Download Motors, generators and alternators

Motors, generators and alternators
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1.
Chapter
Investigate and explain the operation of simple DC motors consisting of
- one coil, containing a number of loops of wire, which is free to rotate
about an axis
- two magnets (not including radial magnets)
- a commutator
- a DC power supply;
Apply a field model to define magnetic flux , using  = BA and the qualitative
effect of differing angles between the area and the field;
Investigate and analyse the generation of emf, including AC voltage and
calculations using induced emf,  = -n/t; in terms of
- the rate of change of magnetic flux (Faraday’s Law)
- the direction of the induced current (Lenz’s Law)
- number of loops through which the flux passes;
Explain the production of DC voltage in DC generators and AC voltage in
alternators, including in the use of commutators and slip rings respectively;
Compare DC motors, DC generators and AC alternators;
Investigate and compare sinusoidal AC voltages produced as a result of the
uniform rotation of a loop in a constant magnetic flux in terms of frequency,
period, amplitude, peak-to-peak voltage (VP-P) and peak-to-peak current (IP-P);
[2 marks]
2
(2001 Q9)
Two students examine a DC motor. They find that it has an armature consisting of a rectangular
coil with 50 turns, which is shown below.
They observe that the armature is in the field of a two-pole magnet, and can rotate about an axis
as shown below. The magnetic field is produced by the current flowing through field coils as
shown in below. The armature windings and the field coils are connected in series, so that the
same current flows through each. The current to the armature flows through a commutator
which is not shown. When the motor is operating, the current flowing is 1.5 A.
An enlarged diagram of one field coil is shown above.
In which direction must the current flow through the field coil to produce a field as indicated by
the arrows?
A.
in at X and out at Z
B.
in at Z and out at X
C.
it is an AC current, so the direction is always changing
D.
in either direction, the field direction does not depend on the current direction
2.
[2 marks]
(2001 Q10)
With the armature oriented as shown above, the magnetic field in the region of side JK is 0.10 T.
A current of 1.5 A flows through the armature.
What is the magnitude of the force on the side JK of the armature?
3.
[2 marks]
(2001 Q11)
The armature is at rest in the orientation as shown, and the current begins to flow in the
direction shown by the arrows.
Which one of the following statements best describes the initial motion of the armature?
A.
It will start to rotate anticlockwise.
B.
It will start to rotate clockwise.
C.
It will start to rotate but the direction cannot be predicted.
D.
It will oscillate about the position shown.
4.
[2 marks]
(2002 Q11)
The circuit of a simple DC electric motor is shown below. It consists of a current-carrying coil of
50 turns as the armature. The coil is square with sides of 5.0 cm. The coil is in a uniform
magnetic field of strength 0.005 T.
A current of 3.0 A flows through the coil in the direction shown.
Calculate the magnitude of the force exerted on the 50 wires of side P of the coil.
5.
[1 mark]
(2002 Q12)
When the coil is in the position shown, which of the directions (A – D) below best shows the
direction of the force exerted on side P of the coil?
6.
[3 marks]
(2002 Q13)
The ends of the coil are connected to the commutator, as shown above, so that it is free to rotate
with the coil. Explain
•
why the commutator must be free to rotate in this manner
•
how this is fundamental to the operation of the DC electric motor.
7.
[2 marks]
(2003 Q8)
The figure below is a diagram of a simple alternator. A coil (UVWX) 0.30 m by 0.40 m, consists
of 20 turns of wire. It is in a uniform magnetic field of strength 0.25 T, and can rotate as shown.
With the coil oriented as above, what is the magnitude of the magnetic flux through each turn
of the coil?
8.
[3 marks]
(2003 Q9)
The coil is rotated at a constant rate of 50 revolutions per second in the direction shown.
What is the average voltage developed across the resistor R when the coil rotates through 90
degrees from the orientation shown in the figure above?
9.
[2 marks]
(2003 Q10)
The graphs below show possible variations of the magnetic flux through the coil as a function
of time as it rotates. They all begin at time t = 0, when the coil is oriented as in the diagram
above.
Which of these graphs (A – D) best shows the variation of the magnetic flux through the coil as
a function of time? Take the direction from N to S in the figure as positive.
10.
[3 marks]
(2003 Q11)
Assuming the same conditions as in the question above, which of the graphs (A – D) best shows
the variation of the current flowing from U to V in the coil, as a function of time? Explain the
logic of your choice.
11.
[2 marks]
(2004 Q9)
Mary and Shin have constructed a simple alternator. It consists of a single rectangular coil of
wire, 0.40 m × 0.30 m, which is connected to slip rings, as shown below. The coil is in a uniform
magnetic field of 0.12 T, and can be turned in the direction as shown below.
What is the magnitude of the flux through the coil when oriented as shown above?
12.
