The brushless motor is now popular in sectors including automotive (particularly electric vehicles (EV)), HVAC, white goods and industrial because it does away using the mechanical commutator used in traditional motors, replacing it with an electronic device that enhances the reliability and sturdiness of the unit.
An additional benefit of any BLDC motor is it can be done smaller and lighter compared to a brush type with the same power output, making the former ideal for applications where space is tight.
The downside is the fact that BLDC motors do need electronic management to run. For instance, a microcontroller – using input from sensors indicating the positioning of the rotor – is needed to energize the stator coils in the correct moment. Precise timing allows for accurate speed and torque control, along with ensuring the motor runs at peak efficiency.
This informative article explains the basic principles of BLDC motor operation and describes typical control circuit to the operation of the three-phase unit. This article also considers several of the integrated modules – how the designer can choose to relieve the circuit design – that happen to be specifically made for BLDC motor control.
The brushes of your conventional motor transmit ability to the rotor windings which, when energized, turn within a fixed magnetic field. Friction involving the stationary brushes along with a rotating metal contact around the spinning rotor causes wear. In addition, power can be lost as a result 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 eliminating this way to obtain wear and power loss. Additionally, BLDC motors boast numerous 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, which makes it a good choice for applications such as automatic washers and EVs, where high power is needed but compactness and lightness are critical factors. (However, it ought to be noted that brush-type DC motors have a higher starting torque.)
A BLDC motor is known as a “synchronous” type because the magnetic field generated from the stator and the rotor revolve on the same frequency. One advantage of this arrangement is that BLDC motors do not enjoy the “slip” typical of induction motors.
Whilst the motors comes in one-, two-, or three-phase types, the latter is regarded as the common type and is also the version which will be discussed here.
The stator of your BLDC motor comprises steel laminations, slotted axially to support an even quantity of windings down the inner periphery (Figure 1). As the BLDC motor stator resembles that from an induction motor, the windings are distributed differently.
The rotor is made out 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 energy delivery in the motor. The downside is actually a more complex control system, increased cost, and minimize maximum speed.
Traditionally, ferrite magnets were used to make the permanent magnets, but contemporary units have a tendency to use rare earth magnets. While these magnets are more expensive, they generate 49dexlpky flux density, allowing the rotor to become made smaller for the given torque. Using these powerful magnets can be a key good reason that BLDC motors deliver higher power than the usual brush-type DC motor of the identical size.
Details regarding the construction and operation of BLDC motors may be found in an interesting application note (AN885) released by Microchip Technology.
The BLDC motor’s electronic commutator sequentially energizes the stator coils establishing a rotating electric field that ‘drags’ the rotor around from it. N “electrical revolutions” equates to one mechanical revolution, where N is the number of magnet pairs.
As soon as the rotor magnetic poles pass the Hall sensors, a higher (for one pole) or low (for your opposite pole) signal is generated. As discussed in detail below, the exact sequence of commutation can be determined by combining the signals from the three sensors.
All electric motors produce a voltage potential due to the movement in the windings from the associated magnetic field. This potential is known as an electromotive force (EMF) and, according to Lenz’s law, it gives rise into a current inside the windings with a magnetic field that opposes the initial alternation in magnetic flux. In simpler terms, this means the EMF has a tendency to resist the rotation of the motor and it is therefore referred to as “back” EMF. To get a given motor of fixed magnetic flux and variety of windings, the EMF is proportional on the angular velocity in the rotor.
Nevertheless the back EMF, while adding some “drag” to the motor, can be used as an advantage. By monitoring the back EMF, a microcontroller can determine the relative positions of stator and rotor without making use of Hall-effect sensors. This simplifies motor construction, reducing its cost in addition to eliminating the extra wiring and connections on the motor that will otherwise be necessary to secure the sensors. This improves reliability when dirt and humidity can be found.
However, a stationary motor generates no back EMF, making it impossible for the microcontroller to ascertain the position of the motor parts at start-up. The perfect solution is always to start the motor within an open loop configuration until sufficient EMF is generated for your microcontroller to take 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 problem of getting through 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 power over a sensorless BLDC motor using back EMF measurement will be covered in a future article.) This product shows three of the coils from the motor arranged in 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 for that high-power switching). The output from your microcontroller (mirrored by the IGBT driver) comprises pulse width modulated (PWM) signals that determine the average voltage and average current on 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.
Some 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, and ultimately, 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 looking after coil W.
At each step, two phases have with one phase feeding current towards the motor, as well as the other providing a current return path. Another phase is open. The microcontroller controls which a couple of the switches in the three-phase inverter must be closed to positively or negatively energize the two active coils. As an example, 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 test out 8-bit microcontroller-based development kits to experience control regimes before committing on the appearance of a full-size motor. For instance, Atmel has produced an inexpensive basic starter kit, the ATAVRMC323, for BLDC motor control in line with the ATxmega128A1 8-bit microcontroller.4 A few other vendors offer similar kits.
While an 8-bit microcontroller allied to a three-phase inverter is a superb start, it is not necessarily enough for an entire BLDC motor control system. To accomplish the task takes a regulated power supply to drive the IGBT or MOSFETs (the “IGBT Driver” shown in Figure 3). Fortunately, the position is created easier because several major semiconductor vendors have specially designed integrated driver chips for the job.
These products typically comprise a step-down (“buck”) converter (to power the microcontroller as well as 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 approximately 2.3 A sink and 1.7 A source peak current capability, and needs just one power supply with the input voltage of 8 to 60 V. These devices uses automatic hand shaking when high-side or low-side IGBTs or MOSFETs are switching to avoid current shoot through.
ON Semiconductor delivers a similar chip, the LB11696V. In such a case, a motor driver circuit with all 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, which makes it ideal for applications that has to exhibit high reliability. This device is ideal for large BLDC motors like those used in ac units and also on-demand hot 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. In addition, the development of powerful rare earth magnets has allowed producing BLDC motors that may make the same power as brush type motors while fitting into a smaller space.
One perceived disadvantage is 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 having the electronics, the motors cannot operate.
However, the proliferation of inexpensive, robust gadgets engineered for motor control signifies that designing a circuit is comparatively easy and inexpensive. In reality, a BLDC motor might be established to run in a basic configuration without even by using a microcontroller by making use of a modest three-phase sine- or square-wave generator. Fairchild Semiconductor, by way of example, offers its FCM8201 chip for this application, and it 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 on the chip, so there is not any need for microcontroller to finish the program. 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 the developer’s own software) adds very little cost for the control system, yet gives the user much greater control of the motor to make sure it runs with optimum efficiency, as well as offering more precise positional-, speed-, or torque-output.