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Chapter 13 DC Motors

Terms in this set (87)

Motors operate on the principle that when a ___-carrying conductor is placed in a magnetic field, a force that tends to move the conductor out of the field is exerted on the conductor.
current
A DC series motor is a DC motor that has the series field connected in series with the ___.
armature
The ___ that is produced by a motor depends on the strength of the magnetic field in the motor.
torque
A DC ___ motor should always be connected directly to the load and not through belts, chains, etc.
series
In a DC shunt motor, if the ___ to the armature is reduced, the speed is also reduced.
voltage
A(n) ___ is used to increase or decrease the strength of the field or armature in a DC shunt motor.
rheostat
The wires extending from the shunt field of a DC shunt motor are marked ___.
F1 and F2
Stepper motors are used in applications that require precise control of the position of the ___.
motor shaft
In a stepper motor, the permanent magnet is called the ___ or armature.
rotor
Because the stator coils are turned on and off rapidly to control a stepper motor, ___ switches are used.
solid-state transistor
To help reduce damage to switch contacts, a(n) ___ circuit is used to protect the switch.
snubber
The electron flow motor rule states that the ___ points in the direction of the electron current flow in the conductor.
middle finger
A(n) ___ is a sensor (transducer) that produces discrete electrical pulses during each increment of shaft rotation.
encoder
The direction of shaft rotation is controlled by the polarity of the produced magnetic field in the ___.
stator coil
___ is produced when a force overcomes a resistance.
Work
___ is the force that produces rotation.
Torque
Horsepower (HP) is a unit of power equal to ___ lb-ft per min.
33,000
The ___ is the base unit of electrical power.
watt
The ___ of a motor determines the size of a load that the motor can operate and the speed at which the load turns.
horsepower
The requirements a motor must meet in controlling a load include work, force, torque, and horsepower in relation to ___.
speed
Most DC motors are designed so that the brushes and the ___ can be inspected without disassembling the motor.
commutator
A(n) ___ circuit is a circuit in which current takes a shortcut around the normal path of current flow.
short
The intensity of the magnetic field and the amount of current in a conductor are normally changed to increase the ___ on the conductor.
force
A(n) ___ should be used for more accurate testing of insulation.
megohmmeter
Terminal markings S1 and S2 always indicate the ___ field leads.
series
Full-wave rectified power is obtained by placing a(n) ___ in an AC power line.
bridge rectifier
___ wear faster than any other component of a DC motor.
Brushes
A DC motor consists of ___ windings, an armature, a commutator, and brushes.
field
A commutator connects each armature winding to the brushes using ___ bars (segments) that are insulated from each other with pieces of mica.
copper
A spring placed behind the brush forces the brush to make contact with the ___.
commutator
A commutator rotates in ___° increments.
180
DC motors require more maintenance than AC motors because they have ___ that wear.
brushes
___ is exerted on an armature when it is positioned so that the plane of the armature loop is parallel to the field, and the armature loop sides are at right angles to the magnetic field.
Torque
What is a machine that converts electrical energy into mechanical energy by means of electromagnetic induction?
A motor
Conductors in a motor tend to move at what angle relative to the field?
right angles
What does the direction of the movement of the conductor depend on?
on the direction of the current and the magnetic field
What rule is used to determine the direction of motion of a current-carrying conductor in a magnetic field?
The electron flow motor rule
The electron flow motor rule states that with the thumb, index finger, and middle finger of the right hand set at right angles to each other___________
the index finger points in the direction of the magnetic field (N to S), the thumb points in the direction of the induced conductor motion, and the middle finger points in the direction of the electron current flow in the conductor.
The result of two magnetic fields intersecting create a rotating force on the loop called what?
Torque
What are brushes made of in a DC Motor?
carbon or graphite material.
What is a pigtail?
an extended, flexible connection or a braided copper conductor.
Connecting voltage directly to the field and armature of a DC motor allow the motor to produce _____________ in a smaller frame than AC motors.
higher torque
DC motors provide excellent speed control for ______________ with effective and simple ____________
acceleration and deceleration, torque control
Are AC or DC motors better at traction equipment applications?
