Ac/dc motors

Pataasin ang iyong marka sa homework at exams ngayon gamit ang Quizwiz!

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


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