The motor is what drives the propellers, which ultimately give the drone lift and control. The general idea of a motor is to convert electrical energy into mechanical kinetic energy. Motors use the forces of electromagnetism to turn an axle. There are many ways of doing with, but the two main types of motors are brushed and brushless motors.
I will explain how an electric motor works. In essence small electromagnets are created which interact with permanent magnets that will pull an arm around an axle. This diagram shows a brushed motor:
As you can see there is coils of wire wrapped round an armature. The ends of the coil are attached to the commutator which is basically a drum with one side as one terminal and the other side the remaining terminal. Then there are two stationary metallic brushes which slide along the commutator which acts as a switch. This diagram shows this:
As the electricity flows along the coils it creates a magnetic field of an opposite polarity to that of the permanent magnets at the ends of the coil (shown as the north and south here). Every tried putting two north ends of a magnet together, they repel. This causes the arm to spin around to the other pole, so north is aligned with south, but just as they align the brushes reach the other side of the commutator which switches the direction of the current going through the coils. When switching the direction of the current the polarity of the electromagnet swaps around and the north of the electromagnet becomes the south. Then again it is South on South, North on North and they repel. The process repeats.
This video shows a simple homemade motor in action.
This setup works and is simple and cheap to manufacture, but it has a lot of problems:
- The brushes eventually wear out.
- Because the brushes are making/breaking connections, you get sparking and electrical noise.
- The brushes limit the maximum speed of the motor.
- Having the electromagnet in the center of the motor makes it harder to cool.
- The use of brushes puts a limit on how many poles the armature can have.
A brushless motor is a motor without brushes, it eliminates these problems. It does this by “turning the motor inside out”. In the case of brushless motors the permanent magnets move instead of the coils. Then the coils are switched electronically using transistors instead of mechanically using the commutator. Of course you can have many many poles switching diagram explains:
In this motor diagram the magnet rotates in the center, it is known as an inrunner. As you can see the is multiple coils, these are switch polarity as the motor reaches them. The idea is the magnet is pulled around as the coils change polarity. So it would work like this:
- B1 would become a South terminal and B2 a North terminal. A1 would become North and A2 south. C1 & C2 would be off.
- The magnet would rotate around to B1 & B2
- Then C1 would become south, C2 would become north. B1 would become north, B2 would become south. A1 and A2 would be off.
- The magnet would rotate around to C1 & C2
- The process moves on to A2 & A1 and so on as the magnet rotates around.
A wave corresponds to each set of coils. These signals would come from an electronic speed controller.
An outrunner motor is the same except the magnets rotate around the outside, this usually allows more leverage, therefor outrunner motors offer more torque.
This diagram shows an outrunner motor.
Some type of brushless motors include hall and/or temperature sensors. The hall effect sensors detect the position of axle as it spins around. This allows better switching timing for the magnetic coils. Motors with hall sensors are a lot smoother at low rpm and respond better under load. The temperature sensor measures the temperature of the motor which can help to diagnose and manage overheating.
Brushless motor specifications
There are many many different types of brushless motors with different specifications. I shall explain the specifications below:
Kv basically means theoretically how many times the motor will turn with one volt, at no load. So it represents how fast the motor turns per volt. If the kv of a motor is 1,500 the motor will spin 1,500 times per minute per volt. If your battery pack is 10volts, then the motor will spin (without propeller at 15,000rpm.
This is important because it gives a good idea to the torque of a motor, a motor with a low Kv will tern only a few times for the same voltage as one with a high Kv, the difference being the one which only terns a few times will have a much higher torque. This means a bigger load can be used, such as a larger propeller.
This is the working range of a motor, it is the voltage which the motor is designed to run at. For example a battery with a voltage rating of 3-10 volts is designed so that it can run a maxium of 10 volts and a minimum of 3 volts. A higher voltage can short out the motor and a voltage too low might not allow the motor to turn.
This is the highest amount of current that you can pass through the motor at any voltage. Higher currents than stated can burn out the motor.
This is a very important aspect of the motor, it is the amount of power that the motor can dissipate. A motor with a high power will create a large amount of turning force, where as a low power motor will create a small amount of turning force. Motor power is rated in Wattage. Wattage is the voltage times current. So 100 watts could be 10 amps at 10 volt. Normally a manufacture states wattage at specific voltages. For example 500 watts at 10 volts, this means you could supply up to 50 amps to the motor. A higher voltage would mean you have a lower maximum Amps limit. The reason for this limit is because a higher wattage would generate too much heat in the motor and cause it to fail.
Number of poles
This basically means the number of separate permanent magnets. So for example the brushless outrunner motor above has 14 poles. More poles for the same size motor means a lover KV. It is also usually more efficient to have more poles.
Number of turns
This is how many times the coils are round around the arms of the motor. A higher number of winds creates a lower Kv and a small number of turns a lower Kv.
How a manufacture controls Kv ratings and current ratings?
There is one route a Motor Manufacture may take in order to bring the KV back up to a usable amount. The best option is to decrease the amount of winds in the motor. A 2 Pole motor for example will typically have many more winding than a 4 Pole motor. When a 4 Pole motor has the amount of windings reduced, KV will increase back to the amount needed. When a winding from the motor is removed, the physical size of the motor remains the same resulting in a void. The best solution to this created void is to add more copper. More Copper is added to the winding. This is done in such a way similar to moving down in the Gauge type. In other words the wire used as a wind is now increased in diamater.
As we know from electrical theory, when the cross section of a wire is increased the resistance decreases. This decreased resistance would allow a greater current load to pass through the motor.
Voltage rating would be controlled by the amount of enameling on the winding, more enameling would keep the windings more insulated from each other and allow a higher voltage, but of course make the motor heavier.
Other variables include the weight and dimensions of the motor, mounting thread sizes, etc. These are useful when comparing say the shaft size of a propeller to a motor etc. Some motors have special adaptions such as cooling fins and sensors built in which is important to consider. Other things to consider are the types of bearings used in the motor.
Motor load and power relationship
Now it is important to calculate the load placed on the motor, this is because a motor with a little load will not draw much power, but a motor with a high load will have to work harder and will draw more power. You must make sure that the load placed on the motor does not cause the motor to work harder than it is capable of handling. For example, a large 20 in prop will require a lot of force to swing and move all the air. A high KV motor such as 1000 kv with a 500w rating on a 10v battery will not be able to handle a 20 inch prop. In fact it would draw something in the realm of 200+ amps, that is 2000 watt draw which is far higher than the motor could handle. Using a smaller proppeler such as a 8 inch prop would require less wattage, and the motor would be able to work with that.
It is very important to match the motor and propeller to find the perfect ratio. You don’t want to over speed or under speed the propeller or not using an excessively large motor or one that isn’t big enough to cope with the wattage. Later I will demonstrate my calculations used for my drone project.
Cooling is also important, when running a motor near to it’s limit it will not be able to dissipate the heat generated if the cooling is poor. I poorly cooled motor could still overheat even if running within specification. Therefore ensuring adequate airflow is a must.
The rest of the power system must be able to deal with the demands of the motor. The Electronic Speed Controller must be up to the job of powering the motor, and then the batteries powering the ESC’s. The motor must also be able to give the drone enough power and control to fly safely. I will talk about Electronic Speed Controllers in my next post.
Thanks for reading!