HVAC Electric Review 2
Which of the following motors is most energy efficient?
. ECM
In a schematic, what voltage would be found across the contacts when energized in line
0 volts
In a schematic, what voltage would be found from terminal G to the load side.of.the transformer with the system operating in cooling mode?
0 volts
What voltage would be found across a fuse if the system is working normally and the thermostat closed?
0 volts
Which of the following is equal to 100 milliamps?
0.10 amperes
Three 300 ohm resistors are wired in parallel. The total resistance equals?
100 ohms
If the fuse in L2 is blown the in a 220 volt circuit you would read how many volts in the circuit?
120 to ground from L1
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
Which of the following is equal to 2.1KW?
2,1OO Watts
The voltage to ground of a "high Leg" or "Stinger Leg" on a three phase open Delta system will be approximately?
208
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
In a schematic, what voltage would be found across the transformer primary on a 240 volt system?
240 volts
What voltage would be found across L1 to L2?
240 volts
Utility companies usually supply residential homes with___________service?
240V, single phase, three wire
Relays are generally used to control inductive loads up to ________ amps.
25
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
What is the minimum number of wires required between two Start/Stop push button controls?
3
Which of the following is an acceptable temporary replacement for a run capacitor rated at 35MFD@440VAC?
37.5MFD @ 440VAC.
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 a 9 ohm resistor. The total resistance equal?
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 = 30 ohms R2 = 15 ohms R3 = 10 ohms. If the current through R2 = 4 amperes, what is the source voltage?
60 volts
A transformer has 1000 turns of wire in the primary coil and 500 turns in the secondary coil. If 120 volts are applied, the output voltage will be?
60 volts
One horsepower is equal to _______ watts
746
An ohmmeter should never be connected to?
A charged capacitor until it has been discharged
Any material that will readily release electrons is called?
A conductor
When troubleshooting the A/C system, 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. This could be caused by?
A faulty blower motor or blower relay
When troubleshooting the A/C system, 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 I FAN ON switch
An electron has?
A negative charge
In a series circuit, the voltage drop across each resistance (load) 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.
The blower motor is most likely controlled by the?
Blower motor relay
To change direction of rotation of a three phase motor?
Change the position of any two supply wires
Contactors are rated according to the maximum amperage through the?
Contacts
A diode is used to?
Control the direction of electron flow
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
Resistance can be calculated by?
E divided by I
A solenoid valve coil operates on the principal of?
Electromagnetism
Current flow is?
Electron flow in a circuit
Direct Digital Control (DDC) is?
Electronic controls use for equipment operation and monitoring
If the control system is set for heating and the temperature in the structure drops below the set point, the first stage of the heating thermostat opens. True or False?
False
The schematic wiring diagram uses photos to represent electrical components in the system. True or False
False
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 color coded?
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-capacitor run (CSCR) motor provides?
High starting and running torque.
The symbol for current is?
I
Voltage can be calculated by?
I times R
Capacitors are always connected_____________to the start winding?
In series
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 motor is a?
Line starter
What connects to the terminals T1, T2, &T3 on a contactor or motor starter?
Load connections
A(n) ____________________ system is a control system that is operated by less voltage than that applied to the equipment.
Low-voltage control system
The thermal overload contacts on a line starter are usually_________ reset.
Manual
When testing a capacitor with an analog or digital ohmmeter, the meter should?
Measure low resistance then return to high reading
Capacitors are rated in?
Microfarads & Volts
Single phase motor speed is determined by the?
Number of stater poles.
The crankcase heater will normally operate when the compressor is?
Off
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
What would be the result, if a 24 volt, 20-ohm low voltage contractor coil were wired in series with a 24 volt, 200-0hm control relay coil?
Only the control relay would operate.
Impedance is?
Opposition to alternating current flow
Resistance is?
Opposition to current flow
The____________drawing illustrates where the components are located in small appliances.
Pictorial
A transducer can create a signal based on?
Pressure.
Circuit breakers &fuses are primarily used to provide?
Protection from an overload of current
An electronically commmutated 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 the contacts or control
On a good single-phase compressor motor, which winding terminals have the highest resistance?
Run to Start
The ____________________ diagram not only identifies the electrical components that are in the unit, but also illustrates how the unit works and electrical connections
Schematic Diagram
What is the term used to describe the output side of a transformer?
Secondary side
Excessive current can be caused by?
Short circuit
Which of the following materials would be best suited for use as a semi-conductor?
Silicon
A typical thermocouple used on a gas furnace can generates a?
Small DC electrical current
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
Direction of rotation of a single phase induction motor is determined by?
The direction of current flow through the start winding
In electrical wiring, a black colored wire in an alternating current circuit usually indicates?
The hot conductor
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 of?
Three supply lines
On an electronic oil safety switch what changes the pressure signal to electrical signal?
Transducer
An electric motor is an inductive load True or False?
True
Current flow will increase when voltage increases True or False?
True
To discharge a capacitor prior to ohmmeter testing?
Use a 15,000 or 20,000 ohm, 2 to 5 watt resistor across the terminals
Transformers are rated according to?
VA at the secondary
A step up transformer increases?
Voltage
Fuses are sized by?
Volts & Amps
The correct formula for determining wattage in an electrical circuit?
W = E times I
The electric meter on a house measures the ...............passing through it
Wattage
Which of the following is the symbol for Impedance in an AC circuit?
Z
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.
