provides Class 20 operation in either single or three-phase applications.
Type A relays are provided with field selectable manual or automatic reset modes. The relay is typically supplied from the manufacturer set for manual reset operation. However, it may be adjusted in the field for automatic reset by loosening the hold-down clamp at the base of the relay, repositioning the reset rod, and re-tightening the clamp.
The Type A relay has an inverse time delay trip with adjustable trip rating of the heater element over a + 15% range (approximately 85% to 115%) of its rating. This feature permits adjustment of the desired protection level and is accomplished by turning an adjustment knob located on the relay body.
The Type A relay is available as either ambient-compensated or non-compensated. Ambient-compensated relays have the advantage of providing the same trip characteristics in ambient temperature from -40oC to +77oC. Compensated and non-compensated relays are generally identified by the color of their reset rod.
For the Type A overload relays, interchangeable thermal heater elements for single-pole and block-type relays are available to cover motor full-load currents from 0.29 to 133 amperes in approximately 10% steps.
Type B
Using a block-type, bi-metallic design that provides Class 20 operation in either single or three-phase applications, the Type B overload relay is similar to the Type A overload relay in that it is also designed to protect industrial motors against overload conditions.
Additional similarities of the Type B with the Type A relay include: available ambient-compensated and non-ambient-compensated models, inverse time delay trip operation, standard SPST NC snap-action control contact, factory-available SPDT NO-NC contacts, visual trip indicator and available interchangeable thermal heater elements rated to cover motor full-load currents from 0.29 to 133 amperes in approximately 10% steps.
The basic differences of the Type B relay with respect to the Type A relay is that Type B relays are furnished only with manual reset capabilities, they have no trip adjustment knob, they provide a mechanical trip bar to manually check the contact operation of the relay, and they use a different reset-bar color code to indicate compensated and non-compensated relays.
Temperature Compensation Criteria Environmental Conditions
In the selection of overload relays, it is important to note and consider temperature environmental conditions. Following are conditions that should be considered.
Motor-Ambient - In accordance with NEMA MG-1, the ambient temperature rating of the motor is the maximum temperature of the medium and gases surrounding the motor that the motor is designed to operate in and to meet the ratings of its nameplate. Increased ambient temperature will cause an increase in motor operating temperature, which in turn presents a risk to the motor.
In the selection of overload relays, NEC Article 430 addresses the consideration of motor temperature rise and thus motor ambient temperature by requiring that overload relay trip ratings be limited based on rated motor temperature rise. In accordance with NEC Article 430, overload trip settings are to be limited to a maximum of 115% of motor full-load current for motors with a temperature rise greater than 40oC.
Starter-Ambient - The ambient operating temperature of the starter should also be considered. Starters operating in a constant ambient temperature that is within the rating of the overload relay will allow the relay to operate properly. This operation will provide for consistent and acceptable protection of the motor. For this condition, it is not a requirement to use a temperature compensated overload relay.
Severe Environments - For some cases, a starter and its overload relay may be located in one area where the ambient temperature varies, while the motor is located in a different area where the ambient is constant. The varying ambient temperature at the starter can result in improper operation of the overload relay. This operation will cause the protection of the motor to be affected. For this condition and similar conditions, where ambient operating temperature for the starter and the overload relay vary, it is important to use an overload relay that has ambient temperature compensation.
Ambient
In accordance with NEMA ICS-2, an overload relay identified as ambient temperature-compensated indicates that the ultimate current that causes the relay to trip remains essentially unchanged over a designated range of ambient temperatures.
The important feature of an ambient-compensated overload relay is that motor overload protection is provided with substantially the same trip characteristics in ambient temperatures that vary. Overload relays typically provide ambient temperature compensation for temperatures ranging from -40oC to +75oC.
In thermally actuated overload relays, temperature compensation is typically accomplished through use of a compensating bi-metal that is responsive only to heat generated by motor current that is passing through the heater element. The bi-metal maintains a constant “travel to trip” distance that is independent of ambient conditions. In this way, the operation of the relay remains essentially unchanged by any change in ambient temperature. The compensating feature is fully automatic, and no adjustments are required for its use when it is supplied with the overload relay.
