Brushed vs Brushless Motors : What are the advantages and disadvantages of each type of motor. Welcome. In this article you will learn the Facts about Electric Motors.
The electric motor is one of the most important inventions of our time. I can’t imagine a world without them. Every day we rely on the power of electro-mechanical energy to make our tasks easier and our jobs more efficient. From opening the window in your car, using an elevator, or mowing the lawn, we rely on these devices.
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Despite being such an integral part of modern life, most people have only a vague understanding of electric motors. We kind of take them for granted. Though most of our appliances and power equipment use a motor, and a better understanding of how an electric motor works will enable us to make better decisions when buying, using, and maintaining these devices. Electric motors can be AC or DC and may use brushes or can be brushless. If we’re going to be looking at brushless vs brushed electric motors, we first have to understand how an electric motor works and what brushes are used for.
Brushless vs Brushed Motor
What’s the difference?
I suppose the answer to the question; what’s the difference between a brushless and brushed electric motor, is quite obvious. One has brushes and the other doesn’t. But what does this mean in practice? I’ll go into all the details through the course of the article, but for those who want a basic understanding as to the pros and cons of brushless vs brushed motors, here it is.
Brushed Motors
Brushes are simple old-fashioned technology, which makes them cheap and simple to maintain. Though they do need more frequent maintenance, the brushes need to be replaced as they wear. This is not a serious drawback as most brushes are designed for easy DIY replacement.
The main reason why brushed motors are not the best, is their inefficiency and comparatively short lifespan. Friction caused by brushes making contact with the commutator not only wears the brushes; but generates heat and places extra stress on the bearings. This makes a brushed motor less efficient and they generally don’t last as long as motors without brushes. In some instances, the noise produced by these motors can be an unwanted distraction.
Brushless Motors
By eliminating the brushes, a motor will be more efficient and won’t require as much maintenance. Because there is no physical contact between moving components, brushless motors are much quieter.
VIDEO | Makita Brushless Motor Technology
However, these motors require additional components – transistors, switches, and usually capacitors. This makes the motor more expensive to manufacture. While these motors tend to last longer and don’t require much maintenance, replacement parts are more expensive. Being more sophisticated, diagnostics and repairs are more complicated and required technicians with a greater level of skill, further adding to long term maintenance costs. Though the motor should have a longer lifespan, which will offset the additional, occasional, maintenance costs.
What are Brushes?
Brushes are used to conduct electricity to a rotating shaft, a rotor, also known as an armature. Since the rotor is in constant motion, it is impossible to permanently attach an electric wire to it, the wire will twist and snap. Early electric motors used brushes made of copper to conduct electricity to the rotor. This was later replaced by solid copper rods and then carbon. It is a simple device that maintains constant contact with a rotating shaft and has a spring to ensure that the brush presses firmly against this shaft as the brush wears down.
Knowing what a brush is, doesn’t mean much if we don’t understand how an electric motor works. So let’s take a look at how the electric motor came into existence.
Diagram | Brushless Motor
How does an electric motor work?
The basic definition of an electric motor is a device that converts electric energy into mechanical energy. In other words, using electricity to create motion. The principles involved, were discovered long before the first electric motor was invented.
History
The discovery of electrochemical energy, by Alessandro Volta in 1799, sparked a revolution – the invention of a battery that could deliver an electric charge. It took another twenty-years before Danish physicist, Hans Christian Ørsted, realized the interaction between an electric current an magnetic field. Through the course of the following year, understanding of the principles governing electromagnetic interaction progressed rapidly, ultimately leading to Michael Faraday demonstrating the magnetic effect of rotary motion. Faraday’s law of induction and electromotive force (EMF) was the basis of what was to become the first DC electric motor. He showed, by means of a simple experiment, that when a conductor was placed under a magnet it would wind around the magnet as soon as an electric current was introduced. This showed the relation between the electromagnetic field and a fixed magnet.
DC Electric Motor
It took the invention of the commutator in 1832 by British inventor, William Sturgeon, before the commutator (or brushed) DC electric motor would become a reality. A commutator forms the basis of any brushed electric motor. In order to maintain constant motion, an electromagnet placed between fixed magnets needs to constantly change polarity. The electromagnetic force of the windings around the stator needs to alternate from north to south as it rotates. Basically, the magnets have to keep pushing away from one another.
To achieve continuous magnetic repulsion, the stator has multiple windings, each generating an opposing magnetic force. As the rotor turns, current is conducted to the next winding. The commutator is a solid copper drum at the top of the rotor with segments, separated by grooves. Brushes conduct an electric current to the rotating commutator. Each section of the commutator is connected to a winding, generating a magnetic field. As it turns, current is conducted to a winding and is broken for a short time by the slot in the commutator, before moving onto the next winding.
So, an electric motor works by placing a shaft with copper windings, producing a magnetic field (rotor), between fixed magnets on either side (Stator). By constantly changing the polarity of the magnetic field, the rotor and stator are crating a force that spins the rotor shaft. This shaft is used to propel a mechanical device.
