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Motor Selection Basics: Types of AC/DC Motors
Selecting the appropriate electric motor for your conveyor, XYZ table, or robotic application can be challenging. Before making a decision, it’s essential to understand the unique characteristics of each motor type available in the market.
Electric motors fall into two broad categories based on their input voltage: AC (Alternating Current) or DC (Direct Current). AC motors use alternating current to power wound coils, while DC motors utilize direct current to drive carbon brushes or electrically commutate the windings. Generally, DC motors are more energy-efficient and compact than AC motors.
Understanding the differences between AC and DC motors is crucial, but it’s equally important to explore the specific types within these categories. Certain manufacturers, like Oriental Motor, offer both motors and drivers. Even if the motor is DC, its driver might include an internal power supply, enabling it to run with an AC power source.
Let’s delve deeper into AC and DC motors.
**Ideal for Constant Speed: AC Motors**
AC motors can be divided into several subcategories, including shaded-pole, split-phase, capacitor-start, capacitor-start/capacitor-run, and permanent split capacitor (PSC) types. Oriental Motor specializes in PSC AC motors ranging from 1/2 HP down to 1/750 HP. PSC motors share a similar structure, featuring wound coils in the stator and a squirrel cage rotor for rotation. Capacitors are vital for single-phase motors to generate a polyphase power supply. These motors are simple to operate and do not require a driver or controller. Minor modifications to the basic AC induction motor cater to different performance needs, such as adding various types of brakes.
**Induction Motors / Asynchronous Motors**
Induction motors are the most prevalent and are suitable for continuous operation across a wide range of output powers, from fractions of a horsepower to thousands. Known as "asynchronous" motors, they exhibit a lag or slip between the rotating magnetic field produced by the stator and the rotor. Their name stems from the fact that they operate by inducing a current onto the rotor. Without significant friction, induction motors can experience an overrun of approximately 30 revolutions after power removal (before any gearing).
The following image illustrates the design and construction of an induction motor:
[Image description]
Key components include:
- **Flange Bracket**: A die-cast aluminum bracket press-fitted into the motor case.
- **Stator**: Composed of an electromagnetic steel core, polyester-coated copper coil, and insulation film.
- **Motor Case**: Made of die-cast aluminum with a machined finish.
- **Rotor**: Electromagnetic steel plates with die-cast aluminum.
- **Output Shaft**: Available in round and pinion types, made of S45C metal.
- **Ball Bearings**: Only ball bearings are used.
- **Lead Wires**: Heat-resistant polyethylene-coated wires.
- **Painting**: Acrylic or melamine resin baked finish.
**How Do They Work?**
When powered, an induction motor generates a rotating magnetic field in the stator. According to Faraday's Law of electromagnetic induction, current is induced onto the rotor, creating a magnetic field that interacts with the rotating magnetic field to produce rotation. The interaction can be explained further using Lenz's Law and Fleming's Left and Right Hand Rules. Overrun depends on load inertia and can reach up to 30 revolutions. For those interested in more technical details, here’s a comprehensive guide on AC induction motors.
**Speed-Torque Curve Depicts Expected Motor Performance**
A motor’s performance is illustrated on a speed-torque curve. An AC induction motor starts from zero speed at torque "Ts," accelerates past the unstable region, and stabilizes at "P" where the load and torque are balanced. Load changes shift the position of "P" along the curve, and the motor stalls if it operates in the unstable region. Each motor has its unique speed-torque curve and a "rated torque" specification.
Induction motors are robust and suited for general-purpose applications requiring continuous duty and where stop accuracy is not critical. Single-phase motors meet fixed speed requirements, while variable speed needs can be fulfilled by pairing a three-phase induction motor with a VFD (Variable Frequency Drive) or a single-phase motor with a TRIAC controller. Some manufacturers also offer waterproof and dustproof motors by enclosing induction motors in sealed cases.
**Reversible Motors**
Reversible motors, by definition, can reverse direction while in motion and are ideal for start/stop operations. A reversible motor resembles an induction motor but incorporates a friction brake and more balanced windings. The friction brake reduces motor overrun from ~30 revolutions to ~6 revolutions after power removal. Balanced windings enhance starting torque for frequent start/stop operations.
Reversible motors generate more heat, limiting their recommended duty cycle to 30 minutes or 50%. An example application is an indexing conveyor that isn’t overly demanding on throughput or stop accuracy.
**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. These motors offer an overrun of just 2~3 revolutions (before gearing) and can handle up to 50 activations per minute. They are designed to hold their rated load during vertical operations or lock the motor in place when power is removed.
The electromagnetic brake mechanism is more advanced than that of reversible motors. Instead of relying on a brake shoe and coil spring, the electromagnetic brake engages and disengages through an electromagnet and spring mechanism.
**How Do They Work?**
This is a power-off-activated type of brake. When voltage is applied to the magnet coil during normal operation, it becomes an electromagnet, attracting the armature with the brake lining against the force of the spring and away from the brake hub, thus 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, locking the motor shaft in place.
Electromagnetic brake motors are used in vertical applications requiring load holding or where loads must remain locked in position upon power removal.
