Brushless Direct Current (DC) motors are quiet, light, and quite versatile. They are used in a plethora of devices and situations, from electric cars and wind turbines to hobbyist experiments.
A brushless motor can generate electricity. When generating power from kinetic sources, there are many factors to consider: the speeds, power, variation, and ranges of the abovementioned all go into designing a solution. A brushless motor can be as effective as a brushed motor in generating power but will require more maintenance.
I’ll take a closer look at brushless motors in this article and how they can double up as generators.
Where Do Brushless Motors Fit?
The work of all electric motors is basically the same — turn the electric power provided to the motor into mechanical power that can be used by an implement, tool, toy, etc. There are a number of different motors to choose from, and they all come with their own pros and cons.
The major classification difference has to do with the type of power provided to the motor — AC or DC. Neither AC induction motors nor DC brushless motors require a mechanical system of brushes to control current.
AC vs. DC
Both direct and alternating current have different properties, despite both transferring electric work. DC is simpler, but it is difficult to change the voltage. DC is difficult to use safely as well as being more inefficient in most long-distance power transfer applications.
DC typically provides varying amounts of current of opposing polarities down two lines — one of these is usually called ground and is referenced as 0V.
We can visualize this as having water run down from a water tower; the higher up the water tower, the greater the potential or voltage. When a greater potential is applied to the same pipe, there will be a faster water current.
If you have too much resistance — if the pipe is too small — the flow of water will be hindered, just the same as how closing a tap reduces the amount of water flowing through it.
The amount of work that can be done by the motor per hour is calculated by the W/h, and it is broken up into components — voltage and current (P=VI). Resistance can also play a role and influence your voltage, meaning V=IR. In other words, your power can also be calculated as P=I2R.
Then, we have alternating current (AC) that can be compared to an ocean’s waves. Water is constantly being pushed back and forth in the ocean, transferring energy by moving above the 0V ground, then below it again. This movement is cyclic and can be described using a sine wave.
When a magnet is passed across a wire, it creates a current using the rule of thumb. When this magnet is spinning, the current is pulled in one direction as one magnetic pole moves over the wire. Then, the current is reversed when the opposite pole moves over the wire, effectively creating an AC current (source).
AC Motors vs. DC Motors
The power source for these two types of motors is obviously different. They can be further subdivided into synchronous or asynchronous AC motors and brushed or brushless DC motors. Some of the most significant differences are discussed below.
In AC motors, the power is always supplied to the non-moving part of the motor, the stator, which means that it does not need carbon brushes — blocks of carbon attached to an electrical rotary switch called a commutator — to control current.
In brushed DC motors, power is supplied to the rotor, and, therefore, the mechanical system of carbon brushes and a commutator is required to control current (source).
Brushed motors are unique because they receive DC, but it is converted into AC and provided to the stator, making brushes unnecessary. It aids in creating a higher-performing and more efficient motor.
AC motors experience a decrease in torque when the speed increases, while DC motors have a constant torque regardless of speed changes. AC motors are cheaper and need less maintenance, while the wear and tear from the brushes require more maintenance in DC motors. Brushless motors are like AC motors in this regard.
All in all, AC motors have a lower efficiency than DC motors, and they are used more often for both domestic and industrial equipment. However, DC motors are used when easier speed control, high torque, and precise positions are required (source).
How Can You Tell if a Motor Is Brushless?
Brushed and brushless motors have fairly similar internal mechanisms. The differences, however, vastly changes the way in which they work. When it is not possible to look inside a motor to check the internal composition, how do you tell the difference between a brushed or brushless motor?
Difference Between Brushed and Brushless Motors
A brushed motor’s main parts are the brushes, commutator, electromagnet, and permanent magnet. The electromagnet, or armature, is a configuration of coiled wires encased within the permanent magnet and is also the rotating element within the motor.
