Drive Technology Alternatives

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 Ð and not infrequently a misunderstanding Ð 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
Typical Applications

• AC dynamometers and test stands
• Steel casting lines
  
• Plastic extrusion
  
• Wire drawing
  
• Crane and hoist
  
• Material handling
  
• Winding equipment
  
• Machine tools
  
• Shuttles and stackers
  
• Printing and bookbinding
  
• Windmills

Advantages

• Wide speed range, 1000:1
• Servo performance
• 100 per cent torque @ 0 rpm
• Positioning capabilities to within 1/4 of an encoder count
• Precise speed and torque control
• 0.05 per cent speed regulation
• Less than 3 per cent torque linearity of the full speed range
• No static radial forces on motor
• No magnetics

• Higher costs
• Higher PWM losses
• Requires feedback sensor

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:

• Full Torque at zero speed
• Ensures load positioning capacity
• Full torque at rated speed
• Precludes the need for special voltage motors
• Full-range torque control
• Adjustable torque control, from 150 per cent motoring to zero to 150 per cent braking, with no deadband
• Instantaneous torque transient response
• There is a zero lag in the torque control loop

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
Typical Applications

• Conveyors
• Grinders
• Pumps and fans
• Blenders
• Mixers
• Process Lines
• Material Handling
• Crane and Hoist

• Less complex hardware and software algorithms
• Lower cost
• No static radial forces on motor
• No feedback sensor required

• Less accurate speed control, 3 per cent typical
• High PWM losses

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
Typical Applications

• Canning and bottling
• Machine tools
• Material Handling
• Robotics

• Wide speed range, 1000:1
• Servo performance
• 100 per cent torque @ 0 rpm
• Positioning capabilities to within 1/4 of an encoder count
• Precise speed and torque contro
• 0.05 per cent speed regulation
• Less than 3 per cent torque linearity of the full speed range

• Higher costs
• High PWM losses
• Requires feedback sensor
• High radial forces on moto
• Limited over-speed range

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
Typical Applications

• Compressors
• Fans
• Treadmills