Motors Exam 1 Study

Exam study flashcards for Exam 1. Chap. 1-3

What is Electromagnetism?

When electrical current flows through a conductor, a magnetic field is created around the conductor.

What three properties determine the magnetic strength of an ELECTROMAGNET

• Intesity of the current

• Number of turns in a coil

• Type of core material

What is meant by the term, “ampere turns.”

It is a measurement describing the magnetic strength of an electromagnet based on current and the number of wire turns

What is the priniciple of electromagnetic induction?

A changing magnetic field can induce a voltage into an electrical conductor

Which three things are necesary for magnetic induction to occur ?

• Electrical Conductor

• Magnetic Lines of Flux

• Relative motion between Lines of Flux and Conductor

The Strength of an induced voltage is proportional to which 3 factors

• # of wire turns

• Magnetic lines of flux density

• The speed of cutting action

What is it called when there is only one wire coil on which both electromagnetism and magnetic induction act

Self Induction

What are the two important points about self induction?

• Inductors oppose change in current

• Both resistance and reactance oppose current flow

What does it mean when inductors oppose a change in current?

Inductors have a stabalizing effect on current

What is another name for CEMF?

Counter Voltage

Lenz’s law says that an induced voltage will have the opposite polarity of-?

The electrical source that created it

How can the resistance of an inductor coil be determined?

With an Ohmmeter

What happens whenever electrical current flows through resistance?

Heat dissipates from the conductor

Reactance is the CEMF that does what to current flow in an inductor?

Delays it when voltage is applied to the inductor

The Volts-per-Turn principle is established in which coil?

The energized coil (Primary Coil)

An inductor coil with 200 turns is energized with a source of 600V. What is the Volts-Per-Turn ratio?

3 Volts per turn

600V/200 = 3v/ turn

Why can the induced voltage never be as great as the source voltage?

Becauses losses occur in the inductor coil

What will generally increase the strength of an electromagnet?

• Increasing the current

• increasing the # of turns

If the number of turns in an energized coil is increased while the source voltage remanins the same, what happens to the Volts-Per-Turn ratio?

It decreases

What is CEMF in an inductor?

It is the voltage that opposes the change that created it

What basic electrical component would an electric motor primarily be considered?

An Inductor

How can you tell the difference between a single and 3-phase motor?

SIngle Phase motors will have a Capacitor

Why does In-rush happen when energizing a motor?

Because the only thing that is holding back current is the Resistance of the wire.

• Current levels off due to Self Induction (CEMF)

What are the 3 overcurrent conditions?

• Ground Fault

• Short Circuit

• Overload

Relay Contacts are called out when the relay is ________

De-energized

Why are the iron cores in motors and transformers made from thin, laminated sheets of iron instead of one solid piece of Iron?

To reduce Eddy Currents and to Reduce Heat

What is the name of the part of a motor that rotates?

The Rotor

What is the name of the part of a motor that remains stationary?

The Stator

What are the types of losses that can occur in energized transformers and motors?

• Noise

• Heat

• Vibration

What is the main purpose of the conduit box on an Induction Motor

To connect the stator leads to the power supply

What is the main purpose of the motor end bells

To seal the motor housing and hold the outer bearings races

Why is the drive-end bearing usually larger than the opposite-end bearing?

It handles additional mechanical loads from the equipment being driven

Why are the stator coils placed into slots in the stator core?

To keep the coils in position and surrounded by iron

What is the purpose of the varnish placed around the stator coils?

To electrically insulate the coils

How are the rotor bars commonly formed inside the rotor coils?

Aluminum is poured into the caveties in the rotor core

What is the purpose of the shorting rings on a squirrel-cage rotor

To connect all the rotor bars together

Why is this type of rotor called a squirrel-cage rotor?

Its bars and shorting rings form a cage-like shape

How many watts are in 1 Hp?

746W

What is synchronous speed?

the speed of the magnetic field rotating around a stator

How do you find the Power Factor in a motor?

Watts/Volt-Amps

What are the 4 torque measurements?

• Locked Rotor Torque

• Pull-Up Torque

• Breakdown Torque

• Full Load Torque

What is the most common NEMA design letter induction motor?

Design B - It is applicable to many situations

What is the purpose of the shorting rings on each end of the rotor?

To form current paths between the rotor bars

When all of the rotor bars are shorted together, what does the turns equivalent become?

The VA in the rotor equal the VA in the stator, creating high current with the low voltage

What is energy?

The total VA that determines the potential or capactiy for work accomplished

What is Work?

When electrical energy is applies and is converted into another form of energy

What is Torque?

The force created when turning something

• It can determine what mechanical work can be preformed

What is Power?

How much work is accomplished in a given amount of time

What is the formula for Calculating Synchronouse Speed?

(120 x Hz) / # of Poles

When counting the poles in a three-phase motor, the coils of how many phases are counted?

Only poles of a single phase are counted

How is synchronous speed affected by the mechanical load of a motor?

The speed of a rotor will slow down to create more torque. Slowing down increases more voltage which causes current to flow more creating a stronger magnetic field

What is Slip?

The RPM difference between the stator rotating magnetic field and the actual rotating speed of the Rotor