Which type of electric motor do you size for your conveyor, XYZ table, or robot? Before you select one, you must understand the characteristics of each type of motor in the market.
Types of Electric Motors
There are two obvious types of electric motors as determined by input voltage: AC (Alternating Current) or DC (Direct Current).
While AC motors use alternating current to power a series of wound coils, DC motors use direct current to power either carbon brushes or electrical commutation. DC motors are generally more efficient and compact than AC motors.
It's not only important to understand the differences between the characteristics of AC and DC motors but also the specific types within these categories.
Remember that certain manufacturers have the ability to offer both motors and drivers. Even if the motor is DC, its driver can house an internal power supply, so AC input drivers can easily run DC motors with an AC power supply.
Now let's dig deeper into AC and DC motors.
Ideal for Constant Speed: AC Motors
AC motors can be separated into four main categories: shaded-pole, split-phase, capacitor-start, capacitor-start/capacitor-run, and permanent split capacitor. Since Oriental Motor only manufactures permanent split capacitor type AC motors from 1/2 HP down to 1/750 HP, we will generally cover this segment in more detail.
Each type of PSC motor is similar in structure. There are wound coils in the stator and a squirrel cage rotor is used for rotation. Capacitors are required for single-phase motors to generate a polyphase power supply. These motors are very easy to control and require no driver or controller to operate. Minor differences change the characteristics of the basic AC induction motor to suit different performance needs, such as various types of brakes.
Induction Motors / Asynchronous Motors
Induction motors are the most common and are rated for continuous duty operation for a wide range of output power from a fraction to thousands of horsepower. They're considered "asynchronous" motors due to the existence of a lag, or slip, between the rotating magnetic field produced by the stator and its rotor. The reason why they're called "induction" motors is that they operate by inducing a current onto the rotor. Since there's no friction besides the ball bearings, they offer an overrun of approximately 30 revolutions after power is removed (before gearing).
The below image describes the design and construction of an induction motor.
① Flange Bracket
Die-cast aluminum bracket with a machined finish, press-fitted into the motor case
② Stator
Comprised of a stator core made from electromagnetic steel plates, a polyester-coated copper coil and insulation film
③ Motor case
Die-cast aluminum with a machined finish inside
④ Rotor
Electromagnetic steel plates with die-cast aluminum
⑤ Output shaft
Available in round shaft type and pinion shaft type. The metal used in the shaft is S45C. Round shaft type has a shaft flat (output power of 25 W 1/30 HP or more), while pinion shaft type undergoes precision gear finishing.
⑥ Ball bearing
Oriental Motor only uses ball bearings.
⑦ Lead wires
Lead wires with heat-resistant polyethylene coating
⑧ Painting
Baked finish of acrylic resin or melamine resin
How Do They Work?
When the motor is powered, it generates a rotating magnetic field in the wound stator. By Faraday's Law of electromagnetic induction, current is induced onto the rotor, and the magnetic field created by the induced current interacts with the rotating magnetic field to produce rotation. Its characteristics can be further understood by Lenz's Law and Fleming's Left Hand and Right Hand Rules. However, overrun, depending on load inertia, can be up to 30 revs. For anyone who wants to know more, here's a blog post for more background and technical information on AC induction motors.
Speed-torque curve depicts expected motor performance
A motor's performance is plotted on a speed-torque curve. An AC induction motor will start from zero speed at torque "Ts", then gradually accelerate its speed past the unstable region, and settle on "P" in the stable region where the load and torque are balanced. Any changes to its load will cause the position of "P" to move along the curve, and the motor will stall if it operates in the unstable region. Each motor has its own speed torque curve and a "rated torque" specification.
Induction motors are robust and can be used for a variety of general-purpose applications where continuous duty is necessary, and stop accuracy isn't critical. Single-phase motors are offered for fixed speed requirements. Variable speed requirements can be met by combining a three-phase induction motor with a VFD (variable frequency drive) or a single-phase motor with a TRIAC controller. Some manufacturers also offer wateright, dust-proof motors by enclosing an induction motor in a sealed case.
