Transformers for Variable Frequency Drives

WR transformers for variable speed applications are designed to integrate into systems requiring the control of induction motors. The configuration described by WR enables adjustment of the voltage and frequency applied to the motor to control its speed. This type of equipment is notably used in applications such as conveyor systems and pumps, where the speed must be able to vary according to the facility's needs.

WR's approach relies on custom engineering tailored to the project's specifications. Electrical characteristics, winding construction, terminations, and mechanical integration can thus be defined based on the application and the equipment to which the transformer will be connected.

Design

The variable speed drive transformer described by WR includes a set of windings connected in series and parallel to the motor. This configuration allows for adjusting voltage and frequency to control the induction motor's speed.

WR also manufactures autotransformers, which utilize a single winding with multiple tap points to step up or step down the voltage. The company notes that this type of transformer is particularly used in variable speed drives as well as in certain voltage stabilization and power conditioning applications.

The selected configuration therefore depends on the characteristics of the application. WR specializes in manufacturing dry-type electrical transformers to customer specifications, with capacities ranging from 15 kVA to 30 MVA, voltages up to 49 kV, and a 220°C insulation class.

Windings

The windings can be manufactured using copper or aluminum, and their construction varies depending on the voltage level and the required insulation class.

WR coils are vacuum pressure impregnated using the VPI process with a polyester resin. This impregnation limits moisture ingress into the windings and their insulation, contributing to their durability in demanding industrial environments.

Depending on the transformer's characteristics, the windings can be barrel- or disc-wound. For certain low-voltage applications, WR can also use strip/foil conductors. This construction helps reduce eddy current losses and provides enhanced short-circuit strength. Adjustment taps are located on the front of the transformer for easy access.

Core

WR cores are manufactured using grain-oriented steel that offers high magnetic permeability.

The magnetic flux density is maintained below the saturation point to limit core losses and help reduce noise levels. The steel laminations are precision-cut and carefully stacked, and the core is then vacuum impregnated with resin to help prevent corrosion.

The core is electrically insulated, with the exception of a single ground point connected to the frame using a copper ground strap. On larger capacity units, an air gap can also be integrated to prevent saturation and allow air to circulate through the core to assist with transformer cooling.

Integration

In a variable speed application, the integration of the transformer with other electrical system components must be taken into account right from the design stage.

WR offers a standard "stub-up" connection using NEMA hole patterns, with or without mechanical lugs. The company can also manufacture and install busbars coordinated with low- or medium-voltage switchgear. Flexible braid connectors can be added to limit the transmission of transformer vibrations to connected equipment.

Transition compartments can also be integrated to facilitate the interface between high- and low-voltage cables. They provide a protected space for connections and allow for the separation of different sections. Cable supports can be added to facilitate an organized and safe installation.

The enclosure can ultimately be selected according to operating conditions. WR notably offers NEMA 1 and NEMA 3R configurations, with options to add filters against dust, water, or snow. Stainless steel enclosures are also available for corrosive environments.

Testing and Compliance

Every transformer manufactured by WR undergoes a testing process prior to delivery.

WR's manufacturing facilities in Canada and the United States feature ISO/IEC 17025-certified testing laboratories. Standard tests include dielectric tests, no-load losses and excitation current, polarity, turns ratio, resistance, as well as load losses and impedance.

Optional tests can also be performed based on project requirements, including temperature rise, BIL, partial discharge, and sound level. Both of WR's manufacturing facilities are also ISO 9001:2015 certified and produce the same product lines of dry-type electrical transformers according to the company's North American manufacturing standards.