DC motor fundamentals
DIRECT-CURRENT MOTORS are used extensively in indu strial applica tions because of their ability to meet a wide range of torque and speed requirements. They are especially ap propriate for applications requiring smooth acceleration over a broad range, accurate speed change and/or speed matching, and close control of torque or tensioning.
Although AC induction motors furnished with variable-frequency adjust able-speed drives are gaining in popu larity, the DC motor will maintain its desirability for certain applications because of its special speed/torque characteristics. For example, A C mo tors driving heavy loads at about twice their rated torque will usually stall. Direct-current motors, on the other hand, can deliver about three times their rated torque for short periods; and for very short periods, say 3 to 4 seconds, they can deliver up to five times rated torque.
The DC motor consists of two major components-the yoke or frame that contains the .field windings and the rotating component called the arma ture. A very important part of the armature is the commutator and brush assembly.
The yoke, also called the stator, is a cylindrical frame of high-permeability iron alloy to which are bolted the pole structures. The poles are arranged in an alternating north and south pat tern, and the frame provides a return flux path as well as mechanical support for the poles, bearings, and brush hold ers.
The armature consists of a shaft and laminated punchings of silicon steel slotted to accept the armature conduc tors or coils. The coil ends are connect ed to the segments of the commutator, which in turn connects the armature conductors to the power source through the brushes.
Alternating current flows in the armature conductors, the frequency of which is a function of the speed of rotation and the number of field poles. The laminated armature of the poles reduces core loss. Direct current flows only in the circuits external to the machine, because the armature conduc tors are successively connected to and disconnected from the external circuits by the commutator.
Direct-current motors are classified in accordance with the types and con nections of the various windings.
Shunt motors
The most widely used type is the shunt-wound motor. The name origi nated with early operation of these machines where the armature and field circuits were connected in parallel (shunt) to a constant-potential power supply. While the term “shunt” is still used, relatively few motors are now applied in this way. Shunt motors as now applied have their field circuits excited from a source of power sepa rate from the armature power supply. The excitation voltage level is usually the same as the armature voltage, but special shunt-field voltage ratings of 15 to 600 V are available as a modifica tion. The shunt motor is characterized by its relatively small speed change under changing load; rarely will the drop exceed 5% .
The speed of the shunt-wound motor can be changed by varying the shunt field current or armature voltage. Speed control by changing the arma ture resistance is unsatisfactory be cause “the speed regulation is objection able. Whenever the load changes slow ly, the flux changes as a result of armature reaction and speed will remain constant. However, if the load changes more rapidly than the self induction of the field windings will allow the flux to change, then the speed will change rapidly. A speed range of approximately 4:1 with reasonable run ning stability is possible for loads up to full-load torque. Care must be taken never to open the field of a shunt wound motor that is running unloaded. The loss of field flux causes motor speed to increase to dangerously high levels.Fig. 1shows speed/torque char acteristic of a DC shunt-wound motor.
Series motors
In series-wound motors, the field flux is created by coils that are electri cally in series with the armature (see Fig. 2). When the motor starts, the current, and consequently the magnetic field, are at maximum values, produc ing a large starting torque. As the motor speeds up and the current is reduced, the field flux is reduced. The torque will vary as the square of the armature current, neglecting satura tion of the field poles, which reduces this relationship. The torque and speed are very sensitive to the load current (which is also the field current) because of the corresponding change in flux. The speed of the series motor may be adjusted by shunting out the series winding, short-circuiting some field turns, or inserting resistance in series with the field and/or armature. How ever, speed adjustment is not easily accomplished. This motor has one dis advantage in that it tends to “run away” at light loads. The overspeed can reach a destructive value if the load is suddenly removed. For this rea son, series-wound motors should be used only where the load is directly connected or geared to the shaft.
Compound motors
A motor built with both shunt and series fields is termed a “compound wound” motor (Fig. 3). By proportion ing the relative amounts of series and shunt windings, the designer may shift the motor characteristics to be more nearly shunt or more nearly series in nature.
Each winding has turns and wire sizes similar to the shunt-wound and series-wound motor field windings. The proportion of the total flux supplied by the series winding determines the amount of “compounding,” which can be varied to suit the speed characteris tics desired. A strong series field will give speed characteristics approaching those of a series motor. A weak series field will give characteristics approach ing those of a shunt motor. Motors with series fields producing 40 to 75% to the total flux are often used, with a value of 50% most commonly provided. Compound motors having series fields producing 10 to 25% of the total flux are also used for some industrial appli cations. Generally, the speed charac teristics lie between those of shunt wound and series-wound motors. Com pound-wound motors can be used when speed variation with load variation is permitted.
The starting torque of the com pound-wound motor is high, although not so high as that of a series-wound motor. The torque will increase rapidly with load because the series field will increase the flux. The speed will decrease rapidly for the same reason. However, the motor will not run away at light loads because of the shunt-field flux.
The speed of the compound-wound motor can be adjusted with a shunt field rheostat. Modern DC motor con trols are somewhat complex; however they provide excellent control.