By Raymond Erbe, P.Eng. and John Reichard
It is unfortunate that so many motor users, and even OEMs, do not really understand the strengths and deficiencies of drive technology alternatives. This lack of understanding – and not infrequently a misunderstanding – often can result in products that could be better, products requiring expensive maintenance, or products with lower up-front costs and larger long-term support costs. Obviously, when selecting a motor drive for a particular application, not only the costs and motor efficiencies are involved, but typically a full comparison must be made based upon the particular application.
In this brief article, we shall examine the advantages, costs and efficiencies, and identify the typical applications, as we compare the operation of the following motor drive technologies:
- AC Flux Vector
- AC Open Loop
- Brushless DC Servo
- Brushless DC Trapezoidal
- Switched Reluctance
Today, technological developments have made it feasible to replace DC servo motors, and even hydraulics, with AC motors in many applications. Such retrofitting of old, but mechanically sound equipment, with conventional induction motors and/or variable frequency drives or servo controls affect safety, production capability and productivity, and environmental issues, and also extends that equipment's useful economic life by bringing performance up to the standards of new machines.
For this reason, it becomes increasingly, economically important that the strengths and deficiencies of alternate drive technologies be considered, whether for new motor applications, in maintenance operations, and anytime efficiency or savings are sought.
We begin our examination by describing the benefits of AC motor technology over DC motors. The application of AC motors in place of DC motors affects the reliability, serviceability, and life span of equipment. In addition, deploying AC motors also impacts safety and environmental issues.
AC Flux Vector
The application of AC motors in place of DC motors affects the reliability, serviceability, and life span of equipment. In addition, deploying AC motors also impacts safety and environmental issues. Many of the situations in which DC motors and controls are now being used will benefit when retrofitted with AC induction motors and/or control systems. AC motors are less expensive to maintain than DC motors and do at least as good a job of positioning and regulating, at a lower initial cost.
For high horsepower positioning applications, you can successfully apply induction servo solutions to motors up to 2500 HP. In addition, they may have overload capacities exceeding 130,000 lb-in at 1800 RPM. This positioning or regulating market is unserved today by standard AC control solutions.
AC Flux Vector Summary
AC Flux Vector Servo Control
Vector control using flux vector drives is not generally understood. We can quickly summarize the benefits of vector control by listing these features:
Features available in these high-performance controls include positioning-indexing, load sharing, 100 per cent torque at 0-RPM with a 1000:1 Constant Torque Speed Range, 150 per cent torque for one minute at zero speed, and brake interface allowing precise positioning and unique torque control capabilities for many applications. All of this can be achieved using a standard, inexpensive induction motor with an encoder.
AC Flux Vector controls are efficient and cost-effective replacements for applications demanding high starting torque, wide speed range and fast dynamic response. These include closed loop DC, eddy current, mechanical, and most servo drives, as well as hydraulic drives.
AC Open Loop
AC open loop technology covers the same horsepower and torque ranges as the AC vector but lacks the servo grade performance. In general, the control electronics and algorithms for open loop drives are significantly less complicated than a vector solution. This leads to a cost advantage, especially at the lower end of the horsepower range. Since a rotor position sensor is also not required, further cost saving and system simplification results. Other characteristics of an open loop system are a typical speed range of 50:1 or less and a nominal speed regulation of 3 per cent. Again, standard, inexpensive AC induction motors are the machine of choice.
AC open loop drive systems are ideal for cost sensitive applications that have less demanding dynamic requirements or where a feedback device is impractical.
AC Open Loop Summary
Brushless DC Servo
Brushless DC Servo drive systems have essentially the same performance as the AC vector with the exception of the extended constant horsepower region above the base speed of the motor. These systems can also have extremely high RPM slew rates with the proper choice of a low inertia rare-earth magnet motor. At present, brushless servo system horsepower ratings are restricted by the limited availability of higher horsepower motors. One decided disadvantage of this technology is the motor itself, since the selection is somewhat limited and the cost tends to be higher than the induction motor alternative. Feedback devices are required and are normally an integral part of the motor.
Brushless DC Servos are typically used in applications under 20hp that require extremely high motor acceleration and deceleration rates.
Brushless DC Servo
Brushless DC Trapezoidal
Brushless DC trapezoidal drives offer a unique mixture of performance features while maintaining a low overall system cost. Since this is a closed loop system, speed accuracy is excellent and full torque is available at zero speed. Dynamic response with standard commutation hall sensor feedback is somewhat limited, as is the operating speed range. This can be improved however, with the addition of an additional feedback device for better speed sensing. Motors used in this type of system are normally constructed with ceramic magnets to reduce cost at the expense of the motor inertia. Trapezoidal drives also do not have an extended constant horsepower range above base speed.
Trapezoidal drives are an excellent choice for cost sensitive applications where closed loop performance is necessary but servo grade dynamic response is not required.
Brushless DC Trapezoidal Summary
Switched reluctance technology employs a unique motor construction and power conversion arrangement to drive what is the simplest, most rugged motor of all the options considered in this paper. While not a solution for most industrial applications, it does fill certain specialized niches. These systems can either be open or closed loop, since sensorless feedback is relatively easy to implement with SR motors. Advantages include low motor cost and very high-speed capability while torque ripple, system efficiency, and audible noise can be problematic. Standard motors are also not available so proprietary designs are the norm.
Switched Reluctance Summary
Determine the technology for your application
When considering an application, consider the advantages and disadvantages discussed above. Specifying AC flux vector controls in bridge applications requires answers to some relatively simple questions.
Different drive technologies may have seemingly subtle differences, yet in operation may provide quite different results. Choosing a drive depends upon many factors and it is unfortunate that so many motor users, and even OEMs, do not really understand the strengths and deficiencies of alternative drive technologies. Obviously, when selecting a motor drive for a particular application, not only the costs and motor efficiencies are involved; a full comparison must be made based upon the particular application. Without such a comparison, products or applications could result in substandard performance, require unnecessary, expensive maintenance, or hide larger long-term support costs behind the appeal of lower up front costs.
Understanding the strengths and deficiencies of alternate drive technologies is paramount to developing better, products requiring less and lower cost maintenance, or lower long-term support costs. As you've seen, when selecting a motor drive for a particular application, not only the costs and motor efficiencies are involved. A full comparison of drive and motor capabilities must be made based upon the particular application and its life span.
There are choices today in servo applications: DC servo, brushless DC, AC induction motors. The DC and brushless DC servo motors are used primarily in machine applications, usually involving smaller size motors.
Using flux vector drives offers two principal advantages: the same type of response, but in larger horsepower ratings; and through the use of standard, off-the-shelf motors.
The most current methodology uses AC variable frequency drives for varying speeds on electric motors. This permits varying both the voltage and frequency, to obtain the desired output without damaging the motor.
Flux vector drives are cost effective with smaller horsepower motors as well. The flux vector drives with AC induction motors are compatible in many applications where other motor types are unsatisfactory or ineffective.
Raymond Erbe and John Reichard are with Thor Technologies, Inc. located in Milwaukee, Wisconsin.