Unit 11 - Contactors, Relays, and Overloads

what are the three major parts of a contactor or relay

- coil - contacts - mechanical linkage

what is a magnetic starter

- four sets of contacts, a magnetic coil to close contacts when coil is energized and a set of overloads - overload protection to the load it controls - overload de-energizes the coil

a contactor or relay coil could be electrically diagnosed in which of the following conditions

- open - shorted - good

what are the two types of armatures used in contactor relays

- sliding armature - swing armature

what are the major reasons for replacing the contacts of a contactor

Proper voltage must reach the load - any voltage above 5% is excessive Dirty, pitted contacts must be replaced

the pilot duty overload breaks .......

auxiliary set of contacts connected to control circuit

what are the three types of overloads used in magnetic starters in the industry. how does each type work

bi metal relay - two metals welded together with different expansion qualities thermal relay - operates on heat going through a wire to determine current flow and contacts will open on overload molten alloy relay - ratchet wheel soldered to conductor. When higher than normal temp, solder melts allowing ratchet wheel to open contacts

what is the advantage of an internal compressor overload

can sense current draw of motor as well as winding temp

name two basic types of pilot duty overloads and describe how they work

current overload - similar to line break overloads except that a pilot duty set of contacts is opened rather than the line magnetic (heinemann) overload - moveable metal core that is pulled up, opening the pilot contacts

what is the purpose of the overload

electric device that protects a load from a high ampere draw by breaking a set of contacts

which of the following types of overloads gives the fastest response time to an overload condition

electronic

true or false = the contacts in a relay are easily checked by a visual inspection

false - contacts are usually hidden and cannot be visually inspected without disassembly

true or false = the most popular line break overload used in small equipment is the two wire klixon overload

false - internal compressor overload

true or false = push button stations are widely used in the industry

false - motor control, circuit control and magnetic starters

true or false = magnetic starters are primarily used on single phase motors

false - three phase because they contain overloads and can effectively control the operation of loads in heating, cooling, refrigeration system

which of the following is the simplest overload in the industry

fuse

major difference between a magnetic starter and a contactor

magnetic starter house its own overload

what is the purpose of a contactor or relay

make or break a set of contacts that control the voltage applied to some load in cooling systems - used to control an electric load in a control system

the easiest faults to diagnose with a contactor are usually problems with the .......

mechanical linkage

how does a line break overload operate

metal disc mounted between 2 contacts (bi metal) - if current draw or temp of motor cause disc to overheat and expand, the contacts open

what is the proper procedure for checking an internal compressor overload

motor - open starting or run windings, chances are the common or the internal overload is open - compressor housing cool, motor might be damaged - compressor housing warm or hot, overload could be open

true or false = the size of the relay coil is smaller than the contactor coil

true

true or false = a voltage reading taken across closed contacts of the same pole will show the voltage drop across the contacts

true - contactor must be closed

what is the largest electrical load in an air conditioning or refrigeration system

compressor

major difference between a relay and a contactor

current carrying capacity (contactor)

why is it difficult to repair a contactor

difficult finding necessary parts

what is the characteristics of the current draw of an electric motor from locked rotor (start) until the motor reaches full speed

higher amp draw on startup than while running

an electric motor is an ..... load

inductive

what do the terms "normally open" and "normally closed" mean in reference to relays and contactors

normally open - opens when de energized and closes when energized normally closed - closes when de energized and opens when energized

the line break overload breaks .......

power to motor

an electric heater is an ..... load

resistive

contacts are usually made of ......

silver and cadmium alloy