The speed-torque relationship for a typical shunt-wound motor is shown in Figure 8.
Figure 8 Torque-vs-Speed for a Shunt-Wound DC Motor
A shunt-wound DC motor has a decreasing torque when speed increases. The decreasing torquevs-speed is caused by the armature resistance voltage drop and armature reaction. At a value of speed near 2.5 times the rated speed, armature reaction becomes excessive, causing a rapid decrease in field flux, and a rapid decline in torque until a stall condition is reached.
Shunt-Wound Motor Applications
The characteristics of a shunt-wound motor give it very good speed regulation, and it is classified as a constant speed motor, even though the speed does slightly decrease as load is increased. Shunt-wound motors are used in industrial and automotive applications where precise control of speed and torque are required.
Since the armature and field in a series-wound motor are connected in series, the armature and field currents become identical, and the torque can be expressed as shown in Equation (6-8).
The torque-vs-speed characteristics of a series-wound motor with a constant voltage source are shown
In Figure 9. As the speed decreases, the torque for a series wound motor increases sharply. As load is removed from a series motor, the speed will increase sharply. For these reasons, series-wound motors must have a load connected to prevent damage from high speed conditions.
Figure 9 Torque-vs-Speed for a Series-Wound Motor
Series-Wound Motor Applications
The advantage of a series-wound motor is that it develops a large torque and can be operated at low speed. It is a motor that is well-suited for starting heavy loads; it is often used for industrial cranes and winches where very heavy loads must be moved slowly and lighter loads moved more rapidly.
The compounded motor is desirable for a variety of applications because it combines the characteristics of a series-wound motor and a shunt-wound motor. The compounded motor has a greater torque than a shunt motor due to the series field; however, it has a fairly constant speed due to the shunt field winding. Loads such as presses, shears, and reciprocating machines are often driven by compounded motors.
The types of DC motors are summarized below.
Types of DC Motors Summary
In a shunt-wound motor, the field is in parallel, or "shunts" the armature.
In a series-wound motor, the field is in series with the armature.
A compounded DC motor is constructed so that it contains both a shunt and a series field.
A shunt-wound DC motor has a decreasing torque as speed increases.
The characteristics of a shunt-wound motor give it very good speed regulation, and it is classified as a constant speed motor, even though the speed does slightly decrease as load is increased.
A series-wound motor has a rapidly increasing torque when speed decreases. As load is removed from a series-wound motor, the speed will increase sharply. The advantages of a series-wound motor are that it develops a large torque and can be operated at low speed. It is a motor that is well-suited for starting heavy loads.