Driving a brushless motor is relatively simple from a schematic perspective; however, there is a large amount of theory about brushless direct current motors that you need to understand to be successful. If you only have experience with brushed motors or no experience with driving motors at all, we will be covering everything you need to know.
You can find the Altium Project with schematic and board design on GitHub as always, released as open source under the highly permissive MIT license.
Brushless motors cover a wide variety of motor constructions, but the term has become synonymous with PM BLDC (Permanent Magnet Brushless Direct Current) motors in recent years. For this article, we'll focus on these and brush over the rest. Suppose you're looking for a low-cost PMBLDC motor to experiment with. In that case, radio-controlled model electric motors are typically very high-quality brushless motors that can deliver astounding amounts of torque and power when driven correctly. What's even better is that they are very cheap from companies such as HobbyKing.
In general terms, electrical motors require a rotating magnetic field to generate movement. In classical DC motors, this was achieved using a commutator. A commutator is essentially a ring with multiple conducting bands. Each of these bands is one of the motor's contacts, and graphite contacts called brushes then press against these conducting bands and are, in turn, connected to a DC power source. The motor's rotation changes the point where the brushes make contact, creating an alternating current going into the motor windings. The obvious drawback is that we have a soft point of contact that wears away and brings with it all the disadvantages of mechanical contacts (EMI, sparks, degrading conductance...).
Several factors have conspired in modern times to make these types of motor a thing of the past:
Cheap and powerful rare earth magnets
Availability of cheap but capable microcontrollers
Advances in solid-state power switching (Power MOSFETs)
After all, why use a mechanical device to switch the current when we can do it ourselves using electronics?
Higher power-to-weight ratio
Higher speeds
Much higher torque
Durability
Lower prices in more powerful motors
As a bonus, BLDC motor controllers can be electronically controlled without the need for direct motion feedback (i.e., an encoder) in what is known as sensorless configuration. However, there are some limitations to using that approach that we'll talk about later. For now, let's review the basics of BLDC motor controllers.
How to Drive a BLDC Motor
The most common configuration for BLDC motors is a three-phase star setup (also known as a wye configuration).
Each of the resistors in the image above represents each of the windings of the motor. We'll use a similar diagram to explore the inner workings of the motor.
While it's not an accurate description of modern brushless motors' real-world construction and electrical properties, it's a good enough mental model to think about what's happening.
Each axis on the image above represents the current flowing through each winding. The vector sum of the three gives us a representation of the motor's magnetic field. The magnetic field and current can be considered as being always synchronized for the purpose of this explanation. Keep in mind that IX is not a real current flowing in the system, or the total current provided by our driver, but rather a representation of the equivalent current in terms of the magnetic field.
An additional vector of constant length, not represented in the above figure for clarity, will represent our rotor's angular position. Our rotor will try to align itself with the IX vector the same way that a compass needle tries to point north. To make the rotor continuously spin, we just need to ensure that the magnetic field we create with the motor windings is rotating around at the correct speed.
A lot of interesting physical parameters of our system can be determined through this representation.
Our driving speed is the number of revolutions that the IX vector completes in a specified length of time.
Our real speed is the number of revolutions the rotor vector completes in a specified length of time.
Our torque is proportional to the IX vector's angular distance and the rotor vector (up to a limit).
BLDC Driving Methods
When talking about BLDC motor controllers, a lot of different terms are used for very similar control schemes. This guide is not an attempt at a thorough explanation but rather a simplification to help you start learning how to drive motors as quickly as possible while keeping you aware of what's happening (i.e., no black boxes or hidden magic).
Driving a brushless motor is relatively simple from a schematic perspective; however, there is a large amount of theory about brushless direct current motors that you need to understand to be successful. If you only have experience with brushed motors or no experience with driving motors at all, we will be covering everything you need to know.
You can find the Altium Project with schematic and board design on GitHub as always, released as open source under the highly permissive MIT license.
Brushless motors cover a wide variety of motor constructions, but the term has become synonymous with PM BLDC (Permanent Magnet Brushless Direct Current) motors in recent years. For this article, we'll focus on these and brush over the rest. Suppose you're looking for a low-cost PMBLDC motor to experiment with. In that case, radio-controlled model electric motors are typically very high-quality brushless motors that can deliver astounding amounts of torque and power when driven correctly. What's even better is that they are very cheap from companies such as HobbyKing.
In general terms, electrical motors require a rotating magnetic field to generate movement. In classical DC motors, this was achieved using a commutator. A commutator is essentially a ring with multiple conducting bands. Each of these bands is one of the motor's contacts, and graphite contacts called brushes then press against these conducting bands and are, in turn, connected to a DC power source. The motor's rotation changes the point where the brushes make contact, creating an alternating current going into the motor windings. The obvious drawback is that we have a soft point of contact that wears away and brings with it all the disadvantages of mechanical contacts (EMI, sparks, degrading conductance...).
Several factors have conspired in modern times to make these types of motor a thing of the past:
Cheap and powerful rare earth magnets
Availability of cheap but capable microcontrollers
Advances in solid-state power switching (Power MOSFETs)
After all, why use a mechanical device to switch the current when we can do it ourselves using electronics?
Higher power-to-weight ratio
Higher speeds
Much higher torque
Durability
Lower prices in more powerful motors
As a bonus, BLDC motor controllers can be electronically controlled without the need for direct motion feedback (i.e., an encoder) in what is known as sensorless configuration. However, there are some limitations to using that approach that we'll talk about later. For now, let's review the basics of BLDC motor controllers.
How to Drive a BLDC Motor
The most common configuration for BLDC motors is a three-phase star setup (also known as a wye configuration).
Each of the resistors in the image above represents each of the windings of the motor. We'll use a similar diagram to explore the inner workings of the motor.
While it's not an accurate description of modern brushless motors' real-world construction and electrical properties, it's a good enough mental model to think about what's happening.
Each axis on the image above represents the current flowing through each winding. The vector sum of the three gives us a representation of the motor's magnetic field. The magnetic field and current can be considered as being always synchronized for the purpose of this explanation. Keep in mind that IX is not a real current flowing in the system, or the total current provided by our driver, but rather a representation of the equivalent current in terms of the magnetic field.
An additional vector of constant length, not represented in the above figure for clarity, will represent our rotor's angular position. Our rotor will try to align itself with the IX vector the same way that a compass needle tries to point north. To make the rotor continuously spin, we just need to ensure that the magnetic field we create with the motor windings is rotating around at the correct speed.
A lot of interesting physical parameters of our system can be determined through this representation.
Our driving speed is the number of revolutions that the IX vector completes in a specified length of time.
Our real speed is the number of revolutions the rotor vector completes in a specified length of time.
Our torque is proportional to the IX vector's angular distance and the rotor vector (up to a limit).
BLDC Driving Methods
When talking about BLDC motor controllers, a lot of different terms are used for very similar control schemes. This guide is not an attempt at a thorough explanation but rather a simplification to help you start learning how to drive motors as quickly as possible while keeping you aware of what's happening (i.e., no black boxes or hidden magic).
No comments:
Post a Comment