Flashcard Set Preview
Side ASide B
What are the 2 main functions a motor controller preforms.
To disconnect the electrical power and protect a motor from an overload condition.
Common examples of fractional horsepower motors are...?
Exaust fans, garbage disposals, blower motors, small pumps, small drill presses.
What material is used for the surface of motor contacts and why?
Silver Cadmium has good conductivity and a long contact life
Normal motor operation consists of what 3 stages?
Resting, starting and operating under load.
What happens when a contact is opened under load?
An arcing occurs.
What is a magnetic motor starter?
An electromechanical device that provides a means of starting and stopping a motor.
Why is a magnetic motor starter not required to be located in close proximity to the machine operator?
Because pilot devices allow the starter to be located where it poses no danger to the operator.
List 4 examples of processes controlled by magnetic motor starters...
conveyor systems, pumping operations, packaging systems, paint booths and chemical mixing.
Why is it preferable to use a control voltage that is derived from the motor voltage?
Because both control and motor voltage will become de-energized when the motor controller disconnect is opened.
List 8 common control voltages...
24, 48, 120, 220, 208, 277, 480, 600
Addtion to main motor contacts other contacts are known as what?
Auxiliary contacts and electrical interlocks
Configurations available for auxiliary contacts are what?
N/O, N/C and combinations of both.
The word "normally" means?
It refers to the condition of the contact in a "power off" or "de-energized" state.
What is the difference between a contactor and a motor starter?
A motor starter has overload relays.
What is the main function of an overload relay?
To protect the motor winding from excessive heat caused by overload currents.
What are 3 basic types of overload relays?
Electronic, thermal, magnetic.
A thermal overload relay consists of what 3 parts?
Heaters, contacts and a tripping mechanism w/reset button.
What is the cause of most motor failures?
What are 2 types of overload relays?
Melting Alloy and Bi -metallic
What are 2 advantages of of using magnetic overload relays?
Greater flexability and quicker reset times.
What is (FLC) Full Load Current?
It is the MAXimum current in amps a motor is expected to draw while running under nameplate voltage, speed and torque conditions.
Where can you find the information reguarding (FLC)?
On the motor nameplate that is permanently attached to the motor housing.
What is the difference between a momentary and a maintained contact?
Momentary - the contacts will return to their origional position once the operator is released. Maintained - the contacts will stay in the same postion even after the operator...
Using pilot devices with maintained contacts to control a motor starter is known as what?
Two wire control.
Using pilot devices with momentary contacts to control a motor starter is known as what?
Three wire control.
When the control voltage differs from the voltage utilized by the motor how is the control voltage supplied?
Through the use of a seperate control transformer.
In most cases the stop pushbutton uses what type of contacts?
In most cases the start pushbutton uses what type of contacts?
General purpose relays are also know as what?
Ice Cube Relays
Explain the construction of a general purpose relay....
Consists of a coil and contains up to 4 sets of contacts sealed in a plastic enclosure.
Explain how the control wiring is physically connected to the male pins of the relay...
The matching male pins are inserted into a matching female base platform where the control wiring is physically connected.
Why are general purpose relays considered a fast and simple replacement device?
Because all of the external control wiring is terminated on the base platform that matches the relay.
What types of loads would be typical for general purpose relays?
Control circuits where small loads require precice control.
General purpose relays generally have a contact rating of how many amps?
General purpose relays are available in what different pole configurations?
2,3, and 4
Reed swiches have how many contacts?
1 in a hermetically sealed enclosure.
What material is the normally closed contact in a shielding activated reed switch is based from?
The distance required to activate a reed switch varies depending on the ? ? stregnth and the ? of the reed switch.
magnetic field, sensitvity
What is a reed relay?
It is a reed switch mounted in a sealed plastic enclosure...similar to an ice cube relay.
What is the difference between a reed switch and a reed relay?
- reed switch uses a permanent magnet -reed relay uses an electromagnet for changing contact positions.
Reed relays comes with ratings ranging from?
fractional to 3 amps.
The most common type of manual operator interface device used in motor control applications are called what?
The two main componets of a standard pushbuttons are called?
The operator and the contact block.
What causes the contacts of a pushbutton to change state?
The operator mechanially forces the contacts on the block.
What are the two methods standard pushbuttons use to operate contact blocks?
Momentary and maintained.
Describe the most common configuration for the contact block of a pushbutton...
N/O N/C and one of each.
How do the contacts return to their origional position once the pushbutton is released?
When contact blocks are attached to strandard pushbuttons, the contact block inherits the characteristics of the ? ? and are identified as etheir ? ? or ? ?.
Pushbutton operator, momentary contacts, maintained contacts.
Explain the operational difference of a contact block when it is attached to a maintained operator as opposed to a momentary operator.
The contacts function similarly except with maintained the contacts do not return to their origional position when the operator removes thir hand from the actuator.
List 3 basic methods that timers use to control timing...
Dashpot, motor driven, electronically.
What types of timing functions are timers capable of?
On-Delay, Off-Delay, One shot, and recycle timing.
List two specific examples of applications where a synchronous clock timer might be used...
Site lighting and lawn sprinkler systems.
What happens when the power is lost to a synchronous clock timer?
The timimg will be delayed by the ammount of time that the power is lost and the correct time will have to be reset manually.
What are 2 basic types of dashpot timers?
Pneumatic and fluid.
How is timing managed in a dashpot timer?
By controling fluid flow, air or oil, through a small orifice.
A part of a pneumatic dashpot timer that air enters or leaves durring the timing function is called a?
Diaphragm or bellows
Pneumatic timers have narrow trimmimg ranges from ? ? seconds.
.1 to 180
Dashpot timers use ? ? to initiate the timing process.
Explain the timing process for a dashpot timer...
As soon as the coil is energized or de-energized, the timing process begins and the rate of fluid flow determines the legnth of the timed delay.
Most dashpot timers are designed to control two types of contacts, what are they?
instantanous and timed
Durring the operation of a dashpot timer, when do the instantanous contacts change state?
When the coil is energized or de-energized.
What are the two categories of solid state timers?
Electronic and digital
Compaired to dashpot timers, what can be said about the accuracy solid state timers?
They are much more accurate.
Why are solid state timers more accurate?
No moving parts. Not effected by heat, humidity or pressure.
The timing operations of digital timers are normally set with ? ? or ?
Dioed switches or keypads.
Compaire solid state timers with dashpot timers, and explain the difference in the requirements for initiation of the timing function...
Solid state timers are constantly powered and need only an external triggering signal to initiate their timing functions.
Name 3 types of solid state mounting configurations...
Plug, Blade, Round.
Explain the difference between 8 and 11 pin solid state timers.
- 8 pin generally only preform "on-delay" functions -11 pin preform "off-delay" and multi-functions.
What are the current carring capacites for most solid state relays?
Less than 10 amps.