In order to make the operation more reliable, more efficient, and less noisy the recent trend has been to use brushless D.C motors. They are also lighter compared to brushed motors with the same power output. This article gives an illustrative introduction on the working of BLDC motors.
Thus the brushed D.C motor should never be used for operations that demand long life and reliability. Fort this reason and the other reasons listed in the introduction, BLDC motors are used in most of the modern devices. Efficiency of a BLDC motor is typically around 85-90%, whereas the conventional brushed motors are only 75-80% efficient. BLDC motors are also suitable for high speed applications ( 10000 rpm or above). The BLDC motors are also well known for their better speed control.
The stator has a coil arrangement, as illustrated; The internal winding of the rotor is illustrated in the Fig.3 (core of the rotor is hidden here). The rotor has 3 coils, named A, B and C.
Out of these 3 coils, only one coil is illustrated in the Fig.4 for simplicity. By applying DC power to the coil, the coil will energize and become an electromagnet.
The operation of a BLDC is based on the simple force interaction between the permanent magnet and the electromagnet. In this condition, when the coil A is energized, the opposite poles of the rotor and stator are attracted to each other (The attractive force is shown in green arrow). As a result the rotor poles move near to the energized stator.
As the rotor nears coil A, coil B is energized. As the rotor nears coil B, coil C is energized. After that, coil A is energized with the opposite polarity (compare the last part of Fig.6 with Fig.5).
This process is repeated, and the rotor continues to rotate. The DC current required in the each coil is shown in the following graph.
A humorous analogy help to remember it is to think of BLDC operation like the story of the donkey and the carrot, where the rabbit tries hard to reach the carrot, but the carrot keeps moving out of reach.
For this instant, a same polarity current is through the second coil. The combined effect produces more torque and power output from the motor. The combined force also makes sure that a BLDC has a beautiful, constant torque nature. Such torque nature is difficult to achieve in any other type of motors.
The current form required for the complete 360 degree rotation is shown in the graph below.
With this configuration 2 coils need to be energized separately, but by making a small modification to the stator coil, we can simplify this process. Just connect one free end of the coils together, as shown in the Fig.10.
When the power is applied between coils A and B, let’s note the current flow through the coils. By comparing second part of the Fig.13 with Fig.9, it is clear that, the current flow is just like the separately energized state.
Why BLDC motors ?
The brushes in conventional D.C motors wear out over the time and may cause sparking. This is illustrated in the Fig.1. As a result the conventional D.C motors require occasional maintainance. Controlling the brush sparking in them is also a difficult affair.
Fig.1 The brushes in a conventional D.C motor might cause sparking as shown
The Basic working
The rotor and stator of a BLDC motor are shown in the Fig.2. It is clear that, the rotor of a BLDC motor is a permanent magnet.
Fig.2 The Rotor of a BLDC is a permanent magnet; the stator has a winding arrangment
Fig.3 The coil arrangement in a BLDC is shown here, with different color for different coils
Fig.4 The coil energized by a DC power source becomes an electromagnet
Fig.5 The rotor moves towards the energized coil, due to the attractive force
Fig.6 In a BLDC, as the rotor nears the energized coil, the next coils is energized; this will make the rotor continuously rotate
Fig.7 The DC voltage required in each coil is shown in this graph
Fig.8 Just like the donkey runs after the carrot, in a BLDC the rotor runs after the moving magnetic flux
Further improving the BLDC Performance
Even though this motor works, it has one drawback. You can notice that, at any instant only one coil is energized. The 2 dead coils greatly reduce the power output of the motor. Here is the trick to overcome this problem. When the rotor is in this position, along with the first coil, which pulls the rotor, you can energize the coil behind it such a way that, it will push the rotor.
Fig.9 One more coil is energized in practical motors; this will result in a push force apart from the pull force
Fig.10 The BLDC has a constant torque nature as shown.
Fig.11 The voltage form required in each of the coil
Fig.12 Connecting one free ends of the coil together makes the BLDC voltage regulation much simpler
