Motor Controls - Ch 5 - Electric Motors

Summarize the torque and speed characteristics of a DC series motor

the speed varies widely between no load and rated load. Therefore, these motors cannot be used where a constant speed is required with variable loads.

rotational force

torque

What two main criteria are used to classify motors

type of power supply used and the principle of operation.

T/F- Consequent pole single-winding motors have their stator windings arranged so that the number of poles can be changed by reversing some of the coil currents.

T

T/F- High locked-rotor motor current can create voltage sags or dips in the power lines, which may cause objectionable light flicker and problems with other operating equipment

T

T/F: Maintenance of the brush/commutator assembly found on DC motors is significant compared to that of AC motor designs

T

T/F: The no-load speed of a series motor can increase to the point of damaging the motor. For this reason, it should never be operated without a load of some type coupled to it

T

Two-speed, single-winding motors are called ____________ pole motors

consequent

The advantage of high ____________ is that the magnetic fields inside the motor are strong, producing high torque (turning force), so it is ideal for starting very heavy mechanical loads.

current

Unlike motor starters, motor _________ are designed to provide, in addition to protection, precise control of the speed, torque, acceleration, deceleration, and direction of rotation of motors

drives

When a motor reaches its full no-load speed, it is designed to be generating a CEMF nearly _____ to the applied line voltage

equal

A three-phase motor stator winding consists of three separate groups of coils, called _________, and designated A, B, and C.

phases

for a typical DC electronic variable-speed motor drive, this drive is made up of two basic sections: the _______ section and the ______ section.

power , control

is defined as the ratio of the actual (or true) power (watts) to the apparent power (volt-amperes) and is a measure of how effectively the current drawn by a motor is converted into useful work.

power factor (PF)

A device called a _____ mover provides the mechanical energy input, which physically turns the armature coil

prime

What name is given to the device that drives a generator?

prime mover Some common examples of prime movers include: Steam Turbine: In power plants that use steam as a working fluid, a steam turbine is often used as the prime mover to drive large generators. Gas Turbine: Gas turbines are commonly used in combined cycle power plants and aircraft to drive generators. Diesel Engine: Diesel generators utilize diesel engines as prime movers to generate electrical power. Hydroelectric Turbine: In hydroelectric power plants, water flowing through turbines serves as the prime mover for generators. Wind Turbine: In wind power systems, the wind acts as the prime mover, driving the rotor of a wind turbine, which generates electricity. Internal Combustion Engine: In portable generators and smaller power plants, internal combustion engines (e.g., gasoline engines) are used as prime movers. Gasoline Engine: Gasoline engines are also used as prime movers in some portable generators and small power plants.

The ______ of the squirrel-cage rotor has an important effect on the operation of the motor.. Why?

resistance A high-resistance rotor develops a high starting torque at low starting current. A low-resistance rotor develops low slip and high efficiency at full load

A simple method used to determine the direction of movement of a conductor carrying current in a magnetic field is the:

right-hand motor rule

is key to the operation of all AC motors.

rotating magnetic field

The most common type of _______ has cast-aluminum conductors and short-circuiting end rings.

rotor

With __________ ____________ motors, a separate winding is installed in the motor for each desired speed *really easy, c'mon

separate winding

What special types of processes may warrant the use of a DC motor?

- Variable speed applications -High starting torque -Regenerative braking -Precision servo control -Battery powered devices

Electric motors are used to convert electric energy into mechanical energy. The motor represents one of the most useful and labor- saving inventions in the electrical industry. Over __ percent of the electricity produced in the United States is used to power motors.

50

Field loss protection must be provided for DC motors. Why

If a DC motor suffers a loss of field excitation current while operating, the motor will immediately begin to accelerate to the top speed that the loading will allow. This can result in the motor virtually flying apart if it is lightly loaded.

State the operating principle of a generator

The operating principle of a generator is based on Faraday's law of electromagnetic induction, which describes the generation of an electromotive force (EMF) or voltage in a conductor when it cuts through magnetic lines of force.

The ________ _________ listed on a DC motor's nameplate is an indication of how fast the motor will run with rated armature voltage and rated load amperes at rated field current

base speed

There are two ways to define AC motor speed

Synchronous speed, actual speed

T/F: Any DC machine can act either as a generator or as a motor.

True

Perhaps the greatest advantage of DC motors is

speed control.

There stator windings of a three-phase motor are placed ____ electrical degrees apart

120 electrical degrees

Common induction motors, started at rated voltage, have locked-rotor starting currents of up to _?_ times their name- plate full-load current.

6x

One horsepower is also equivalent to ____ watts of electrical power.

