Starting up separately excited dc motor

The rotor resistance of my Separately excited dc motor is approx. 3.5R with a voltage of 120V. I need a way of limiting the current on start up .What is the conventional way of doing this. I have thought of adding resistance in series with the armature winding but at what point do you remove this resistance and how?

There is a device that was used in color televisions that had the CRT display. It was called "In rush limiter."
Looked somewhat like a thermistor. It starts out with a high resistance and then goes to a lower resistance.
But that was used in an AC circuit on the input to the power transformer.

Are you going to apply the full rated voltage (from instance, from a battery) from a dead-stop, or can you change the armature voltage (via SCRs)?

-If the latter, you only have to ramp up slowly the voltage from zero, via SCR control.
-If the former, the best way is to use a power resistor with an ohmic value equal to the input voltage divided by the FL armature current. In parallel with the resistor, there should be a N.O. relay contact, which will close after a brief period of time. This period of time can be set up with a simple timer, or via an armature current sensor.

In all instances make sure that the field is fully excited. This will provide the proper back-EMF when the motor gets up to speed.

Because of the amount of power involved, NTC thermistors are not advisable.