[2 marks]
(2004 Q10)
As a test, Shin rotates the coil at a constant rate in a time of 0.15 s through an angle of 90° from
the orientation shown above. Mary observes the voltage on an oscilloscope.
Which of the graphs below best represents the variation as a function of time of the voltage at
point Q relative to point P (VQP)?
13.
[2 marks]
(2004 Q11)
What is the average voltage measured between points Q and P due to this rotation?
14.
[3 marks]
(2004 Q12)
Mary now rotates the coil at a constant rate of 5 revolutions per second, and the students
observe that the voltage between points Q and P varies with time as shown below.
Shin decides to test the effect of rotation speed, and turns the coil at a rate of 10 revolutions per
second.
On the figure above, sketch a graph that shows the variation with time of the voltage between
points Q and P that the students would now see.
15.
[2 marks]
(2004 Sample Q9)
Payel is investigating the operation of a simple alternator, as shown below. She has found that
if the coil is rotated anticlockwise as shown, the light glows dimly.
Sketch the voltage produced in the coil on the figure above. Note that initially the coil is
positioned to intercept the maximum number of field lines.
16.
[2 marks]
(2004 Sample Q10)
Does the voltage output of the alternator increase or decrease when it is spun faster?
17.
[1 + 2 = 3 marks]
i.
State Lenz’s law.
ii.
Use Lenz’s law to explain your answer to Question 16.
(2004 Sample Q11)
18.
[2 marks]
(2004 Pilot Q7)
Kris is investigating a generator. The magnets are attached to a shaft and are free to rotate
around the fixed coil as shown in below. In this test, the shaft is connected to an electric motor
and rotated at a constant speed.
The figure below shows the magnetic flux inside the coil. At a speed of 10 revolutions
(rotations) per second (rps), the 40-turn coil produces an emf of 2.0 VRMS, and the light globe
glows dimly.
What is the time difference between points P and Q?
19.
[4 marks]
(2004 Pilot Q8)
Complete the figure above by sketching the voltage output of the coil. Label your sketch to
indicate the maximum and minimum voltages.
20.
[2 marks]
(2004 Pilot Q9)
The electric motor is now turned off, and the shaft gradually slows to a halt.
Which one of the following diagrams best describes the voltage output of the generator as it
comes to rest?
21.
[2 marks]
(2004 Pilot Q10)
Which one or more of the following actions would increase the emf of this generator to 4.0VRMS?
A. increase the number of turns to 80
B. increase the thickness of the wire used
C. decrease the number of turns to 20
D. decrease the thickness of the wire used
22.
[2 marks]
Briefly describe the function performed by the commutator in a DC generator.
(2004 Pilot Q11)
23.
[3 marks]
(2004 Pilot Q12)
John is using a small portable generator which has both a 240 VRMS and 12 VRMS output. He
wants to power two identical 12 V lamps as shown. The first lamp is connected directly to the
low voltage output of the generator. The second lamp is some distance away and requires a 75
m extension lead. The total resistance of the extension lead is 1.0 Ω. When the generator is
started, both lamps glow, but lamp 1 is considerably brighter than lamp 2. John measures the
current at R to be 4.0 A. Assume both lamps have constant resistance and obey Ohm’s law.
Why is lamp 1 brighter than lamp 2? Justify your answer.
24.
[4 marks]
(2004 Pilot Q13)
Calculate the power dissipated in lamp 2.
25.
[3 marks]
Which one of the following statements is true?
A. the current at Q is less than 4 A
B. the current at Q is 4 A
C. the current at Q is 8 A
D. the current at Q is greater than 8 A
(2004 Pilot Q14)
26.
[4 marks]
(2004 Pilot Q15)
John plans to use the 240 VRMS output of the generator to supply lamp 2 with 12 VRMS.
Complete the figure below by designing a step-down transformer to deliver 12 VRMS to lamp 2.
Make sure that you clearly label the diagram.
27.
[2 marks]
(2004 Pilot Q16)
For lamp 2 it is preferable to use the 240 VRMS and then step it down to 12 VRMS rather than
using the 12 VRMS output. Explain the reason for this.
28.
[2 marks]
(2005 Q2)
Some students are studying the emf induced by a magnetic field in a coil of wire. Their
experimental apparatus consists of a coil of 100 turns of wire in a magnetic field of 2.0 × 10-2 T
as shown.
With the coil vertical as shown above, the flux through the coil is 8 × 10-6 Wb. What is the area
of the coil?
29.
[2 marks]
(2005 Q3)
The coil is rotated at a rate of 10 revolutions per second, and the output is observed on an
oscilloscope (CRO), as shown above. What is the time interval, TQR, between Q and R?
30.
[3 marks]
(2005 Q4)
Calculate the average emf observed over the time interval TQR.
31.
[3 marks]
(2005 Q5)
The rotation speed of the coil is increased to 20 revolutions per second.
On the figure above, sketch the output from the oscilloscope that would be observed now.
