Transformers


	Transformers

	Introduction
	Physical Characteristics
	Electrical Characteristics
	Selection Guidelines
	Typical Electrical Connections

INTRODUCTION

To understand the complex, we must first understand the basics. This article will explain transformers as they relate to the HVAC trade in general . Remember that in combination with the following information, local code related issues must be adhered to.

From the basics to the utterly incomprehensible, the main purpose of a transformer is to alter a supply voltage (from here in called the primary or line side) to a desired voltage (from here in called the secondary or load side). A second purpose; which is sometimes seen in the trade, is for the transformer to maintain the same voltage characteristics from the primary side to the secondary side, but to isolate the secondary side to protect the load or control circuitry of a unit.

PHYSICAL CHARACTERISTICS

There are two main physical characteristics for transformers.

Open

Open Transformers

Open transformers can be easily identified due to the quite visible and open electrical connections. The electrical connections are usually screwed, or slip-on spades. Occasionally they may have wire pigtails for wire nut connections. These transformers must only be used where they are to be mounted inside a unit cabinet or electrical enclosure. Thus protecting it, and anyone around the unit from damage or electrocution.

Totally Enclosed Transformers
These transformers are totally contained in a metal or plastic enclosure and can be mounted anywhere with some exceptions. Weather tight units must be used if they are to be mounted in an exterior location or locations that are moderately damp. Sealed explosion proof enclosure transformers are also available for special application areas. Another thing to remember is if any electrical component is to be mounted on the inside of an exterior wall where no insulation and/or vapor barrier is present, a solid insulation base (usually plywood) must be applied to the wall first, the component is then mounted to the insulation base. This is to prevent possible condensation damage to the electrical component

ELECTRICAL CHARACTERISTICS

Transformers in the trade are either auto transformers (non isolating) or isolation transformers. The main difference between the two is that isolating transformers protect the load from any problems associated with the supply voltage. These are ideally suited for loads that are finicky to voltage spikes and dips, or there is a concern for damage to expensive electronic components.

Speaking of protection, the majority of transformers have inherent protection for the secondary through a fuse or thermal link. The fuse may or may not be easily replaceable. When installing transformers, if there is any doubt, at the least install secondary fusing to protect the transformer and/or load on the transformer. It is also recommended to install primary line side fusing for maximum protection.

There are three basic configurations of transformers used in the trade. Primary voltages are typically AC (alternating current), while the secondary voltage might be AC or DC (direct current). For the purposes of this explanation, we will use AC throughout, but you may substitute DC where warranted. Also there are single phase and three phase versions, multi-tap primary and secondary. For this discussion we will keep it simple for now.

.Step Down Transformers

This is used where a lower voltage is required to supply a load or control voltage. Example: A unit has a 230 volt supply, and the control circuit requires 24 volts. A unit has 600 volt supply and a damper motor requires 120 volts.

Step Up Transformers

Rarely seen in the trade but is sometimes used, it is used where a higher voltage is required. Example: Unit supply voltage is 120 volts and a crankcase heater requires a 230 volt supply.

Isolation Transformers

Another transformer that is sometimes used . Isolation transformers have the same primary and secondary voltages. They are used to protect the secondary load from supply voltage irregularities. Please note that isolation transformers are also available in step up and step down configurations.

SELECTION GUIDELINES

To select a transformer for your job or system you need to know the following:

Open or Enclosed:
As explained above, you must know where it is to be installed.

Auto or Isolation:
Does the load require some protection from the supply voltage side of the transformer?

Primary voltage:
the supply voltage from the unit or building. You must also know if it is single phase or three phase.

Secondary voltage:
the voltage requirements of the load you are supplying.

Transformer load :
transformer load ratings can be indicated for the primary and/or secondary side. The majority of the time, we size transformers by the secondary rating. Typically the transformer will be rated by VA (volt x amperes) or KVA (1000 x volt amperes). Volt amperes is actually the wattage of your load, so if the load of your secondary is 4 amps and the secondary voltage is 120 volts, the minimum VA rating you require is going to be 120V x 4A= 480 VA (watts). Therefore the minimum VA you require is 480VA or the next size up available.

TYPICAL ELECTRICAL CONNECTIONS

	Single Phase
	Three Phase

To be continued

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