746 W

Explain how counter EMF is produced in a DC motor.

As the armature rotates in a DC motor, the armature coils cut the magnetic field of the stator and induce a voltage, or electromotive force (EMF), in these coils. This occurs in a motor as a by-product of motor rotation and is sometimes referred to as the generator action of a motor. This induced voltage OPPOSES the applied terminal voltage

NEMA Design __ —Considered a standard type with normal starting torque, low starting current, and low slip at full load. Suitable for a broad variety of applications, such as fans and blowers, that require normal starting torque

B

With armature voltage control of a DC shunt motor, what is the effect on the rated torque and horsepower when the armature voltage is increased?

By raising or lowering the armature voltage, the motor speed will rise or fall proportionally.

NEMA Design ____—This type has higher than standard rotor resistance, which improves the rotor power factor at start, providing more starting torque. When loaded, however, this extra resistance causes a greater amount of slip. Used for equipment, such as a pump, that requires a high starting torque.

C

is directly proportional to the speed of the armature and the field strength

CEMF

How can the direction of rotation of a wound DC motor be changed?

Changing the polarity of either the armature or series field winding changes the direction of rotation. If you simply changed the polarity of the applied voltage, you would be changing the polarity of both series field and armature windings and the motor's rotation would remain the same

With field current control of a shunt DC motor, what is the effect on the rated torque and horsepower when the armature voltage is increased?

If it is controlled, it can be held stable

Explain the essential difference between construction of an AC and a DC generator.

Commutator vs. Slip Rings: DC Generator: In a DC generator, the armature windings are connected to a commutator, which is a rotating split-ring device mounted on the rotor shaft. The commutator serves to reverse the current direction in the armature coils as the rotor rotates, ensuring that the output voltage remains in one direction (DC). AC Generator: An AC generator, also known as an alternator, does not have a commutator. Instead, it uses slip rings, which are continuous circular rings mounted on the rotor shaft. The armature windings are connected to the slip rings, and the electrical output is induced in the stator windings as the magnetic field rotates with the rotor. Use of Brushes: DC Generator: DC generators require brushes to maintain electrical contact with the commutator segments as it rotates. The brushes allow the electrical current to flow from the rotating armature windings to the stationary external circuit. AC Generator: Alternators (AC generators) do not use brushes since they utilize slip rings for electrical connection. The slip rings provide continuous electrical contact with the stationary external circuit, eliminating the need for brushes.

These motors are generally used where BOTH severe starting conditions are met and constant speed is required at the same time.

Compound DC Motor

Compare the torque and speed characteristics of a compound motor with those of the series and shunt motors.

Compound have high starting torque, more than shunt but less than series. The speed of a compound varies a little more than shunt, but not much. While it varies much less than the wandering speeds of a series. You usually use these motors when sever starting conditions are met, and you also need the motor to run at a constant speed.

As the armature rotates in a DC motor, the armature coils cut the magnetic field of the stator and induce a voltage, or electromotive force (EMF), in these coils. This occurs in a motor as a by-product of motor rotation and is sometimes referred to as the generator action of a motor. Because this induced voltage opposes the applied terminal voltage, it is called:

Counter Electromotive Force (CEMF)

NEMA Design __—The even higher rotor resistance of this type produces a maximum amount of starting torque. This type is suitable for equipment with very high inertia starts such as cranes and hoists.

D

in general, __ magnetic motor starters are intended to start and accelerate motors to normal speed and to provide protection against overloads

DC

How is the speed of a DC motor controlled below base speed? above base speed?

DC motors can be operated below base speed by reducing the amount of voltage applied to the armature and above base speed by reducing the field current. In addition, the maximum motor speed may also be listed on the nameplate.

How the starter coils F and R of a wound DC motor keep from being energized at the same time.

Electrical interlocking byway of the NC R and F auxillary control contacts

How does electricity produce magnetism

Electricity and magnetism are interconnected phenomena, and one of the fundamental principles of electromagnetism is that an electric current flowing through a conductor produces a magnetic field around the conductor. This relationship is described by Ampere's law, one of the fundamental laws of electromagnetism.

T/F: the load of a motor regulates the speed by affecting the CEMF and voltage

F It's CEMF and current flow.

In general, what causes an electric motor to rotate?

In general, an electric motor rotates due to the interaction between the magnetic fields produced by the stator (the stationary part) and the rotor (the rotating part) of the motor. When an electric current flows through the stator windings, it generates a magnetic field around the stator. Simultaneously, the rotor, which is made of ferromagnetic material, also produces its own magnetic field.

