DC motors
_______ is a protective device used to disconnect power from the armature if the current flow through the shunt field decreases below a certain level.
Field loss relay (FLR)
What is the primary difference between the brush-type and the brushless-type motor?
1) Brushless type motors do not have a commutator or brushes; 2) Brushless type motor are generally smaller and cost less 3) Brushless type motors require a converter; 4) Brushless type motors dissipate heat more quickly; 5) Brushless type motors are more efficient, require less maintenance and have less downtime.
What happens when the armature slows down as a result of load being added?
1) Counter-EMF is reduced 2) torque increases; 3) resistance decreases; 4) current flow increases
What are the characteristics of a ServoDisc motor?
1) no cogging action 2) extremely fast acceleration 3) speed can be varied by changing the amount of voltage supplied to the armature
The direction of a shunt motor can be changed by ________ OR ________, but not both.
1) reversing the shunt field leads 2) reversing the armature leads
What two factors determine the amount of torque produced by a motor?
1) the magnetic field of the pole pieces 2) the magnetic field of the armature
Servomotors generally contain how many poles?
2 to 6
The torque-to-weight ratio of PM motors can exceed those of conventional field-wound motors by ______ percent
40-90%
The power-to-weight ratio of PM motors can exceed those of conventional field-wound motors by ______ percent
50-200%
1 horsepower equals _______ watts
745.7 watts
What is a motor?
A device that converts electrical energy to mechanical energy
_______ is used to create a rotating magnetic field inside the stator of a brushless motor.
AC
The amount of CEMF is produced in the ________ and proportional to what three (3) factors?
Armature 1) # of turns of wire in the armature 2) strength of the magnetic field 3) speed of the armature's rotation
The amount of torque produced by the shunt motor is proportional to ______.
Armature current
_______ motors that have both a series field and a shunt field.
Compound motor
Induced voltage opposite to polarity of applied voltage is called ______
Counter-EMF
_______ limits the flow of current through the armature when the motor is running.
Counter-EMF
What are the two types of a compound motor?
Cumulative-compound Differential-compound
When load is added to a series DC motor, current ______ (increases or decreases) and speed (increases or decreases)?
Current Increases; voltage decreases; speed decreases
When CEMF opposes applied voltage, current _______ (increases or decreases)
Decreases
When full-load voltage is applied to both the armature and the shunt field, the motor will operate at what speed?
Normal (base) speed
In a DC motor, what do the brushes do?
Provide power to the armature from an external source
DC motors operate on the principle of _______ and _______.
Repulsion and attraction of magnetism
What is the standard way of reversing the direction of a shunt motor? Why?
Reverse the armature leads Because if the shunt field leads of a compound motor are reversed, the motor will change from a cumulative-compound motor to a differential-compound motor.
A _______ type motor only has a series field connected in series with the armature.
Series
Which motor type (series, shunt or compound) produces the most torque?
Series
What functions do the series field and the shunt field perform in a compound motor?
Series field: permits the motor to develop high torque Shunt field: permits speed control and regulation
What is characteristics of a series motor that a shunt motor does not have?
Series has the ability to develop extremely high torque (on average 450% of full torque)
What are the three basic types of DC motors?
Series, shunt, compound
What does the commutator do if used as a motor?
Serves as a rotary switch and maintains the correct direction for current to flow.
A _______ motor is a type of DC servomotor that uses permanent magnets to provide a constant magnetic field like conventional servomotors, but has a different armature design.
ServoDisc motor
What are the two connection types of a compound motor?
Short shunt Long shut
Which motor type (series, shunt or compound) produces the most speed regulation?
Shunt
What is used to add current resistance to a shunt motor?
Shunt field rheostat
The DC shunt motor has the _________ connected in _________ to the ________.
Shunt field windings Parallel Armature
The pushing and turning force caused by the magnetic lines of flux around the conductors is called ______.
Torque
T/F: Motors with a large amount of inertial exhibit superior speed-regulation characteristics.
True
T/F: Permanent magnet motors have a higher efficiency than conventional field-wound motors because power is supplied to the armature circuit only.
True
T/F: Series motors have no natural speed limit, so it should never be connected in a no-load (without a load) condition.
True
T/F: Series motors should always be "coupled" to a load and not with chains or belts.
True
T/F: The armature will continue to increase in speed until CEMF is almost the same value as the applied voltage at which time the motor produces only enough torque to overcome its own losses.
True
T/F: The stationary armature or stator winding in a brushless motor is generally 3-phase, but can, in some motors, be designed to operate on 4-phase and 2-phase power.
True
T/F: the use of magnet gears eliminates the need for mechanical gears and other speed-rotating equipment which in turn eliminates friction and backlash associated with mechanical gears.
True
How can the direction of a PM motor be reversed?
by reversing the polarity of voltage applied to the armature leads
_____ is caused by the reaction of the iron armature with the field of the permanent magnets in a ServoDisc motor.
cogging
______ are used to change DC into multiphase AC to produce a rotating magnetic field in the stationary amamature or stator of a brushless motor.
converters
in general, what type of compound motor connection should be avoided?
differential-compound
How do permanent magnets affect the field flux in a PM motor?
field flux remains stable at all times
The rotor in a brushless motor is surrounded by _______.
fixed stator windings
The average voltage supplied to the armature of a ServoDisc motor is determined by the _____
length of time the voltage is turned vs. time it is turned off.
