Types of Motors in Motion Control

We might not manufacture motors at ADVANCED Motion Controls, but we work with them every day and build servo drives to make them run, so you might say we know a thing or two about them. We take a lot of pride in being able to serve any application, and that means we have a drive for just about every kind of motor you can find. How many motor types are there and where are they each used? Let’s take a look at each one.

Servo Motors

In general, all servo motors fall under the umbrella of permanent magnet motors, which have permanent magnets in either the stator (the part that stays still) and the rotor (the part that spins in the middle of the stator). Over time, however, the boundaries of what is a servo motor and what isn’t have become a little blurred. This ambiguity is partially due to the expanded uses of servo drives beyond servo control.

Decades ago, servo drives did little more than amplify a command signal, hence their alternate name, servo amplifiers. As motion control technology evolved, engineers discovered that servo amplifiers could be modified and set up to control other motor types the same way they control servo motors. The servo drives of today are much more versatile and can easily be configured for a wide variety of motor types, yet still keep “servo” in their title.

Servo motors are used in millions of applications from remotely operated vehicles to CNC milling machines to surgical robots. Servo motors are popular for a number of reasons, including their power efficiency and small size. But most importantly, they can offer very precise control when they have a feedback device and a servo drive.  ADVANCED Motion Controls’ drives are capable of controlling just about any servo motor you can find.

Brushed DC (single phase)

Brushed DC motors are perhaps the simplest motor type out there. You have permanent magnets fixed in the stator and looped coils of wire in the rotor. When electrical current runs through the coils, the magnetic field creates a force that causes the rotor to move. The question is, how do you constantly get DC current to a spinning object without wires getting tangled up? The answer is conductive brushes and a commutator.

The commutator is a round piece with metallic contact points connected to each of the rotor’s coil loops. The conductive brushes aren’t like hair brushes; they are typically pieces of a graphite mixture that are spring loaded to make electrical contact with the motor’s commutator as it spins to provide the electrical current. They offer great performance at an economical price.  However one drawback is the physical contact between the brushes and commutator creates friction, which wears down the brushes and creates lots of dust particles over time. Because of this, brushed motors are more susceptible to maintenance costs over a long period of time than their brushless counterparts, despite lower initial costs. For low cost applications, short life applications, or applications where the motor is easily accessible to be repaired or replaced, then a DC brushed motor can do the trick.

Servo motor control is our game, which is why every single ADVANCED Motion Controls servo drive can run a DC brushed motor, provided it is in the appropriate power range of course.

Brushless DC (three phase)

Brushless motors take the opposite approach as brushed motors. They put the permanent magnets in the rotor and run the electricity through the stator. There is no longer a mechanical commutator. Instead, the stator’s three motor phases are powered by DC current and their coils interact with the magnetic field from the rotor’s magnets. By alternating which of the two motor phases are active at once in a controlled fashion, the magnetic fields cause the rotor to spin.

DC Brushless motors typically cost more to purchase than their brushed counterparts. However, the reduced mechanical contact (AKA, lack of brushes) in DC brushless motors means excellent heat dissipation, less of a need for maintenance, and greater electrical efficiency, all of which can reduce costs in the long run.

For applications where the motor may not be easily accessible for maintenance or higher efficiency than a brushed motor is desired, then a three phase brushless DC motor can be the way to go.

Brushless servo motor control is easy with our products. With few exceptions all of our actively marketed servo drives can run DC brushless motors.

Brushless AC

Also known as permanent magnet synchronous or PMAC motors, AC brushless servo motors are incredibly efficient. Like brushless DC motors, the electrical current runs through the stator and the permanent magnets are in the rotor.

These days, the distinction is less about the motors themselves when comparing Brushless AC motors with Brushless DC motors, but rather how they are driven by the servo drive.  With Brushless AC, the current is constantly running through the three phases, but alternating back and forth in a sinusoid fashion as you would see in the AC supply out of the wall.  This phenomenon creates a net rotating magnetic field, rotating much more smoothly than the magnetic field achieved by turning on and off motor phases in a DC brushless motor.

Much like DC brushless motors, they rarely require maintenance due to the lack of mechanical brushes. The drawback to AC brushless motors when compared to DC brushless motors (and AC induction motors, which will be discussed later) is their even greater initial cost. However, their performance efficiency and minimal maintenance cost can make up for it in the long run.

AMC DigiFlex® Performance and FlexPro servo drives can be configured to run brushless AC motors as long as there is a feedback device to close the loop.

