What are BLDC motors

What are BLDC motors?

What are BLDC motors?

They are made with permanent magnets driven by the windings in the stator. The principle is basically “rotate the magnet”. It is equipped with absolute position sensor, required for phase commutation. They do not employ brushes in the commutation for the transfer of electrical energy to mechanical energy, but the commutation is performed electronically by means of power transistors. The incorporation of semiconductors eliminates many problems that brushed electric motors have such as friction, which decreases performance and generates heat, noise and maintenance to a large extent. However, BLDC motors have two disadvantages: they have a higher cost and require significantly more complex control. 

Benefits

  • Greater dynamic response.
  • Higher efficiency.
  • Longer service life (over 10,000 hours).
  • Greater speed range.
  • No arcing.
  • No electrical noise.
  • Much higher torque-size ratio.
  • Better engine speed-torque ratio.

Control of BLDC Motors

Brushless DC motors use electronic switches to commutate the stator phases. They are connected in an H-bridge for a single-phase BLDC motor or in a three-phase H-bridge for a three-phase BLDC motor, as seen in Figure 13. The commutators use the pulse width modulation (PWM) technique to convert the DC voltage to a modulated voltage, which allows controlled motor starts and motor speed regulation. Generally, increasing the switching frequency increases the switching losses in electronic switches due to PWM. However, lowering the frequency too much limits the system bandwidth and raises the current ripple to undesirable levels. [4]

Schematic of H bridge and three-phase bridge. Respectfully borrowed from MPS Monolithic Power. [4]

The BLDC motor requires a Hall effect sensor to detect the rotor position and phase shift for every 60° traveled. By combining these 3 phase signals ‘U’, ‘V’ and ‘W’, the exact sequence of commutation can be determined. [4]

The next illustration shows 3 Hall effect sensors ‘a’, ‘b’ and ‘c’ mounted on the stator at 60° intervals and the 3 phases are connected with wye diagram. Every 60° of rotation, one of the Hall sensors changes state. It takes 6 steps to complete an entire electrical cycle.

Commutation sequence. Respectfully borrowed from MPS Monolithic Power. [4]

At each step, the polarity of the motor terminals is alternated, one becomes positive, one becomes negative and the third remains floating, as shown here. [4]

Commutation sequence. Respectfully borrowed from MPS Monolithic Power. [4]

The phase windings ‘U’, ‘V’ and ‘W’ float or are energized according to the values of the ‘a’, ‘b’, ‘c’ signals from the Hall effect sensors. In this example, the phases are 120° apart and the motor rotates counterclockwise. If the DC voltage on the inverter bus is higher than its rated voltage of the motor, then by varying the PWM duty cycle it can be adjusted to the appropriate value and thus prevent damage. [4]

If in the application it is not appropriate to use sensors, then a sensorless BLDC motor should be used and monitor the counter-electromotive force (BEMF) signals instead of the position detected by the Hall sensor, as seen in Figure 16. The commutation signal changes state when the polarity of the BEMF voltage crosses zero and provides accurate position information. [4]

Commutation sequence by zero-crossing technique. Respectfully borrowed from MPS Monolithic Power. [4]

Why is it considered a ‘DC motor’ if it alternates current?

There is some discussion on why it is called a DC motor if it looks and acts similarly to a AC induction motor. The main point that you should know (to save you time) it’s because it requires DC to work. Even when switching, as it rotates, it is still powered by DC current, just in other direction. Actually many DC brushed motors uses a conmutation system anyways. So, there is no point in calling it a AC motor or anything similar.

Kinds of BLDC Motors

These motors are used in 2 ways: direct or indirect drive. In direct drive the motor is inside the application and has no speed converter/reducer, for example: fans, hub motors, cutters, etc. In indirect drive the motor is outside the application and is connected to a speed reducer via pulleys, chains, gimbals, gears, etc. Examples of this drive can be mills, conveyor belts or compressors. [5]

In 2-wheeled vehicles such as bicycles, it is common to find hub motors, because the motor is inside the wheel itself. There are 2 types of hub motors: direct drive and with pinions or gears. [5]

Direct Drive BLDCs

Direct drive motors are faster, but have less torque. They are more durable, but also heavier. Their range with a full load is a little shorter than with pinion motors. Direct-drive motors are quiet. When the motor is driven, it drives the wheel directly. It is a simple system, but to produce enough power, the motor must be large and heavy. A direct drive motor without a cover is shown. [5]

BLDC Motor with direct drive. Respectfully borrowed from electricbikereport.com. [5]

Indirect Drive BLDCs

Sprocket or geared motors produce a slight noise, but are light and small. They could almost fit through a normal bicycle rim and offer very little resistance when pedaling. Their top speed will be lower, although they offer more torque, allowing you to climb uphill on an incline more easily. They are more complex, but this feature makes them a lighter and smaller option. Nylon sprockets reduce speed and increase torque to push it forward. These parts wear out with heavy use and make more noise as they rotate, requiring replacement after a short time. Eventually they will wear out and begin to rub, creak, jump, and slip. [5]
They possess a clutch for protection of the pinions, which prevents the machine from being driven as a generator. Consequently, these machines cannot be used for regenerative braking.