Reversible Motors
Reversible motors, by definition, can reverse on the fly and are ideal for start/stop operation. A reversible motor is similar to an induction motor but with a friction brake and more balanced windings. Due to a friction brake mechanism, its overrun is significantly reduced after power is removed. The motor winding is also more balanced to increase its starting torque for start/stop operation.
Due to the additional heat generated from reversible motors, their recommended duty cycle is only 30 minutes or 50%. An example of a reversible motor application is an indexing conveyor that isn't too demanding on throughput or stop accuracy.
How Do They Work?
A friction brake mechanism is installed at the rear of a reversible motor. The coil spring applies constant pressure to allow the brake shoe to slide toward the brake plate.
When the motor stops, the friction from the brake reduces the motor overrun from ~30 revs to ~6 revs.
The brake force produced by the brake mechanism of an Oriental Motor's reversible motor is approximately 10% of the motor's output torque.
The graph shows the difference between speed-torque curves of an induction motor vs a reversible motor. A reversible motor has highter starting torque characteristics than an induction motor.
Electromagnetic Brake Motors
Electromagnetic brake motors combine either a three-phase induction motor or a single-phase reversible motor with a built-in power-off-activated electromagnetic brake. Compared to reversible motors, these motors offer an overrun of just 2~3 revolutions (before gearing) and can be used up to 50 times a minute. These motors are designed to hold their rated load during a vertical operation, or just to lock the motor in place when power is removed.
The brake mechanism inside an electromagnetic brake motor is more advanced than the reversible motor. Instead of a brake shoe and a coil spring that constantly applies pressure, the electromagnetic brake is engaged and disengaged by an electromagnet and spring mechanism.
How Do They Work?
This is a power-off-activated type of brake, which means the brake engages and stops the motor when power is removed from its lead wires. When voltage is applied to the magnet coil during normal operation, it becomes an electromagnet and attracts the armature, with the brake lining, against the force of the spring and away from the brake hub, thereby releasing the brake and allowing the motor shaft to run freely. When no voltage is applied, the spring presses the armature onto the brake hub and holds the motor's shaft in place, thereby actuating the brake.
Electromagnetic brake motors are used in vertical applications where the load must be held, or in applications where the load must be locked into position when the power is removed.
Torque Motors
Torque motors are designed to provide high starting torque and sloping characteristics (torque is highest at zero speed and decreases steadily with increasing speed), along with operating over a wide speed range. Due to their ability to alter torque output based on input voltage, they provide stable operation under a locked rotor or stall condition, such as a winding/tensioning application.
How Do They Work?
A torque motor can vary its torque and speed according to the load torque.
Easy torque adjustment for tensioning
A voltage controller, such as the TMP-1, can be used to vary voltage to a torque motor to control its torque. Just like a speed controller, a voltage can be set using a potentiometer or external DC voltage.
Synchronous Motors
Synchronous motors are called "synchronous" because they use a special rotor to synchronize their speed with the input power frequency. For a 4-pole synchronous motor running at 60 Hz power, it will rotate at 1800 RPM (AKA "synchronous speed"). My earliest memory of a synchronous motor application was someone using it to drive the clock hands of a tower clock.
How Do They Work?
This type of motor offers a bit more responsiveness and precision on the speed. Its synchronous speed is determined by how many poles the motor has and the frequency of input voltage.
Another type of synchronous motor called the low-speed synchronous motor provides highly precise speed regulation, low-speed rotation, and quick bi-directional rotation. These motors actually use the rotor and stator laminations from a stepper motor design but are driven by AC power supply. Therefore, they're more responsive, but the higher number of poles also decreases the synchronous speed to 72 RPM at 60 Hz. Low-speed synchronous motors can stop within 0.025 seconds at 60 Hz if operated within the permissible load inertia.
The basic construction of low-speed synchronous motors is the same as that of stepper motors. Since they can be driven by an AC power supply and offer superb starting and stopping characteristics, they are sometimes called "AC stepper motors".