__________ refers to the use of large number of poles for low speed and high torque.
magnetic gearing
How are the permanent magnets of a ServoDisc motor mounted?
on either side of the disc armature and parallel to the shaft
_____ motors that do not require an external force to supply the magnetism in the field poles instead of using series and shunt windings.
permanent magnet (PM) motors
The armature of a brushless motor is made up of ______.
permanent magnets
What is another name for the ServoDisc motor? Why?
printed-circuit motor Because the conductors are printed on a fiberglass material like a printed circuit.
How is voltage varied in a ServoDisc motor?
pulsating-width modulation
______ refers to the length of time a ServoDisc is turned on compared to the length of time it is turned off.
pulse width
What factor determine speed regulation of a DC motor?
resistance of the armature
What type of magnets are used in permanent magnet motors?
samarium-cobalt neodymium
What type of motor should never be operated without a load?
series motor
______ are PM motors that have small, lightweight armatures that contain very little inertia.
servomotor
What type of motor is considered a "constant speed" motor?
shunt motor
The amount by which the speed decreases when load is added is called the ______.
speed regulation
The speed of a PM motor can be controlled by ______ .
the amount of voltage applied to the armature
The speed of a brushless motor is determined by _______.
the number of stator poles per phase the frequency of the AC voltage
What are two (2) characteristics of a shunt motor?
1) very good speed characteristics 2) full load speed remains within 10% of the no-load speed
Motors powered by the trapezoid waveform produce about how much more torque than those powered by sine waves.
10%
When full-load voltage is applied to the armature and reduced speed is applied to the shunt field, the motor speed will be ______.
Above normal speed
Magnetic field strength is proportional to _______.
Amount of Current flow
How are each of the windings labeled?
Armature leads: A1, A2 Series leads: S1, S2 Shunt leads: F1, F2 Commutator leads: C1, C2 (or S3, S4)
The motor's own losses overcome by the torque produced include ______, _______, ________, _______, and ______.
Bearing friction Bush friction I^2 R (power) loss in the armature windings Winding loss
Why might the inrush current be high when a motor first starts?
Because no Counter-EMF is being produced
In large DC motors, full-load is generally applied to the shunt field at all times and variable voltage to the armature. How does this help the large DC motor?
Because voltage is applied all the time to the shunt field, the shunt field acts like a heater to prevent moisture from forming in the motor.
How is the speed controlled in a series motor?
By the amount of load connected to the motor.
Which motor ( series, shunt, or compound) is the most widely used DC motor in the industry?
Compound ( specifically the long shunt compound motor)
When a load is added to a DC shunt motor, current _______ (increases or decreases), torque ______(increases or decreases), speed ______ (increases or decreases)
Current decreases Torque increases Speed decreases
When load is added to a DC motor, the armature speed will _______ ( increase or decrease). Why?
Decrease not enough torque to overcome added load
What is the formula for horsepower
HP = (1.59)(torque)(RPM) / 100,000
________ is a measure of power for electric and mechanical devices
Horsepower
When load is added to a DC shunt motor, current _____ (increases or decreases)
Increases
When a compound motor is connected as a differential-compound, which field winding determines the direction of the motor?
Initially shunt field until enough load is added then series field takes over and speed in no longer being controlled.
A brushless DC motor that uses a rotor shaped like a hollow cylinder or cup is called a _______ motor.
Inside-out motor
How is the ServoDisc motor's armature different from the conventional servomotor?
Instead of iron, the ServoDisc motor's armature is made of 2 to 4 layers of copper conductors formed into a thin disc.
In a series motor, how much does torque increase?
It increases by the current squared (e.g., if current doubles, torque quadruples)
Which connection type of a compound motor is most widely used? Why.
Long shunt Because it has superior speed regulation
The compound motor produces more speed regulation than the _____ motor, but less speed regulation than the ______ motor.
More speed regulation than the Series motor, but less speed regulation than the shunt motor
The compound motor produces more torque than the _____ motor, but less torque than the ______ motor.
More torque than the shunt motor, but less torque than the series motor
Why are terminal leads labeled?
So they can be identified when they are brought outside the motor housing to the terminal box
What is the process for determining a motor's efficiency?
Step 1) calculate horsepower output of the motor Step 2) convert horsepower output to watts Step 3) calculate input power W = V x A Step 4) calculate efficiency using the efficiency formula
What is the only current-limiting factor when a motor first starts?
The amount of wire resistance in the armature windings
T/F: Motors powered by sine waves operate more smoothly and with less torque ripple at low speeds than trapezoid waveform motors.
True
T/F:PM motors of comparable horsepower ratings are smaller and lighter in weight than conventional field-wound motors.
True
How is torque produced in an ServoDisc motor?
it's produced when current flows through the copper conductors o f the disc, which produces a magnetic field that reacts with the magnetic field of the permanent magnets.
What does RPM stand for?
revolutions per minute
Motors that use high pole count for low-speed, high-torque applications are often called ______
ring motors (or ring torquers)
How is an inside-out motor assembled?
with the stator windings wound inside the rotor and a permanent magnet rotor mounted on the outside of the stator winding.
When full-load voltage is applied to the shunt field and reduced voltage is applied to the armature, the motor will operate at what speed?
Below normal speed
What is the difference between a conventional DC motor and a brushless DC motor?
Brushless DC motors do not have a wound armature, a commutator, or brushes.
List the several ways of controlling speed in a DC motor.
1) by connecting resistance in series with the armature thereby reducing armature current 2) by using an electronic controller to supply variable voltage to the armature circuit separately from the field 3) by increasing frequency
What is the equation for motor efficiency as a percent
Efficiency = (power out (HP) / power in (line voltage) X 100
