As we known,DC motor types such as fractional-horsepower shunt, series, and compound DC motors can be started by connecting the motor directly to DC line voltage.
But we must take a note if our DC motor is rated more than 2 HP,it must starting with reduced voltage starter to ensure it run properly and can avoid serious damage.
DC motor starting information.
It have two requirements for starting DC motors with higher horsepower ratings, especially if they are started under load.We must take attention and precaution about this matter.
a) Both the motor and the supply lines must be protected from excess current during the starting period.
b) The motor starting torque should be made as large as possible to bring the motor up to full speed in a minimum amount of time.
DC motor starting description
At starting, when the motor armature is stationary, no internal counter voltage is generated. Thus the only limit on the current being drawn from the supply is the armature resistance.
In most DC motors this resistance is very low, about 1 Ω or less. To meet the first starting requirement, an external resistance can be placed in series with the motor armature during the starting period.
Starting resistors are variable resistors whose resistance value in the circuit can be controlled manually or automatically. As the motor speeds up, the counter voltage increases, decreasing the armature current.The starting resistance can then be reduced in successive increments until the motor reaches its full speed.
The second DC motor starting requirement can be met by maximizing the value of magnetic field flux, and by allowing the maximum safe value of armature current to flow when the motor is being started.
When starting shunt and compound motors,maximum field flux is obtained by cutting out the shunt-field rheostat.In series motors, field flux is at maximum due to the heavy starting current flowing through the field winding.
If the DC motor is in an adjustable-voltage, adjustable-speed drive, it can be started by advancing the speed control from zero to the desired speed.
If a DC motor in a motion-control system is to achieve smooth, high-speed motion without over stressing it, the motion controller must direct the motor amplifier to ramp motor velocity up gradually until it reaches its preset speed.
When the motor is to be turned off, it should be ramped down gradually until it stops. This is done by programming a trapezoidal velocity command profile into the motion controller to keep motor accelerations and decelerations within their required limits.