Conclusiones. Conclusiones

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.)

Note: Work Aid 1 describes the procedures for using the motor service factor to select both

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).

Modules - In place of the type of heater elements used by thermally actuated overload relays,

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.