Lesson video
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Hello.
My name's Mrs. Taylor.
And I'm really pleased you can be here to join me for our lesson today.
Our lesson today is motors, and this is part of the systems approach to design: sustainable futures unit.
The outcome: I can explain the function of different motors and use them as output components in a control system.
There are four key words, direct current, which is abbreviated to DC.
Direct current is the electric current which flows in one direction only.
Torque is a force that makes something rotate.
Servo motor is a motor that uses a closed loop system to ensure accurate and precise control of their position, speed, and torque.
Uniform magnetic field, a magnetic field that has the same strength wherever with evenly spaced field lines.
There are three parts to the lesson today.
DC motors, controlling DC motors, and servo motors.
Let's begin with DC motors.
Motors are used to make objects move.
Examples of products that use motors include a roller door and a cordless drill.
Here we have a check for understanding.
Which of the following use electrical motors? Is it A, a bicycle, B, an electric fan, C, a hammer, or D, a candle? Pause the video and have a go.
Wonderful.
Let's check.
That's right, it's an electric fan.
Well done.
When a current flows through a wire, there is a magnetic field around it.
A plotting compass can be used to plot the shape and direction of this magnetic field.
There is a uniform magnetic field between the two magnets below.
The magnetic field lines are evenly spaced.
The strength of the magnetic field is the same everywhere.
This part is called the yoke.
Here we have a check for understanding.
Which of these show a uniform magnetic field between the motor magnets? Is it A, B, or C? Pause the video and have a go.
Wonderful.
Let's check.
That's right, it's C.
Well done.
A current carrying wire in a uniform magnetic field is forced to move.
Here, the wire is forced upwards.
We use this movement to make motors.
A DC motor contains a coil of wire that spins when a current flows through it.
It spins because the coil of wire is cutting through a uniform magnetic field.
The magnets are either side of the coil.
The coil is in the centre.
The coil of wire is insulated with exposed ends to make connections with the electrical contacts.
When the coil is horizontal to the magnetic, the contacts connect and current flows through the coil causing it to start to spin.
When the coil is vertical, the electrical contact is broken.
No current flows around the coil.
There are no forces making it spin, but the coil continues to move as there is very little friction.
When the coil is next horizontal, the contact is made again.
The current flows again and the coil is pushed around in the same direction.
This results in a spin in action.
Which we can see here in the three different images.
Here we have a check for understanding.
True or false, current flows through the coil when the coil is vertical to the magnetic field? Pause the video and have a go.
Wonderful.
Let's check.
That's right, it's false.
When the coil is vertical, the contacts are broken and no current flows around the coil.
Well done.
The parts of a DC motor can be identified when it is disassembled.
We can see a photograph on the left of a DC motor and the illustration on the right.
The yoke is the outer part with the electrical contacts inside.
We can see the axle in both the photograph and the illustration, and the position of the magnets inside the yoke, and the coil.
Here we have a check for understanding.
Which part of a DC motor is the arrow pointing to? Is it A, the coil, B, magnets, C, electrical contacts, or D, axle? Pause the video and have a go.
Wonderful.
Let's check.
That's right, it's the coil.
Well done.
We now move to task A.
Label the parts of the DC motor and explain what each part does.
Pause the video.
Fabulous.
Let's check.
We can see the magnets which create a uniform magnetic field, a coil which carries the current, which creates a magnetic field around it, the axle which allows the coil to turn, the yoke which holds the magnets in place, and the electrical contacts which connect with the exposed ends from the coil of wire and allow current to flow through the coil.
Well done.
We now move to part two of our lesson, controlling DC motors.
Motors can work with direct current, DC, or alternating current, AC.
Direct current flows in one direction only as we can see here on the graph.
Alternating current periodically reverses, as we can see here on a graph.
When turned on, DC motors usually rotate continuously.
They operate as open loop systems. This means they don't have feedback loops to monitor the position or speed.
DC motors are most suitable for applications where continuous rotation is needed such as fans or electric vehicles.
An example of a circuit used to operate a motor, a slide switch, a DC motor, and a nine volt battery.
The motor spins when the slide switch makes the circuit.
The motor spins at 17,568 revolutions per minute.
This is abbreviated to RPM.
We can control the speed of a motor by changing the current or adding a gearbox.
We can change the current by adding a resistor.
Here we have a check for understanding.
How can we change the speed at which a DC motor spins? Is it A, open the switch, B, add a gearbox, or C, remove a resistor? Pause the video.
Wonderful.
Let's check.
That's right, it's B, add a gearbox.
Well done.
Using a resistor to control a motor reduces torque.
Torque is a turning force which makes something rotate.
This is simple and inexpensive to add.
Using gears to control a motor adds additional mechanical parts and complexity, but does not reduce torque.
Here we have a check for understanding.
What is torque? Is it A, a type of resistor, B, a turning force, or C, a type of gear? Pause the video.
