The brushless motor has become popular in sectors including automotive (particularly electric vehicles (EV)), HVAC, white goods and industrial mainly because it does away using the mechanical commutator employed in traditional motors, replacing it having an electronic device that improves the reliability and sturdiness of the unit.
An additional benefit of a BLDC motor is that it can be created smaller and lighter when compared to a brush type using the same power output, making the previous ideal for applications where space is tight.
The downside is that BLDC motors do need electronic management to run. By way of example, a microcontroller – using input from sensors indicating the positioning of the rotor – is needed to energize the stator coils at the correct moment. Precise timing provides for 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 the operation of a three-phase unit. This content also considers several of the integrated modules – that this designer can make to ease the circuit design – that are specifically designed for BLDC motor control.
The brushes of the conventional motor transmit capability to the rotor windings which, when energized, turn in a fixed magnetic field. Friction between the stationary brushes along with 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 with the brushes – instead employing an “electronic commutator” – the motor’s reliability and efficiency is improved by reducing this supply of 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 dimension is higher, rendering it a great choice for applications like washing machines and EVs, where high power is required but compactness and lightness are critical factors. (However, it ought to be noted that brush-type DC motors do have a greater starting torque.)
A BLDC motor is known as a “synchronous” type for the reason that magnetic field generated through the stator and also the rotor revolve at the same frequency. One benefit of this arrangement is BLDC motors do not experience the “slip” typical of induction motors.
While the motors comes in one-, two-, or three-phase types, the second is considered the most common type and is the version that will be discussed here.
The stator of a BLDC motor comprises steel laminations, slotted axially to fit a much variety of windings over the inner periphery (Figure 1). Whilst the BLDC motor stator resembles those of an induction motor, the windings are distributed differently.
The rotor is manufactured 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 ability delivery through the motor. The down-side is really a more complex control system, increased cost, and lower maximum speed.
Traditionally, ferrite magnets were utilized to create the permanent magnets, but contemporary units tend to use rare earth magnets. While these magnets can be more expensive, they generate 49dexlpky flux density, allowing the rotor to be made smaller to get a given torque. Using these powerful magnets is a key good reason why BLDC motors deliver higher power than the usual brush-type DC motor of the identical size.
More information regarding the construction and operation of BLDC motors are available in an appealing 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 along with it. N “electrical revolutions” equates to just one mechanical revolution, where N is the quantity of magnet pairs.
When the rotor magnetic poles pass the Hall sensors, a very high (for just one pole) or low (to the opposite pole) signal is generated. As discussed at length below, the precise sequence of commutation can be based on combining the signals from your three sensors.
All electric motors produce a voltage potential because of the movement from the windings through the associated magnetic field. This potential is recognized as an electromotive force (EMF) and, in accordance with Lenz’s law, it gives rise to your current from the windings having a magnetic field that opposes the original change in magnetic flux. In simpler terms, this simply means the EMF will resist the rotation in the motor and it is therefore known as “back” EMF. For the given motor of fixed magnetic flux and quantity of windings, the EMF is proportional to the angular velocity from the rotor.
But the back EMF, while adding some “drag” on the motor, can be used as an edge. By monitoring the 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 any additional wiring and connections towards the motor that might otherwise be needed to support the sensors. This improves reliability when dirt and humidity can be found.
However, a stationary motor generates no back EMF, which makes it impossible for the microcontroller to determine the position of the motor parts at start-up. The remedy is to start the motor within an open loop configuration until sufficient EMF is generated to the microcontroller to take over motor supervision. These so-called “sensorless” BLDC motors are gaining in popularity.
While BLDC motors are mechanically relatively simple, they are doing require sophisticated control electronics and regulated power supplies. The designer is confronted by the process of dealing with a three-phase high-power system that demands precise control to perform efficiently.
Figure 3 shows a standard arrangement for driving a BLDC motor with Hall-effect sensors. (The control over a sensorless BLDC motor using back EMF measurement will probably be covered in a future article.) This system shows the 3 coils of your motor arranged in a “Y” formation, a Microchip PIC18F2431 microcontroller, an insulated-gate bipolar transistor (IGBT) driver, plus 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 from your microcontroller (mirrored from the IGBT driver) comprises pulse width modulated (PWM) signals that determine the standard voltage and average current towards the coils (thus 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 when 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 finally, 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 every step, two phases are stored on with one phase feeding current towards the motor, and the other providing a current return path. One other phase is open. The microcontroller controls which two of the switches from the three-phase inverter needs to be closed to positively or negatively energize both the active coils. By way of example, switching Q1 in Figure 3 positively energizes coil A and switching Q2 negatively energizes coil B to provide the return path. Coil C remains open.
Designers can try 8-bit microcontroller-based development kits to experience control regimes before committing on the design of a complete-size motor. By way of example, Atmel has produced a cheap starter kit, the ATAVRMC323, for BLDC motor control depending on the ATxmega128A1 8-bit microcontroller.4 A number of other vendors offer similar kits.
While an 8-bit microcontroller allied to some three-phase inverter is an excellent start, it is not enough for a whole BLDC motor control system. To accomplish the work requires a regulated power source to get the IGBT or MOSFETs (the “IGBT Driver” shown in Figure 3). Fortunately, the job is manufactured easier because several major semiconductor vendors have specially designed integrated driver chips for the job.
These devices typically comprise a 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 great example (Figure 6).
This pre-driver supports up to 2.3 A sink and 1.7 A source peak current capability, and needs a single power source by 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 to stop current shoot through.
ON Semiconductor offers a similar chip, the LB11696V. In this instance, a motor driver circuit with all the desired output power (voltage and current) could be implemented with the addition of discrete transistors from the output circuits. The chip offers a whole complement of protection circuits, rendering it suited to applications that must exhibit high reliability. This device is ideal for large BLDC motors like those found in ac units as well as on-demand water heaters.
BLDC motors offer a number of advantages over conventional motors. The removing of brushes coming from a 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 production of BLDC motors that will produce the same power as brush type motors while fitting right into a smaller space.
One perceived disadvantage is that BLDC motors, unlike the brush type, require an electronic system to supervise the energizing sequence from the coils and give other control functions. With no electronics, the motors cannot operate.
However, the proliferation of inexpensive, robust electronics specifically created for motor control ensures that designing a circuit is pretty easy and inexpensive. Actually, a BLDC motor may be established to run inside a basic configuration without even using a microcontroller by employing a modest three-phase sine- or square-wave generator. Fairchild Semiconductor, as an example, offers its FCM8201 chip just for this application, and has published an application note on 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 desire for microcontroller to finish the program. These devices enables you to control a 3-phase or four-phase BLDC motor.
However, employing an 8-bit microcontroller (programmed with factory-supplied code or the developer’s own software) adds minimal cost to the control system, yet provides the user much greater power over the motor to make sure it runs with optimum efficiency, together with offering more precise positional-, speed-, or torque-output.