The motor driver is now popular in sectors such as automotive (particularly electric vehicles (EV)), HVAC, white goods and industrial mainly because it does away with the mechanical commutator found in traditional motors, replacing it having an electronic device that increases the reliability and durability of the unit.
Another benefit of any BLDC motor is it can be produced smaller and lighter when compared to a brush type with the same power output, making the first kind appropriate for applications where space is tight.
The down-side is the fact BLDC motors do need electronic management to operate. For instance, a microcontroller – using input from sensors indicating the positioning of the rotor – is needed to energize the stator coils on the correct moment. Precise timing provides for accurate speed and torque control, along with ensuring the motor runs at peak efficiency.
This article explains the fundamentals of BLDC motor operation and describes typical control circuit for that operation of a three-phase unit. The article also considers a few of the integrated modules – the designer can make to relieve the circuit design – that happen to be created specifically for BLDC motor control.
The brushes of a conventional motor transmit capability to the rotor windings which, when energized, turn inside a fixed magnetic field. Friction between the stationary brushes plus a rotating metal contact on the spinning rotor causes wear. Furthermore, power could 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 eliminating this source 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 better speed ranges.1
Moreover, the ratio of torque delivered in accordance with the motor’s size is higher, which makes it a great choice for applications like automatic washers 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 may have a greater starting torque.)
A BLDC motor is known as a “synchronous” type for the reason that magnetic field generated with the stator and the rotor revolve at the same frequency. One benefit of this arrangement is the fact that BLDC motors tend not to experience the “slip” typical of induction motors.
As the motors comes in one-, two-, or three-phase types, the second is regarded as the common type which is the version that will be discussed here.
The stator of your BLDC motor comprises steel laminations, slotted axially to accommodate an even number of windings down the inner periphery (Figure 1). Even though the BLDC motor stator resembles that from 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 ability delivery from the motor. The down-side is a more complex control system, increased cost, and minimize maximum speed.
Traditionally, ferrite magnets were utilised to help make the permanent magnets, but contemporary units often 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. Using these powerful magnets is actually a key good reason that BLDC motors deliver higher power than a brush-type DC motor the exact same size.
Detailed information in regards to the construction and operation of BLDC motors are available in an intriguing 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 a single mechanical revolution, where N is the volume of magnet pairs.
If the rotor magnetic poles pass the Hall sensors, a very high (first pole) or low (for that opposite pole) signal is generated. As discussed in depth below, the specific sequence of commutation can be dependant on combining the signals from your three sensors.
All electric motors generate a voltage potential due to movement in the windings throughout the associated magnetic field. This potential is called an electromotive force (EMF) and, according to Lenz’s law, it gives rise to your current in the windings having 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 of the motor and is also therefore referred to as “back” EMF. To get a given motor of fixed magnetic flux and number of windings, the EMF is proportional on the angular velocity of the rotor.
Nevertheless the back EMF, while adding some “drag” on the motor, can be used a benefit. 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 and also eliminating the extra wiring and connections for the motor that could otherwise be found it necessary to retain the sensors. This improves reliability when dirt and humidity can be found.
However, a stationary motor generates no back EMF, which makes it impossible to the microcontroller to determine the position of the motor parts at start-up. The answer is to start the motor in a open loop configuration until sufficient EMF is generated for that microcontroller to adopt 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 confronted by the task of dealing with 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 probably be covered inside a future article.) This product shows three of the coils in 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) may also be used to the high-power switching). The output from the microcontroller (mirrored by 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 generate 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 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 caring for coil W.
Each and every step, two phases are saved to with one phase feeding current towards the motor, and also 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 should 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 offer the return path. Coil C remains open.
Designers can try out 8-bit microcontroller-based development kits to try out control regimes before committing on the design of a complete-size motor. As an example, Atmel has produced an economical basic starter kit, the ATAVRMC323, for BLDC motor control based on the ATxmega128A1 8-bit microcontroller.4 Other vendors offer similar kits.
While an 8-bit microcontroller allied into a three-phase inverter is a great start, it is far from enough for an entire BLDC motor control system. To perform the position demands a regulated power source to operate the IGBT or MOSFETs (the “IGBT Driver” shown in Figure 3). Fortunately, the work is created easier because several major semiconductor vendors have specially designed integrated driver chips to do the job.
These products typically comprise one 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 good example (Figure 6).
This pre-driver supports around 2.3 A sink and 1.7 A source peak current capability, and requires an individual power source with the input voltage of 8 to 60 V. The unit 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 cases like this, a motor driver circuit using the desired output power (voltage and current) might be implemented with the addition of discrete transistors inside the output circuits. The chip offers a complete complement of protection circuits, so that it is ideal for applications that must exhibit high reliability. This piece of equipment is designed for large BLDC motors for example those utilized in air conditioning units as well as on-demand hot water heaters.
BLDC motors offer numerous advantages over conventional motors. The removal of brushes from the motor eliminates a mechanical part that otherwise reduces efficiency, wears out, or can fail catastrophically. Additionally, the introduction of powerful rare earth magnets has allowed the creation 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 a digital system to supervise the energizing sequence of your coils and offer other control functions. Without having the electronics, the motors cannot operate.
However, the proliferation of inexpensive, robust electronics specially designed for motor control implies that designing a circuit is relatively simple and easy inexpensive. Actually, a BLDC motor could be established to run within a basic configuration without utilizing a microcontroller by using a modest three-phase sine- or square-wave generator. Fairchild Semiconductor, as an example, offers its FCM8201 chip just for this application, and it has published an application note regarding 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 no desire for microcontroller to accomplish the machine. The product enables you to control a three-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 minimal cost on the control system, yet supplies the user much greater power over the motor to make certain it runs with optimum efficiency, along with offering more precise positional-, speed-, or torque-output.