AC DC Full Load Current Calculation Formula

In this article, we will discuss the AC DC full load current calculation formula and calculation procedure of full load current.

WELDING PREHEAT CALCULATION
WELDING PREHEAT CALCULATION

The full load current of an electrical machine is the maximum current that equipment can safely carry without damage. The conductor of an electrical machine has a certain current carrying capacity according to the machine rating. The excess current above the rated full load current produces additional heat in the electric machine. And, this additional heat can not be dissipated efficiently because the cooling system of the machine is designed to disparate the heat generated at full load current.

If the current above the full load current(FLC) flows in a winding the additional heat loss increase in the proportionality of the square of the current. The heat generation in the winding is equal to the product of resistance and square of the current(I2R). The insulation system of the electrical system is most vulnerable to increased heating of the machine. The increased temperature deteriorates the insulation life of the machine.

The insulation life becomes half with every additional increase in temperature by 10 Deg C. The increased temperature of the machine may lead to insulation failure. Therefore, no electrical machine must run above the full load current(FLC) of the machine.

Alternating Current AC Loads

The AC loads are of three types.

  1. Resistive Load – Heater, room heater, and resistive elements
  2. Inductive Load – Induction furnace, single-phase induction motor, three-phase induction motor
  3. Capacitive Load – Capacitor banks, Open circuited transmission line

Now, we will discuss the AC DC Full Load Current Calculation Formula for three-phase induction motor, single-phase induction motor, heater, and DC Machines.

Full Load Current Calculation of 3-phase Induction Motor

The stator of a 3-phase induction motor may have a connection in the star or delta configuration. The input power remains the same irrespective of types of connection, star, or delta.

Full Load Current(FLC) Calculation formula for Star Connected Motor

Let the phase voltage and current of the motor be Vph & Iph . The phase power(Pp) is the product of current, voltage, and power factor.

Thus, the total power (Pt)of all the three phases will be;

AC DC Full Load Current Calculation Formula

From the above formula, we can calculate the motor’s full load current(FLC).

It is not possible to measure the phase voltage of the motor because the neutral point is formed inside the motor and it is not brought out of the machine. The relationship between the phase voltage and line voltage(Phase to phase voltage) in star connection is;

phase voltage and phase current for star connected load

Therefore, the full load current of a star-connected 3-phase induction motor is;

Full Load Current(FLC) Calculation formula for Star Connected Motor

Full Load Current Calculation formula for Delta Connected Motor

In delta connected stator, the phase voltage is equal to the line voltage, and the phase current is 58 % of the line current.

formula of phase voltage and phase current for delta connected load

Therefore, the full load current of a delta-connected 3-phase induction motor is;

formula for full load current of a delta connected 3-phase induction motor

Full Load Current Calculation of a Single-phase Motor

The full load current(FLC) of a single-phase induction motor is;

formula for Full Load Current Calculation of a Single-phase Motor

Full Load Current Calculation of a Single-phase Heater

The heater is a resistive type load and the power factor of the resistive load is unity. The full load current(FLC) of a single phase heater is;

formula of Full Load Current Calculation of a Single-phase Heater

Calculation of Heater FLC by measuring Resistance and Power

  • Measure the resistance of the heater by a multimeter. Let it is R.
  • Measure the power after connecting the heater to the circuit.
  • Calculate the full load current of the heater using the below-given formula.
formula for Calculation of Heater FLC by measuring Resistance and Power

Full Load Current Calculation of a Three-phase Heater

The full load current of a three-phase heater is;

Full Load Current Calculation of DC Machine

In a DC circuit power factor does not happen and the power of the DC machine is simply the product of voltage and current.

formula of Full Load Current Calculation of DC Machine

The supply voltage(V) of the DC shunt and series machines are given below.

formula of supply voltage(V) of the DC shunt and series machines

The back EMF of the DC machine is;

formula of back EMF of DC machine
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