BLDC with gears. Respectfully borrowed from electricbikereport.com. [5]

External Rotor BLDCs

Brushless motors can also be classified by their rotor. If it has an external rotor, the motor windings are on the inside of the frame. It is easier to obtain torque, thanks to the air gap radius of the external rotor. The speed is stable, it has more inertia and the design of the magnets reduces stalling and jerking, especially at low speeds. This also facilitates lower machine noise and regulates torque disturbances in variable torque applications, such as pumping. They can accommodate more poles in the design which allows motors with lower rated speed. The motor generally operates at lower rated current. Require more safety measures at the time of installation. Smaller in size compared to the brushless internal rotor motor. Its construction is more complex due to the magnets. [3] [6] [7]

Internal Rotor BLDCs

In contrast, if the brushless motor has an internal rotor, the motor windings are on the outside of the frame. The rotor is smaller compared to the external rotor, which allows it to rotate and respond quickly, since it has a small inertia. It significantly improves heat dissipation, which increases its output torque. A disadvantage is that it is difficult to obtain high torque and the magnets can be damaged by centrifugal force. The rotor construction demands that the magnets must necessarily be of high density. [3] [6 [7]

BLDC with external rotor. Respectfully borrowed from www.nidec.com. [3]
BLDC with internal rotor. Respectfully borrowed from www.nidec.com. [3]

Anatomy of a Hub Wheel Motor

In-wheel electric drive motors represent an effective method of providing propulsion for vehicles that would not otherwise be designed to have motor-driven wheels. They are compact, do not require the support of rotating axles from the main vehicle, and can be designed around the vehicle to propel them. Thanks to this technology, most vehicles can be converted to electric vehicles. Figure 11 shows the stator of a brushless electric motor in its design stage. Hub wheel motors, or “Hub Wheel Motor” as they are known in English, also have their disadvantages dictated by laws of physics, which are described below. [8]

Design of a hub motor; essentially, a brushless DC motor attached to a wheel. Respectfully borrowed from www.instructables.com [8]

Wheel hub motors are inherently heavier and bulkier than the driving wheels.

Motors are machines constructed of steel and copper, both of which are very heavy. When you increase the weight of a wheel by two or three times, expect a drastic increase in the weight to be carried by the suspension that was not designed for such performance. If these motors are placed in vehicles previously equipped with indirectly driven wheels, a change in driving performance is to be expected. A hub motor will inevitably take up more space on the vehicle wheel and will lose mechanical advantage because the wheel must be larger in diameter than the motor. This matters to a lesser extent for large vehicle wheels. [8]

Wheel hub motors will generally produce less torque than an indirect drive system.

An indirect drive engine, such as one oriented to the wheels through a transmission, has the advantage of torque multiplication. This is how a truck with a 400 kW diesel engine can carry much more cargo than a 400 kW sports car. A complex system of gears transmits the torque generated to the drive wheels. A sports car is light and fast because its 400 kW engine is mainly calibrated for speed. [8]
From mechanical physics, power output is a product of torque and speed. Due to the laws of physics, it is simpler and more economical to manufacture a fast low torque motor than a slow high torque motor, the power output levels being equal. The hub motor has a direct drive. There are no pinions to convert its rotational speed into torque. [8]

Hub motor drives will generally be less electrically efficient than an indirect drive system.

A motor is a transducer, where the input power is electrical and the output power is mechanical. Electrical power P in [kW] is defined as:

𝑃 = 𝑉∗𝐼

where V is the voltage at the motor in volts [V] and I is the current flowing into the motor in amperes [A]. Mechanical power is defined as:

