Specialty Motors and Drives
This topic is going to deal on a very limited basis with three (3) specialty drive types. Each has its following and each also has specific features and uses in the industrial realm.
We'll talk about "Servo Motors and Drives", "Stepper Motors and Drives", and "Synchronous Motors and Drives". They are NOT types that we see in our marketplace on a heavy usage basis, but we want to at least touch on the topics so you know what they are, some normal applications, how they should be applied, and the fact that A.R.&E. personnel can help you with the application, selection, and purchase of these special items.
Servo Motors and Drives
A servo motor is a rotary actuator or motor that allows for precise control in terms of angular position, acceleration and velocity, capabilities that a regular motor does not have. It makes use of a regular motor and pairs it with a sensor for position feedback. The controller is the most sophisticated part of the servo motor system, as it is specifically designed for the purpose.
Servo motors are not a specific class of motor but are a combination of specific parts, which happen to include a DC or AC motor, and are suitable for use in a "Closed-Loop Control System". Applications include robotics, automated manufacturing, and computer numerical control (CNC) machining applications.
The servo motor is a closed-loop servomechanism that uses position feedback to control its rotational speed and position. The control signal is the input, either analog or digital, which represents the final position command for the shaft. A type of encoder serves as a sensor, providing speed and position feedback. In most cases, only the position is reported. The final position is reported to the controller and this is compared to the initial position input. Then, if there is a discrepancy, the motor is operated to get to the correct position.
The simplest servo motors use DC motors and position sensing through a potentiometer. These systems have the motor move at maximum speed until it stops at the designated position or is stopped. This is NOT widely used in industrial motion control as it can be quite inaccurate, but these kinds of servo motors are popular in radio-controlled devices such as model aircraft and toy cars.
Sophisticated servo motors for industrial use have both position and speed sensing as well as implement proportional-integral-derivative control algorithms, allowing the motor to be brought to its position quickly and precisely without overshooting, as the speed of the shaft can also be controlled.
Stepper Motors and Drives Systems
A stepper motor system consists of three basic elements, often combined with some type of user interfaces such as a host computer, PLC or dumb terminal. The three elements are:
- An "Indexer", or controller, is a microprocessor capable of generating step pulses and directional signals for the driver.
- The "Driver" is an "amplifier" that increases the controller signals sufficiently to energize the motor windings. There are numerous types of drivers, all with different voltage and current ratings, and construction technology. Not all drivers are suitable to run all motors, so when designing a motion control system the driver selection process is critical.
- Finally, a "Stepper Motor". This is an electromagnetic device that converts digital pulses into mechanical shaft rotation. The advantages of step motors are low cost, high reliability, high torque at low speeds and simple, rugged construction that operates in almost any environment. The main disadvantages of using a stepper motor are the resonance effect (pulses due to steps), often exhibited at low speeds and decreasing torque with increasing speed.
- Low cost for control achieved
- High torque at startup and low speeds
- Simplicity of construction
- Can operate in an open-loop control system
- Low maintenance
- Less likely to stall or slip
- Will work in any environment
- It can be used in robotics on a wide scale.
- High reliability
- The rotation angle of the motor is proportional to the input pulse.
- The motor has full torque at standstill (if the windings are energized)
- Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3–5% of a step and this error is non-cumulative from one step to the next.
- Excellent response to starting/stopping/reversing.
- Very reliable since there are no contact brushes in the motor. Therefore, the life of the motor is simply dependent on the life of the bearing.
- The motor's response to digital input pulses provides open-loop control, making the motor simpler and less costly to control.
- It is possible to achieve very low-speed synchronous rotation with a load that is directly coupled to the shaft.
- A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses.
A stepper motor, also known as a stepping motor, is a brushless DC electric motor that divides a full rotation into many equal steps. The motor's position can then be commanded to move and hold at one of these steps without any position sensor for feedback (an open-loop controller), as long as the motor is carefully sized to the application concerning torque and speed.
Stepper Motor Controllers are capable of producing accurate speed and positioning of the stepper motor shaft, and therefore the driven load. As you can see in the photos above, one controller is simplistic in design and information while the second is a full-fledged computer program with high-resolution graphics that allows full and total control of the process, complete with position readout.
Stepper motor controls and systems are used in numerous items you use every day. The hard drive in your computer, an inkjet printer, scanners, CD drives, CNC machines, and camera lenses, just to name a few.
Synchronous Motors and Drives
Synchronous Motors and Drive Systems are used throughout the retail, commercial and industrial markets in a wide range of applications. The short definition of a synchronous motor and drive system would be that the design of the motor is such that the rotation of the motor is in "synchronization" with the frequency of the AC power line that feeds it and is very stable and steady. This is why an analog clock uses a synchronous motor. The time will be kept steady based on the utility power being a "steady" 60 Hz power supply.
A synchronous motor, like the common induction motor, has a stationary winding that is fed power, normally from the utility company power lines and produces a "rotating magnetic field". The rotating portion of the motor is constructed such that it produces a second magnetic field. This field is produced either by "permanent magnets" on the rotor, or additional "wound fields" that are connected to an external DC power supply to produce the actual magnetic field. In some designs, like the drawing above, the main motor has an "exciter" built right on the shaft of the rotor. This exciter creates the "excitation" power for the rotating winding thus negating the need for an external DC power source.
Once the synchronous motor is energized and the stationary field is produced, the rotating portion "locks" into synchronization with the rotating of the main field based on the frequency of the power being supplied to that main field. When this "locking" occurs, there is no "slip", like there is with an induction motor. This is one of the benefits of the synchronous motor that allows it to drive a load at a "continuous" and "steady" speed, without consideration to a fluctuating load.
Another big advantage is that the synchronous motor is VERY efficient, compared to the standard induction motor. In normal operation, the induction motor has a varying power factor (depending on load) and that causes inefficiencies, along with higher energy bills. The synchronous motor runs at a "unity" power factor and doesn't change. So it is highly efficient. But, if the rotor is "over-excited", the power factor can become a "leading" power factor and assist in improving the grid's power factor. It's a fact that some industrial plants that have a large number of induction motors consuming power, thus causing a low power factor, will install a synchronous motor and let it run without being connected to any load! The high power factor of the synchronous motor brings the plant power factor into a more manageable number and saves the company money.
A synchronous motor controller is not too complex but does have a few characteristics that should be mentioned. The synchronous motor has very low starting torque due to its design, so it may be necessary to add resistance to the rotating winding to help in the start process. The controller may have relays and contactors to add and remove this resistance during the start process. Then once the motor is turning, either speed or current is monitored to establish when the motor is nearing operating (synchronous) speed. Once it reaches that speed (or close to it), a relay will energize the field winding and cause the motor to go "into synchronization". And of course, there are overload relays connected in the stator circuit to protect the motor.
And finally, applications associated with synchronous motors today include AC analog clocks, power factor correction in industrial plants, robotics, elevators, compressors, record player turntable, fans, blowers, dc generators, line shafts, centrifugal pumps, compressors, reciprocating pumps, rubber, and paper mills. Basically, any application in which a constant speed is preferable.
Specialty drives and motors are a small portion of the everyday menu of industrial control products, but you may still run into them and we wanted to make certain you knew that A.R.&E. can help you with them. Don't hesitate to call us if you run into any of these, or other, Specialty Drive Products. We'll be happy to discuss your needs.