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