𝑃 = 𝜏∗𝜔

where τ is the torque output in newton-meters [Nm] and ω is the rotational speed in radians per second [rad/s].
Hub motors avoid virtually all of the mechanical losses associated with a clutch, transmission, shafts, and gears normally found in a vehicle powertrain. These components consume 15-20% of the power produced by the engine. The engine torque only has to pass through the tire, with its frictional and deformation forces. However, there are other shortcomings to consider specific to the direct drive. The hub motor does not have a high electrical efficiency and this naturally impacts the efficiency of the system and the duration of the battery state of charge. [8]
It is feasible to input electrical power to the motor, but not obtain rotation because the torque is not sufficient to overcome the counter-rotating forces on the motor. In this case, the efficiency is zero, there is no mechanical output power. This is called a locked rotor condition and if in this condition for an extended time it can cause permanent damage, rendering the motors unusable. All motors must start the vehicle at idle and all will have zero efficiency for a fraction of a second. During most of their use, hub motors are relatively close to a locked rotor state, more so than an indirectly driven gear motor. A hub motor has to draw a higher current for the same torque output and the current is what causes heating in the leads. The more current flowing, the more heat will be generated. For this reason, hub motors are less electrically efficient than indirect drive motors. [8]

Hub brushless electric motor torque equations

The torque (τ) per phase in a BLDC motor is:

𝜏=4∗𝑁∗𝐵∗𝐿∗𝑅∗𝑖

where N is the number of complete turns, B is the permanent magnet field strength in Teslas [T], L is the stator length, R is the motor armature radius and i is the current in amperes. [8]
The constant 4 at the beginning of the equation essentially represents the number of turns of wire which is 2 passes per cycle multiplied by 2 active phases times number of teeth per phase. The full derivation of this constant 4 implies that each wire loop is composed of two sections of wire, each of length L passing one turn on one side of the stator and again on the opposite side (at 180°). We can observe in the next image that 2 phases of 2-phase windings are present. One winding is separated at 180° from the other, corresponding to the same phase. [8]

Construction of the stator’s windings. Respectfully borrowed from www.instructables.com [8]

Two characteristic parameters of the motor are Km and Kv, which are used for torque and speed calculations, respectively. Km is a construction constant that determines the torque achieved per ampere applied and Kv determines the speed achieved per voltage applied, according to the following expressions: [8]

𝜏 = 𝐾𝑚 ∗ 𝑖

𝑉 = 𝐾𝑣 ∗ 𝜔.

How does it compare to its closest brother: three-phase Induction Motor

Induction motors have some shortcomings overcome by the BLDC brushless motor and those are: speed control is limited and very complicated, especially with methods that do not involve semiconductors. The torque-speed characteristic curve is not linear and presents lower torque at lower speed, while the BLDC presents a linear torque-speed characteristic. However, this deficiency disappears if the induction motor is used with a variable frequency drive. The power output to size ratio is higher for the BLDC than for the induction motor. The response is faster to speed changes in the BLDC than in the induction motor. In contrast to the above, the major disadvantage of the BLDC is its higher cost due to its electronic controller that must always be incorporated. If the application requires precise speed control, then it will generally have a lower cost than the induction motor and variable frequency drive, but this must be analyzed specifically for each application. Figure 6 shows the stators and rotors for each motor. The rotors are fundamentally the difference. In brushless motors, the rotor has a permanent magnet, while in induction motors, they are wound. [4]

Click in the link below to read more about what is a three-phase induction motor and a variable frequency drive, its equivalent controller in AC voltage.

Resources

  1. Departamento DSIE de la Universidad Politécnica de Cartagena, «Informe sobre motores».
  2. Microchip Technology Inc., «Appl. Note 885. Brushless DC Motor Fundamentals».
  3. Nidec Corporation, «Brushless Motors,» [En línea]. Available: https://www.nidec.com/en-NA/technology/capability/brushless/. [Último acceso: 19 I 2019].
  4. MPS Monolithic Power, «Appl. Note 047. Brushless DC Motor Fundamentals,» 2014.
  5. A. B. «Understanding the Differences Between Direct Drive & Geared Electric Bike Hub Motors,» 5 VI 2013. [En línea]. Available: https://electricbikereport.com/electric-bike- direct-drive-geared-hub-motors/. [Último acceso: 19 I 2019].
  6. Allied Motion, «Outer Rotor Brushless DC Motors,» [En línea]. Available:
    http://www.koshindenki.com/img/
    file/KinetiMax_TechnologyOvr_R3(scrn).pdf
  7. SDM Magnetics, «Magnetic Motor Components,» 2016. [En línea]. Available: https://www.magnet-sdm.com/magnetic-motor-components/. [Último acceso: 19 I 2019].
  8. C. Z. G. “teamtestbot”, «Make Your Own Miniature Electric Hub Motor,» 2010. [En línea]. Available: https://www.instructables.com/id/Make-Your-Own-Miniature-Electric-Hub- Motor/. [Último acceso: 19 I 2019].

Conclusions

In this post, a description of BLDC or brushless DC motors was given, how it is controlled, different kinds of motors and a anatomy of a hub wheel motor. Finally, a small comparison to its ‘brother’ motor was briefly shown: the three-phase AC induction motor.

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