final exam hvacr
which of the following is equal to 100 milliamps
.10 ampres
Relays are generally used to control inductive loads up to ________ amps.
10
Three 300 ohm resistors are wired in parallel. The total resistance equals?
100 ohms
utility companies usually supply residential homes with
115, single phase, three wire
how many watts of power are used by a heater operating at 240 volts and drawing 5 amps
1200 watts
The synchronous speed of a four pole, 60 Hz single phase motor is?
1800 rpm
what is the minimum number if wires required between two start/ stop push button controls
2
The voltage to ground of a "high Leg" or "Stinger Leg" on a three phase open Delta system will be approximately?
208
which of the following is equal to 2.1 kw
2100 Watts
A transformer has 1000 turns of wire in the primary coil and 2000 turns in the secondary coil. If 120 volts are applied, the output voltage will be?
240 volts
Three defrost heaters, each 15 ohms are wired in parallel to a 120 volt power supply. How many watts of power will be used?
2880 watts
which of the following is an acceptable temporary replacement for a run capacitor rated at 35uf @ 440 vac
37.5 uf @440 vac
which of the following is generally considered to be within the maximum allowable plus or minus supply voltage variation
5%
a parallel circuit contains a 18 ohm and 9 ohm resistor the total resistance equals
6 ohms
how much resistance must be placed in a 120 volt circuit in order to limit the current to 2 amps
60 ohms
a parallel circuit consists of three resistors r1= 30ohms r2= 15ohms r3=10ohms if the current through r2= 4 amps then what is the source voltage
60 volts
a transformer has 1000 turns of wire in the primary coil and 500 turns in the secondary if 120 volts are applied the output would voltage would be
60 volts
one horse power is equal to
746 watts
An ohmmeter should never be connected to?
A charged capacitor until it has been discharged
resistance can be calculated by
E divided by I
which of the following motors is most energy efficient
ECM
direct digital control (DDC) is
Electronic controls use for equipment operation and monitoring
a typical thermocouple used on a gas furnace can
Generate a small DC electrical current
symbol for current
I
voltage can be calculated by
I times R
capacitors are rated in
Microfarads & Volts
Single phase motor speed is determined by the?
Number of stater poles.
a capacitor analyzer is designed to read
The capacitance of a capacitor
when testing the windings of a single phase motor
The common to run terminals show the lowest resistance
Current flow is
The flow of electrons
resistance is
The opposition of current flow.
when resistors are wired in parallel
The voltage drop across each resistor will be the same as the applied source voltage
a three phase load requires a minimum
Three supply lines
transformers are rated according to
VA at the secondary
which of the following is the symbol for impedance in an AC circuit
Z
any material that will readily release electrons is called
a conductor
when trouble shooting the a/c system in the schematic the following conditions are found: the thermostat contacts are closed, the compressor and condenser fan motor are short cycling the blower motor is not running this is caused by
a faulty blower motor or blower relay
when trouble shooting the A/C system in the schematic the following conditions are found. the thermostat contacts are closed the compressor and condenser fan motors are short cycling the blower motor is not running. when the fan switch at the thermostat is moved from the auto position to the fan on position the blower runs this could be caused by
a faulty thermostat- auto/ fan on switch
an electron has
a negative charge
in a series circuit the voltage drop across each resistance will
add up to equal the source voltage
when checking the windings of a three phase motor
all three terminals should show the same resistance
fuses are sized by
amps and volts
to change direction of the rotation of a three phase motor
change the position of any two supply wires
contractors are rated according to the maximum amperage through the
contacts
a diode is used to
control the direction of electron flow
which of the following statements about current is true
current flow will increase when voltage increases
a ___ is commonly used on a single phase capacitor start fractional horse power compressor motor to disconnect the start winding after the motor starts
current relay
when the temperature of a negative temperature coefficient NTC thermistor increases its resistance will
decrease
which of the following would be considered an inductive load
electric motor
a solenoid valve coil operates on the principle of
electromagnetism
With a motor running at full load, voltmeter leads are placed on L1. and T1 terminals of the contactor. The voltmeter indicates no measurable voltage. This is an indication of?
good clean contacts
the safety ground conductor is usually
green or bare
When testing a motor with an ohmmeter, the following measurements are recorded, Common to run 4 ohms, Common to start 6.2 ohms, Run to start 10.2 ohms, and Common to ground 20.4; This indicates a/an?
grounded condition
the capacitor start run motor provides (cscr)
high starting and running torque
when the temperature of a positive temperature coefficient PTC thermistor increases its resistance will
increase
If the conductor in an AC circuit is wound into a coil, the resulting EMF opposes the supply voltage. This opposition is called?
inductive reactance
the total resistance of a parallel circuit will be
less than the smallest resistance
a device with overload protection used to control a monitor is a
line starter
what connects terminals T1, T2, T3, on a contractor or motor starter
load connections
The thermal overload contacts on a line starter are usually ___ reset.
manually
when testing a capacitor with an analog or digital ohm meter the meter should
measure low resistance then return to a high reading
what would be the result if a 24 volt 20 ohm low voltage contractor coil were wired in series with a 24 volt 200 ohm control relay
neither the contractor or relay would operate
which meter is used to bench check a motor for shorts or open windings
ohmmeter
which of the following meters must use its internal power to check a load or circuit
ohmmeter
impedance is
opposition to alternating current flow
the ___ drawing illustrates where the components are located in small appliances
pictorial
circuit breakers and fuses are primarily used to provide
protection from an overload of current
an electronically commented motor ECM is designed to
provide proper air flow under various conditions
like charges
repel each other
If contacts become badly pitted, it is acceptable to
replace contacts or control
On a good single-phase compressor motor, which winding terminals have the highest resistance?
run to start
what is the term used to describe the output side of a transformer
secondary side
capacitors are always connnected in
series to the start winding
excessive current can be caused by
short circuit
which of the following materials would best be used as a semi conductor
silicon
direction of rotation if a single phase induction motor is determined by
the direction of current flow through the start winding
in electrical wiring the black wire in an alternating current circuit usually indicates
the hot conductor
when trouble shooting the A/C system schematic the following conditions are found. the thermostat contacts are closed. the compressor and condenser fan are not running. the blower motor is running this could be caused by:
the hp,lp, overloads, or oil pressure switch are open
On a electronic oil safety switch what changes the pressure signal to electrical signal?
transducer
To discharge a capacitor prior to ohmmeter testing?
use a 15,000 or 20,000 ohm 2 to 5 watt resistor across the terminals
a step up transformer increases
voltage
which of the following is the correct formula for determining wattage in an electrical current
w=e times i
The electric meter on a house measures the _______ passing through it.
wattage
when current flows in a two wire circuit (such as 120 volts) if a clamp on amp meter is clamped around both wires what measurement would you expect
zero amps due to cancellation of the two fields
