DC Wound-field motors use an electromagnet called the field winding to generate the magnetic field. The only other way to generate a magnetic field is with permanent magnets, The speed of wound-field motors is controlled by varying the voltage to the armature or field windings.When the motor is unloaded, it will go faster than the rated speed.
These motors are designed to run at 90 Vdc because 90 V is about what a practical rectifier circuit can produce from standard 120 Vac.
There are three basic types of wound field motors: series wound, shunt wound, and compound.
DC Series-Wound Motors
High starting torque. Field coil is in series with armature.
In a series-wound motor, the armature and field windings are connected in series. This configuration gives the motor a large starting torque, which is useful in many situations.
The explanation for this large initial torque is as follows: When the motor is stopped, there is no CEMF, and the full line voltage is available to the windings. Therefore, the initial armature current is large; and being in series with the field windings, this same current creates a large magnetic field as well.
The combination of a large armature current and a large field flux is what produces the large start-up torque . Once the motor starts turning, the increasing CEMF reduces the motor currents and hence the torque. Because the field coil carries the full armature current, it must have a low resistance; thus, it consists of a few turns of heavy-gauge wire.
S = CEMF/(kXφ)
S = speed of the motor (rpm)
CEMF = voltage generated within the motor
K = a motor constant
φ = magnetic flux
Another characteristic of the series-wound motor is that it tends to “run away” (go faster and faster) under no-load conditions. As the field current diminishes from the increasing CEMF, the magnetic field flux also decreases, which according to Equation tends to speed up the motor, which increases the CEMF even more.
The overall effect of this seemingly circular logic is that the motor will continue to accelerate until the torque is balanced by friction forces. Most smaller motors can handle the high speeds without causing any damage, but larger motors may literally fly apart if operated with no load.
Better Speed regulation. Field coil is in parallel position with armature.
In the shunt-wound motor, the armature and field windings are connected in parallel. With this configuration, the current in the field is dependent only on the supply voltage.
In other words, the field flux is not affected by variations in current due to the CEMF. This results in a motor with a more natural speed regulation.
Because the shunt motor tends to run at a relatively constant speed, it has traditionally been used in such applications as fans, blowers, conveyer belts, and machine tools.
The compound motor has both shunt and series field windings, although they are not necessarily the same size. There are two configurations of the compound motor, the short shunt and the long shunt, as shown in Figure.
Typically, the series and shunt coils are wound in the same direction so that the field fluxes add. The main purpose of the series winding is to give the motor a higher starting torque. Once the motor is running, the CEMF reduces the strength of the series field, leaving the shunt winding to be the primary source of field flux and thus providing some speed regulation.
Also, the combination of both fields acting together tends to straighten out (linearized) a portion of the torque-speed curve.
The motor discussed so far, where the fields add, is called a cumulative compound motor. Less common is the differential compound motor, where the field coils are wound in opposite directions.
The differential compound motor has very low starting torque but excellent speed regulation. However, because it can be unstable at higher loads, it is rarely used. The compound motor direction of rotation is reversed by reversing the polarity of the armature windings.
DC Stepper Motor:
A stepper motor is a unique type of DC motor that rotates in fixed steps of a certain number of degrees. Step size can range from 0.9 to 90°. Figure 8.1 illustrates a basic stepper motor, which consists of a rotor and stator.
In this case, the rotor is a permanent magnet, and the stator is made up of electromagnets (field poles). The rotor will move (or step) to align itself with an energized field magnet. If the field magnets are
energized one after the other around the circle, the rotor can be made to move in a complete circle.
Application of DC Stepper Motor:
Stepper motors are particularly useful in control applications because the controller can know the exact position of the motor shaft without the need of position sensors.This is done by simply counting the number of steps taken from a known reference position. Step size is determined by the number of rotor and stator poles, and there is no cumulative error (the angle error does not increase, regardless of the number of steps taken).
In fact, most stepper motor systems operate open-loop—that is, sends the motor a determined number of step commands and assumes the motor the controller goes to the right place.