Explain the function of the commutator in the operation of a DC motor

It is a rotating switch-like device that reverses the direction of current flow through the motor's windings, allowing the motor to continuously rotate in the same direction. When a coil is in the most effective position for generating torque (aligned with the magnetic field), the commutator ensures that the current flows in the correct direction through the coil.

in what way is the shunt field winding of a shunt motor different from that of the series field winding of a series motor?

It is in parallel with the armature

According to power requirements, in what ways are motors classified?

Low Voltage Motors vs. High Voltage Motors: Single-Phase Motors vs. Three-Phase Motors: AC Motors vs. DC Motors: Low Power Motors vs. High Power Motors: Fractional Horsepower Motors (FHP) vs. Integral Horsepower Motors (IHP): Synchronous Motors vs. Asynchronous Motors:

Give two reasons why DC motors are seldom the first motor of choice for some applications

Maintenance and Wear, Complex Control and Power Electronics

________ _________ regulation is a measure of a motor's ability to maintain its speed from no load to full load without a change in the applied voltage to the armature or fields.

Motor speed

Why should a DC series motor never be operated without some sort of load coupled to it?

Operating a motor above its rated maximum speed can cause damage to equipment and personnel

[(No-Load Speed - Full Load speed) / Full Load speed ] x 100 this is the formula for:

Percent Speed Regulation

______-_______________DC motors use permanent magnets to supply the main field flux and electromagnets to provide the armature flux

Permanent-magnet

Applying the right-hand motor rule to given current-carrying conductor placed in a magnetic field indicates movement in the downward direction. What could be done to reverse the direction in which the conductor moves

Reverse the Direction of the Current Change the Orientation of the Magnetic Field Adjust the Angle Between the Conductor and the Magnetic Field Modify the Physical Properties of the Conductor

_________ motors can be made to operate above base speed by field weakening

Shunt

Compare the starting torque and load versus speed characteristics of the series motor to those of shunt wound motor.

Shunts start out slow and build up, series can also start large loads. The main perk of a shunt is it's constant speed, even with no load attached to it it will not speed up more and damage itself.

The rotor does not revolve at synchronous speed, but tends to slip behind. ____ is what allows a motor to turn. If the rotor turned at the same speed at which the field rotates, there would be no relative motion between the rotor and the field and no voltage induced. Because the rotor slips with respect to the rotating magnetic field of the stator, voltage and current are induced in the rotor.

Slip

Since the series field winding is connected in series with the armature, it will carry [THE SAME/ A DIFFERENT] amount of current that passes through the armature

The Same

How is the base speed of a DC motor defined?

The base speed listed on a DC motor's nameplate is an indication of how fast the motor will run with rated armature voltage and rated load amperes at rated field current

What is the basic purpose of an electric motor?

The basic purpose of an electric motor is to convert electrical energy into mechanical energy to perform useful work

In what direction are the lines of flux of a magnet assumed to travel

The lines of flux of a magnet are assumed to travel from the North pole to the South pole of the magnet. These lines represent the magnetic field produced by the magnet and form closed loops, indicating the path of the magnetic field lines as they emerge from the North pole, curve around the magnet, and re-enter the South pole.

What is motor armature reaction? b. State three effects that armature reaction has on the operation of a DC motor

The magnetic field produced by current flow through the armature conductors distorts and weakens the flux com- ing from the main field poles. This distortion and field weakening of the stator field of the motor are known as armature reaction. Shifting the neutral plane in a direction opposite to the direction of rotation of the armature. • Reducing motor torque as a result of the weakening of the magnetic field. • Arcing at the brushes due to short-circuiting of the voltage being induced in the coils undergoing commutation

armature reaction

The magnetic field produced by current flow through the armature conductors distorts and weakens the flux coming from the main field poles. This distortion and field weakening of the stator field of the motor are known as _________ __________

Why is the motor stator coil constructed with an iron core?

The motor stator coil is constructed with an iron core because iron is a ferromagnetic material, which significantly enhances the efficiency and performance of the electric motor. The presence of an iron core in the stator helps to concentrate and guide the magnetic field produced by the stator windings, leading to several advantages

How is the polarity of the poles of a given coil reversed?

The polarity of the poles of a coil can be reversed by changing the direction of the electric current flowing through the coil. The magnetic field generated by the coil is directly related to the direction of the current flow in the windings. Reversing the current direction will change the magnetic field's polarity and, consequently, the polarity of the poles.

How is the speed of a permanent-magnet motor controlled?

The speed of a permanent-magnet DC motor can be controlled by adjusting the voltage applied to the motor's armature (rotor).

Explain how interpoles minimize the effects of armature reaction.