Ideal for Speed Control: DC Brushed and Brushless Motors
DC motors are generally much smaller than AC motors and use direct current to power the carbon brushes and commutator, or electrically commutate the windings with a driver. DC motors are about 30% more efficient than AC motors since they do not have to induce current to create magnetic fields. Instead, they use permanent magnets in the rotor. Oriental Motor's DC motors are generally fractional horsepower; up to 400 watts (1/2 HP).
Within DC motors, there are two main types: brushed and brushless. While brushed motors are designed for general-purpose variable speed applications, brushless motors are designed for more advanced requirements.
Different Types of DC motors
Brushed Motors
The brushes and commutator inside a brushed motor mechanically commutate the motor windings as it runs, and it continues rotation as long as its power supply is connected. Brushed motors are easy to control (by varying the voltage for speed and torque), but the brushes require periodic maintenance and replacement and therefore have an estimated lifespan of 1,000~1,500 hours (more or less due to operating conditions). While they're considered more efficient than AC motors, they suffer losses in efficiency compared to brushless motors due to resistance in the winding, brush friction, and eddy-current losses.
Brushed motors are offered in multiple types: permanent magnet brush type, shunt-wound type, series-wound type, and compound-wound type. A typical application for a brushed motor includes RC cars and windshield wipers.
Since Oriental Motor doesn't manufacture brushed motors, we offer limited information on brushed motors.
Brushless Motors
Brushless motor systems offer better speed control and performance than brushed motors due to electrical commutation and closed-loop feedback but require drivers to work. This raises the overall cost per axis, but it may be a necessary cost for applications requiring more advanced speed control features or closed-loop functions, such as continuous duty conveyors requiring multiple speeds or status monitoring.
Brushless motor and driver systems are often compared with AC motor and VFD systems for their advantages in size, weight, and efficiency especially for applications such as conveyors or mobile robotics.
Here's a comparison between a 200 W AC motor and VFD vs a BLE2 Series brushless motor and driver.
We also show a speed torque curve of a brushless motor system compared to an AC motor and VFD system of equivalent frame size.
Brushless Motor + Driver
AC Motor + VFD
How Do They Work?
Compared to a brushed motor, a brushless motor simply requires a driver to understand its feedback signals and commutate the motor windings in the right sequence and timing.
For Oriental Motor's brushless motors, a three-phase winding in a "star" connection is used on a radially segmented permanent magnet rotor. A built-in Hall effect sensor IC or optical encoder sends signals to the drive circuit to determine rotor position for the purpose of phase excitation timing.
On brushless motors with Hall effect IC, three Hall effect sensors are placed within the stator at 120 degrees apart and send digital signals as they sense the north and south poles go by as the rotor rotates. These signals tell the driver what speed the motor is running at and when to energize the next set of winding coils at exactly the right time.
Want to know more? Learn about the differences between brushed vs brushless motors.
Oriental Motor's brushless motor systems are paired with their own dedicated drivers for guaranteed specifications and quick setup. Various gearing options are offered for flexibility. Closed-loop feedback is done by either encoder or hall-effect sensors, and each driver offers different features and functions to suit various applications.
The Brushless Motor Advantage
Advantages vs Brushed Motors
Advantages vs AC Motors
- Longer life
- Lower electrical noise (EMI)
- Lower audible noise
- No arcing or sparks
- More available torque
- Lower temperature
- Cleaner
- Smaller size
- More efficient
- Smaller size
- Lower temperature
- Constant torque
Ideal for Positioning: Stepper Motors
Stepper Motors
Technically, brushless motors also include stepper motors, which are designed for positioning applications due to their high pole count, holding torque, and superior stop accuracy. Compared to 10 or 12 poles on a brushles motor rotor, a stepper motor rotor has at least 50 poles or even 100 poles. Similar to a brushless motor, a stepper motor requires a driver to operate. Unlike a brushless motor, a stepper motor can operate without feedback.
Also, duty cycles for open-loop stepper motor systems must be limited since they generate high heat. Generally, stepper motors use full current at all times, while brushless motors only use what it needs according to the load, speed, or acceleration/deceleration parameters. Oriental Motor provides 2-phase (1.8°) and 5-phase (0.72°) motors as well as unipolar and bipolar constant current chopper drivers.
