Starting of DC Motors:

At the moment a DC motor is beginning the armature is stationary and there is no counter EMF being produced. The only component to limit beginning current is the armature resistance, that, in most DC motors is a extremely low value (approximately one ohm or less), as describe in Equation (6-9).

I_a = E_t - E_CEMF /R_a                                                                          (6-9)

In sequence to decrease this extremely high beginning current, an external resistance has to be placed in series along with the armature during the starting period.  To display why this is essential, let us consider a 10-hp motor within an armature resistance of 0.4 ohms.  The resulting current would be as display in Equation (6-9) if the motor were supplied by a 260 VDC source.

I_a = E_t - E_CEMF /R_a

I_a = 260 VDC - 0/0.4?

I_a = 650 amps

This large current is approximately twelve times greater than fact full-load current for this motor. That high current would, in all  possibility, cause severe damage to the brushes, windings, or commutator. Beginning resistors are commonly incorporated into the motor design to limit starting current to 125 to 200 % of full load current.

An amount of starting resistance necessary to limit starting current to a more desirable value is calculated by using Equation (6-10).

R_s =  E_t / I_s -R_a                                                         (6-10)

where

R_s = starting resistance

E_t = terminal voltage

I_s = desired armature starting current

R_a = armature resistance