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Get To Know your DC Motor

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We shall start with the basics ~ Understanding the DC-Motor

Although this lens is primarily about DC motors, it is necessary to cover a little groundwork before you read on. If I have under-estimated your knowledge then please go ahead and scroll down to get to the juicy stuff.

Using electrical energy, an electric motor produces mechanical energy. An electric motor uses magnets to create the kinetic energy which is represented by it's output. Using the simple physics that we all learnt in school, we know that opposites attract and that likes repel. We all remember our amazement at the fact that we could move a magnet without even touching it, simply by applying the like field of another magnet to it. These principles have been applied to the 'working parts' of the motor, using the repel and attract properties, we get the rotational motion.

But how does it work?


I hear you ask, well, there are two types of magnet in play here, one is a permanent magnet (a field magnet) the other is an electromagnet.

A very Basic electric Motor diagram


Now, if you look at the above picture you will notice the three fundamental parts. The outer magnet (coloured portion) is the permanent field magnet. The crudely motion blurred part in the centre is the electromagnet, which is held in place by the Axle, (which is the black circle in the centre.)


The working principle is when the North pole of the field pulls the south pole of the electromagnet towards it and vice versa, the rotation then reverses the poles of the electro magnet and the pole is then repelled by the field pole, pushing the electro magnet away and towards it's opposite pole in the field magnet. Then the process repeats.

And just how, pray tell, are the poles reversed?


Indeed, this isn't some magic that occurs when you turn a magnet upside down, a fridge magnet still sticks to the fridge whether it's the right way up or upside down.

A very Basic electric Motor diagram


The next parts of our motor's anatomy to concentrate on are the brushes (shown in red) and the commutator (in green). These are the most essential part of the motor as they supply the power to the electromagnet. Each part of the commutator supplies power to it's end of the electromagnet. Because of the gap in the commutator ring, when the pole of the electro magnet reaches it's opposite pole of the field magnet, the power supply switches. This supply switch reverses the polarity of the electro magnet. In effect this makes north south and south north, and causes the field poles to repel the electro magnet, thus completing the cycle.


This polarity reverse happens just past the point when the electro magnet is perpendicular to the field magnet, assuring that the electro magnet is not sent back the way it came and creating a continuous circle of movement.

After all this we have an engineering masterpiece which can be used in anything from a CD player to an electric motor car.

While this was somewhat of a hefty introduction, it is worth knowing as it will help you read on, and it is after all quite interesting just how these things work. I was particularly astonished by the simplicity behind the workings and I definitely felt a sense of 'of course...'.

DC and AC - Any difference?

So what exactly is the difference between AC and DC or were they both simply invented to help name the band?

Direct Current (DC) is quite simply a constant source of current with almost no variation in voltage or power. DC also refers to the constance of polarity of voltage or current. This is particularly useful in motors as the polarity of the current is what creates the polarity in the electro magnet. DC supply is mainly used for low voltage applications.

Products that use solar panels or batteries always use a DC current as these power supplies are only capable of creating a direct current. The vast majority of power in automotive applications is obtained through a direct current, and applications using fuel cells are operating under DC power.

Alternating Current (AC) is a form of current that has a cycle whose direction reverses polarity. A good example of the use of AC power is in street lamps, where there is a very slight flicker in the light, this is due to the power alternating.

DC Motors

Why not try and make your own - it's cheap, safe and relatively easy!

A DC motor is, as you can imagine, a motor that runs on DC electric power. Faraday's homopolar motor along with the ball bearing motor are two examples of pure DC motor design, both of which are rare and relatively unused.

YOU CAN BUILD YOUR OWN!!



Farday's homopolar
This motor works on the pretence that in this instance the magnetic field is not parallel to the axis of rotation of the electric field. This causes a torque in the magnet which can be released through an object attached to it by the magnetic field. A simple, home made, example of this can be made using a length of copper wire, a neodymium disk magnet and a regular alkaline battery.

The magnet should be placed under the battery (the batteries negative terminal), you then bend the wire so that each end is touching the positive terminal of the battery and it is looped around the magnet, loosely.
When done correctly (takes some experimenting) the wire is then placed on the battery and it should start to spin, fast.

Homopolar Motor

Create your own homopolar motor

This video carries on the ideas laid out above allowing you to create your own motor, they are not functional, economic or a good use of energy but it is fun!
3 Part Homopolar Motor
by DangerouslyFunCom | video info

1,278 ratings | 1,108,342 views
curated content from YouTube

Get the components for your motor

Magnets & Wire

The neodymium disk magnets are particularly hard to get hold of on the high street so I would suggest buying them on-line. I have found some on ebay.

