To change the speed as per the requirements, it is not possible to increase the voltage or currents beyond certain limit. These limits are called ratings of the motor.
The maximum voltage that can be applied to the motor, safely is called rated voltage or normal voltage of the motor. While changing the applied voltage, one should not apply the voltage more than the rated voltage of the motor.
Similarly maximum current that field winding can carry, safely is called rated field current of the notor. Hence while changing the flux, one should not increase field current beyond its rated value. This is important rating as far as shunt motor is concerned. In a series motor, the entire armature current flows through the series field winding. The armature current is decided by the load and it can not be changed by changing the resistance of the armature circuit. So the maximum current that armature winding can carry safely is decided by the load called full load current or full load rating of the motor. Motor should not be loaded more than its full load capacity indicated by its full load armature current.
Exceeding the rating is dangerous from the motor point of view as due to high currents, the heat produced, which is proportional to the square of the current is very large. This may damage the windings electrically.
Now N α V/Φ
So for V = Vrated and Φ α If rated i.e. when there is no external resistance in the armature and field circuit and motor is excited by normal rated voltage, the speed obtained is called rated speed or normal speed.
... Vrated α Vrated/ If rated
Note : Note that the rated or normal speed is not the maximum speed with which motor can run safely but it is the speed when the electrical parameters controlling the speed are at their rated values.
Practically a motor speed can be increased to approximately twice its normal speed safely.
Thus while controlling the speed, the voltage applied should not be more than rated voltage of a motor, the field current should not be more than its rated value and the current carried by armature should not be more than its full load value. All the ratings are provided by the manufacturer in the form of name plate of a d.c. motor.