The current is delivered to the electromagnet via the brushes and commutator. The commutator ensures the polarity changes in the electromagnet and rotates with it, while the stationary brushes ensure that the commutator is always in contact with the provided current.
The electromagnet’s polarity causes a push-pull effect with the stationary permanent magnet around it as the corresponding poles repel and the opposite poles attract.
The electromagnet rotates, and the commutator causes a change in polarity and a never-ending push-pull movement with opposite poles never actually meeting.
Electric energy is effectively transformed into mechanical energy by utilizing magnetism. Brushless motors make use of commutation but in a different, electronic configuration. Obviously, the motor doesn’t have brushes, but it also doesn’t have a mechanical commutator.
Another major difference is that it is not the electromagnet that rotates but, rather, the permanent magnet.
The motor can be constructed with the electromagnet encased within the rotating permanent magnet or as the stationary magnet encasing the rotating permanent magnet attached to the rotor. Regardless, the electromagnet is stationary in this type of DC motor.
There are three electromagnets in the brushless motor, each individually connected to the provided current. In other words, there are three phases. The same basic principle applies in that the electromagnet’s polarity changes, causing a push-pull effect with the permanent magnet.
In this case, the commutation happens electrically as a sensor monitors the rotor’s position and indicates to a controller when the polarity in the different phases need to change, one after the other. The speed of the motor can then also be controlled by controlling when the polarity changes.
For the most part, brushless motors are preferred over brushed ones as they require less maintenance. They are less noisy and more efficient with a high output power-to-size ratio. However, they do require an external controller for speed control while a brushed motor doesn’t (source).
Identifying a Brushless Motor
Identifying which motor you have can be quite simple — it should be clearly indicated on the motor’s label. Unfortunately, this is not always the case.
If you can have a peek inside the motor, a brushed motor will have one set of contacts per line in the motor. This means there will be at least two points of contact in the form of small conductive blocks or copper braided wire between the wires entering the motor and the commutator inside.
As a brushless motor’s commutation doesn’t happen mechanically through a commutator but electronically, it will have no point of connection (source).
Can You Run a Brushless Motor Without an ESC?
The ESC is an electronic speed controller, and while it is always necessary for a brushless motor to have some type of electronic control, it does not necessarily have to be an ESC.
The electronic controller in a brushless motor is the device that does the commutation and is an essential part of the motor. ESC is a name widely used for a low-cost version of this device, but there are also motor controllers, sensorless motor controllers, and motor drivers.
In a brushed motor, the commutator is mechanical and, along with the brushes, responsible for the change in polarity of the electromagnet and, thus, the continuous rotation and working of the motor. In the brushless motor, this function is taken over by the electronic controller.
It is the electrical component that receives the information on where the rotor is at any given time, and, based on that, it controls which phases are activated, causing the rotor to rotate.
The frequency at which the different phases are activated also controls the speed of the motor. The higher the frequency, the faster the speed.
Where an ESC comes in is when you actually want to control the speed of your motor. In electric cars, for example, you will obviously have to be able to control speed, whereas a washing machine, on the other hand, does not need speed control and, thus, the motor does not have to be an ESC.
Brushless Motors Generate Energy
If the work of a motor is to turn electric power into mechanical power, the work of a generator is the exact opposite. It turns mechanical power into electric power.
A generator would usually be powered by fuel or coals, but some motors can be coupled to a renewable power source to generate electric power from the source’s kinetic energy.
This ability of a motor to reverse its function is often used for small-scale experiments. On a larger scale, it is also used in regenerative braking to recover energy that would normally be lost as heat due to friction or in resistors. It has also been used in elevator systems to ensure the smooth running of the elevator.
When using a brushless motor as a generator, it has to be connected to a number of other bits and bobs to effectively produce a DC current that can be used by the final load attached to the current, whether that be a battery you want to charge or some LED lights you want to run.
Other than building a circuit that will allow you to deliver the required power output, you will also need an efficient power supply to turn the shaft of your brushless motor.
