Ac/dc motors
What is a Megger?
(Megohmmeter) An ohm meter which uses higher voltages to test insulation in a motor
What is the purpose of a UPS system in the plant?
(Uninterruptible Power System) A power supply that provides constant on-line power when the primary power supply is interrupted. Used if an outage will cause equipment, production or security issues.
A full wave rectifier is subjected to a 50Hz AC voltage. What is the frequency of the output voltage?
100
Max distance for a conduit run for a pull box or condulet?
100 feet
A DC motor, driven by a battery charged by a solar cell, is rated at 12 volts and 36 watts. It will draw close to _____ amps?
3 amps Formula: Power= Current x Voltage
What is the full scale current of a panel ammeter?
5 amps
A rotary switch that is used in electrical machines to reverse the direction of the current in the rotor coils is
A commutator
What is a Thermocouple?
A device that converts heat energy directly into electrical energy. Uses two dissimilar metals and puts out a millivolt output.
What is a Dynamo?
A generator that produces direct current using a commutator
What is the signal out of a Rotary Encoder?
A square wave
A rectifier (diode) is used to convert what?
AC to DC
The field windings of a DC shunt motor are burnt. The cause of this can be:
AC voltage applied to the winding
Name 2 ways a Transistor can be used?
As an Amplifier or as a Switch
A Float Switch is an example of what type of control switch?
Automatic
What's another name for Chinese Fingers (wire pulling aid)?
Basket Grip or Kellem Grip
What is meant by Plugging a crane?
Braking by reverse rotation
What's the most common way to turn off an SCR (silicon controlled rectifier)?
Break current between Cathode and Anode
How is speed controlled in a DC motor?
By the amount of voltage to the motor. Controlled by the amount of resistance to the field.
How is the Gate isolated from a Cathode in a Gate Pulse Generator using an SCR?
By using a transformer
In a PLC where are the calculations made?
CPU (Central Processing Unit)
How do you change the speed in an AC motor?
Change the frequency of the supply voltage.
What happens in a DC generator when the excitation coils are reversed?
Changes polarity of output voltage
What is the easiest way to blow a fuse in a meter?
Check amps on voltage setting
Before installing a wet motor, that has been dried out, what should you do first?
Check insulation
This will trip a 480V 3-phase breaker?
Continuous high current, Intermittent high current, Ground fault (All of the above)
Which of the following is not suitable for the core material in electromagnetic devices
Copper
What is the main current limiting factor in a DC Motor?
Counter EMF
An Ammeter is a device used for measuring?
Current
The ampere is the unit of measurement for?
Current
Which of the following can be used to measure torque of DC motor.
Current
When a short happens what voltage change is it followed by?
Decrease in voltage
breakdown maintenance cost ______ with the increase in preventative maintenance cost.
Decreases
Bearing failure in motors can be caused due to wrong selection of bearings. A motor mounted horizontally or vertically needs ____ types of bearings.
Different
Which of the following is a fault that is caused by the rotor of a synchronous motor
Eccentricity
Name 3 things that will saturate the core of a transformer?
Excessive voltage, Low frequency, DC Current
To make this output energize ----l l---l l---l/l---( )---
False, False, True
In a dual voltage 3-phase motor how are the wirings hooked up?
For High Voltage: Windings hooked up in Series For Low Voltage: Windings hooked up in Parallel
____ is zero in transformers even at full load.
Friction loss
When wiring a panel, what is the one thing that is forgotten most?
Grounding
The Primary coil of a power transformer has 100 turns and the Secondary coil has 50 turns. The voltage across the Secondary will be?
Half that of the Primary
What is used for a dielectric withstand test
Hitpot tester
What does HTTP stand for?
Hypertext Transfer Protocol (HTTP)
How do you increase the surge breakdown voltage for an SCR?
Increase value of Snubber Capacitor
Which of the following instruments can be used to measure the alternating current?
Induction meter
What is the formula for Inductive Reactance and Capacitive Reactance?
Inductive Reactance: XL=2piFL Capacitive Reactance: XC= 1/2piFC pi=the symbol for Pi Also, for XC, that's 1 over 2piFC
The cross sectional area of a copper wire ___ related to its resistance
Inversely
When a circuit has too much gain, what happens to the circuit?
