Monday, December 7, 2015

Brushless DC Motor Control Introduction Fundamental Operation, Controlling and Driving

Eltronicschool. - Steven Keeping has written about the article in Digi-Key article library with the the title An Introduction to Brushless DC Motor Control. This article describe about the advantages of BLDC motor , fundamental operation of BLDC motor, Controlling a BLDC motor, and the last about Driving a BLDC motor.

In here we will give you the summary of this article according the part of this article and we will give you link that allow you to visit the original source of this article. 

Advantages of BLDC motor

The reliability and durability of the unit of BLDC motor make this motor is becoming increasingly popular in sectors such as automotive (particularly electric vehicles (EV)), HVAC, white goods and industrial. An electronic device which used in BLDC motor surely can improve the reliability and durability of the unit than used  the mechanical commutator used in traditional motors.

In application this BLDC motor can be used for applications where space is tight. It is because that BLDC motor can be made smaller and lighter than a brush type with the same power output.

The BLDC motor also give the advantages in brushless motor operation. The brushes of a conventional motor transmit power to the rotor windings which, when energized, turn in a fixed magnetic field. Friction between the stationary brushes and a rotating metal contact on the spinning rotor causes wear. In addition, power can be lost due to poor brush to metal contact and arcing. 

Fundamental Operation

To describe the fundamental operation of BLDC motor, you can see the Figure 1. below. According Figure 1, that The BLDC motor’s electronic commutator sequentially energizes the stator coils generating a rotating electric field that ‘drags’ the rotor around with it. N “electrical revolutions” equates to one mechanical revolution, where N is the number of magnet pairs. 

Figure 1. Hall sensors are embedded in the stator of a BLDC motor to determine the winding energizing sequence. (Courtesy of Microchip.)

For a three-phase motor, three Hall-effect sensors are embedded in the stator to indicate the relative positions of stator and rotor to the controller so that it can energize the windings in the correct sequence and at the correct time.

When the rotor magnetic poles pass the Hall sensors, a high (for one pole) or low (for the 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 generate a voltage potential due to the movement of the windings through the associated magnetic field. This potential is known as an electromotive force (EMF) and, according to Lenz’s law, it gives rise to a current in the windings with a magnetic field that opposes the original change in magnetic flux. In simpler terms, this means the EMF tends to resist the rotation of the motor and is therefore referred to as “back” EMF. For a given motor of fixed magnetic flux and number of windings, the EMF is proportional to the angular velocity of the rotor.

Controlling BLDC Motor

Figure 2. BDLC power supply control system using an 8-bit microcontroller. (Courtesy of Microchip.) 

Figure 2 above show us about BDLC power supply control system using an 8-bit microcontroller that presented by Microchip. According this Figure 2 that a typical arrangement for driving a BLDC motor with Hall-effect sensors. (The control of a sensorless BLDC motor using back EMF measurement will be covered in a future article.) This system shows the three coils of the motor arranged in a “Y” formation, a Microchip PIC18F2431 microcontroller, an insulated-gate bipolar transistor (IGBT) driver, and a three-phase inverter comprising six IGBTs (metal oxide semiconductor field effect transistors (MOSFETs) can also be used for the 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 to the coils (and hence 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.

The system employs a six-step commutation sequence for each electrical revolution. Because the motor has two pairs of magnets, two electrical revolutions are required to spin the motor once.

Driving a BLDC motor 

One driver IC for BLDC motor is coming from Texas Instruments’ DRV8301 motor driver which integrates a buck regulator, gate driver, and control logic in a single package like in Figure 3 below. (You also can read: Circuit Schematic a Sensor-less Brushless DC Motor Driver based on NE555 and DRV10866 IC)

Figure 3. Texas Instruments’ DRV8301 motor driver integrates a buck regulator, gate driver, and control logic in a single package

This pre-driver supports up to 2.3 A sink and 1.7 A source peak current capability, and requires a single power supply with 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 prevent current shoot through. 

ON Semiconductor offers a similar chip, the LB11696V. In this case, a motor driver circuit with the desired output power (voltage and current) can be implemented by adding discrete transistors in the output circuits. The chip also provides a full complement of protection circuits, making it suitable for applications that must exhibit high reliability. This device is designed for large BLDC motors such as those used in air conditioners and on-demand water heaters. 

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