LINE FREQUENCY
The majority of toroidal power transformers are designed to operate in 50/60Hz, 60Hz or 400Hz applications. As the frequency increases the thickness of the strip steel is decreased to improve efficiency. The core size and/or the winding also decreases, making for a smaller transformer. This reduction in the physical size of the transformer, as a function of frequency, should be considered when packaging a transformer in the product. A 60Hz transformer will be 20% smaller than a 50/60Hz transformer.
PRIMARY VOLTAGE
A transformer operates using magnetic induction. The basic transformer consists of two coils of wire wound on a steel core. When a voltage is applied to one of the coils, it magnetizes the core and a voltage is induced in the second coil. The ratio of the primary voltage to the secondary voltage depends on the turns ratio of the two coils:
V_P / V_S = T_P / S
where V = voltage and T = turns
Taps may be provided on the transformer to compensate for different country requirements. Figure 6 shows typical primary voltage configurations.
Note: Multiple primary windings must be connected in parallel or series to maintain rated power.
SECONDARY VOLTAGE
The secondary voltage(s) of the transformer is specified with rated primary voltage and full load secondary current.
SECONDARY VOLTAGE REGULATION
The voltage regulation of the transformer is the relationship of the open circuit (no load condition) to the rated voltage (full load condition). This condition can be expressed as:
Reg = (V_NL - V_FL) / V_FL
where:
V_NL = no load AC voltage and V_FL = full load AC voltage
Regulation can be improved by decreasing the Wcu losses or by specifying a transformer with a larger VA ratiing.
SECONDARY DUTY CYCLE
The secondary VA requirements can be reduced if the load is intermittent and the "on" time is shorter than the transformer thermal time constant. Thermal time constants for transformers are typically a few minutes to fifteen minutes, depending on physical mass of the transformer.
Duty Cycle = [T_ON / ( T_ON + T_OFF )]^–2
where TON = time transformer is powering load and
TOFF = time transformer is not powering load
SECONDARY VA REQUIREMENTS
The secondary winding capacity is defined in terms of voltage, current and
duty cycle:
VA = V_FL x I_FL x (Duty Cycle)
where:
V_FL = AC secondary voltage at specified current requirements and
I_FL = AC secondary current at specified maximum requirement