Why you should be turning on to brushless DC motors
More engineers are specifying BLDCs for automation applications.
But with no brushes, commutation has its own unique demands.
That’s why analog driver circuits for brushless commutation now rely more heavily on the software domain.
Brushed DC motors use carbon to conduct current between a rotating commutator ring and the rotor winding. They physically reverse the direction of flow at the right time to keep the rotor turning. A brushless DC (BLDC) motor commutates electronically using stator windings to attract magnets attached to a rotor. The attraction causes the rotor to turn. The stator windings are usually placed around the outside of the rotor.
The key to BLDC motor control is applying the right level of power to the stator coils at the right time. This is usually expressed as a speed setting. Speed can be controlled programmatically in application software or by using manual controls that are regulated in software.
The operating principle of a BLDC motor defines it as a synchronous motor. In a synchronous motor, the rotor turns at the same speed as the rotating electromagnetic field energizing the stator windings. This contrasts with an asynchronous motor, where the rotor field lags the stator field by a factor called slip. The amount of slip varies depending on the motor’s load.
Operating principles of BLDC motors
One disadvantage of using BLDCs has been the relatively higher cost of the commutation circuitry, compared to the cost of a simple brushed motor system. However, today’s focus on minimizing energy consumption places a high value on the flexible speed control afforded by electronic commutation. Hence the BLDC has become a popular choice for energy-conscious applications.
The most common BLDC motors are single phase and three phase. A single-phase motor can be viewed as a single winding along four arms of the stator to form four magnetic poles. The external rotor contains permanent magnets arranged to form two pole pairs.
Current is applied to the stator through transistors in an H-bridge. When Q1 and Q4 are on, current flows through the winding from left to right. This induces electromagnetic poles that interact with the rotor magnetic field to produce a force, causing the rotor to turn. When the rotor has turned through 180°, Q1 and Q4 are turned off while Q2 and Q3 turn on. This effectively reverses the direction of current in the stator winding. This induces a stator field of opposite polarity that interacts with the rotor magnetic field, which has also reversed after being rotated 180°. This causes the rotor to continue turning through 180° and complete a full revolution.
Single-phase BLDC controlled through an H-bridge.
A three-phase motor has three separate stator windings, commonly referenced as u, v and w. The current delivered to each winding is controlled using three half bridges. The control circuit comprises six power transistors. The rotor begins to turn when current flows through one of the three stator windings to produce an electromagnetic pole. The magnetic field generated attracts the closest rotor magnet of opposing polarity. The controller turns on successive adjacent phases in sequence causing the rotor to revolve.
A six-transistor bridge circuit for three-phase BLDC motor control.
The torque produced depends on several design parameters:
- current amplitude
- number of stator-winding turns
- strength of the permanent magnets
- size of the permanent magnets
- air gap between the rotor and the windings
- length of the rotating arm
Accurate motor control relies on the timing of the commutation signals and the magnitude of the applied voltage. Together, these factors set the speed. The timing is dependent on the rotor position. The voltage is usually controlled using pulse-width modulation (PWM) of the high-side devices in the power bridge connected to the stator windings.
The PWM frequency is recommended to be about 10 times the maximum motor rotation frequency. If the DC bus voltage is higher than the motor’s rated voltage, the PWM duty cycle can be limited to meet the motor's rated voltage.
The control algorithm for a variable-speed BLDC drive has an inner loop to manage the commutation timing and an outer loop for managing speed. The system includes a microcontroller (MCU) to run the control algorithm, a power module (analog front end, or AFE) that contains gate drivers (discrete gate drivers may also be used), and the three-phase power bridge.
This block diagram shows a speed-controlled 3-Phase BLDC motor control circuit.