An ambient-compensated overload relay should be used whenever the control is located in a varying ambient temperature area and whenever the motor that it protects is in a constant ambient temperature.
Non-Ambient
Non-ambient compensated overload relays are relays that do not have built-in features to automatically compensate for varying ambient temperatures. Whenever the overload relay is located in an area with a constant temperature, or whenever it is located in the same area as the motor, compensation may not be necessary.
Pole Arrangements Single-Pole
Thermally activated overload relays are available as single-pole or three-pole arangements. Single-pole overload relays can be used for application on single-phase circuits, or three individual single-pole units can be combined for use on a three-phase application.
The single-pole unit works as an independent overload relay with its own heater element and its own NC contact to open the starter coil circuit. Selection of a single-pole unit is accomplished in the same manner as selection of a three-pole block unit, with the selected relay rating and heater rating being based on full-load current. When three single-pole units are applied to a three-phase application, the individual NC contacts of the three units are connected in series to allow any one of the three to open the starter coil circuit.
The major advantage of selecting three single-pole units for a three-phase application is that the arrangement provides improved protection against a single-phasing condition, where one phase of the three-phase circuit becomes open. The disadvantages of using three single-pole units for a three-phase application in place of a single three-pole block are increased cost and increased space requirements.
Note: 16-SAMSS-503.5 requires thermally actuated overload relays to be three-pole block type.
Three-Pole
The use of a single three-pole overload relay for three-phase applications is the arrangement that is commonly used. This arrangement provides for the three current carrying poles of the relay to be mounted in the same insulated housing. The relay contains only one NC contact for use in opening the starter coil on a relay trip.
With the three-pole arrangement, overload relays can be designed to work with one, two, or three heater elements. Most modern thermally activated overload relays are designed to use three separate heater elements. The body of the relay is designed to allow mounting and connection of each heater in its own compartment, with the heat generated by all three heaters acting on the bi-metallic strip that operates the relay NC contact.
Other Considerations Single-Phasing
Single-phasing is a conditions that occurs when one phase of a three-phase circuit supplying a motor becomes open and allows the motor to operate as a single-phase motor. For this condition, the current in the phase that opens goes to zero while the current in the other two phases increases. Operating in this unbalanced condition results in overheating of the motor and can lead to damage or failure of the insulation if not detected quickly enough.
The potential problem when this condition occurs and a single three-pole block-type thermal overload relay is connected in the circuit is that the relay may not be able to detect the condition and operate. The operation of the relay depends on the combined heat generated by all three heater elements. With the relay operating with one phase open, the heater in the open phase will not generate any heat, and even though the current in the other two phases has increased, the increased heat of the two elements may not be sufficient to result in a total amount of heat that will activate the bimetallic strip and operate the relay.
Alternatively, if the single-phasing (or open-phase) condition occurred in a circuit using three single-pole relays, each of the relays in the two conducting phases would immediately detect the increase in current and, in accordance with its time-current curve, cause its relay to operate.
The three-pole solid-state relay does not have the same problem as the thermal relay in detecting an unbalanced current condition. For the solid-state relay, a special function plug-in module is available to detect unbalanced current conditions. Modules are available to trip on detecting either a maximum of 10% current unbalance or 20% current unbalance.
Process Criticality
When overload relays are selected and applied to nominally non-critical operating processes, the usual choice is to select the less accurate, but more economical thermally activated type of overload relay. For these applications, an occasional false trip due to the less accurate relay will not result in significant expense or loss of production.
However, when an overload relay is selected for a critical process, where priority must be given to maintaining the process operational, the selection should not be made on economy. For this type of application, the more expensive but more accurate solid-state overload relay should be selected. In this case, owing to the accuracy of the overload relay, the advantage will be a minimum of false trips.
Solid-state relays, owing to their more complex design using solid-state components, are more expensive to purchase than are the more simply constructed thermal relays. However, the solid-state overload relay operates with greater accuracy than does the thermal type.
Remote Access Sites
Another consideration when selecting relays is to determine whether the overload relay will be placed in a local or in a remote location. This determination can contribute to whether a manual or automatic reset is selected for the relay.