DC motors
Why do DC motors require more maintenance than AC motors?
because they have brushes that wear.
What are the four basic types of DC motors?
Series, Shunt, Compound, Permanent-magnet
What are the wires extending from the series coil in a Series DC motor labeled with?
S1 and S2
In a DC series motor, what are the wires extending from the armature labeled with?
A1 and A2
What DC motor is used for starting with high torque?
Series motor
What type of DC motor has poor speed control but very high starting torque?
Series motor
What are DC series motors ideal for?
applications in which the starting load is large
What does the amount of current that flows through a motor depend on?
the size of the load
In DC series motors, what changes rapidly when torque changes?
speed
Torque and Speed have what kind of relationship?
Inverse
When current increases, flux increases, which produces a large _____________, which greatly decreases the speed of the motor
CEMF
What happens when there is no load on a series motor?
They will run away
What is the speed regulation formula
(m@NL-m@FL)/m@FL x100
m = motor
How do you change the speed of a series motor?
Adjust the voltage to filed windings
How do you change the direction of a DC motor?
change the direction of the armature or the coil (field windings) but not both. Typically you don't swap leads on the windings in series.
How do you adjust the speed of a DC shunt motor?
adjust voltage on the field windings or current on the armature
When does run away occur on a DC shunt motor?
When no voltage is on the field windings
What relationship does the armature voltage and speed have in a DC shunt motor?
direct relationship
What is another name for a DC generator?
TACH
What relationship does the field winding voltage and the speed have in a DC shunt motor?
inverse
What happens if you reduce the field winding voltage to much in a DC shunt motor?
the motor will run away
DC series motors specs are?
-High Torque
-Reduced Speed
-Bad voltage regulation
DC shunt motors specs are?
-Moderate torque (linear)
-Linear regulated speed
-better voltage regulation
DC compound motors specs are?
-High starting torque
-constant speed
Which wire leads matter in determining the direction that a DC motor spins?
A1 and A2/ or F1 and F2
DC permanent-magnet motors specs are?
-relatively high torque
-low speeds
-some self breaking when removed from power
What is the difference between a Stepper Motor and a regular motor?
unlike other motors that rotate their shafts when power is applied, stepper motors only move their shafts in small controlled increments called steps
The shaft of a stepper motor rotates at fixed angles when it receives what?
electrical pulses
Torque equation
T = W x r
Work equation
W = F x D
What is one HP in watts?
746W
What is one HP in lb-ft per min and per sec?
33,000 lb-ft per min and 550 lb-ft per sec
What is the equation for HP using efficiency?
HP = (IxExEff)/746
What is the equation for HP using torque and rpm?
HP = (rpm x T)/5252
What is the relationship between torque, horsepower and speed?
If the torque remains constant, speed and horsepower are proportional. If speed remains constant, torque and horsepower are proportional. If torque and speed vary simultaneously but in opposite directions, the horsepower remains constant
What are the steps for troubleshooting brushes?
1. Turn the handle of the safety switch or combination starter to the OFF position.
2. Measure the voltage at the motor terminals to ensure that the power is OFF
3. Check the brush movement and tension and remove the brushes.
4. Check the length of the brushes.
What are the steps for troubleshooting commutators?
1. Visually check the commutator
2. Check the mica insulation between the commutator segments
What are the steps for troubleshooting for Grounded, Open, or Short Circuits?
1. Check for grounded circuit, check for continuity between ground and one of the leads.
2. Check for an open circuit with the correct leads for the particular motor (series, shunt, compound). Check for continuity.
3. Check for a short between windings (A1 to S1 or A1 to F1) check for continuity
How do you remark the leads on DC motor terminals?
using a DMM by measuring the resistance of each pair of wires.
What determines if a pair of leads truly are a pair?
A pair of wires must have a resistance reading or they are not a pair.
Which windings have a greater resistance, series or armature?
Series have less and armature have greater
Which windings have a greater resistance, shunt or armature?
shunt have considerably greater and armature has less
What is an easy methoding of finding the armature leads?
rotate the shaft while reading resistance, the value will vary or lifting/putting a paper underneath the brushes so that they armature leads will read infinite resistance
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