The motor driver is now ever more popular in sectors including automotive (particularly electric vehicles (EV)), HVAC, white goods and industrial because it does away using the mechanical commutator employed in traditional motors, replacing it having an electronic device that raises the reliability and sturdiness from the unit.
An additional advantage of your BLDC motor is that it can be done smaller and lighter when compared to a brush type using the same power output, making the previous appropriate for applications where space is tight.
The downside is the fact BLDC motors do need electronic management to work. For example, a microcontroller – using input from sensors indicating the position of the rotor – is necessary to energize the stator coils at the correct moment. Precise timing allows for accurate speed and torque control, as well as ensuring the motor runs at peak efficiency.
This short article explains the fundamentals of BLDC motor operation and describes typical control circuit for the operation of a three-phase unit. The article also considers a few of the integrated modules – how the designer can select to alleviate the circuit design – which can be specifically made for BLDC motor control.
The brushes of any conventional motor transmit power to the rotor windings which, when energized, turn in a fixed magnetic field. Friction between your stationary brushes plus a rotating metal contact around the spinning rotor causes wear. Moreover, power can be lost because of poor brush to metal contact and arcing.
Since a BLDC motor dispenses together with 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. Furthermore, 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 in accordance with the motor’s size is higher, making it the ideal choice for applications including washing machines and EVs, where high power is essential but compactness and lightness are critical factors. (However, it needs to be noted that brush-type DC motors have a greater starting torque.)
A BLDC motor is regarded as a “synchronous” type as the magnetic field generated by the stator and the rotor revolve in the same frequency. One benefit from this arrangement is that BLDC motors usually do not go through the “slip” typical of induction motors.
While the motors come in one-, two-, or three-phase types, the second is easily the most common type and is the version that can be discussed here.
The stator of a BLDC motor comprises steel laminations, slotted axially to accommodate a level amount of windings along the inner periphery (Figure 1). Even though the BLDC motor stator resembles those of an induction motor, the windings are distributed differently.
The rotor is constructed from 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 is really a more technical control system, increased cost, and minimize maximum speed.
Traditionally, ferrite magnets were utilised to make the permanent magnets, but contemporary units usually 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 application of these powerful magnets is actually a key reason why BLDC motors deliver higher power when compared to a brush-type DC motor of the same size.
Detailed information concerning 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 along with it. N “electrical revolutions” equates to a single mechanical revolution, where N is the amount of magnet pairs.
Once the rotor magnetic poles pass the Hall sensors, a very high (for starters pole) or low (for your opposite pole) signal is generated. As discussed in detail below, the specific sequence of commutation could be determined by combining the signals from your three sensors.
All electric motors produce a voltage potential due to movement of your windings from 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 from the windings by using a magnetic field that opposes the original improvement in magnetic flux. In simpler terms, this means the EMF has a tendency to resist the rotation in the motor and is also therefore called “back” EMF. For any given motor of fixed magnetic flux and amount of windings, the EMF is proportional to the angular velocity of your rotor.
Nevertheless the back EMF, while adding some “drag” for the motor, can be used for an advantage. By monitoring the back EMF, a microcontroller can determine the relative positions of stator and rotor without resorting to Hall-effect sensors. This simplifies motor construction, reducing its cost as well as eliminating an added wiring and connections on the motor that will otherwise be required to keep the sensors. This improves reliability when dirt and humidity are present.
However, a stationary motor generates no back EMF, which makes it impossible for your microcontroller to ascertain the position of the motor parts at start-up. The answer is usually to start the motor in an open loop configuration until sufficient EMF is generated for your microcontroller to consider over motor supervision. These so-called “sensorless” BLDC motors are gaining in popularity.
While BLDC motors are mechanically relatively simple, they generally do require sophisticated control electronics and regulated power supplies. The designer is faced with the task of working with a three-phase high-power system that demands precise control to perform efficiently.
Figure 3 shows an average arrangement for driving a BLDC motor with Hall-effect sensors. (The power over a sensorless BLDC motor using back EMF measurement will likely be covered in the future article.) This technique shows three of the coils of the motor arranged within a “Y” formation, a Microchip PIC18F2431 microcontroller, an insulated-gate bipolar transistor (IGBT) driver, along with a three-phase inverter comprising six IGBTs (metal oxide semiconductor field effect transistors (MOSFETs) can also be used for that high-power switching). The output from your microcontroller (mirrored through the IGBT driver) comprises pulse width modulated (PWM) signals that determine the standard voltage and average current to the coils (so 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 set of Hall-effect sensors determines if the microcontroller energizes a coil. In this particular 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 every step, two phases are on with one phase feeding current on the motor, along with the other providing a current return path. The other phase is open. The microcontroller controls which two of the switches within the three-phase inverter has to be closed to positively or negatively energize both the active coils. As an example, switching Q1 in Figure 3 positively energizes coil A and switching Q2 negatively energizes coil B to offer the return path. Coil C remains open.
Designers can test out 8-bit microcontroller-based development kits to test out control regimes before committing on the appearance of a full-size motor. For instance, Atmel has produced a cheap starter kit, the ATAVRMC323, for BLDC motor control in accordance with the ATxmega128A1 8-bit microcontroller.4 Several other vendors offer similar kits.
While an 8-bit microcontroller allied into a three-phase inverter is a good start, it is not enough for an entire BLDC motor control system. To perform the task 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 specially engineered integrated driver chips for the job.
These products typically comprise one step-down (“buck”) converter (to power the microcontroller and 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 around 2.3 A sink and 1.7 A source peak current capability, and needs an individual power supply having an input voltage of 8 to 60 V. The device uses automatic hand shaking when high-side or low-side IGBTs or MOSFETs are switching in order to avoid current shoot through.
ON Semiconductor offers a similar chip, the LB11696V. In this case, a motor driver circuit using the desired output power (voltage and current) may be implemented with the help of discrete transistors from the output circuits. The chip offers a complete complement of protection circuits, making it appropriate for applications that must exhibit high reliability. This device is designed for large BLDC motors including those utilized in air conditioners and also on-demand water heaters.
BLDC motors offer a variety of advantages over conventional motors. Removing brushes from the motor eliminates a mechanical part that otherwise reduces efficiency, wears out, or can fail catastrophically. Additionally, the growth of powerful rare earth magnets has allowed the creation of BLDC motors that can make the same power as brush type motors while fitting right into a smaller space.
One perceived disadvantage is the fact BLDC motors, unlike the brush type, require a digital system to supervise the energizing sequence from the coils and supply other control functions. Without having the electronics, the motors cannot operate.
However, the proliferation of inexpensive, robust electronics specially engineered for motor control implies that designing a circuit is comparatively basic and inexpensive. In fact, a BLDC motor might be established to run in the basic configuration without even by using a microcontroller by employing a modest three-phase sine- or square-wave generator. Fairchild Semiconductor, by way of example, offers its FCM8201 chip with this application, and has published an application note concerning how to set things up.5
Similarly, ON Semiconductor’s MC33033 BLDC motor controller integrates a rotor position decoder about the chip, so there is not any desire for microcontroller to finish the program. The unit may be used to control a three-phase or four-phase BLDC motor.
However, employing an 8-bit microcontroller (programmed with factory-supplied code or perhaps the developer’s own software) adds minimal cost on the control system, yet offers the user much greater control over the motor to guarantee it runs with optimum efficiency, together with offering more precise positional-, speed-, or torque-output.