D.C. ELECTRIC MOTORS

1. Motors use the effect of forces on current-carrying conductors in magnetic fields

F

d

Define torque as the turning moment of a force using:

Solve problems and analyse information about simple motors using:

cosnBIA

Fd

Increasing F or d increases the turning effect

(torque)

BIlF Substituting: BIld Torque from

each side

So total Torque, )2( dBIl

BIA Torque for each loop

and for n loops nBIAIf the coil is at an angle to a uniform magnetic field

Describe the forces experienced by a current-carrying loop in a magnetic field and describe the net result of the forces

F

F

d

M.Edwards 25/4/02

Below is a diagram of a coil of wire attached to a split-ring commutator and a power source.

A student placed some permanent magnets at A and B and the motor started spinning. Attaching a torque meter to the axle, they were able to determine the torqueat various angles (the angle as shown in the diagram is 0o). The results are shown at right.

(a) Graph the results in such a way that there is a linear relationship between the two variables. (b) Given that there were 200 turns of wire, a current of 3 A, the coils were 2cm by 2cm, and using the graph, determine the magnetic field strength. (c) If the student did not have permanent magnets, how could they have created the magnetic field?

cosnBIA

Angle

(degrees)

Torque

(Nm)

0 0.018

15 0.017

30 0.016

45 0.012

60 0.007

75 0.002

(a) 2 marks

(b) 2 marksgradient = (0.018 +0.004)/(1-0) = 0.022gradient = nBIAB = 0.022/nIAB = 0.022/200 x 3 x 0.02 x 0.02B = 0.09 T

(c) 2 marksThey could have created an electromagnet by wrapping a wire around an iron core many times and attaching the ends so that it was in parallel with the motor.

Angle

(degrees)

Torque

(Nm)

0 0.018

15 0.017

30 0.016

45 0.012

60 0.007

75 0.002

cosnBIA-0.005

0.000

0.005

0.010

0.015

0.020

0.00 0.20 0.40 0.60 0.80 1.00 1.20

cos(angle)

To

rqu

e (

Nm

)

Identify that the required magnetic fields in DC motors can be produced either by current-carrying coils or permanent magnets

Where are the permanent magnets placed? Why?

Can we place an electromagnet in the same circuit as the motor? How?

What are the advantages and disadvantages of permanent and electromagnets?

What is the shape and direction of a magnetic field produced by a current-carrying coil?

Identify data sources, gather and process information to qualitatively describe the application of the motor effect in:

– the galvanometer– the loudspeaker

Jacaranda

PHYSICS IN FOCUS P.112

Student exploration of large galvanometers

Student exploration of car stereo speakers

E.g. ammeter:The galvanometer can be converted into an ammeter by using a very small resistor in parallel with the galvanometer and changing the scale appropriately. Most of the current will flow through the small resistor, with the remainder through the galvanometer. This allows larger currents to be measured without the needle moving off the scale.

voltmeter: Make sure you find the answer to this too!

Question 1

Below is a diagram of a galvanometer that is capable of measuring a small current.

(a) Explain how the galvanometer works?

(b) Qualitatively describe how the galvanometer could be converted into an ammeter.

1 (a) 4 marks

The magnetic field induces a force in the current carrying wire of the armature. The

force is up for one side of the wire (where the current comes out of the page) and down

for the other side of the wire (where the current goes into the page). This creates a

torque on the armature that is proportional to the magnitude of the current. The mass

on the end of the needle produces a torque which opposes the induced torque and

increases with greater deflection. When the torques equal each other in magnitude, the

needle is stationary and points on the scale to the value of the current.

(b) 2 marksThe galvanometer can be converted into an ammeter by placing a very small resistor in parallel with the galvanometer and changing the scale appropriately. Most of the current will flow through the very small resistor and only a small amount of the current will flow through the galvanometer. This allows larger currents to be measured.

Describe the main features of a DC electric motor and

the role of each feature

armatureN

F

d

M.Edwards 25/4/02

magnets (stator)

S

coil

axle

The magnets are stationary and are known as the stator.

The coil, armature and axle rotate and are known as the rotor.

brush brush

Split-ring commutatorSource of EMF

HSC Practical Research TaskInternet DC motor animation by Walter Fendt

N S

F

M.Edwards 25/4/02

The right hand push rule tells us this rotor will move anticlockwise.

side Aside B

Conventional current in side B is currently out of the page

I

I

When reaches 90 degrees, the split ring will contact the other brush, reverse polarity and current in side B will be into the page

F

A

B

So the rotor continues to rotate in an anticlockwise direction

AF

B

HSC Practical Research Task

M.Edwards 25/4/02

M.Edwards 25/4/02

Describe the main features of a DC electric motor and

the role of each feature

Question 1The diagram below shows a DC motor.

(a) Explain why the motor spins.(b) Explain why the split-ring commutator is employed.(c) If the coil was 2 cm long and 2 cm wide, consisted of 50 turns of wire carrying a current of 5 mA in a magnetic field of magnitude 0.6 T, what would be the magnitude of the maximum torque generated and the direction the coil turned?(d) How could this device be modified in order to create a galvanometer?

1 (a) 1 markDue to flow of electrons in the magnetic field, side AB will experience a downwards force. Similarly, CD

will experience an upwards force. This will cause the coil to spin in an anticlockwise direction.

(b) 3 marksIf there was no split ring commutator, side AB would always experience a downwards force and side CD

would always experience an upwards force. After a quarter of a turn, the force would no longer cause the coil to spin anticlockwise but clockwise resulting in an oscillating coil. To make the device useful, the coil must spin in one direction continuously. This can only be obtained by changing the direction of the force every half cycle. The split ring commutator allows the current to be changed every half cycle, hence the torque is always in the same direction.

(c) 1 mark = nBIAcos= 50 x 0.6 x 0.005 x 0.02 x 0.02cos90=6 x 10-5 Nm spinning in an anticlockwise direction.

(d) 2 marksA spring could be connected to the axle of the motor so that the torque generated by the motor effect is

countered by the opposing torque of the spring. The more the coil is turned, the greater the torque of the spring. The coil would come to rest when these torques were in equilibrium. The degree to which the coil had moved would be proportional to the magnitude of the current. A pointer attached to the axle could indicate on a scale the magnitude of the current.