These narrow poles have a few turns of larger-gauge wire connected in series with the armature. The strength of the interpole field varies with the armature current. The magnetic field generated by the interpoles is designed to be equal to and opposite that produced by the armature reaction for all values of load current and improves commutation.

How are the series and shunt field windings of the compound-wound DC motor connected relative to the armature?

This type of DC motor has two field windings. One is a shunt field connected in parallel with the armature; the other is a series field that is connected in series with the armature

how is the direction of rotation of a permanent-magnet motor changed?

To reverse the direction of rotation, the polarity of the external power source connected to the brushes must be reversed. This causes the current to flow in the opposite direction through the armature coils. As a result, the interaction between the coil's magnetic field and the permanent magnets' field also reverses, leading to a change in the direction of torque and, consequently, the rotation direction of the motor.

List several control functions found on a DC motor drive that would not normally be provided by a traditional DC magnetic motor starter

Unlike motor starters, motor drives are designed to provide, in addition to protection, precise control of the speed, torque, acceleration, decelera- tion, and direction of rotation of motors. In addition, many motor drive units are capable of high-speed communication with programmable logic controllers (PLCs) and other industrial controllers.

. In what direction will a current-carrying conductor, placed in and at right angles to a magnetic field, move

When a current-carrying conductor is placed in a magnetic field at right angles to the field, the conductor experiences a force that causes it to move perpendicular to both the current direction and the magnetic field lines. This phenomenon is described by the right-hand rule for the force on a current-carrying conductor in a magnetic field.

This is the speed at which the shaft rotates.

actual speed

DC motors are not used as much as AC types because:

all electric utility systems deliver AC

Movement of the magnetic field of the armature is achieved by switching current between coils within the motor. This action is called ______________________

commutation.

Variable-speed control of a PM motor is accomplished by varying the value of the voltage applied to the armature. The speed of the motor varies directly with the amount of armature voltage applied. The higher the value of the armature voltage, the ___________the motor will run

faster

[Compound DC Motors] This motor is normally connected cumulative-compound so that under load the series fieldflux and shunt field act [IN THE SAME / A DIFFERENT] direction to strengthen the total field flux

in the same direction

An _________ motor rotor can be either wound rotor or a squirrel-cage rotor

induction

The AC _________ motor is by far the most commonly used motor because it is relatively simple and can be built at less cost than other types

induction

Direct current motors are ideally suited to a multitude of ____________- and __________applications in which high torque and variable speed are required.

industrial and marine

An electric motor converts electric energy into mechanical energy by using ___________ ___________ ___________-

interacting magnetic fields

In larger DC motors, ______________ (also called commutating poles) are placed between the main field poles to minimize the effects of armature reaction.

interpoles

The moment a motor is started, during the acceleration period, the motor draws a high inrush current. This inrush current is also called the:

locked-rotor current

A series-wound DC motor has a ____resistance field and ____ resistance armature circuit. Because of this, when voltage is first applied to it, the current is HIGH.

low

Because of the large diameter of the series winding, the winding will have only a few turns of wire and a very ______ resistance value

low

The series delta connection results in ____ speed and the parallel wye in ____ speed. The torque rating is the same at both speeds. **If wiring speeds are reversed, then it is the HP that will remain the same at all speeds

low, high

The induction motor is so named because:

no external voltage is applied to its rotor. There are no slip rings or any DC excitation supplied to the rotor. Instead, the AC current in the sta- tor induces a voltage across an air gap and into the rotor winding to produce rotor current and associated magnetic field

A ___________ excited DC motor has the armature and field coils fed from separate supply sources. This type of motor has a field coil similar to that of a self-excited shunt motor

separately

[Compound DC Motors] The _______ field gives it the advantage of being able to develop a large torque when the motor is started under a heavy load.

series

One of the main advantages of a ______ motor is its constant speed. It runs almost as fast fully loaded as it does with no load

shunt

[Compound DC Motors] The _______ field gives this type of motor the constant-speed advantage of a regular shunt motor.

shunt

The difference between the speed of the rotating magnetic field and the rotor in an induction motor is known as _____ and is expressed as a percentage of the synchronous speed

slip

In what way is a cumulative-compound motor connected?

so that under load the series field flux and shunt field act in the same direction to strengthen the total field flux

Once started, the _________ _________ motor will continue to run, with a phase loss, as a single-phase motor. The current drawn from the remaining two lines will almost double, and the motor will overheat. The motor will not start from standstill if it has lost a phase

squirrel cage

The force that rotates the motor armature is the result of the interaction between two magnetic fields (the ________ field and the armature field)

stator

The ___________ speed of an AC motor is the speed of the stator's magnetic field rotation

synchronous