A stepper motor's precise stopping ability comes from a toothed and magnetized rotor and a toothed electromagnetic stator.
A standard 1.8° stepper motor has 50 poles from 50 teeth in the rotor and 8 poles in the stator.
How Do They Work?
Like a brushless motor, a stepper motor also requires a driver to electrically commutate its windings. With higher pole count, comes better control. Using a two-phase excitation method for maximum torque, a driver excites two motor phases at a time and operates by pulses and steps. This means that each command pulse received by the driver will make the motor take a small "step", and the frequency of these command pulses determines the motor speed. Also, a stepper motor driver can energize specific poles in the motor, generate holding torque at standstill, and hold the rotor at specific positions.
Imagine stopping at position "1" in the simplified 4-step rotation diagram below. If the driver can provide a steady amount of current and energize phases "A" and "B" continuously, the motor will hold at position "1". If the driver excites the next two phases, then the motor will move to the next step. If the driver follows the step sequence and switches faster, the stepper motor will seem like it's rotating continuously.
In the real world, a 2-phase standard 1.8° stepper motor moves a quarter of a tooth pitch on a 50 tooth rotor for every command pulse its driver receives and therefore needs 200 steps to rotate one full revolution. The ability to generate holding torque at standstill helps to maintain stop position accuracy.
Open-loop stepper motors may suffice for general repeated positioning applications. However, closed-loop stepper motors are available for advanced positioning applications requiring higher reliability and efficiency.
A stepper motor's speed torque curve is typically downward sloping; with the highest torque occurring during low speed, which means that it can be used for acceleration and deceleration. Unlike a brushless motor system, a stepper motor does not have a limited-duty region.
If you'd like to learn more, I have written separate notes about stepper motors. Please enjoy.
Learn about the differences between hybrid, PM, and VR stepper motors
Learn more about the differences between servo motors and stepper motors
Motor Selection Tip: Rule of Thumb
- Use AC motors for constant speed, DC brushed and brushless motors for speed control, and stepper motors for positioning applications.
- Calculate the required torque, load inertia, and speed by using our motor sizing tool.
- Select a motor that can satisfy the application's torque, load inertia, and speed requirements.
- Select a motor or a motor and driver series that uses your available power supply. Stepper motor systems with AC input drivers output more torque in the high-speed region than DC input drivers.
- Select a motor or a motor and driver combination that satisfies other application requirements, such as stop accuracy, speed range, electromagnetic brake control, or networking capabilities.
This blog post provides a general understanding of the many types of AC/DC motors in the market. In addition to performance differences, quality, cost, product breadth, lead times, and support can also be deciding factors. Finding a quality motor supplier that can guarantee performance, provide expert support for a wide range of products, and ship reliably can also be important.
Ready for a little practice? Which type of motor would you use for these applications?
Click the application GIFs below to see the recommended motors for these applications.
Washdown Conveyor
XYZ Table
Need immediate answers? Contact our team!
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What Makes A Motor Move?
The most vague and simple answer is magnetism! Ok, now let's take this simple force and turn it into a super car!
To keep things simple, we will need to look at some concepts through the lens of the thought experiment. Some liberties will be taken, but if you want to get down and dirty with the details, you can consult Dr. Griffiths. For our thought experiment, we are going to state that a magnetic field is produced by a moving electron i.e. current. While this creates a classical model for us to use, things break down when we reach the atomic level. To understand the atomic level of magnetism more, Griffiths explains that in another book...
Electromagnetism
To create a magnet or magnetic field, we are going to have to look at how they are generated. The relationship between current and magnetics field behave according to the right-hand rule. As current passes through a wire, a magnetic field forms around the wire in the direction of your fingers as they wrap around it. This is a simplification of Ampère's force law as it acts on a current carrying wire. Now, if you place that same wire in a pre-existing magnetic field, you can generate a force. This force is referred to as the Lorentz force.
The right-hand rule shows the direction of the magnetic field in relation to the current path.