Other Rotary Motors

Stepper

Stepper motors are similar to brushless motors, but the movement is defined in incremental “steps”. How is this accomplished? The rotor and stator are shaped with “teeth,” but unlike gears, the teeth don’t mesh – they are used for magnetic alignment. There are fewer teeth in the stator than there are in the rotor, so not all the teeth can be aligned at once. By magnetizing different stator phases as north or south, the rotor will shift ever so slightly to align or counter-align with the active phases.

Even with only two or three motor phases, stepper motors can move in very tightly controlled increments with each step, which can be less than a degree of movement. With half-stepping (alternating between using 1 and 2 aligned phases at once) and microstepping (activating and deactivating the phases more gradually), the resolution can be doubled at the expense of torque. This makes stepper motors great for high precision applications.

You see stepper motor control in printers (2D and 3D), optics, low end desktop CNC machines, computer components, camera lenses, and other devices that need precise position control. ADVANCED Motion Controls FlexPro and DigiFlex® Performance servo drives can operate closed-loop stepper motors.  This means the motors need encoder feedback so the servo drives can operate them like servo motors.

AC induction

Invented by Nikola Tesla, induction motors are possibly the most common motor type in the world. Unlike the other rotary motors we’ve discussed, AC induction motors do not use permanent magnets in either the stator or rotor.

Like Brushless AC servo motors, they rely on three-phase AC loop structure, which produces a net rotating magnetic field. However, instead of using the field to move permanent magnets in the rotor, the magnetic flux induces a current in the rotor, which is contstructed with a squirrel-cage design. This current then produces a magnetic field that interacts with the magnetic field from the stator and makes the rotor spin. In a sense, the rotor is constantly trying to catch up with the stator’s rotating magnetic field. The difference in speed is known as slip.

The speed of an AC induction motor can be adjusted simply by adjusting the AC frequency. In addition, the usage of AC as opposed to DC in itself can make these motors very appealing for very high power applications. As a result. AC induction motors are commonly for large devices such as cranes, elevators, electric cars, and other heavy machinery. All of ADVANCED Motion Controls’ digital servo drives can operate closed loop vector motors, essentially AC induction motors with encoder feedback.

Non-Rotary Motors

Not all motion control applications involve spinning something around or articulating joints.

Linear Motors

Linear motors can be thought of as an “unrolled” DC brushless motor where the stator and rotor are swapped. There is a long track of permanent magnets alternating in polarity and a moving carriage with three phases of coils. The direction of current through these coils magnetizes the phases north or south, which pulls or pushes it along the motor track respectively.

These motors are used for applications where precise and high speed linear motion is necessary, such as industrial 3D printers or AMC’s ball toss demos. They can be oriented horizontally, vertically, or at an angle. These features quite literally come at a price though; linear motors are much more expensive than other motor types.

All ADVANCED Motion Controls DigiFlex® Performance and FlexPro digital servo drives can operate linear motors.

Linear Actuator

Linear actuators are an alternative to linear motors. Technically, they are not really a different type of motor. They are a rotary motor, such as a servo, induction, or stepper motor, coupled with a ball and screw mechanism to create linear motion. Because this design is very susceptible to backlash, dual loop feedback is often used when precision is needed. The loops use one feedback device on the rotary motor and one device on the linear-moving component.

Linear actuators are found in many devices, such as large shop machines, desktop 3D printers, and large gantry systems. They are usually a less costly alternative to DC linear motors, the trade off being slower top speeds and more space required for integration (due to the rotary motor). However, a motion control system with a linear actuator can be incredibly efficient and produce greater forces than those using a linear motor.

Voice Coils

Voice coils are single phase linear motors with a limited travel of less than one electrical cycle. They are useful for their response speed, precision, and accuracy.

Voice coils are most commonly found in audio speakers. The back and forth motion coupled with the short range of movement and precision allow  voice coils to create the vibrations that make audible noise from electrical signals. In motion control applications voice coil motors are used for linear motion over a short range such as accelerated life testing machines where materials can be subjected to many strain cycles in a controlled fashion, active dampening devices where a weight can be rapidly moved back and forth to counteract unwanted vibrations, or any short-stroke positioning application where high speed and precision are needed.

Using a servo drive to control a voice coil is analogous to brushed DC servo motor control, the main difference being motion is linear instead of rotational. In the case of a servo motor control, current is proportional to torque, whereas in a voice coil control, current is proportional to force. And because all of our servo drives can control brushed motors, they can all control voice coils as well.

Final Thoughts

Tech_Motor-Characteristics_inductanceHopefully this blog gives you an idea of the different motor types out there and how they can each be used. If you aren’t sure what kind of motor you need for your motion control application, contact us. Again, we don’t make motors, but we know all aspects of motion control and are more than happy to make recommendations. We’ll work with you to put together a motion control solution that meets all of the needs for your application.

 

by Jackson McKay, Marketing Engineer