For a long time, brushes were seen as the only way to conduct electricity to a rotating commutator, thereby energizing the rotor. Though brushes are not ideal. Friction causes them to wear and they have to replaced periodically. The friction also generates heat which reduces the efficiency of the motor. Contact between the brushes and the commutator is noisy and this also transfers some of the energy, further reducing efficiency.
The biggest advantage to using brushes is their simplicity. Apart from wear on the actual brushes, there is little to go wrong and repairing these motors is not complicated. The simplicity of this technology also makes it relatively cheap.
Brushless AC Electric motors
Multi-phase brushless AC motors have been around the longest. Nikolas Tesla is credited with the invention of the induction AC motor in 1887. Though it seems that he was not the first, Galileo Ferraris built a working prototype of the AC electric motor in 1885. These early electric motors relied on an inductive current, using more than one phase. Single phase inductive and synchronous motors came later.
What is an inductive current?
Induction is the energy transferred by a magnetic field rather than a solid conductor. Basically put, any type of inductive current is transferred wirelessly, using an Electromagnetic Force (EMF) instead of a metal conductor, like copper wire. Since the rotor, or armature, of an electric motor is in constant motion, it makes sense to use this principle. A brushless inductive electric motor uses the magnetic force from the stators to energize the rotor, eliminating the need for brushes which would otherwise conduct this current.
Inductive electric motors rely on constantly switching polarity of an alternating current to create opposing magnetic fields between the rotor and stator. In doing so, the attraction and repulsion effect of a magnetic force is changing as a rotor spins between fixed electromagnets (stators).
In order to achieve this, early electric motors needed more than phase. Each phase was connected to an independent stator with opposing field windings. This meant opposing magnetic fields would affect the rotor as it passed a particular stator, depending on the phase. The electromotive field created in the rotor will, in effect, oppose the stator that is creating it.
A single phase AC electric motor is more complicated as it needs additional windings, or a split phase design. For low torque AC induction motors, a copper ring is used to create a shaded pole, covering a portion of each pole. When the stator winding is energized by an AC current, the shaded pole creates an opposing field to start the rotation of the stator.
The energy derived from a shaded pole is not high enough to overcome resistance from a force being placed on a motor. This means that these motors can only be used for light-duty applications, generally less than 250W (⅓HP), like a small fan.
If the motor has to start under a load that is resisting the rotation of the rotor, like a pump that has the force of water acting on the impellers, more torque is needed to initiate the electromagnetic force. Electric motors, typically more than 250W, with high torque startup use a split phase. These motors have split stator windings, with the second winding being out of phase with the primary winding. This is usually controlled by a capacitor which switches to allow current to flow to the primary and secondary windings at startup. Once the motor is spinning, current is switched to supply only the primary windings.
Polyphase electric motors (using more than one phase) will produce torque even when not in motion and there are a number of ways to start the motor. Traditionally, these would be reduced voltage, auto transformer, or star-delta starting starters. Though Variable Frequency Drives (VFD) have become the preferred method. Because of their high-torque startup capabilities, polyphase AC motors are preferred for heavy-duty industrial applications and heavy lifting equipment, like elevators. These motors also use a higher voltage which means lower amperage, which is advantageous as the watt output increases.
Brushless DC Motors
As brushless DC motors use relatively new technology, the questions surrounding brushless vs brushed motors is most relevant to DC equipment. This has become increasingly important with the growing popularity of battery-powered tools and appliances.
Essentially, a brushless DC motor is no different to an AC electric motor. An inverter or switching power supply is used to create an AC current, or simulated AC current. A closed loop controller is used to drive each phase of the motor. It was only through the discovery of the semiconductor that this technology was made possible, allowing for electronic control of the electric pulses that supply the stator.
Because a brushless DC motor is expensive to manufacture, it was not immediately implemented on a large scale. The real value was realized when battery technology improved, paving the way for battery powered machinery to become more commonplace.
Brushless vs Brushed DC motors
Although brushed DC motors are cheaper to produce and simpler to repair, the advantages of brushless technology make the cost (and complications) viable when considering efficiency vs battery life.
Brushless DC motors have no commutator or brushes and, therefore, have an improved power to weight ratio. By removing the brushes, friction is eliminated, improving motor efficiency, and reducing wear. This means brushless DC motors produce more torque at a lower amperage and require little maintenance, along with improved durability. These motors are considerably quieter than those that use brushes.
Probably the most significant advantage to a brushless motor, when using battery power, is the ability to control the power electronically. Not only does this allow for improved speed control, it enables better power management, which becomes is important as the battery voltage is reduced.
Typically, brushed DC motors are only effective when the battery is fully charged. As the battery discharges and the voltage drops, the motor loses power. The electronic inverter or switched power supply in a brushless DC motor is able to compensate for power loss as the battery discharges. This allows a brushless DC motor to perform optimally for longer. Torque loss through battery discharge is gradual and is hardly noticeable until the battery is almost fully discharged.
Improved efficiency, makes for improved runtime for battery powered equipment, and electronic power management allows us to maximize the use of these devices as the battery discharges.