When overload relays are placed in an area that is local and accessible, the normal and accepted practice is to select a manual reset for the relay. Selection of a manual reset provides an opportunity, following a relay trip, for an operator to inspect a motor installation to determine the cause of the trip, and to establish that conditions are safe and acceptable for resetting and restarting.
However, there may be cases, where the overload relay is placed at a remote and inconvenient-to-reach location. In addition, operating conditions for the motor may be such that no danger or hazard is presented for an automatic restart following both a relay trip and a cooling period. Under these conditions, it may be an advantage to select an overload relay with an automatic reset.
SELECTING A LOW VOLTAGE MOTOR CONTACTOR
A major component in all motor starters is the contactor. The contactor is essentially an on-off switch that is operated by electromechanical means and that controls the flow of current to the motor. When selecting a contactor for application in a motor starter, several factors must be considered. These factors include the type of contactor to be selected (air-magnetic or vacuum), the size of contactor required for the application, the need of contactor auxiliary devices for operation of the control circuit, and the proper contactor coil voltage rating. This Information Sheet describes these contactor selection factors. Note: Work Aid 2 has been developed to help the Participant select a contactor.
Motor Contactor Types Air-Magnetic
The air-magnetic contactor is the most common type of contactor selected for motor starter applications. Figure 20 shows a typical NEMA air-magnetic contactor with an overload relay connected to its load terminals. This type of contactor is generally selected because it is economical and easy to maintain and because it has a versatile design that provides for accommodating a great many variations in the method of control.
The electrical portion of the contactor consists of an electromagnet, a coil, and a moving armature or crossbar. Moving and stationary contacts, arranged in sets or poles, carry the motor current. Air-magnetic contactors are often provided with three poles or sets of contacts. However, other configurations, such as two, four, or five poles are available.
When power is applied to the contactor coil, magnetic flux is created in the electromagnet. The magnet then attracts the armature, pulling the moving contacts into the stationary contacts and allowing power to flow through the contacts to the motor.
The air-magnetic contactor must be able to close, carry, and open normal motor current. As a result, the contactor is rated in accordance with the size of load that it must control. NEMA standards provide two methods of rating the air-magnetic contactor. One is a rating based on horsepower and the other is a rating based on motor full-load and locked-rotor current.
Vacuum
When selecting the type of contactor to use in a low-voltage motor starter, another choice is a vacuum type. Figure 21 shows a typical three-pole vacuum contactor. The vacuum type contactor offers several advantages. These advantages include a compact, lightweight design and a long service life. The most important of these advantages to consider is the long service life. With respect to air-magnetic contactors, service life is typically measured in
tens-of-thousands of operations. But in the case of vacuum contactors, service life is typically
measured in hundreds-of-thousands of operations. However, the comparision of a vacuum contactor with an air-magnetic contactor of the same rating, reveals that the vacuum contactors cost more.
The vacuum contactor is constructed with its main contacts sealed inside ceramic tubes from which all air has been evacuated (i.e. the contacts are in a vacuum). No arc boxes are required, because any arc formed between opening contacts in a vacuum has no ionized air to sustain it. The arc simply stops when the current goes through zero as it alternates at line frequency. The arc usually does not survive beyond the first half-cycle after the contacts separate. As a result of the vacuum’s limiting the amount of arcing, the rate of contact wear is reduced and contact life is increased.
The ceramic tube with the moving and stationary tubes enclosed is called a vacuum interrupter, or bottle. There is one bottle for each pole of the contactor. A two-pole contactor has two bottles, and a three-pole contactor has three bottles. A metal bellows (like a small, circular accordion) allows the moving contact to be closed and pulled open from the outside without letting air into the vacuum chamber of the bottle. Both the bellows and the metal-to-ceramic seals of modern bottles have been improved to the point that loss of vacuum is no longer a cause for excessive concern.
Aside from the difference in contact and interrupting medium (vacuum versus air) design, the vacuum contactor is used and applied in the same manner as an air-magnetic contactor. As a result, low-voltage vacuum contactors are designated by NEMA according to the same tables as used to size and rate air-magnetic contactors. NEMA sizing and rating criteria are described in the following section.