The brushed motor is becoming popular in sectors such as 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 having an electronic device that raises the reliability and durability from the unit.
An additional advantage of a BLDC motor is that it can be done smaller and lighter when compared to a brush type with the same power output, making the former 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 required to energize the stator coils on the correct moment. Precise timing permits accurate speed and torque control, as well as ensuring the motor runs at peak efficiency.
This short article explains basic principles of BLDC motor operation and describes typical control circuit for your operation of a three-phase unit. This content also considers some of the integrated modules – the designer can choose to ease the circuit design – that happen to be specifically designed for BLDC motor control.
The brushes of your conventional motor transmit power to the rotor windings which, when energized, turn within a fixed magnetic field. Friction involving the stationary brushes and a rotating metal contact in the spinning rotor causes wear. In addition, power could be lost on account of poor brush to metal contact and arcing.
Because a BLDC motor dispenses 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. Moreover, BLDC motors boast a variety 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 size is higher, rendering it a great choice for applications including automatic washers and EVs, where high power is essential but compactness and lightness are critical factors. (However, it must be noted that brush-type DC motors will have a better starting torque.)
A BLDC motor is known as a “synchronous” type because the magnetic field generated with the stator and the rotor revolve in the same frequency. One good thing about this arrangement is that BLDC motors will not experience the “slip” typical of induction motors.
As 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 any BLDC motor comprises steel laminations, slotted axially to accommodate an even variety of windings down the inner periphery (Figure 1). While the BLDC motor stator resembles that 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 strength delivery through the motor. The down-side is really a more complicated control system, increased cost, minimizing maximum speed.
Traditionally, ferrite magnets were used to make the permanent magnets, but contemporary units tend to use rare earth magnets. While these magnets are more expensive, they generate 49dexlpky flux density, allowing the rotor to be made smaller for any given torque. Using these powerful magnets is a key reasons why BLDC motors deliver higher power when compared to a brush-type DC motor of the same size.
More information in regards to the construction and operation of BLDC motors are available in a fascinating application note (AN885) released by Microchip Technology.
The BLDC motor’s electronic commutator sequentially energizes the stator coils generating a rotating electric field that ‘drags’ the rotor around from it. N “electrical revolutions” equates to one mechanical revolution, where N is the amount of magnet pairs.
When the rotor magnetic poles pass the Hall sensors, an increased (for just one pole) or low (for the opposite pole) signal is generated. As discussed at length below, the actual sequence of commutation might be based on combining the signals from your three sensors.
All electric motors generate a voltage potential as a result of movement of the windings with the associated magnetic field. This potential is recognized as an electromotive force (EMF) and, based on Lenz’s law, it gives rise into a current in the windings using a magnetic field that opposes the initial change in magnetic flux. In simpler terms, this simply means the EMF is likely to resist the rotation from the motor and it is therefore called “back” EMF. For the given motor of fixed magnetic flux and number of windings, the EMF is proportional to the angular velocity in the rotor.
But the back EMF, while adding some “drag” towards 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 along with eliminating the additional wiring and connections on the motor that would otherwise be required to secure the sensors. This improves reliability when dirt and humidity exist.
However, a stationary motor generates no back EMF, rendering it impossible to the microcontroller to figure out the positioning of the motor parts at start-up. The remedy is always to start the motor inside an open loop configuration until sufficient EMF is generated to the microcontroller to adopt over motor supervision. These so-called “sensorless” BLDC motors are becoming more popular.
While BLDC motors are mechanically relatively simple, they are doing require sophisticated control electronics and regulated power supplies. The designer is faced with the challenge of handling a three-phase high-power system that demands precise control to work 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 inside a future article.) This product shows the 3 coils from the 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) can also be used for that high-power switching). The output from the microcontroller (mirrored by the IGBT driver) comprises pulse width modulated (PWM) signals that determine the average voltage and average current for 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 produce the magnetic flux.
A set of Hall-effect sensors determines once 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 caring for coil W.
At each step, two phases are stored on with one phase feeding current for the motor, and also the other providing a current return path. One other phase is open. The microcontroller controls which 2 of the switches from the three-phase inverter needs to be closed to positively or negatively energize the 2 active coils. For 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 try 8-bit microcontroller-based development kits to test out control regimes before committing on the design of a complete-size motor. By way of example, Atmel has produced an affordable basic starter kit, the ATAVRMC323, for BLDC motor control based on 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 a whole BLDC motor control system. To complete the position demands a regulated power supply to get the IGBT or MOSFETs (the “IGBT Driver” shown in Figure 3). Fortunately, the work is created easier because several major semiconductor vendors have specially engineered integrated driver chips for the task.
These products typically comprise a 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 great example (Figure 6).
This pre-driver supports up to 2.3 A sink and 1.7 A source peak current capability, and requires one particular power supply having an input voltage of 8 to 60 V. The product 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 such a case, a motor driver circuit together with the desired output power (voltage and current) might be implemented with the help of discrete transistors from the output circuits. The chip also provides a complete complement of protection circuits, making it suitable for applications that has to exhibit high reliability. This device is for large BLDC motors including those found in air conditioning units and also on-demand hot water heaters.
BLDC motors offer a variety 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. In addition, the introduction of powerful rare earth magnets has allowed producing 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 that BLDC motors, unlike the brush type, require an electronic system to supervise the energizing sequence of the coils and provide other control functions. Minus the electronics, the motors cannot operate.
However, the proliferation of inexpensive, robust gadgets engineered for motor control implies that designing a circuit is fairly basic and inexpensive. In reality, a BLDC motor can be established to run within a basic configuration without by using a microcontroller by making use of a modest three-phase sine- or square-wave generator. Fairchild Semiconductor, for instance, offers its FCM8201 chip with this application, and possesses published a software note on 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 absolutely no necessity for microcontroller to perform the program. The product enables you to control a 3-phase or four-phase BLDC motor.
However, employing an 8-bit microcontroller (programmed with factory-supplied code or maybe the developer’s own software) adds very little cost on the control system, yet gives the user much greater control over the motor to make sure it runs with optimum efficiency, together with offering more precise positional-, speed-, or torque-output.