The benefits of variable speed pumping were recognized decades before the invention and successful commercialization of variable frequency drives (VFD). Despite superior reliability and low cost, AC induction motors are limited to certain specific speeds, which are a function of the applied power frequency and internal windings of the motor. This limitation has been problematic for pump specifiers and users ever since centrifugal pumps were first used.
One early idea for variable speed pumping that remains relevant today is the eddy current drive. Alternately known as magnetic drives, or electromagnetic drives, these cleverly simple machines use the mechanical energy of a suitably rated constant speed motor and adjustable-strength magnetic flux to regulate the torque (and thus, speed) available to the driven load.
Image 1 illustrates this process. The motor drives a cylindrical drum (light blue) at constant rated speed. A magnetically active rotor (red) rotates on a separate shaft and drives the pump at variable speed. The magnetic rotor is equipped with a low voltage coil that is energized by DC excitation current from the controller. Variation of the excitation current results in a proportional variation in torque, enabling the system to control speed to within 0.5 percent of demand.
Because of their simplicity, rugged construction and ease of maintenance, eddy current drives are expected to last as long as their associated equipment, (i.e. the motor and pump). It is not uncommon to see units remaining in active service after 40 or 50 years of duty.
Controls Are Vulnerable
Older mechanical units are almost always repairable using widely available standard parts, such as ball bearings, carbon brushes, slip rings and speed feedback tachometers. Despite their long life of service, some controllers are vulnerable to failure and parts obsolescence. For a relatively low cost, the entire controller can be replaced.
Modern adaptations of the controller can utilize the rugged and reliable silicon controlled rectifier (SCR) conversion technology to produce variable DC excitation from a single-phase AC input. This old-school technology is mated to a 21st-century digital-microprocessor-based control platform, giving this device a modern feel and far more flexibility than any preceding design. This makes possible a series of new features for the eddy current drive, like compact size, keypad monitoring and a single control platform.
In most pumping applications, the speed is controlled locally on the control panel by an adjustment potentiometer or up/down arrows on the digital keypad. For remote or automated operation, a switch or keypad selection transfers the control of the unit to an outside source such as a Programmable Logic Controller (PLC) or supervisory control and data acquisition (SCADA) system. The controller exchanges these commands via current (4-20 mA) or voltage (0-10 VDC) analog signals and relay-based status signals for start, stop and fault indication, among others.
Some units offer compatibility with a broad spectrum of high-level control equipment, enabling the exchange of command signals, status, speed, torque, and fault information over an Ethernet cable. This has the potential to reduce the wire routing and connection requirements for the installation.
An added capability with the addition of an Ethernet IP option is the ability to remotely monitor and control the associated drive unit over the internet, from any location. Drive units can have their own password protected website accessible on a user’s desktop, tablet or smartphone. Another feature is the ability to send “distress signals” (faulted status) to email or text addresses to alert operators or plant managers of a unit failing in service.
The development of the digitally controlled, universal SCR exciter/controller brings even more relevance to the eddy current drive. Pump users who choose this option can enjoy the assurance of decades of reliable service for their variable speed pumping requirements, coupled with the flexibility of a digital control interface well suited to today’s automation environment.