32.
[1 mark]
(2005 Q6)
Pat and Kris are discussing DC generators. Pat says that slip rings are used in a DC generator.
Kris disagrees, and says that DC generators cannot use slip rings because they must produce
DC, and therefore a commutator is essential.
Who is correct, Kris or Pat?
33.
[2 marks]
(2005 Q7)
Explain the operation of a commutator.
34.
[3 marks]
(2005 Q8)
Which one of the following diagrams (A..D.) best describes the output of a DC generator?
35.
[2 marks]
(2006 Q4)
Jo and Effe have found a design for a small electric motor, shown below. It consists of two large
permanent magnets, a coil of wire and a commutator. They plan to build the motor as part of
their Physics experiment. The area of the coil is 5.0 cm2 and the magnetic field strength (which
can be assumed to be uniform) between the poles is 2.0 × 10-4 T (Wb m-2).
What is the magnetic flux, B, threading the loop when the face of the coil is aligned as shown
below? Include the unit.
36.
[2 marks]
(2006 Q5)
What is the magnetic flux, B, threading the loop when the face of the coil is aligned as shown
below? Include the unit.
37.
[2 marks]
(2006 Q6)
Jo tests the motor using a 6.0 V battery as a power source and it turns very slowly. Effe suggests
that they should replace the commutator with slip rings to make it turn faster.
What is the role of the commutator? Can Effe’s idea work? Explain your answer.
38.
[2 marks]
(2006 Q7)
A loop of wire in the form of a 2.0 cm square is moved through a uniform magnetic field at a
constant speed of 4.0 cm s-1. This is shown below.
The figure below shows an end-on view of the loop as it passes across the face of the north pole
of the magnet. Current flowing through the loop is measured by a micro-ammeter (µA).
You may assume the magnetic field exists only in the region between the north and south poles.
Initially, the loop is placed well outside of the magnetic field and the micro-ammeter confirms
that no current is flowing through the loop. The loop is now moved at a constant rate through
the magnetic field. The figure below shows the flux through the loop as a function of position of
the leading edge of the loop.
Which one of the following (A - D below) best describes the current in the loop as it passes
through the magnetic field?
39.
[4 marks]
(2006 Q8)
The field strength between the poles is 3.7 × 10-3 T (Wb m-2).
Calculate the maximum induced voltage in the loop.
40.
Use the following information to answer Questions 40 and 41.
[1 + 2 = 3 marks]
(2007 Q4)
The figure below shows a schematic diagram of a DC motor.
The motor is initially stationary as shown above.
a. In which direction, A (clockwise) or B (anticlockwise), will the motor rotate when the switch
is closed?
b. Explain your answer.
41.
[3 marks]
(2007 Q5)
Why is the split ring commutator necessary for the motor to operate correctly? Explain the
operation of the commutator.
Use the following information to answer Questions 42 and 43.
42.
[3 marks]
(2007 Q6)
The figure below shows an alternator consisting of a rectangular coil with sides of 0.20 m × 0.30
m, and 1000 turns rotating in a uniform magnetic field. The magnetic flux through the coil in
the position shown is 3.0 × 10-4 Wb.
What is the magnitude of the magnetic field? Include a unit.
43.
[3 marks]
(2007 Q7)
The coil rotates a quarter of a revolution in 0.01 s from the starting position shown above.
Calculate the magnitude of the average induced emf in the coil in this time.
You must show your working.
Use the following information to answer Questions 44 and 45.
44.
[3 marks]
(2007 Q8)
At a particular speed of rotation, the output of another alternator is as shown below.
What is the frequency of rotation of the alternator?
45.
[2 marks]
What is the RMS value of the output voltage?
(2007 Q9)
46.
[2 marks]
(2007 Q10)
Another alternator is driven by a water turbine, as shown below.
When the valve is opened, water begins to flow and the alternator gradually speeds up from
stationary.
Which one of the following graphs (A - D) best represents the shape of the output voltage as the
alternator speeds up from rest?
Use the following information to answer Questions 47 and 48.
47.
[2 marks]
(2007 Q14)
To study Lenz’s law, students set up the following experiment using the circuit shown below.
Initially switch S is open.
Which one of the following (A - D) will best describe the current through the milliammeter A,
when the switch S is closed?
A. current flows momentarily in the direction X to Y
B. current flows momentarily in the direction Y to X
C. current flows continuously in the direction X to Y
D. current flows continuously in the direction Y to X
48.
[3 marks]
(2007 Q15)
Explain how Lenz's law enables you to determine the direction of the current flow in this
experiment.
49.
[2 marks]
(2008 Q5)
50.
[2 marks]
(2008 Q6)
51.
[2 marks]
(2008 Q7)
52.
[2 marks]
(2008 Q8)
53.
[3 marks]
(2008 Q9)
54.
[4 marks]
(2008 Q10)