The BLDC motor is now ever more popular in sectors for example automotive (particularly electric vehicles (EV)), HVAC, white goods and industrial as it does away together with the mechanical commutator utilized in traditional motors, replacing it with the electronic device that improves the reliability and durability from the unit.
An additional advantage of any BLDC motor is it can be produced smaller and lighter when compared to a brush type with the exact same power output, making the first kind ideal for applications where space is tight.
The downside is the fact that BLDC motors do need electronic management to operate. For instance, a microcontroller – using input from sensors indicating the positioning of the rotor – is required to energize the stator coils on the correct moment. Precise timing permits accurate speed and torque control, along with ensuring the motor runs at peak efficiency.
This post explains the basic principles of BLDC motor operation and describes typical control circuit for your operation of any three-phase unit. The content also considers several of the integrated modules – that this designer can select to alleviate the circuit design – which are specifically made for BLDC motor control.
The brushes of your conventional motor transmit power to the rotor windings which, when energized, turn in the fixed magnetic field. Friction involving the stationary brushes as well as a rotating metal contact in the spinning rotor causes wear. Furthermore, power might be lost because of poor brush to metal contact and arcing.
Since a BLDC motor dispenses using the brushes – instead employing an “electronic commutator” – the motor’s reliability and efficiency is improved by reducing this method to obtain wear and power loss. Moreover, BLDC motors boast a number of other advantages over brush DC motors and induction motors, including better speed versus torque characteristics; faster dynamic response; noiseless operation; and higher speed ranges.1
Moreover, the ratio of torque delivered relative to the motor’s dimensions are higher, which makes it a good choice for applications like automatic washers and EVs, where high power is required but compactness and lightness are critical factors. (However, it must be noted that brush-type DC motors do have a greater starting torque.)
A BLDC motor is regarded as a “synchronous” type since the magnetic field generated by the stator and also the rotor revolve with the same frequency. One benefit of this arrangement is BLDC motors will not feel the “slip” typical of induction motors.
As the motors comes in one-, two-, or three-phase types, the second is easily the most common type which is the version which will be discussed here.
The stator of your BLDC motor comprises steel laminations, slotted axially to allow for an even amount of windings down the inner periphery (Figure 1). Whilst the BLDC motor stator resembles that of an induction motor, the windings are distributed differently.
The rotor is made of permanent magnets with two-to-eight N-S pole pairs. More magnet pairs increase torque and smooth out so-called torque ripple, evening the power delivery from the motor. The down-side can be a more advanced control system, increased cost, minimizing maximum speed.
Traditionally, ferrite magnets were used to make the permanent magnets, but contemporary units often use rare earth magnets. While these magnets can be more expensive, they generate 49dexlpky flux density, allowing the rotor to be made smaller for a given torque. The usage of these powerful magnets is really a key good reason that BLDC motors deliver higher power than a brush-type DC motor of the identical size.
Detailed information regarding the construction and operation of BLDC motors may be found in an appealing application note (AN885) released by Microchip Technology.
The BLDC motor’s electronic commutator sequentially energizes the stator coils creating a rotating electric field that ‘drags’ the rotor around from it. N “electrical revolutions” equates to a single mechanical revolution, where N is the amount of magnet pairs.
When the rotor magnetic poles pass the Hall sensors, a very high (for one pole) or low (for that opposite pole) signal is generated. As discussed in more detail below, the specific sequence of commutation may be dependant upon combining the signals through the three sensors.
All electric motors produce a voltage potential as a result of movement from the windings throughout the associated magnetic field. This potential is recognized as an electromotive force (EMF) and, as outlined by Lenz’s law, it gives rise to a current in the windings by using a magnetic field that opposes the first alternation in magnetic flux. In simpler terms, this implies the EMF is likely to resist the rotation of your motor which is therefore known as “back” EMF. For any given motor of fixed magnetic flux and variety of windings, the EMF is proportional to the angular velocity from the rotor.
However the back EMF, while adding some “drag” towards the motor, can be used as a plus. By monitoring your back EMF, a microcontroller can determine the relative positions of stator and rotor without the need for Hall-effect sensors. This simplifies motor construction, reducing its cost along with eliminating the extra wiring and connections on the motor that could otherwise be needed to secure the sensors. This improves reliability when dirt and humidity are present.
However, a stationary motor generates no back EMF, rendering it impossible for the microcontroller to determine the position of the motor parts at start-up. The answer is to start the motor in an open loop configuration until sufficient EMF is generated for the microcontroller to consider over motor supervision. These so-called “sensorless” BLDC motors are rising in popularity.
