MM0704, Lesson 3
If another coil is added at an angle of 90 (figure 3-6) from the original coil (figure 3-5), the coil assembly will then
turn regardless of its position. In figure 3-6, coil CD has a torque applied to it in a clockwise direction. As it rotates, it
turns the commutator. This turns coil AB and as coil CD moves toward its neutral position, the current is conducted
into coil AB by the commutator. The continuation of this cycle reverses the current in CD so that it continues to
receive a force in clockwise direction.
Figure 3-6. T-Section, High-Pass Filter.
The rotational force which turns the armature is called torque. Torque in a motor depends on two factors: the current
flowing in the armature conductors and the flux density of the field. Mathematically, this relationship is expressed as:
T = 0xIA, where T is torque, 0 (phi) is flux density, and IA is armature current.
Series DC Motor
In the series DC motor, the armature and field are connected in series as shown in figure 3-7. This method of
connection requires that the field windings be large enough to carry heavy armature current. Consequently, the wire in
a series field is large and the winding contains few turns. The one serious disadvantage of the series motor is that if its
load is disconnected, it will race to destruction because the motor speed varies inversely with the load. Therefore, this
type of motor is not good for constant-speed applications. It is ideal where it is continuously under the control of an
operator and where a high starting torque is required.
The torque of a series motor varies as the square of the current. That is, if the current could be doubled in the armature
while the field flux remained constant, the torque would be doubled. Also, if the current remained constant in the
armature and the field current were doubled, the torque would be doubled. Since the field and armature are in series,
the current in both must be the same; therefore, with each winding contributing twice as much torque, there is four
times as much torque in the combination. (The torque will increase as the square of the current, up to the point where
the core material reaches saturation.) Because of its speed characteristics, the series motor is not suitable for any load
where the required torque might drop below 15 percent of full-load torque.
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