Motion Control Resources
Motor Types: What You Need to Know
by Leah Zitter, Contributing Editor
Motion Control & Motor Association Posted 02/13/2020
Motors are everywhere, running from pumps, drive compressors and conveyors to fans, blowers, drills, and mixers. They help deliver different types of movement: fast, precise, continuous, with or without gear shifting, and so much more. With all these variations for motors, it can be difficult to narrow down which motor is best for your application.
There are three main types of electric motors:
|Image courtesy of Moons Industries|
If you’re building a robot or any application for which you need precise electronic control, you’ll want a stepper. Invented by Frank W. Wood in 1918 (and described in his patented design, “Art of producing step-by-step movements”), the stepper uses magnetic poles and copper/ metal winding to take steps - or well-defined angles - around a motor. Electricity pulses through the wires prodding it to “pulse” in. The stepper motor can be used for heavy and one way (unidirectional), moves.
As Sales Director of Moons' Industries, Andy Sklierenko explained, “Simply put – you’re passing an electric current through a metal wire wound in poles that electromechanically interact with magnets to take steps of “motion”. With a rotary stepper, it will “step” as you pass the current through a sequence of phases (imagine a traditional analog clock hand). You can divide these into smaller steps by micro-stepping with your drive electronics.”
Stepper Applications: Stepper motors can be used for any application that requires accurate positioning, speed control, and low-speed torque.
In contrast to the stepper motor, the servo is an integrated motor, feedback device, and drive system that gives you predictable and repeatable motion. While the stepper is “open-loop”, namely data going one way (or unidirectional), and closing the loop requires additional hardware and software, the servo is closed loop, meaning that the system is self-adjusting.
“The servo”, Robbie Queen, Sales Manager of Moog Industrial Group, North Carolina, explained, “makes corrections based upon internal feedback.” This means, when required, the servo runs forward, finishes its round, snaps to position, and reverses in the opposite direction at the same speed.
The servo uses one of three motors:
- AC servo motors (brushless) - Run on alternating current (AC) at a specific single speed, e.g., 130 volt
- DC servo motors (brush type) Are battery or rectified AC that runs on direct current (DC), where the electricity flows in a single direction. You can vary its speed by varying the voltage plugged to its motor.
- DC servo motors (brushless) - Have permanent magnets and very high torque-to-volume ratio. They’re smaller, lighter and more efficient than AC.
AC v. DC: Typical AC motors are inexpensive, but are powered by frequency, which limits their applications. In contrast, DC batteries that use the ubiquitous voltage can also be run in portable or remote locations.
Servo applications: Servos are used in a wide variety of applications where high accuracy, efficiency, speed, high torque-to-inertia ratio, and low noise are desired.
|Image courtesy of Dart Controls|
In its simplest form, the DC motor has an armature, a commuter (an attachment connected to the armature through which current pulses), brushes and magnets all tucked within a self-enclosed housing. Rotation occurs when the brushes interact with the magnetic field.
DCs are ubiquitous. As Mark Lewis, VP of Marketing and Sales at Dart Controls, Zionsville, Indiana, told us, “The DC motor applications we work with include pumps but extend to conveyors, food equipment, packaging and printing, mobile (trucks/ agricultural), equipment, medical and other devices, where, in many cases, the controls we sell interface to a supervisory controller.”
There are two kinds of DC motors:
- Brushed - Inexpensive to produce, simple to control and have excellent torque at low speed. However, their brushes need to be regularly maintained and replaced.
- Brushless (BLDC) - Use magnets instead of brushes and are more efficient and powerful. They are typically more expensive and more complex.
There are four main types of DC motors:
- Permanent Magnet DC Motors - They provide good speed but limited torque. They’re best with low horsepower applications.
- Series DC Motors - They offer maximum speed and are a good choice for applications requiring high starting torque, as torque will gradually decrease as speed increases.
- Shunt DC Motors - Offer great speed regulation and are perfect for variable speed drive applications.
- Compound DC Motors - These motors have good starting torque and efficient speed regulation.
DC Applications: DC Motors are common in our everyday usage. Examples include lawnmowers, power saws, and hybrid automobiles. The smaller, lighter, and more efficient DC motors are used in medical devices, robotics, computing, military, and are continually expanding their applications.
Questions to Consider When Purchasing a Motor:
- Will you use the motor in a harsh environment?
- Is motor life, efficiency, noise, vibration, etc. critical?
- What are the repercussions if you have a motor failure?
- Does the motor supplier you’re dealing with understand the application?
- Does your vendor have the expertise to size and apply the correct motor for your application as well as support you through the lifecycle of your project?
- Are the motors designed for reliability and performance?
Choosing the Right Motor for Your Application
Steppers are an efficient tool for positioning and speed control. According to Sklierenko, “The performance of a stepper with the developments in manufacturing control has come a long way. By closing the feedback loop on a stepper, you’re getting close to servo performance capability. Steppers, too, are durable and reliable. They have a lot of developed electronics and controls available, they’re simple and offer flexible performance.” On the other hand, steppers can sometimes be jerky, so for flawless precision, you may prefer the servo with its programmed feedback mechanism. Steppers also draw maximum current, could have their speed hampered by small step distance and could skip steps at high loads.
Servos work well in a positioning application. Their feedback design makes them efficient and reliable. They're smoother and typically more precise than the stepper - and a higher price point too. Servos run on continuous power and can run much faster.
Finally, DC motors are great when you need a fairly inexpensive motor with high torque capabilities. As Lewis put it, “DC motors are ideal for applications from sub-fractional up to 1HP, from an economics point of view”. Their versatility with power supply means you can run them in portable or remote locations. On the other hand, DCs don’t give you the precision you need for finely engineered items like robots.
The Bottom Line
There are motors for every type of application. Look carefully at what your application’s requirements are, now and down the road. Consider the capabilities of various motors before you consider the cost. Work closely with your vendor to get the best fit possible for long-term success.