(Credit: HyperPhysics
If the current is increased, the strength of the magnetic field is strengthened. Though, to do something useful with the field, it would take incredible amounts of current. Furthermore, the wire delivering the current would be carrying the same magnetic strength, thus creating uncontrolled fields. By bending the wire into a loop, a directed and concentrated field can be created.
The field has not changed. By bending the wire into a loop, field directions are simply aligned.
(Credit: HyperPhysics
Electromagnets
By looping wire and passing a current, an electromagnet is created. If one loop of wire can concentrate the field, what can you do with more? How about a few hundred more! The more loops you add to the circuit, the stronger the field becomes for a given current. If that's the case, why don't we see thousands **, if not **millions, of windings in motors and electromagnets? Well, the longer the wire the higher resistance it has. Ohm's law (V = I*R) says to maintain the same current as resistance increases, voltage must increase. In some cases it makes sense to use higher voltages; in other cases some use larger wire with less resistance. Using larger wire is more costly and is generally more difficult to work with. These are factors that have to be weighed when designing a motor.
An energized electromagnet producing a magnetic field.
(Credit: HyperPhysics
Experiment Time
To create your own electromagnet, simply find a bolt (or other round steel object), some magnet wire (30-22 gauge works fine), and a battery.
Note: Lithium Batteries are NOT recomended for this experiment.
Wrap between 75-100 turns of wire around the steel. Using a steel center further concentrates the magnetic field, increasing its effective strength. We will go over why this is happens in the next section.
A bit of heat shrink or tape can help keep the coils on the steel center.
Now, using sand paper, remove the insulation from the ends of the wires, and connect each wire to each terminal of the battery. Congratulations! You have built the first component of a motor! To test the strength of your electromagnet, try to pick up paper clips or other small steel objects.
It's not magic, it's SCIENCE!!!
Ferromagnetism
Looking back to the beginning of our thought experiment, magnetic fields may only be produced by a current. Taking the definition of current as a flow of electrons, electrons orbiting an atom should create a current and thus a magnetic field! If every atom has electrons is everything magnetic? YES! All matter, including frogs, can express magnetic properties when given enough energy. But not all magnetism is created equally. The reason I can pick up screws with a refriderator magnent and not a frog is the difference between ferromagnetism and paramagnetism. The way to differentiate the two (and a few more types) is through the study of quantum mechanics.
Ferromagnetism will be our focus, since it is the strongest phenomenon and is what we have the most experience with. Further, to relieve us from having to understanding this at the quantum level, we are going to accept that atoms of ferromagnetic materials tend to align their magnetic fields with their neighbors. Though they tend to align, inconsistencies in material and other factors like crystaline structure create magnetic domains.
When magnetic domains are aligned in a random order, neighboring fields cancel each other out resulting in a non-magnetized material. Once in the presence of an strong external field it is possible to re-align these domains. By aligning these domains, the overall field strengthens, creating a magnet!
(Credit: HyperPhysics
This re-alignment can be permanent depending on the strength of the field. This is great because we'll need these in the next section.
Permanent Magnets
Permanent magnets behave in the same way as electromagnets. The only difference is, well, they are permanent.
In all drawings, arrows will be pointing away from the north pole and towards the south pole. Another convention is to use the color red to represent north and blue to represent south. To identify a magnets polarity, you can use a compass. Since opposites attract, the needle will point north to the south pole of the magnet.
You can perform the same experiment with an electromagnet to determine polarity.
If you reverse the flow of current, you can see how an electromagnet can reverse its poles.
This is a key principle for building motors! Now, let's look at some different motors and how they use magnets and electromagnets.
DC Brush Motors - The Classic
The DC brush motor is one of the simplest motors in use today. You can find these motors just about anywhere. They are in household appliances, toys, and automobiles. Being simple to construct and control, these motors are the go-to solution for professionals and hobbyists alike.
The Anatomy of a Brush Motor
To better understand how one works, let's start by tearing down a simple hobby motor. As you can see, they are simple in construction, comprising of a few key components.
