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.
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, we will only cover PSC motors.
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.
|Different Types of "PSC" AC motors
Induction motors are the most common and are rated for continuous duty operation. 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
How they work
When the motor is powered, it generates a rotating magnetic field in the stator. Current is induced onto the rotor, and the magnetic field created by the induced current interacts with the rotating magnetic field to produce rotation.
Here's a blog post for even more information on AC induction motors.
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.
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.
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 reduced to approximately 6 revolutions after power is removed (before gearing). 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.
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.
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.|
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 50x 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 they work
As shown in the image above, when voltage is applied to the magnet coil, the armature is attracted to the electromagnet against the force of the spring, thereby releasing the brake and allowing the motor shaft to rotate freely. When no voltage is applied, the spring works to press the armature onto the brake hub and hold 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 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.
Easy torque adjustment for tensioning
Synchronous motors are called "synchronous" because they use a special rotor to synchronize its 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.
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 use permanent magnets in the rotor, so they're very responsive. However, they increase the number of poles, so the synchronous speed decreases 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".
DC motors 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.
Within DC motors, there are two main types: brushed and brushless. While brushed motors are designed for general purpose applications, brushless motors are designed for precision applications.
|Different Types of DC motors
The brushes and commutator inside a brushed motor mechanically commutate the motor windings and it continues rotation as long as its power supply is connected. Brushed motors are easy to control, but require periodic maintenance and replacement of brushes, and therefore have an estimated lifespan of 1000~1500 hours (more or less due to operating conditions). While they're considered more efficient than AC motors, they suffer losses in efficiency due to initial 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 include RC cars and windshield wipers.
Since Oriental Motor doesn't manufacture brushed motors, we offer limited information on brushed motors.
Brushless motor systems offer better performance than brushed motors due to electrical commutation and closed-loop feedback but require drivers to electrically commutate the motor windings. 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.
How they work
The brushless motor has a built-in magnetic element or optical encoder for the detection of rotor position. The position sensors send signals to the drive circuit. The brushless motor uses three-phase
windings in a "star" connection. A radially segmented permanent magnet is used in the rotor.
A Hall effect IC is used for the sensor's magnetic element. Three Hall effect ICs are placed within the stator and send digital signals as the motor 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.
Brushless motor and driver systems are often compared with AC motor and VFD systems. 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|
Compared to brushed motors and AC motors, brushless motors present several advantages that can improve operation or offer downsizing for applications such as conveyors and mobile robots.
Oriental Motor's brushless motors 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 (Over AC Motors)
|Advantages vs Brushed Motors||Advantages vs AC Motors|
Technically, brushless motors also include stepper motors and servo motors, which are designed for positioning applications due to their superior ability to stop at precise locations. One thing that makes stepper motors different than brushless motors or servo motors is their ability to operate without feedback.
|Toothed Rotor & Stator with Permanent Magnet||2-Phase Excitation Method (Simplified Diagram)|
A stepper motor's precise stopping ability comes from a toothed and magnetized rotor and a toothed electromagnetic stator. In other words, instead of having just 4 poles in the brushless motor example above, a standard 1.8° stepper motor has 50 poles from 50 teeth.
Due to its rotor and stator design, a stepper motor moves a quarter of a tooth pitch for every square wave pulse its driver receives, and the speed is controlled by pulse input frequency, so it's very easy to control the distance and speed traveled by a stepper motor. A stepper motor also generates holding torque at standstill, which is important to maintain positioning accuracy.
Open-loop stepper motors may suffice for general repeated positioning applications. However, closed-loop stepper motors are available for advanced positioning applications requiring both precision and position verification.
Speed Torque Characteristics of a Stepper Motor
|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.
|Motor Selection Tip: Rule of Thumb (All Motors)|
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 motor supplier that can guarantee quality performance, provide expert support for a wide range of products, and ship with reasonably short lead times is 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.
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