Pages Header Banner 2
Pages Header Banner 1
Pages Header Banner 3

Toll Free: 1.800.387.2840

Frequently Asked Questions

What is a Zig-Zag Grounding Transformer?

A Zig-Zag grounding transformer is a special three phase autotransformer with no secondary or output. It is used to establish a neutral on a 3 wire system. There are several reasons why a zig zag autotransformer may be used include ground fault protection, neutral grounding (solid or via a resistor), and harmonic filtering.

What is an Isolation Transformer?

All transformers other that autotransformers are Isolation Transformer. This is due to the fact that the primary and secondary windings are electrically isolated from each other (they are not physically connected to each other). The transformation in voltage and current between primary and secondary windings occurs as a result of the shared magnetic field in the core (Mutual Inductance).

What is an Autotransformer?

An autotransformer is a transformer that has only one winding per phase, part of which is common to both the primary and secondary circuits. Transformers wired in a “Buck-Boost” configuration are autotransfomers. Autotransformers are designed to adjust the supply voltage when isolation from the line is not necessary and where local electrical codes permit. An autrotransformers can be used in either a step-up or step-down application unlike isolation transformers. Autotransformers can also be used as part of a reduced voltage starter to reduce motor inrush current (see Motor Starting Autotransformers)

What is required clearance around a dry type distribution transformer?

The minimum required clearances of a dry type transformer to walls, floors or other equipment must adhere to the local electrical code requirements.

In the absence of such requirements, Rex Power Magnetics recommends that dry type transformers be mounted so that there is an air space of no less than 150mm (6”) between the enclosures, and between the enclosure and any adjacent surface except floors. When the adjacent surface is a combustible material, the minimum permissible separation between the transformer enclosure and the adjacent surface should be 300mm (12”). Where the adjacent surface is the wall on which the transformer is mounted, the minimum permissible separation between the enclosure and the mounting wall should be 6mm (0.25”) so long as the surface is of a non-combustible material.

What do I need to consider for low ambient temperature conditions?

Low ambient temperatures do not generally affect an energized transformer. The transformer's energization / No-load losses typically generate enough heat to operate effectively in low ambient temperatures down to -20°.

The main issue is when the unit is left de-energized or stored in lower temperatures. Energizing and loading a transformer that has been left in low ambient temperatures can stress the transformer and lead to failure. The insulation can become brittle in low temperature conditions, and as the windings warm up quickly and begin to expand, the insulation can crack, leading to a failure.

Furthermore, low temperatures can cause moisture/frost to form on the unit which can be absorbed into the insulation system. Anti-condensation heaters can be supplied within the enclosure to help keep the ambient temperature above 0°C when the transformer is de-energized, however due to the fact that the enclosures are typically ventilated and uninsulated, they are only effective for ambient down to -10°C. If ambient goes below -30°C, special designs and cold start procedures may be necessary. Care should be taken to store transformers in dry areas with temperature control.

Rex's installation, operation and maintenance manuals recommends that transformers be tested (meggered), brought above 0°C and/or go through a dry-out process if moisture is suspected to be present. Damage and injury can result from energizing a transformer which has had its insulation system compromised by moisture.

What is the terminal temperature rating of ventilated dry type distribution transformers?

Rex Power Magnetics’ ventilated distribution transformer terminals are rated 90°C. Conductors with at least a 90°C insulation rating at or below their 90°C ampacity rating should be utilized.

What is the odor emmiting from my new transformer? 

It`s normal for new transformers to release some harmless odors from the varnish impregnation used in the coils for a week or two after energization. Older Transformers can also release some odor if loaded to a higher level than they have experienced previously in their history.