It becomes unstable
In a pure resistive circuit, if voltage goes up what happens to current?
It goes up also
How can you tell if a motor is a synchronous motor?
It has 2 slip rings. Used to induce DC voltage
What would happen if you phase off a Gate Pulse Generator?
It would become unstable
Typical PLC programs are written using what language?
Ladder logic
A diagram used to show the relationship between circuits and their correspondance to the actual location of the components?
Line Diagram
In an Electric Circuit, where would you put the fuse?
Line side
In a Shunt Generator, Voltage fluctuates with?
Load
How does Dynamic Braking work?
Method of motor braking in which a motor is reconnected to act as a generator immediately after it is turned off. This puts resistance in the armature circuit.
In a 5-digit PLC address, what does the 3rd digit represent?
Module #
In some plant operations, DC current is required where only AC is supplied by the electric company. A device that is used to convert the AC to DC current is called a?
Motor Generator
Most electrical problems involving Voltage, Resistance and Current are solved by applying?
Ohm's law
The unit of measurement for resistance is?
Ohms
Where do you ground a shielded cable?
One end only
Bimetallic strip in motors is used to prevent
Overload
What is the Binary Equivalent of 38?
Please note..you must use the following guide. 128 64 32 16 8 4 2 1 read from left to right. a) Put a 0 on any placement above the number b)Place a 1 on number below, that adds up to the number without going over A. 00100110
What is a Potential Transformer used for? Current Transformer?
Potential Transformer used for metering and relays. Current Transformer measures high current
The current and voltage passing through resistance are doubled. The power will be
Quadrupled
The imaginary part of the impedance of an ac circuit is known as
Reactance
After cutting bent conduit, what should you do next?
Ream it
Which is not a benefit of predictive maitenence
Reduce uptime
Voltage drop in a circuit is usually due to?
Resistance
When putting a 3-phase motor in service, what should you check?
Resistance in windings
What would you use to seat the brushes in a DC motor?
Sandpaper
What 2 items make up an Optical Encoder?
Sensor and LED
If a current transformer is not hooked up to a meter, what must be done to it?
Short (tie) the secondary wires together
In induction motors the difference between the synchronous and asynchronous speed of the rotor is referred to as
Slip
What is the core material made of in a DC coil contactor?
Soft iron
What is a Transducer?
Takes one form of energy and converts it into another form of energy.
What should be done before removing an Ammeter from a current transformer?
The current transformer should be short circuited
What is Slip?
The difference between synchronous speed and the actual speed of an AC motor
Define jogging
The frequent starting and stopping of a motor for short periods of time.
What's a Growler?
The instrument used to check the armature of small DC motors for shorts, grounds or an open circuit.
When brushes are set in the neutral plane how will the motor run?
The motor will run at normal speed
What happens to the motor if the field opens in a DC shunt motor while the motor is running?
The motor will run away
What happens to the motor when you t ake resistance out of the field in a DC shunt motor?
The motor will slow down
If a transformer is not marked, how would you tell which is the primary and which is the secondary?
The secondary would have heavier gauge wire
What is the purpose of interpoles?
They are in the armature circuit to limit arcing on the commutator
If you increase the base voltage of a transistor what happens to the emitter and collector current?
They both increase
The main reason for making wire stranded is?
To make it more flexible
What is a Hickey?
Tool used to bend Conduit
What is the easiest way to determine how much current a motor is drawing?
Use a clamp on ammeter
To clear a PLC after a fault?
Use the PLC programming software
What's the purpose of a Hoist Brake on a Crane?
Used to hold the lifted weight, not to stop it
What is an Optical Pyrometer?
Used to take temperature readings of materials
In the case of a short circuit
Voltage is high, resistance is low
A DC stepper motor is taking one step ahead and one step backward. What can be the possible cause of the behavior?
Wrong sequence of windings
The average value of ac voltage and current is
Zero
The frequency of DC current is defined as
Zero
If you had a load of 50kW with a .8 power factor, what size transformer do you need?
power factor= true power/apparent power apparent power= 50000 watts/.8 = 62,500 or 625kVA
An OhmMeter is a device used to measure?
resistance
What does URL stand for?
uniform resource locator (web address)
When saturated the resistance of a transistor is
zero
What does TCP/IP stand for?
Transmission Control Protocol/ Internet Protocol