**Torque Motors**
Torque motors are engineered to deliver high starting torque and sloping characteristics (highest torque at zero speed and decreasing steadily with increasing speed), along with a wide speed range. Their ability to adjust torque output based on input voltage ensures stable operation under locked rotor or stall conditions, making them ideal for winding/tensioning applications.
**How Do They Work?**
A torque motor adjusts its torque and speed according to the load torque. A voltage controller, such as the TMP-1, can be used to vary voltage to a torque motor for torque control. This is similar to a speed controller, where voltage can be set using a potentiometer or external DC voltage.
**Synchronous Motors**
Synchronous motors are named so because they use a special rotor to synchronize their speed with the input power frequency. For instance, a 4-pole synchronous motor running at 60 Hz power will rotate at 1800 RPM. My earliest memory of a synchronous motor application involved driving the clock hands of a tower clock.
**How Do They Work?**
This type of motor offers improved responsiveness and precision in speed. Its synchronous speed depends on the number of poles and the input voltage frequency.
Another type of synchronous motor, the low-speed synchronous motor, provides precise speed regulation, low-speed rotation, and quick bi-directional rotation. These motors use rotor and stator laminations from a stepper motor design but are driven by AC power supply. Thus, they are more responsive, though the higher number of poles reduces 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.
Low-speed synchronous motors share the same construction as stepper motors. Since they can be driven by AC power supply and offer excellent starting and stopping characteristics, they are sometimes referred to as "AC stepper motors."
**Ideal for Speed Control: DC Brushed and Brushless Motors**
DC motors are generally smaller than AC motors and use direct current to power carbon brushes and commutators or electrically commutate the windings with a driver. DC motors are about 30% more efficient than AC motors since they don’t need to induce current to create magnetic fields. Instead, they employ permanent magnets in the rotor. Oriental Motor’s DC motors are typically fractional horsepower, up to 400 watts (1/2 HP).
Within DC motors, there are two primary types: brushed and brushless. While brushed motors are designed for general-purpose variable speed applications, brushless motors cater to more advanced requirements.
**Brushed Motors**
Brushed motors mechanically commutate the motor windings via brushes and commutators as they run. They are easy to control by varying voltage for speed and torque, but the brushes require periodic maintenance and replacement, giving them an estimated lifespan of 1,000~1,500 hours. Despite being more efficient than AC motors, they lose efficiency compared to brushless motors due to resistance in the winding, brush friction, and eddy-current losses.
Brushed motors come in multiple types: permanent magnet brush type, shunt-wound type, series-wound type, and compound-wound type. Typical applications include RC cars and windshield wipers.
**Brushless Motors**
Brushless motor systems offer better speed control and performance than brushed motors due to electrical commutation and closed-loop feedback. They require drivers to function, increasing the overall cost per axis. However, this investment may be necessary for applications requiring advanced speed control features or closed-loop functions, such as conveyors or mobile robotics.
Brushless motor and driver systems are often compared favorably to AC motor and VFD systems in terms of size, weight, and efficiency, particularly for applications like conveyors or mobile robotics.
Here’s a comparison between a 200 W AC motor and VFD versus a BLE2 Series brushless motor and driver:
[Comparison images]
**How Do They Work?**
Compared to a brushed motor, a brushless motor simply requires a driver to interpret feedback signals and commutate the motor windings in the correct 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 phase excitation timing.
On brushless motors with Hall effect IC, three Hall effect sensors are placed within the stator at 120-degree intervals and send digital signals as they sense the north and south poles passing by as the rotor rotates. These signals inform the driver of the motor’s speed and when to energize the next set of winding coils at the right moment.
**The Brushless Motor Advantage**
Oriental Motor’s brushless motor systems are paired with dedicated drivers for guaranteed specifications and quick setup. Various gearing options are available for flexibility. Closed-loop feedback is achieved by either encoders or Hall-effect sensors, and each driver offers different features and functions to suit various applications.
**Ideal for Positioning: 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 brushless 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. Open-loop stepper motor systems must limit duty cycles due to high heat generation. Generally, stepper motors use full current at all times, whereas brushless motors only use what is needed based on 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.
**How Do They Work?**
Like a brushless motor, a stepper motor also requires a driver to electrically commutate its windings. Higher pole count enables 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. Each command pulse received by the driver makes the motor take a small "step," and the frequency of these command pulses determines the motor speed. Additionally, a stepper motor driver can energize specific poles in the motor, generate holding torque at standstill, and hold the rotor at specific positions.
If you’d like to learn more, I’ve written separate notes about stepper motors. Enjoy!
**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 using our motor sizing tool.
- Select a motor that meets the application’s torque, load inertia, and speed requirements.
- Choose a motor or motor and driver series compatible with 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 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. Besides performance differences, quality, cost, product breadth, lead times, and support are also deciding factors. Finding a quality motor supplier that guarantees performance, provides expert support for a wide range of products, and ships reliably is also crucial.
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Which type of motor would you use for these applications?
Click the application GIFs below to see the recommended motors for these applications.
[Application GIFs]
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