Ice Breaker: Transformer Sizing and Calculating VA Capacity
Joe Marchese Explains How to Size a Step-Down Transformer
A popular electrical component used on many control circuits is the step-down transformer. It is the power source for a separate lower-voltage circuit which typically controls the operation of a system. This lesser voltage-control circuit allows the system to use smaller capacity controls and wire sizes to operate the system. This allows the system to be built using less expensive controls. It also makes it much easier to install as contractors or manufacturers do not need to run heavy-gauge wiring for the control circuit.
A step-down transformer reduces the voltage by means of electromagnetic induction. It has no moving parts; it consists of two windings — a primary winding and a secondary winding — enclosed in one body. The two windings are electrically separated and will be of different sizes. When an ac voltage is applied to the primary winding it will induce a voltage in the secondary winding. In a step-down transformer, the primary winding will have more turns of wire compared to the secondary winding. It is this relationship that determines the change in voltage. For example, if the ratio is five-to-one, the primary winding has five more turns of wire than the secondary winding. If 120 V were applied to the primary winding, 24 V will be induced onto the secondary winding.
Transformers come in various sizes. They are sized by their volt–amp (VA) rating. The VA rating is determined by multiplying the supplied secondary voltage by the maximum amperage draw the transformer can safely handle on its secondary winding (VA = volts x amps). For example: A 40 VA transformer with a 24-V secondary voltage can safely handle 1.66 A (40 VA / 24 V) without causing damage to the transformer.
The maximum amperage draw a transformer can safely handle is determined by the loads connected to its secondary winding. For example, if the secondary side of a transformer supplies a voltage to operate two relays and a contactor, and if each relay draws 0.3 A and the contactor draws 0.5 A when energized, the total amperage draw for the secondary side of the transformer will be 1.1 A (0.3 + 0.3 + 0.5).
The chosen transformer must then be able to safely handle 1.1 A. If the secondary voltage of the transformer is 24 V, then the minimum VA rating of the transformer needs to be 26.4 VA. Typically, the next size up will be chosen, such as a 30 VA transformer. If future loads may be added to the secondary side of the transformer, then perhaps a slightly larger transformer will be chosen, such as a 40 VA. This will allow additional loads to be connected to the secondary winding of the transformer without the need to replace the transformer.
Determining the right transformer for an application is not a difficult procedure, as long as required VA capacity is accurately calculated allowing the right size transformer to be selected.
Publication date: 7/1/2013