You can use any battery on the market, however it is recommended that you stick to AAA or AA as they are smaller. These are available in most high street stores.
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Enough fun - now it's back to the theory.

Once you've created your own motor, what more is there to know? You'd be surprised.

The most common forms of DC Motor are brushed (introduction example) and the brushless motor.
Brushed and Brushless DC motors use (respectively) internal and external commutation to to create an oscillating AC current from a DC supply, because of this they are not purely DC Machines.

Brushed Motors

These motors work on the simple concept of a split ring commutator connected to a current through the use of brushes. The commutator powers the electro-magnet within the motor and the split in it allows for the change in current. Although simple, these motors are far from ideal. They are prone to wear and tear as the brushes and commutator are often worn down by the friction or irregularities in the ring. When a ring develops irregularities it is increasingly difficult for the brushes to achieve contact, this causes intermittent power and failure.

Many of the problems associated with brushed motors are easily rectified, as the motors themselves are generally cheap, in small household gadgets and are easily replaced, the main problems occur when the motor is used in large machinery. In this case, replacing the motor is not an option as they are very expensive, it is necessary to replace the faulty parts which, in itself, is expensive and time consuming.

Brushless Motors

Reversing and inverting the process.

As we've seen, in the more common brushed motors, the brushes give power to the electro magnet through the commutator, this causes the electro magnet to spin on it's axle. In brushless motors it is the permanent magnet that spins while the electro magnets remain stationary. The electro magnets pull the contra-pole of the the permanent magnet towards them, as this happens the opposite electro magnet is repelling it. This process causes the spinning motion and allows the motor to relay, rotational, kinetic energy.

Aside from atmospheric imperfections and axle drag, brushless motors have very little friction and do not rely on brushes and split-ring commutators to bridge the circuit current. This allows the brushless motor to have major superiority over it's brushed counterpart as it is more reliable and generally performs substantially better.

The brushless motor is generally quiter than the brushed motor and it has a much longer performing lifetime as it does not suffer from brush errosion, there is also a vast reduction in electro magnetic interference, which can interrupt, obstruct or degrade the circuit within the brushed motor.

In order for a brushed motor to maintain high performance, it is essential that there is and air flow through the casing to allow the electro magnet to stay cool and not overheat. Due to the placement in a brushless motor this objective is reached through conduction with the motor casing. This allows the motor to be entirely sealed, negating the possibility of foreign matter and dirt obstruction.

There are several advantages that Brushless motors have over their brushed counterparts.

  1. Cool Operation: Due to the lack of commutator friction, brushless motors, are operate at much lower temperatures, extending the life of the motor.
  2. Less Wear: One major issue with brushed motors is the need for the replacement of commutators due to the constant friction during operation. Brushless motors do not use commutators, therefore do not suffer this issue.
  3. Tap into the signal: The commutation produces a tachometer signal that a device can harness allowing for a "motor functioning properly" derivation signal
  4. Precise Speeds: The motor can be synchronised to a devices clock speed allowing for exceptionally precise speed control.
  5. No Spark: There is no danger of the motor causing sparks making them the ideal solution when working with volatile chemicals or liquids.
  6. Remove The Heat: Small brushless motors are used in computer systems, in fans, to remove heat. Due to (1) they do not add to it.
  7. Shhhhhhh!! Brushless motors have a very quiet operation.

Reference

Books on motors

If you want to know / NEED more, have a look at these books.
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Some Extra Information

Get Even more info

Not had enough? Have a look at these links, they will tell you even more and also allow you to buy your dream motor (well maybe very few people have a dream motor, but you can
DC Motor Guide
A guide to different types of DC Motor. Need a good motor? This is your one-stop-shop for finding the perfect motor for your application.
Electric Motor Wizard
Searching the internet for a particular size, brand, type or model can often be very laborious and can also prove to be very difficult. This excellently made directory can do all the hard work for you.
DC Motors
A leading manufacturer of Fractional DC Motors.
Subcontract machining
To make an electric motor/gearbox, there are a lot of high precision engineered components. Often this work isn't carried out by the motor company, but usually by a subcontract machining company. High Precision Machined Parts are crucial to get right, so unless the motor manufacturer has a costly inhouse team & equipment to manufacture said components, it's usually best to subcontract out.
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A Blog for all things Technology and Industry related

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I am a North Wales Based Web Developer working for a company called LiveTech.

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