Chapter 6 Homework
A Class __________ overload relay will trip an overloaded motor offline within 10 seconds at six times full-load amps.
10
AC arc tends to be self-extinguishing when a set of contacts is being opened.
AC
Which contactor operating mode is absolutely silent?
Magnetic contactor
Which of the following NEMA enclosures would most likely be selected to provide a degree of protection against contact with enclosed equipment in a location where unusual service conditions do not exist?
NEMA type 1
The most used high-power switching semiconductor is the SCR .
SCR
A way to provide a magnetic field that will help extinguish an arc as quickly as possible is to use
a blowout coil
An AC contactor coil has
a high in-rush current
Radio-frequency interference (RFI) caused by abrupt switching of SCRs is reduced by
a snubber circuit
To prevent the armature of a contactor coil from being held in by residual magnetism, a permanent __________ __________ must be provided in the magnetic circuit.
air gap
A contactor fails to seat properly when energized. This may result in
all of these
Enclosures are selected according to the environment in which they must operate.
environment
A broken or open shading coil will cause a contactor to become extremely quiet.
f wrong is t
When a motor starter coil is de-energized
gravity and spring tension open the main contacts
Arc chutes not only insulate each set of contacts from each other but also
help contain and quench the arcs
A contactor surge RC suppression module is connected
in parallel with the coil
The purpose of a shading coil on an AC contactor pole tip is to
prevent chattering
The core and armature of AC contactor assemblies are made of laminated steel to
reduce eddy current flow
Contacts are silver-coated to reduce contact resistance .
resistance
As the distance between contacts increases, the
resistance of the arc increases
A furnace or oven would be classified as a(n) resistive load for selecting a heater element.
resistive
Branch and feeder fuses and circuit breakers protect motor circuits against a __________ circuit or a ___________ fault.
short, ground
The purpose of overload protection on a motor is to protect the motor from __________.
sustained overcurrents
A NEMA size 4 contactor has a greater current capacity than a NEMA size 2 contactor.
t
Shading coils are never used with DC contactor coils.
t
A mechanically held contactor
all of these
A solid-state contactor
all of these
Compared to thermal-overload relays, electronic overload relays are
all of these
Dual-element (time delay) fuses, when properly sized, provide
all of these
Motors are subject to
all of these
The main contacts of a contactor
all of these
The current drawn by a motor is
an accurate measurement of motor load
IEC contactor and starter rating are designed to meet the requirements of a number of defined applications .
applications
When switching high current, some type of arc suppression is normally required.
arc
Arc chutes may contain heavy copper coils called __________ coils.
blowout
Compared to IEC-rated contactors and motor starters, equivalent NEMA devices are
built for a higher level of performance
The overload relay contact is connected in series with the __________ of the starter.
coil
A thermal overload relay is designed so that it can be reset immediately upon tripping.
f
Arc chutes are made of copper to provide a good conducting path.
f
Arcing can occur only upon opening of contacts.
f
Blowout coils connect in parallel with the contacts.
f
Contactors like relays, are designed for pilot duty in control circuits.
f
NEMA starters are smaller in size than there IEC equivalents.
f
One heater is required to protect 3-phase motors.
f
Resistive loads such as heating elements are classified as nonlinear loads
f
Solid-state contactors are commonly used to control very high current loads.
f
The bimetallic thermal-overload relay utilizes the principle of heating solder to its melting point.
f
The current flow through an AC contactor coil is limited only by resistance.
f
The current to a motor increases as the motor accelerates.
f
The electrical markings and symbols adopted by various nations conform to the same standards.
f
The least costly type of enclosure is the explosion-proof type.
f
To reduce arcing, the contacts must separate slowly with a small air gap that gradually extinguishes the arc.
f
Two SCRs connected in series are necessary to switch single-phase AC power.
f
With an increase in current, heat will decrease.
f
A capacitor switching contactor uses dampening diodes across each pole designed to minimize the effects of the large charging current.
f t for wrong
Pilot lights operating in conjunction with a contactor are normally controlled by power contacts.
f t for wrong
The resistance reading of an open contactor coil would be zero.
false
The pick-up voltage rating of a contactor coil is normally
greater than the drop-out voltage rating
In AC contactor coils, the current through the coil is limited by the impedance of the circuit and the reactance has a greater effect than the resistance .
impedance resistance
Contactor coils are molded into an epoxy resin to
increase moisture resistance
A thermal overload relay operates on the principle that as the level of current increases, the temperature across the heating element
increases
A motor load would be classified as being
inductive
Power flows through the contactor from the __________ side to the __________ side.
line, load
A shading coil sets up a magnetic field which is out-of-phase with the main field.
out-of-phase
Contactor coils are built to operate
over a range of 85 to 110% of rated voltage
A/an __________ occurs when electrical equipment is required to work harder than it is rated.
overload
Contactors may be used for switching motor loads when separate __________ is provided.
overload protection
Contactors can be automatically controlled by very sensitive pilot devices.
pilot
A magnetic contactor is a magnetically actuated device for repeatedly establishing or interrupting an electric power circuit.
power
The two circuits involved with the operation of a magnetic contactor are the
power and control circuits
Capacitor switching contactors are used to switch
power factor correction loads
A bimetal strip uses two dissimilar metals bonded together.
t
A magnetic blowout coil produces a magnetic field that helps to blow out the arc.
t
AC and DC contactor coils with the same voltage ratings are not normally interchangeable.
t
An overload relay is capable of being reset once the overload is removed.
t
Any device that makes use of a coil of wire can be classed as an inductive load.
t
Arc chutes are used on contactors to help confine the arc.
t
Arc current flow produces additional heat, which can damage contact surfaces.
t
Arcs produced by DC current flow through contacts are considerably more difficult to extinguish than those produced by AC current flow.
t
Auxiliary contactor contacts should never be used to switch high-current load circuits.
t
Heater units are selected from the manufacturer's heater chart by matching the full-load motor current this the heater unit number.
t
High voltage may be controlled by a contactor and kept entirely isolated from the operator.
t
If the control circuit is connected directly to the same line wires as the load than it must be rated for the same voltage.
t
NEMA and IEC use different criteria for rating contactors and motor starters.
t
Silver contacts should never be filed.
t
The anode and cathode leads of an SCR are equivalent to the main contacts of an electromagnetic contactor.
t
The main difference between contactors designed for AC power and those designed to switch DC is the enhanced ability to quench the arc so that it is not sustained any longer than necessary.
t
The overload relay incorporated in a motor starter distinguishes the motor starter from a contactor.
t
The purpose of overload protection is to protect the motor windings from excessive heat.
t
The silver oxide that appears as a black discoloration on contacts is a relatively good conductor of electricity.
t
Use of electronic overload relays reduces the amount of heat generated by a starter.
t
When current to a contactor coil is turned off a high voltage spike is generated.
t
When the magnetic starter reset button does not re-establish the control circuit after an overload, it is most likely because
the overload trip has not cooled sufficiently wrong- the overload heater is burned out
Contactor auxiliary contacts are actuated by the same armature as the power contacts.
true