How to control commutation of BLDC motors
Various strategies can be employed to control commutation, each with its advantages. The most basic is trapezoidal control, which is relatively straightforward. The rotor position can be detected using sensors such as Hall-effect devices, while there is also the option of sensorless detection. The current is controlled through one pair of motor terminals at a time, while the third terminal is always electrically disconnected. At any time, the current in the two energized windings is equal in magnitude.
If the motor is equipped with Hall devices to detect the rotor position, the controller uses the signals from these sensors to switch the current to the motor terminals after every 60 degrees of rotation. However, these sensors are not suited to some applications, such as compressors used in air conditioners and refrigerators. In these situations, sensorless detection can be implemented by measuring the back EMF induced in the unconnected stator winding as the rotor turns.
The current waveform for the three stator windings in trapezoidal control is shown below. See the defining features outlined below.
- In each case, the current transitions from zero to positive, to zero, to negative current.
- The commutation sequence shows that current is only flowing in two of the windings at any time.
- The current-space vector steps between six distinct directions as the rotor turns through a full revolution.
- As a result, the torque contains a ripple, as the lower plot illustrates.
The stator currents and resulting torque ripple experienced when using trapezoidal commutation.
In applications that require smooth and precise motion, particularly at low rotor speeds, sinusoidal commutation may be preferred. In this scheme, all three stator windings of a three-phase motor are driven with currents that vary smoothly and sinusoidally as the rotor turns.
Their relative phases are shifted 120° to produce a smoothly rotating current-space vector that has a constant magnitude. This eliminates torque ripple and commutation spikes. It calls for accurate measurement of the rotor position using an encoder.
The encoder information is used to synthesize two sinusoidal signals, 120° apart, which are multiplied by the torque command to produce sine waves with amplitudes proportional to the desired torque. These phased sinusoidal current command signals are used as inputs to proportional-integral (P-I) controllers that regulate the current in the two appropriate motor windings. The current in the third winding is the negative sum of these currents.
Although this approach ensures smooth motion at low speeds, it is less well suited to working at high speeds that require the current-loop controllers to handle high-frequency sinusoids and overcome a high motor back EMF. Under these conditions, their limited gain and frequency response cause errors that distort the current-space vector leading to poor torque and low efficiency.
What is field oriented control?
The motor torque can be managed more precisely using field-oriented control (FOC), also known as vector control. The principle of FOC is to control the stator’s magnetic field. This can ensure the field remains always at 90° to the rotor field to produce the maximum torque with the applied current.
Advantages of FOC include:
- low torque ripple
- high efficiency
- low audible noise
- fast dynamic response
FOC requires a relatively high-performing MCU. However, as successive generations of MCUs continue to offer a better combination of cost and performance, FOC is becoming more cost-effective for a wider variety of applications.
BLDC motor drive design
The main parts of the motor drive comprise the microcontroller, an integrated or discrete power stage, and the three-phase MOSFET or IGBT bridge.
Take care when selecting power switches suitable for the motor-control bridge circuit. If a MOSFET is used, choose a device with low RDS(on). This gives optimum efficiency. Other critical parameters include the gate capacitance (Cg) and gate charge (Qg). These figures will influence the choice of gate drivers or power module. Other factors such as package size and price must also be considered. The package thermal performance and operating power will affect thermal management and cooling.
These concerns typically require designers to search diligently through many datasheets. Avnet FAEs can help simplify the process using supplier selection tables based on several key design considerations. Many semiconductor vendors offer motor-control evaluation kits that often come with software such as the FOC algorithm. Starting here can trim further time and cost from the project.
Conclusion
The BLDC motor has become popular in markets for e-mobility, appliances, power tools, industrial products and automotive equipment due to:
- flexibility
- energy-efficient operation
- quietness
- low EMI and arcing
- reliability
The necessary electronic commutation enables designers to create systems that deliver high value for users.
Several approaches to commutation control are viable, which can help designers manage system cost and performance to satisfy various target applications and markets. Evaluation kits and component-selection assistance, such as first-order approximation guides, help simplify and accelerate system design and development.