The BLDC motor is becoming increasingly popular in sectors such as automotive (particularly electric vehicles (EV)), HVAC, white goods and industrial as it does away using the mechanical commutator utilized in traditional motors, replacing it having an electronic device that increases the reliability and sturdiness of your unit.
Another advantage of your BLDC motor is that it can be done smaller and lighter than a brush type with the exact same power output, making the previous ideal for applications where space is tight.
The downside is BLDC motors do need electronic management to work. By way of example, 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 enables accurate speed and torque control, in addition to ensuring the motor runs at peak efficiency.
This article explains the fundamentals of BLDC motor operation and describes typical control circuit to the operation of any three-phase unit. The content also considers some of the integrated modules – that this designer can select to ease the circuit design – that happen to be created specifically for BLDC motor control.
The brushes of your conventional motor transmit ability to the rotor windings which, when energized, turn inside a fixed magnetic field. Friction between the stationary brushes plus a rotating metal contact around the spinning rotor causes wear. Moreover, power might be lost due to 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 eliminating this source of wear and power loss. In addition, 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 better speed ranges.1
Moreover, the ratio of torque delivered relative to the motor’s dimensions are higher, so that it is the ideal choice for applications including washing machines and EVs, where high power is required but compactness and lightness are critical factors. (However, it should be noted that brush-type DC motors may have a greater starting torque.)
A BLDC motor is actually a “synchronous” type for the reason that magnetic field generated with the stator along with the rotor revolve in the same frequency. One benefit from this arrangement is the fact BLDC motors tend not to go through the “slip” typical of induction motors.
Whilst the motors can 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 any BLDC motor comprises steel laminations, slotted axially to accommodate a level variety of windings across the inner periphery (Figure 1). Whilst 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 through the motor. The down-side can be a more complex control system, increased cost, and reduce maximum speed.
Traditionally, ferrite magnets were utilised to create the permanent magnets, but contemporary units have a tendency to use rare earth magnets. While these magnets can be more expensive, they generate 49dexlpky flux density, allowing the rotor to become made smaller for any given torque. The application of these powerful magnets can be a key reason why BLDC motors deliver higher power when compared to a brush-type DC motor of the identical size.
Details concerning the construction and operation of BLDC motors are available in an interesting 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 with it. N “electrical revolutions” equates to a single mechanical revolution, where N is the quantity of magnet pairs.
When the rotor magnetic poles pass the Hall sensors, a higher (for one pole) or low (to the opposite pole) signal is generated. As discussed in depth below, the specific sequence of commutation could be dependant upon combining the signals from your three sensors.
All electric motors generate a voltage potential due to movement of the windings through the associated magnetic field. This potential is referred to as an electromotive force (EMF) and, in accordance with Lenz’s law, it gives rise to your current inside the windings with a magnetic field that opposes the initial improvement in magnetic flux. In simpler terms, this means the EMF will resist the rotation of the motor which is therefore known as “back” EMF. For any given motor of fixed magnetic flux and amount of windings, the EMF is proportional on the angular velocity in the rotor.
However the back EMF, while adding some “drag” for the motor, can be used a plus. By monitoring the rear 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 as well as eliminating the extra wiring and connections to the motor that might otherwise be found it necessary to keep the sensors. This improves reliability when dirt and humidity exist.
However, a stationary motor generates no back EMF, so that it is impossible for that microcontroller to determine the positioning of the motor parts at start-up. The solution is usually to start the motor inside an open loop configuration until sufficient EMF is generated for that microcontroller to consider over motor supervision. These so-called “sensorless” BLDC motors are rising in popularity.
While BLDC motors are mechanically relatively simple, they actually do require sophisticated control electronics and regulated power supplies. The designer is up against the process of working with a three-phase high-power system that demands precise control to run efficiently.
Figure 3 shows an average arrangement for driving a BLDC motor with Hall-effect sensors. (The control over a sensorless BLDC motor using back EMF measurement will likely be covered in the future article.) This product shows the 3 coils of your motor arranged inside 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) could also be used to the high-power switching). The output from your microcontroller (mirrored with the IGBT driver) comprises pulse width modulated (PWM) signals that determine the normal 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 produce the magnetic flux.
A couple of Hall-effect sensors determines as soon as the microcontroller energizes a coil. In 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, 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 taking good care of coil W.
At every step, two phases have with one phase feeding current to the motor, and also the other providing a current return path. Other phase is open. The microcontroller controls which 2 of the switches within the three-phase inverter should be closed to positively or negatively energize both active coils. By way of 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 whole-size motor. As an example, Atmel has produced a cheap starter kit, the ATAVRMC323, for BLDC motor control depending on the ATxmega128A1 8-bit microcontroller.4 Several other vendors offer similar kits.
While an 8-bit microcontroller allied to your three-phase inverter is a great start, it is not necessarily enough for an entire BLDC motor control system. To complete the task needs a regulated power supply to get the IGBT or MOSFETs (the “IGBT Driver” shown in Figure 3). Fortunately, the job is produced easier because several major semiconductor vendors have specifically created integrated driver chips for the task.
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 great example (Figure 6).
This pre-driver supports as much as 2.3 A sink and 1.7 A source peak current capability, and needs an individual power supply with the 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 provides 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 addition of discrete transistors from the output circuits. The chip offers a full complement of protection circuits, which makes it ideal for applications that has to exhibit high reliability. This product is for large BLDC motors such as those found in air conditioners and also on-demand water heaters.
BLDC motors offer numerous 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. Furthermore, the development of powerful rare earth magnets has allowed the production of BLDC motors that could 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 the coils and provide other control functions. Without having the electronics, the motors cannot operate.
However, the proliferation of inexpensive, robust electronic devices specially engineered for motor control signifies that designing a circuit is comparatively easy and inexpensive. In fact, a BLDC motor could be established to run within a basic configuration without even using a microcontroller by utilizing a modest three-phase sine- or square-wave generator. Fairchild Semiconductor, for instance, offers its FCM8201 chip for this particular application, and contains published a software note on the way to set things up.5
Similarly, ON Semiconductor’s MC33033 BLDC motor controller integrates a rotor position decoder on the chip, so there is absolutely no desire for microcontroller to accomplish the machine. The device could be used 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 offers 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.