by Greg S. Highfill, P.E., MagnaDrive Corporation
December 17, 2011

Over the past 50 years, advances in technology have provided the primary thrust behind the success and evolution of variable-speed drive systems. Here are some considerations for the proper selection and set up of different types of systems, and what lies ahead in the future.

Pump users have a repertoire of variable-speed systems to choose from that deliver high performance, efficiency, and reliability. However, industry prefers mechanical or electrical adjustable speed drives that ease users' concerns for drive application, maintenance, and the environment. 

Compact in size and weight, the AC variable-frequency drives (VFDs) offer advances in power electronics coupled with induction motors. Utilizing the latest in power-switching transistors, microprocessors, hardware, and software functions, VFDs are the electrical adjustable speed drive most widely adopted by industry.

Permanent magnet drives (PMDs) are a mechanical adjustable speed drive technology that offers innovations in power transmission united with a simple machine design. Offering accurate process control, shaft misalignment tolerant, and a variety of equipment mounting configurations, this technology is a strong competitor with most variable-speed drive systems.

AC Drives

Though DC drives led the way early on for variable-speed motor control, AC drives made a big move to industry in the 1970s via variable-voltage/frequency control using silicon-controlled rectifiers and power switching devices.

Early AC drives operated in open loop but had limited performance. A major step forward was the development of field-oriented (flux vector) control for induction motors by Felix Blaschke at Siemens in 1971, which eventually pushed VFDs to meet or exceed DC drive performance in many applications. Sensorless-vector control (eliminating a shaft encoder) and other drive algorithm advances followed.

Early AC drives (circa 1980s) employed multiple transistors per phase due to their limited voltage and current ratings. This has changed to all-in-one packages, so that a 10-hp drive today has a structure smaller than one transistor pack of the vintage drive. New generations of transistors continue to be improved as manufacturers develop smaller and more efficient power devices. Insulated-gate bipolar transistors (IGBTs) are the present-day workhorse power devices. 

Among major AC drive milestones was the arrival of industrial pulse-width modulated (PWM)-based drives and the introduction of Direct Torque Control (DTC) in 1995. DTC is regarded as advanced technology, able to control motor torque and speed directly without need for separate control of voltage and frequency. With DTC, 100 percent torque is available at zero speed and small torque increments can be controlled at low frequencies.

On the control side, analog was king at first, giving way to digital control initially based on integrated circuits. Microprocessor (MPU)-based digital drives came somewhat later and at first offered only open-loop (V/Hz) control. Continuing advances in MPUs allowed adding multiple control types in the same drive, with only software parameter changes needed to switch control mode.

Today, fast-switching PMW output is considered a prime VFD feature because of its minimal harmonic current production and dynamic motor torque control. Typical selectable features are speed or torque regulation, ability to accept various analog or digital preferences, speed or torque feedback, extra I/O points, as well as control of synchronous and induction motors. 

Permanent Magnet Drives

Two types of magnetic slip couplings are available in industry, and they both function similarly. The heritage (earlier technology) product known as the eddy-current coupling is an electromechanical torque-transmitting device. Permanent Magnet Drives (PMDs), the latest technology product, were introduced in the mid-1990s and are mechanical torque-transmitting devices.

Both work on the principle of magnetic induction. Torque is generated as a function of slip between input and output members. The primary difference between these two types of devices is how the magnetic field is generated - one uses high strength permanent magnets and the other uses a field winding that is excited by direct current. The electrical power source is eliminated when using PMDs.

PMDs are best suited for controlling the speed of centrifugal equipment such as pumps, fans, or blowers. Early PMDs were horizontal design, performance rated up to 250-hp. They were primarily cooled by convection into the surrounding air, making their heat capacity a function of ambient air temperature and rotating speed. New materials and improvements in technology elevated the air-cooled performance rating to 500-hp with higher horsepower drives that utilize water-cooling.

A major high point for PMDs was the introduction of vertical mounting configurations. The addition of an enclosure supports the vertical motor and drive unit and limits overall height to prevent vibration problems.  Pump hydraulic thrust loads can be accommodated with a vertical bearing housing.

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