- Brushes - Delivers power from the contacts to the armature through the commutator
- Contacts - Brings power from the controller to the brushes
- Commutator - Delivers power to the appropriate set of windings as the armature rotates
- Windings - Converts electricity to a magnetic field that drives the axle
- Axle - Transfers the mechanical power of the motor to the user application
- Magnets - Provide a magnetic field for the windings to attract and repel
- Bushing - Minimizes friction for the axle
- Can - Provides a mechanical casing for the motor
Theory of Operation
As the windings are energized, they attract to the magnets located around the motor. This rotates the motor until the brushes make contact with a new set of commutator contacts. This new contact energizes a new set of windings and starts the process again. To reverse the direction of the motor, simply reverse the polarity on the motor contacts. Sparks inside a brush motor are produced by the brush jumping to the next contact. Each wire of a coil is connected to the two closest commutator contacts.
An odd number of windings is always used to prevent the motor from getting locked into a steady state. Larger motors also use more sets of windings to help eliminate "cogging," thus providing smooth control at low revolutions per minute (RPMs). Cogging can be demonstrated by rotating the motor axle by hand. You will feel "bumps" in the motion where the magnets are closest to the exposed stator. Cogging can be eliminated with a few tricks in design, but the most prevalent is removing the stator all together. These types of motors are referred to as ironless or coreless motors.
Pros
- Simple to control
- Excellent torque at low RPM
- Inexpensive and mass produced
Cons
- Brushes can wear out over time
- Brush arcing can generate electromagnetic noise
- Usually limited in speed due to brush heating
Brushless Motors - MORE POWER!
Brushless motors are taking over! Ok, maybe that was an overstatement. However, brushless motors have begun to dominate the hobby markets between aircraft and ground vehicles. Controlling these motors had been a hurdle up until microcontrollers became cheap and powerful enough to handle the task. There is still work being done to develop faster and more efficient controllers to unlock their amazing potential. Without brushes to fail, these motors deliver more power and can do so silently. Most high-end appliances and vehicles are moving to brushless systems. One notable example is the Tesla Model S.
The Anatomy of a Brushless Motor
To better understand how one works, let's start by tearing down a simple brushless motor. These are commonly found on remote control airplanes and helicopters.
- Windings - Converts electricity to a magnetic field that drives the rotor
- Contacts - Brings power from the controller to the windings
- Bearings - Minimizes friction for the axle
- Magnets - Provide a magnetic field for the windings to attract and repel
- Axle - Transfers the mechanical power of the motor to the user application
Theory of Operation
The mechanics of a brushless motor are incredibly simple. The only moving part is the the rotor, which contains the magnets. Where things become complicated is orchestrating the sequence of energizing windings. The polarity of each winding is controlled by the direction of current flow. The animation demonstrates a simple pattern that controllers would follow. Alternating current changes the polarity, giving each winding a "push/pull" effect. The trick is keeping this pattern in sync with the speed of the rotor. There are two (widely used) ways this can be accomplished. Most hobby controllers measure the voltage produced (back EMI) on the un-energized winding. This method is very reliable in high velocity operation. As the motor rotates slower, the voltage produced becomes more difficult to measure and more errors are induced. Newer hobby controllers and many industrial controllers utilize Hall effect sensors to measure the magnets position directly. This is the primary method for controlling computer fans.
Pros
- Reliable
- High speed
- Efficient
- Mass produced and easy to find
Cons
- Difficult to control without specialized controller
- Requires low starting loads
- Typically require specialized gearboxes in drive applications
Stepper Motors - Simply Precise
Stepper motors are great motors for position control. They can be found in desktop printers, plotters, 3d printers, CNC milling machines, and anything else requiring precise position control. Steppers are a special segment of brushless motors. They are purposely built for high-holding torque. This high-holding torque gives the user the ability to incrementally "step" to the next position. This results in a simple positioning system that doesn't require an encoder. This makes stepper motor controllers very simple to build and use.
The Anatomy of a Stepper Motor
To better understand how one works, let's start by tearing down a simple stepper motor. As you can see, these motors are built for direct drive loads containing a few key components.
