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