While BLDC motors are mechanically relatively simple, they generally do require sophisticated control electronics and regulated power supplies. The designer is confronted with the problem of dealing with a three-phase high-power system that demands precise control to perform efficiently.
Figure 3 shows a normal arrangement for driving a BLDC motor with Hall-effect sensors. (The control of a sensorless BLDC motor using back EMF measurement will be covered in the future article.) This method shows the 3 coils of your motor arranged inside a “Y” formation, a Microchip PIC18F2431 microcontroller, an insulated-gate bipolar transistor (IGBT) driver, as well as a three-phase inverter comprising six IGBTs (metal oxide semiconductor field effect transistors (MOSFETs) may also be used to the high-power switching). The output through the microcontroller (mirrored by the IGBT driver) comprises pulse width modulated (PWM) signals that determine the standard voltage and average current on the coils (and therefore motor speed and torque). The motor uses three Hall-effect sensors (A, B, and C) to indicate rotor position. The rotor itself uses two pairs of permanent magnets to create the magnetic flux.
A pair of Hall-effect sensors determines once the microcontroller energizes a coil. With this example, sensors H1 and H2 determine the switching of coil U. When H2 detects a N magnet pole, coil U is positively energized; when H1 detects a N magnet pole, coil U is switched open; when H2 detects a S magnet pole coil U is switched negative, lastly, when H1 detects a S magnet pole, coil U is again switched open. Similarly, sensors H2 and H3 determine the energizing of coil V, with H1 and H3 taking good care of coil W.
At each step, two phases are saved to with one phase feeding current to the motor, along with the other providing a current return path. Another phase is open. The microcontroller controls which a couple of the switches from the three-phase inverter has to be closed to positively or negatively energize the two active coils. For instance, switching Q1 in Figure 3 positively energizes coil A and switching Q2 negatively energizes coil B to deliver the return path. Coil C remains open.
Designers can experiment with 8-bit microcontroller-based development kits to try out control regimes before committing on the style of an entire-size motor. For example, Atmel has produced an economical starter kit, the ATAVRMC323, for BLDC motor control depending on the ATxmega128A1 8-bit microcontroller.4 A few other vendors offer similar kits.
While an 8-bit microcontroller allied into a three-phase inverter is a great start, it is far from enough for an entire BLDC motor control system. To perform the job requires a regulated power supply to operate a vehicle the IGBT or MOSFETs (the “IGBT Driver” shown in Figure 3). Fortunately, the position is manufactured easier because several major semiconductor vendors have engineered integrated driver chips to do the job.
These products typically comprise one step-down (“buck”) converter (to power the microcontroller and also other system power requirements), gate driver control and fault handling, plus some timing and control logic. The DRV8301 three-phase pre-driver from Texas Instruments is a good example (Figure 6).
This pre-driver supports as much as 2.3 A sink and 1.7 A source peak current capability, and requires just one power source with an input voltage of 8 to 60 V. The unit uses automatic hand shaking when high-side or low-side IGBTs or MOSFETs are switching to prevent current shoot through.
ON Semiconductor supplies a similar chip, the LB11696V. In this case, a motor driver circuit with the desired output power (voltage and current) may be implemented with the help of discrete transistors within the output circuits. The chip also provides an entire complement of protection circuits, so that it is suited to applications that has to exhibit high reliability. This gadget is for large BLDC motors including those used in ac units as well as on-demand water heaters.
BLDC motors offer several advantages over conventional motors. The removing of brushes from your motor eliminates a mechanical part that otherwise reduces efficiency, wears out, or can fail catastrophically. In addition, the introduction of powerful rare earth magnets has allowed the production of BLDC motors that may produce the same power as brush type motors while fitting right into a smaller space.
One perceived disadvantage is the fact that BLDC motors, unlike the brush type, require an electronic system to supervise the energizing sequence of your coils and supply other control functions. Without the electronics, the motors cannot operate.
However, the proliferation of inexpensive, robust electronic devices specifically created for motor control ensures that designing a circuit is fairly simple and easy inexpensive. In fact, a BLDC motor could be established to run in a basic configuration without employing a microcontroller by employing a modest three-phase sine- or square-wave generator. Fairchild Semiconductor, as an example, offers its FCM8201 chip for this particular application, and contains published an application note concerning how to set things up.5
Similarly, ON Semiconductor’s MC33033 BLDC motor controller integrates a rotor position decoder on the chip, so there is no necessity for microcontroller to finish the machine. These devices can be used to control a three-phase or four-phase BLDC motor.
However, employing an 8-bit microcontroller (programmed with factory-supplied code or even the developer’s own software) adds almost no cost for the control system, yet provides the user much greater power over the motor to guarantee it runs with optimum efficiency, along with offering more precise positional-, speed-, or torque-output.