Wonderful.
Let's check.
That's right, it's B, a turning force.
Well done.
We now move to task B.
Part one, describe the features of a DC motor.
Include the following, what DC current is, what loop system they use, what they are used for, and how we can control them.
For part two, open Tinkercad and create a circuit with a nine volt battery and a DC motor.
Simulate this and describe what happens.
For part three, now change the wires to be in the opposite connections of the battery.
This changes the polarity.
Simulate this and describe what has changed.
For part four, put the wires back to the correct connections.
Add a slide switch and a resistor.
Simulate this and describe what happens.
And for part five, change the resistor to a photoresistor.
Simulate this and describe what happens.
Pause the video and have a go.
Wonderful.
Let's have a look at some of the answers you may have come up with.
For part one, you may have said, "A DC motor uses direct current, which only flows in one direction.
DC motors have an open loop system, they do not have any feedback.
This makes them suitable for applications where continuous rotation is needed like a fan as they cannot monitor position or speed.
To control the speed of a DC motor, we can use a resistor, but this also reduces torque, so may not be suitable.
Another way to control the speed of a DC motor is to use a gearbox.
This does not reduce torque, but adds cost and time to production." Well done.
For part two, you may have said, "The motor spins when the slide switches on and makes the circuit.
The motor spins at 17,568 revolutions per minute or RPM." And for part three, when you change the polarity of the wires, "The motor spins in the other direction and the RPM is minus 17,568." For part four, you may have said, "The switch connects and makes the circuit.
When it is on, the motor turns.
When it is off, it does not.
The resistor restricts the flow of current to the motor, which means it spins at 1,058 RPM, which is much slower than without a resistor." And for part five, "The photoresistor changes the resistance dependent on the light levels.
This in turn changes the RPM of the motor.
When in the dark, the resistance is high, and the motor RPM is low.
When in the light, the resistance is low and the motor RPM is higher." Well done.
We now move to the third part of our lesson today, servo motors.
A servo motor uses a closed loop system to ensure accurate and precise control of the position, speed, and torque.
The closed loop system is a control circuit with feedback.
This is what enables the position of the motor to be precise and accurate and why there are three connections, the power, ground, and signal connection, which we can see here on the diagram.
Servo motors are suitable for applications such as robot arms, which require precision.
The increments are measured in degrees.
There are 360 degrees in a circle.
Zero is at the top just like north on a compass.
When programming in Tinkercad, the code block rotate servo pin is used to control the signal to the servo motor.
In this example, when button A is pressed, the servo motor moves to zero degrees and displays an arrow on the onboard LEDs.
Here we have a check for understanding, which force makes something rotate? Is it A, compression, B, torque, C, tension, or D, torque? Pause the video.
Great.
Let's check.
That's right, it's B, torque.
Well done.
This control system uses a micro bit and a servo motor.
Here we have a check for understanding.
Which type of motor has a closed loop system and is a control circuit with a feedback mechanism? Is it A, a DC motor, B, an AC motor, or C, a servo motor? Pause the video.
Great.
Let's check.
That's right, it's C, a servo motor.
Well done.
We now move to task C.
There are five parts.
Part one, describe using examples the features of a servo motor.
Part two, open Tinkercad and create a control system with a microbit and a servo motor.
Part three, write a programme to move the servo motor 30 degrees when button A is pressed, and back to zero degrees when button B is pressed.
Part four, simulate the programme and describe what happens.
And part five, plan the motor outputs in your greenhouse control system and add this to your flow chart.
Pause the video and have a go.
Wonderful.
Let's check For part one you may have said, "A servo motor uses direct current, the same as a DC motor.
However, servo motors are a closed loop system, which means they do have feedback.
This is why they have three wires as one is for the feedback signal.
This feedback signal is used to control the precise and accurate position of the motor, which is measured in degrees.
Because they can be controlled so precisely, they are suitable for products such as a robot arm." Here is an example of the code you may have come up with for parts two and three.
For part four, you may have said, "When button A is pressed, the servo motor moves 30 degrees.
When button B is pressed, the servo motor moves back to zero degrees.
I also added the string code block to add the message on the LEDs that the window was opening or closing." And for part five, your flow chart may look something similar to this.
Start, measure the temperature.
Is the temperature above 30 degrees? No.
Display string closing and rotate the server to zero degrees.
If the temperature is above 30 degrees, yes, display string opening and rotate servo 30 degrees.
Well done.
We now have a summary of our learning today.
A current carrying wire in a uniform magnetic field is forced to move.
We use this movement to make motors.
DC motors are most suitable for applications where continuous rotation is needed, such as fans or cars.
The revolutions per minute, or RPM, can be adjusted by changing the current flowing through the motor.
Servo motors can accurately control position, speed, and torque and are suitable for applications where precision is required, such as robot arms. I'm so pleased you could join me for our lesson today.
Thank you, and well done.
