Replacing Your DC Motors? Think AC
Alternating-current (AC) and direct-current (DC) motors have traditionally served distinctly different applications due to their construction and inherent operating characteristics. In general, AC motors were smaller, less expensive, lighter and more rugged than DC motors. DC motors, on the other hand, operated better in variable-speed applications, particularly those requiring wide speed ranges, and provided more precise speed control.
DC motors have been the workhorse of industry in many applications where variable-speed operation is needed. In these applications, DC motors are reliable and provide precise speed control under variable operating conditions. However, DC motors are expensive to purchase and to maintain. In addition, over the past 10 years, AC drives have improved to the point where their speed control is far more precise, rivaling that of servo drives. What's more, the AC motor and drive together often cost about the same price as the DC motor alone.
The development of modern AC motors and drives is blurring the distinctions that once governed the choice between AC and DC drives. The result is that lower-cost, more-reliable AC drives are moving into applications once reserved for DC drives.
Photo 1. A typical AC motor and drive package.
THE RISE OF AC DRIVES
AC drives use a solid-state, adjustable-frequency inverter that adjusts frequency and voltage to vary the speed of an otherwise fixed-speed AC motor. This control is typically produced through pulse width modulation (PWM) of the drive output to the motor. Voltage and frequency are maintained in a constant relationship at any motor speed (known as the volts-to-hertz ratio).
AC drives are preferred when the motor environment is corrosive, potentially explosive, hazardous or wet, and demands special enclosures (explosion-proof, washdown, etc.). AC drives are also a good choice when the motors will receive little maintenance, either because they are inaccessible or because plant maintenance practices are poor.
AC drives generally are smaller and lighter than DC drives for a given torque and speed output, and they are capable of speeds approaching 10,000 rpm. In addition, a single AC drive can control multiple motors.
Modern AC motors and drives provide a number of additional operating benefits that rival those traditionally available only from DC drives. For example, today's drives can now produce full torque at start-up, something that once was impossible. They also are capable of speed ranges of 1,000:1 vs. only around 10:1 with previous AC drives.
Variable-speed AC drives also can adjust to fast-changing loads and provide tight speed regulation. When operated in closed-loop systems, AC drives can regulate speeds to within 0.01 percent and less. This makes them suitable for applications requiring high dynamic response, such as web processes, material handling sorter conveyors, metering pumps, extruders and test stands. The table on the next page compares the capabilities of standard DC motor drives and modern variable-speed AC drives.
Improved control technology is the reason for the vast improvement in AC drive performance. Today's inverters are smaller, less expensive, and they provide more capabilities than a few years ago.
Although AC motors have been capable of operating over huge speed ranges for some time, the drives available could not match their performance. Now, AC drives can produce near-servo-drive performance in a compact, reasonably priced package.
Photo 2. AC motors can provide you with a considerable cost savings when compared to DC motors.
SPEED CONTROL EFFECTS
AC motors have traditionally operated at fixed frequency and speed. At this speed, the built-in cooling system keeps the motor from overheating. Operating an induction motor as a variable-speed motor increases operating temperature and places increased stress on the insulation system. The higher temperatures result from increased motor losses and reduced heat transfer. As a result, many standard-efficient, fixed-frequency motors will not produce their nameplate rating when operated by an adjustable-frequency control. The elevated temperatures may not lead to immediate insulation failure; however, they will shorten life considerably.
In most insulation systems, a 10-degree Celsius increase in operating temperature will reduce expected life by 50 percent. This is one reason why energy-efficient or premium-efficient motors, which operate at cooler temperatures under the same conditions of service, are often recommended for operation with adjustable-frequency drives. Another reason that adjustable-frequency, drive-controlled, fixed-frequency motors operate at higher temperatures is that the cooling fan operates directly off the motor shaft. Thus, as motor speed varies, so does fan speed, resulting in lower cooling at slower speeds.
When operated as adjustable-speed devices, motor cooling will be reduced at slower speeds. In such applications, the motor should be specifically designed for variable-speed operation. AC variable-speed motors usually state their speed range. If you apply the motor within the specified speed range, overheating should not be an issue.
This chart compares the capabilities of standard DC motor drives with those of modern variable-speed AC drives.
When system performance requirements are minimal, a standard AC induction motor often can be applied successfully in adjustable-frequency, variable-speed applications. However, when performance requirements are more demanding, an energy-efficient, premium-efficient or definite-purpose inverter-rated motor must be used. This is particularly true when maximum productivity is required.
While the definition of high-performance is indefinite, one or more of the following factors usually characterizes such applications:
Continuous, constant torque required below 50 percent of the base speed.
Continuous, constant horsepower required above 150 percent of the base speed.
High starting loads or overloads.
High dynamic performance.
A process that cannot be started or run without variable-speed control.
Some AC motors are designed specifically to run on AF power and can provide continuous constant torque down to zero speed (1,000:1 turn-down). These designs are available in traditional totally enclosed, fan-cooled (TEFC) National Electrical Manufacturer's Association (NEMA) frame ratings or in very power-dense designs that look similar to the traditional square-frame DC. These power-dense designs are cooled with an auxiliary blower (force ventilated or blower cooled) or totally enclosed, fan-cooled construction, which eliminates the blower.
In higher horsepower ratings, these power-dense, blower-cooled designs represent a considerable cost saving vs. traditional fixed-speed motors. In addition, motors designed specifically for inverter-duty, high-performance applications have greater speed range, offer higher overload capability, and include thermal protection and mounting provision for speed-regulation devices.
Published July 2007, Reliable Plant magazine