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Frequently Asked Questions
If you would like to ask a question that is not listed here, pleased contact us -we'd be happy to help.
1. How does a transformer work?A transformer is a simple device that transfers electrical energy from one circuit to another by means of magnetic induction. An alternating current (AC) in the primary circuit creates a changing magnetic field in the iron core. This magnetic field induces a changing (AC) voltage in the secondary circuit.
Connection of a load across the secondary will result in an AC current flowing, transferring electrical energy from the primary circuit to the secondary circuit.
The output voltage of a transformer is determined by the ratio of turns between the primary & secondary. The secondary induced voltage Vs, of an ideal transformer, is scaled from the primary Vp by a factor equal to the ratio of the number of turns of wire in their respective windings. Thus the voltage ratio can be expressed:
 For example, a transformer with 240v input and 24v output will have about ten times more turns on the primary than the secondary (disregarding losses).
The number of turns on the primary is determines the strength of the magnetic field created in the core, called the Flux Density.
Three phase transformers operate just like three of the above single phase transformers. For convenience, they may be wound sharing the three legs of one iron core.
2. How do I specify a power transformer?A power transformer may be specified as follows:
3. What is meant by "VA"
4. What is the difference between VA and Watts?
Power is measured in Watts, using the symbol W (named after James Watt - the inventor of the steam engine)
In A DC (Direct Current) circuit W = VA
This is also true in an AC circuit with resistive elements only (no inductance or capacitance).
Inductors & capacitors cause the current in an AC circuit to be out of phase with the voltage (current waveform leading voltage for capacitors & lagging for inductors). The phase angle between voltage & current waves is called the power factor & expressed as cos
 (for most practical transformer related applications cos is usually  0.8)5. How do I convert horsepower (HP) to KW?Power used to be stated in horsepower.
One horsepower (HP) = 0.746 KW (or 746 W)
6. What is "three phase" power? Three phase power consists of three single phase power systems all 120Ã,Ã,° apart (see diagram).Three phase power is particularly useful for applications such as motors, because three phase motors are self-starting, and more efficient than single phase motors.
Electrical power is usually supplied as three phase, and single phase circuits are connected to different phases to balance the load evenly over the three phases.
Power is measured in Watts (W) or kW (One kW = 1,000W) While single phase power: W = V x A x cos Three phase power S = 1.73(V x A x cos
For example: Three phase power drawn by a three phase 415v rated at 5A ?
7. What is the "regulation" of a transformer?The term "regulation" refers to voltage regulation of a transformer.
The voltage regulation is the difference between the open circuit output voltage of the transformer and the full-load output voltage. This is usually expressed as a percentage.
Regulation of small PCB transformer may be as high as 20% , with power transformer up to 1,000VA usually having regulation For example, a typical 100VA control transformer rated at 24v full load (aprox 4A) may have an open circuit output voltage of 25.2v
8. Can a 60Hz rated transformer be run on 50Hz?In Australia we have a power system which operates at a frequency 50Hz. This means that the full sinusoidal voltage waveform repeats 50 times each second (Hz = cycles per second).
Equipment such as transformers & motors rated at 60Hz may not operate safely at 50Hz, due to higher core losses & possible core saturation due to increased flux density. Equipment rated 50/60Hz is, of course, safe to use on 50Hz. Transformers designed to operate at 50Hz will operate perfectly well at 60Hz.
Some electrical equipment will not operate satisfactorily at 50Hz if designed to run on 60Hz - for example, a synchronous motor will run about 20% slower on 50Hz, which may be a problem in a clock or timer.
9. What is "inrush current"?Inrush Current or input surge current is the maximum instantaneous current drawn by electrical devices (transformers, motors etc) when first turned on.
This inrush current only occurs during the first few cycles after turning on, and will have greatly reduced after about 80mS (four cycles). An inrush current as high as 16 x full load primary full-load current would be a peak value that would only occur for about half of one cycle at switch on. A multiple of 10 x full load primary current would be a realistic maximum RMS current over the first few cycles.
An example would be a three phase 50KVA transformer with a 415v primary, which has a full-load primary current of 70A. Inrush current = 10 x 70A = 700A over the first few cycles. Cables, circuit breakers and associated devices must be capable of withstanding these high inrush currents.
10. What is an auto-transformer & when is it used?An auto-transformer is a single winding transformer, providing a smaller & cheaper alternative to a double-wound T/F, useful when isolation from the input mains is not required.
The greatest saving in using an auto-transformer are when the input & output voltages are close in value. For example, a three phase auto-transformer with 415v input / 380v output rated 20KVA would be about the size of a 2KVA isolation transformer. The same savings can be achieved with single phase auto-transformers, with a 240v input / 220v output rated 500VA, could be made equivalent in size to a 50VA isolation transformer.
Imported equipment designed to operate on an overseas voltage can often be adapted for use on local Australian power (240v single phase, 415v three phase). This may be a much cheaper alternative than buying an isolation transformer, or having the equipment rewound.
Three phase auto-transformers 415v input / 200v output are perfect for operating electrical equipment brought in from Japan. As the ratio of output to input voltage is about half, an auto-transformer to run such equipment will be about half the size of an equivalent isolation transformer.
Australian Standard AS 3000 - 1986 (Clause 4.28.4) requires that an auto-transformer that changes voltage by more than 25% must be an integral part of the equipment it is powering. This can be achieved by hard wiring the equipment to the output of the auto-transformer.
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