In A.C motor we have all three phase currents equally or separately supplied for each phase. In D.C motor only one current. The load can appear in the equation in any form.
You can start with the equation that input electrical power is mechanical power delivered to the load by the motor shaft plus losses. For DC, Power (Watts) = Voltage (Volts) X Current (Amperes). For three phase AC, Power (Watts) = RMS Line Voltage (Volts) X RMS Line Current (Amperes) X Square Root of 3 (1.732) X Power Factor (Cos of phase difference between voltage and current). For mechanical power, Power (Watts) = Torque (Newton – Meters) X Rotational Speed (radians per second). Therefore: For a DC motor, Input Current = (Torque X Speed + Losses) / Input Voltage. For a 3-Phase AC motor, Input Current = (Torque X Speed + Losses) / (Input Voltage X 1.732 X pf) The losses include both electrical losses and mechanical losses that are proportional to voltage, current, frequency (for AC), and speed. Speed depends on the speed vs. torque characteristics of the driven load and the speed vs. torque characteristics of the motor. At steady state, the motor operates at the intersection of the motor speed vs. torque curve and the load speed vs. torque curve. The DC motor speed vs. torque curve depends on the machine design and the applied voltage. The AC motor speed vs. torque curve depends on the machine design and the applied frequency and voltage. It is also important to note that the power factor of an AC motor is not constant. It varies with load.