For the most part, this would be hydro- or wind-powered. You have to evaluate and spec your power source before you start installing hydro or wind power sources.
Spec Your System
One of the first things you need to decide and spec is the source of your motor’s power. You will have to consider the physical location in which you want to use your motor/generator and then decide what power source will work the best — water or wind.
Using these power sources can help you live off the grid and potentially save money in the long run. You will have to calculate how well your chosen power source will work. Will it be feasible to invest money into it, and will it then be cost-effective in the long run?
Generally speaking, hydropower is the simplest source to spec, while wind can become a bit more complicated.
On a large scale, hydropower electricity is generated from a dam or reservoir that releases water to run through a turbine. On a smaller scale, it would be ideal if you had a river running downhill and you place a waterwheel or turbine into that river.
To spec your water source, you need to know the head, the vertical distance the water falls, and flow, the quantity of water falling, of your river. To do the calculation, you want the nett head, i.e., the vertical distance, minus the losses from pipe friction. The calculation would look as follows:
Power (watt)=nett head (feet) x flow (gallons per minute) 10
If the river generates enough Watts to make a hydro system feasible, it can provide a fairly stable power source.
Keep in mind that, as with all alternative power sources, problems can occur that will lead to a surge in power that could damage your system for no-power situations. In the case of hydro, you could experience flash floods and/or droughts (source).
Wind power is more complicated to spec than hydro. You will have to estimate your location’s wind resource by consulting wind resource maps, airport wind-speed data, vegetation flagging, local wind systems, or by using measuring devices.
The formula for determining the power output of your wind turbine based on how much wind you have is a bit more complicated:
The elements in this formula are the following:
Cp: Maximum power coefficient (this will be indicated on your turbine)
p: Air density (ld/ft3)
A: Rotor swept area
V: Wind speed (mph)
Wind power as a source for your generator is more volatile in the sense that the wind you have will constantly change, and you need to make allowances for a speed range in your system so that you do not harm your motors and generator.
The calculation above will be used to determine the effectiveness of your chosen power source in the area you wish to implement it. Once you decide which kinetic power source you want to use for your motor, you will need to implement other calculations to determine the compatibility of your motor.
Your Motor’s Efficiency
As explained above, you will need to do some calculations to have your motor running effectively in conjunction with your power source — the hydro or wind turbine. Your turbine and your motor should run at an ideal speed with an ideal torque for maximum power output.
You need to get these two devices to run well together by gearing the rpm up or down in your system. To determine exactly what your ratio is, you can make use of the following formula:
P: Total power — a value you already calculated for your turbines using the formulas above.
n: Rotational speed in revolutions per second.
T: Mechanical torque — Torque can be provided in Newton-meter or foot-pound. To convert Newton-meter to foot-pound, just divide your T value by 1.356.
These mechanical values are not always indicated on the turbine or motor. You can use the formula to determine the missing values and then determine what gear ratio you need. To minimize any power losses, you would want your turbine and motor to run at the same speed and be connected directly, but this is not always possible.
Let’s have a look at a hypothetical example:
If I have a hydro turbine that runs at an optimal speed of 2000 rpm, but my motor wants to run at its ideal speed of 1000 rpm, what gear should I use?
I will need to insert a gear with a 2:1 ratio. This allows both devices to run at their optimal speed.
Some additional information that could be useful is the voltage and current produced by your motor. In ideal situations, this information will be indicated on the nameplate of the motor, but it is not always the case. You can, however, use measuring instruments to determine these values.
If you have either the voltage or current value of your motor, you can calculate the other by using: P=Vx I, where V is your voltage, and I is your current. This is then the voltage, current, and power that will be running through your circuit to the load you wish to power.
The efficiency of your gear, as well as your circuit, also plays a role, however. The power that comes out of your turbine will not be the exact same amount of power that enters your motor, especially if you have a gearbox.
