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Motor Control ch 6

Magnetic Contactors

QuestiAnswer
1. Motor control dates back to the 1800s. Name an early example of a motor controller. Knife switch
2. What is the difference between a contactor and a motor starter? motor starter has overload protection
3. Name the three major factors that led to the discontinuation of using knife switches to control motors. safety concerns for operator, opening and closing reduced its lifespan, exposed live parts.
4. Pilot devices used with manual motor starters are restricted to ___ operations and are connected in ___ with the line or load circuits single phase, series
5. Using pilot devices with maintained contacts to control a motor starter is known as ___ control. two wire
6. Fuses and breakers protect what part of the motor circuit? circuit conductors and controllers
7. Although knife switches are no longer used as motor controllers, a modern version of the knife switch can be found in what device? the disconnect
8. Why are double-break contacts used in contactors instead of single-break contacts? since the contact is broken in two places arcing is reduced.
9.What material is now used for the surface of most starter contacts and why? 90%Silver and 10% Cadmium, because good conductivity, resists arc damage, promotes long contact life
10. Does the oxidation of the silver alloy contacts, caused by arcing, create any conductivity or contact problems? no. It is a good conductor
11. Why are contact surfaces curved? to give a better connection and allows a self cleaning process
12. What happens when a contact is opened under load. 1. Increased risk of arcing 2. contact surfaces start to separate and temp increases 3. contact metal melts some 4. contacts separate, arcing until separates far enough p93
13. Explain the differences between the NEMA and IEC contacts. NEMA - bigger, more possibilities IEC - cheaper, application specific
14. Name three types of contactors, other than motor starters hydraulic elevators, HVAC devices, heaters, and lighting p95
15. Which type of magnetic contactor has options to keep the contacts pulled in when power to the coil is removed? can't find
16. Name three types of contacts found on contactors. normally open power poles, N/O aux contacts, N/C aux contacts
17. What AC coil voltages are typical for IEC contactors 24 600 VAC
18. A lighting load which has a continuous current rating of 250 amperes is to be controlled using a magnetic contactor. What size NEMA contactor would be required for this load?
19. What is the maximum size of motor which can be controlled using a Size 2 NEMA contactor? 15 HP
20. What class of IEC contactor is used to control a motor, when starting, plugging, and inching are typical operations required for the control of this motor? AC 4
21. If you were replacing an AC-3,23 A IEC contactor with a NEMA contactor, what size contactor would be required? size 2
22. Arc chutes are designed to quench the arc by ___, ___, and ___ the arc produced by each set of contacts. dividing, config(?) , and extinguishing
23. What is the maximum number of auxiliary contacts which can normally be installed on a magnetic contactor 8
24. The most common definite use contactor is the ___ contactor lighting
25. Lighting contactors are generally rated for ___ to ___ amps 15 to 20
26. Draw the NEMA symbol for a lighting contactor with four auxilliary contacts and 12 power poles. This had 4 stars on the question page 97
Created by: JoshHampton
 

 



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