Variable Speed Controls Enter the Digital Age

The benefits of variable speed pumping were recognized decades before the invention and successful commercialization of variable frequency drives.  Despite their 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 implemented.

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 utilize the mechanical energy of a suitably rated constant speed motor and use adjustable-strength magnetic flux to regulate the torque (and thusly, speed) available to the driven load. 

Referring to the diagram above:   the motor drives a cylindrical drum (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% 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’s 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.  However, 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.  Drive Source International, makes of the Dynamatic brand, continues to offer replacement controls for a variety of controller models, including the venerable Mark III, used in most pumping applications before 2000.

A more popular choice, though, is a modern adaptation of the controller, the EC-2000.  Like its predecessors, the EC-2000 utilizes the rugged and reliable SCR conversion technology to produce variable DC excitation from a single-phase AC input.  This old-school technology is mated to a 21^st-Century digital microprocessor based control platform, giving this product a modern feel and far more flexibility than any preceding design. This makes possible a series of new features for the eddy current drive:

Compact size

Magnetic pulse pickup and AC or DC tachometer inputs
PLC and SCADA compatible, with digital and analog I/O
Better than 0.5% speed regulation
Keypad monitoring of two selectable variables
4 programmable Run Presets accessible locally and remotely
Outputs do not require separate external D.C. supply
RS232, RS422, or RS485 serial connections
Easily retrofit to any brand or rating of mechanical drive unit
Single control platform across all sizes and brands in user’s facility
Keypad programming for relatively few needed parameters – can be replicated for multiple units
Controlled acceleration and deceleration
Torque limiting

Braking (not used for pumps)

Ethernet IP

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 PLC or SCADA system.  The controller exchanges these commands via current (4-20ma) or voltage (0-10vdc) analog signals and relay-based status signals for start, stop, fault indication, among others.

In the past, few years, Dynamatic has added Ethernet IP communication capability to the EC-2000 family.  This feature offers compatibility with a broad spectrum of high-level control equipment, enabling the exchange of command signals, status, speed, torque, and fault information over a single Ethernet cable.  This has the potential to reduce the wire routing and connection requirements for the installation. 

An added capability made possible with the addition of the Ethernet IP option is the ability to remotely monitor and control the associated drive unit over the internet, from any location.  Each drive unit can have its own password protected website and accessible on a user’s desktop, tablet, or smart phone.  A replica of the unit’s keypad displays the status in real time.  The virtual keys on the replica function just and the hardware keys on the actual keypad. Alternately, one can display and monitor (but not change) the operating status of a unit as a report. Yet another feature of the Ethernet IP option is the ability to send “distress signals” (faulted status) to up to six 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, such as the Dynamatic EC-2000 Ethernet IP brings even more relevance to the eddy current drive than ever before.  Pump users who choose this option will enjoy the assurance of decades of reliable service for their variable speed pumping requirements, coupled with the modern look, feel and flexibility of a digital control interface well suited to today’s automation environment.  In addition, the user is assured of ongoing factory support and parts availability for new and old equipment alike.

Want to learn more? Dynamatic’s team of engineers can help achieve true system efficiency with a Dynamatic variable speed system and digital controls.