You can measure the real power that enters your motor and use the following formula to determine the gearbox’s efficiency:
Your gearbox should be optimized to enable maximum efficiency, or your system might not actually be viable or cost-effective. The same can be said for your motor and the rest of your circuit. If you know the power that enters your motor and you measure the power that leaves it, you can, once again, determine the efficiency.
If you do find that your motor is running inefficiently and there is too much power loss within the motor, you can decide to either replace the motor or to service it to ensure that all the internal components are working optimally.
The circuit that connects your motor to the load you wish to power can also add to possible power losses, and building an efficient circuit is very important.
Things You Need to Build Your Charge Controller
Regardless of your power source, the motor you use, and the power output, there are certain things that you will need to build into your generator’s system.
The rectifier bridge is the first element that will be connected to the motor. The rectifier converts the AC power that is produced by the generator into DC power, and in its most basic form, it consists of four diodes.
When constructing your own rectifier, additional factors like temperature rating and mounting requirements should be taken into account.
Regardless of whether you build your own rectifier or purchase the correctly rated rectifier, it is the part of the circuitry that will adequately convert your output power. The power supply, however, is of a pulsating nature, while we want it to be a smoothed-out, pure DC. For this, we require a capacitor and inductor.
Capacitor and Inductor
A capacitor is two metal plates separated by a dielectric — a non-conducting material. Much like a battery, it is used to store electric energy. In a DC circuit, it is specifically responsible for evening out the voltage within the system.
The capacitor receives a pulsating DC current, and in its cycle of charging and discharging, it evens out the voltage in the system. The inductor does almost exactly the same thing but for the current.
The inductor is a simple component. It is basically just coiled wire and ensures an even supply of current to the load attached to the circuit. It does so because it opposes any change in current. If a spike of current is received, it will effectively counteract that spike and provide even, regulated current.
The last electronic component that can also be added to the system is a voltage regulator. As the name indicates, it regulates the voltage that is supplied to the load. All-in-all, it creates a more stable system that will keep the load from being harmed because it received too much voltage.
Using an MPPT
If you opt to buy a complete system rather than building and connecting all the individual components in your circuit, you should opt to get an MPPT.
A Maximum Power Point Tracker (MPPT) will improve and ensure charge during low and erratic power generation situations. It is a much more fine-tuned system that will include variable capacitors, inductors, and voltage regulators.
The MPPT, with its variable components, will play a huge role in ensuring the efficiency of your system and, ultimately, minimizing power losses.
Let’s take a look at a real-life example:
If you want to charge an ordinary camper battery, you are looking at a 12V 105Ah lead-acid battery.
Using P=V.I we can calculate 12V X 105Ah = 1,260 Wh.
The motor you have is a 14.4V. The motor is labeled as 300W. It has no speed or torque values indicated. You can use your voltmeter to measure the type of power coming from the wires as you turn it by hand.
Determine if the voltage produced across the three wires is AC or DC. It should be AC.
We can then calculate the current the motor produces by dividing the power, which is 300W, by the voltage, 14.4V, which equals 20.8A. Then, use appropriate rectifiers, capacitors, and conductors that can handle that voltage and current and factor in a safety margin of 50% to account for possible spikes in voltage.
The turbine that you connect to the system should not produce more than 300W, or you could damage the load connected to it.
If you want to use a brushless motor as a generator, it is quite possible for it to generate electricity. However, a brushless motor has some specific requirements. Since it does not have brushes and is not mechanically commutated, it will always need some form of electric control, but this does not have to be an electric speed controller.
When you start using your brushless motor as a generator, you will have to connect it to a power source, like wind- or hydropower, to provide the kinetic energy needed to start turning its shaft and ultimately produce electric power.
To provide stable power to the intended load, you will have to wire it into a system with a rectifier, capacitor, inductor, and voltage regulator. You can build this system yourself, but for a highly efficient system, you can opt to purchase an MPPT system that will optimize all the different electronic components in the system to ensure minimal power loss.