In this section will discuss about separately excited DC motor. Direct current (DC) motors converts electrical energy into mechanical energy and they works on dc supply.
Type of DC motors
• Permanent magnet type dc motor
• Separately excited dc motor
• DC series wound motors
• Shunt wound DC motor
• Compound DC motor.
In this section we will discuss about the separately excited dc motor. Like other DC motors, these motors also have both stator and rotor. Stator refers to the static part of motor, which consists of the field windings. And the rotor is the moving armature which contains armature windings or coils. Separately excited dc motor has field coils similar to that of shunt wound dc motor. The name suggests the construction of this type of motor. Usually, in other DC motors, the field coil and the armature coil both are energized from a single source. The field of them does not need any separate excitation. But, in separately excited DC motor, separate supply Provided for excitation of both field coil and armature coil. Figure below shows the separately excited dc motor.
Here, the field coil is energized from a separate DC voltage source and the armature coil is also energized from another source. Armature voltage source may be variable but, independent constant DC voltage is used for energizing the field coil. So, those coils are electrically isolated from each other, and this connection is the specialty of this type of DC motor.
Equations Of Voltage, current and power for DC motors
In a separately excited motor, armature and field windings are excited form two different dc supply voltages. In this motor,
• Armature current Ia = Line current = IL = I
• Back emf developed , Eb = V – I Ra
where V is the supply voltage and Ra is the armature resistance.
• Power drawn from main supply , P = VI
• Mechanical power developed ,
Pm = Power input to armature – power loss in armature
Operating characteristics of Separately excited dc motor
Both in shunt wound dc motor and separately excited dc motor field is supplied from constant voltage so that the field current is constant. Therefore these two motors have similar speed -armature current and torque – armature current characteristics. In this type of motor flux is assumed to be constant.
• Speed – armature current (N – Ia) characteristics: We know that speed of dc motor is proportional to back emf / flux i.e Eb / φ . When load is increased back emf Eb and φ flux decrease due to armature resistance drop and armature reaction respectively .However back emf decreases more than φ so that the speed of the motor slightly decreases with load.
• Torque – armature current ( τ – Ia) characteristics : Here torque is proportional to the flux and armature current . Neglecting armature reaction, flux φ is constant and torque is proportional to the armature current Ia . τ – Ia characteristics is a straight lien passing through the origin. From the curve we can see that huge current is needed to start heavy loads. So this type of motor do not starts on heavy loads.
Speed control of separately excited DC motor
Speed of this type of dc shunt motor is controlled by the following methods:
I. Field control methods: Weakening of field causes increase in speed of the motor while strengthening the field causes decreases the speed. Speed adjustment of this type of motor is achieved from the following methods:
II. Field rheostat control: – Here a variable resistance is connected in series with the field coil. Thus the speed is controlled by means of flux variation.
Reluctance control involving variation of reluctance of magnetic circuit of motor.
Field voltage control by varying the voltage at field circuit while keeping armature terminal voltage constant.
III. Armature control methods: Speed adjustment of separately excited DC motor by armature control may be obtained by any one of the following methods :
i. Armature resistance control: – Here, the speed is controlled by varying the source voltage to armature. Generally, a variable resistance is provided with the armature to vary the armature resistance.
ii. Armature terminal voltage control involving variation of variation of voltage in armature circuit.
Applications OF DC Motors
Separately excited dc motors have industrial applications. They are often used as actuators. This type of motors is used in trains and for automatic traction purposes.