- Axle - Transfers the mechanical power of the motor to the user application
- Bearings - Minimizes friction for the axle
- Magnets - Provide a magnetic field for the windings to attract and repel
- Poles - Increases the resolution of the step distance by focusing the magnetic field
- Windings - Converts electricity to a magnetic field that drives the axle
- Contacts - Brings power from the controller to the windings
Theory of Operation
(Credit: PCB heaven
Stepper motors behave exactly the same as a brushless motor, only the step size is much smaller. The only moving part is the the rotor, which contains the magnets. Where things become complicated is orchestrating the sequence of energizing windings. The polarity of each winding is controlled by the direction of current flow. The animation demonstrates a simple pattern that controllers would follow. Alternating current changes the polarity, giving each winding a "push/pull" effect. A notable difference is how the magnet structure of a stepper is different. It is difficult to get an array of magnets to behave nicely on a small scale. It's also very expensive. To get around this, most stepper motors utilize a stacked plate method to direct the magnetic poles into "teeth".
In a brushless motor, back EMF is used to measure velocity. A stepper relies on the short throw of each winding to "guarantee" it reaches the desired point in time. In highspeed travel, this can lead to stalling where the rotor can't keep up with the sequence. There are ways around this, but they rely on a higher understanding of the relationship between motor windings and inductance.
Pros
- Excellent position accuracy
- High holding torque
- High reliability
- Most steppers come in standard sizes
Cons
- Small step distance limits top speed
- It's possible to "skip" steps with high loads
- Draws maximum current constantly
Linear Motors - The Future!!!
The future is linear! In high-speed pick and place machines speed is everything. With speed comes friction, with friction comes maintanence, with maintanance comes downtime, with downtime comes lost productivity. By removing the components needed to transfer rotary to linear motion, the system becomes much lighter and more efficient. Linear motors are simple to maintain, and, with only one moving part, are incredibly reliable. Did I mention they are incredibly fast?! This is the pick and place machine we are using in production, and it is incredibly fast! This machine also packs such a punch, there is a warning for pacemakers on it. There is an entire row of high-power, rare-earth magnets.
The Anatomy of a Linear Motor
To better understand how one works, let's look inside our pick and place machine downstairs.
- Motion Module - Contains electromagnets and controller.
- Magnets - Provide a magnetic field for the coils to attract and repel
- Linear Bearning - Keeps the motor in alignment with magnets and is the only moving part.
Theory of Operation
The mechanics of a linear motor is nearly identical to a brushless motor. The only difference is if you were to take a brushless motor and unfold it into a straight line you'd have a linear motor. The Motion Module is the only moving part. Where things become complicated is orchestrating the sequence of energizing coils. The polarity of each coil is controlled by the direction of current flow. The animation demonstrates a simple pattern that controllers would follow. Alternating current changes the polarity giving each coil a "push/pull" effect. In a linear motor, there is typically an encoder or some advanced positioning system to keep track of the location of the Motion Module. To reach a high position accuracy, the controllers are much more complicated than anything found on a conventional system. Microstepping is a method to "throttle" the magnets to provide smooth and precise motion. To achieve this though, linear motors require a highly specialized controller tuned for each motor. As controller technology improves, we are likely to see these motors decrease in price. Maybe someday our 3D printers will print in seconds and not hours!
Pros
- Reliable
- High speed
- Efficient
- No rotary to linear conversion required
Cons
- Expensive
- Require custom controllers
- Purpose built for each system
- Did I mention expensive?
Resources and Going Further
So we took a look at some different types of motors and how they might be used. Selecting a motor will require you to first determine the application requirements. With these requirements, you can look at the strengths and weaknesses of each motor type. But more importantly, look for the ratings on each motor. Each motor will have values for input power and output power. You can calulate the load requirements of a system but, sometimes it's easy enough to just try it! To give yourself a headstart integrating motors, take a look as some of these pages:
- Gear Ratios
- Bearings
- Chain drives
- Pulse Width Modulation
- H-Bridges for motor control
- Ardumoto Quickstart Guide
And finally, here is a great place to learn just about everything physics related.