Answer:

Exciting current is the amount of amperage a transformer draws under a no load condition. Another way to look at it is that exciting current is the transformer’s “idling” current. Exciting current could also be referred to as no load current although this is not technically accurate. Exciting current is actually made up of two components: no load losses (normally expressed in watts) and reactive power (normally expresses in kVAR). Exciting current varies as a percent of the transformer’s nameplate rating depending upon the transformer size. It is not unusual to have an exciting current of approximately 10% on very small transformers (under 1 kVA). On larger transformers, exciting current could be as low as a half of one percent.

Peak inrush current is the amount of amperage a transformer draws instantaneously when it is turned on. A transformer has an iron core and works under the principle of magnetic induction. Alternating current flows through a coil of wire (primary winding) and generates a magnetic field. The iron core of the transformer contains most of the magnetism and conducts this magnetism to where it passes through a second coil of wire (secondary winding).

Since alternating current travels in the form of a sine wave, the amount of magnetism will fluctuate depending upon the point in the sine wave. As this magnetism cuts through the path of the second coil of wire, it induces a voltage into it. When the transformer is turned off, the iron core retains an amount of residual magnetism depending on where in the sine wave the unit was when turned off. When the transformer is turned on, the greater the difference in the sine wave from the “turn off” point to the “turn on” point determines the amount of inrush current. Inrush current could be very small if everything was in phase, or it could be as high as 20 to 30 times full load current. Although this inrush condition disappears rapidly (in 6 to 10 electrical cycles - one tenth to one sixth of a second) it is the first half electrical cycle that sees the peak amount of inrush. One other point is the effect of source impedance. Source impedance will help decrease the inrush current. Peak inrush current can cause problems with over current devices. If the fuse or breaker is of a “quick trip” variety or not properly sized according to the National Electric Code, the inrush may cause it to trip falsely.

The technical question and answer above were featured in the December 21, 2011 edition of the SolaHD Wednesday Weekly online newsletter.