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Document Type: Prentice Hall
Author: Theodore Wildi
Book: Electrical Machines, Drives, and Power Systems, Sixth Edition
Copyright: 2006
ISBN: 0-13-177691-6
NI Supported: No
Publish Date: Nov 20, 2007


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DC Motor Calculations, part 3

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Compound Motor

A compound dc motor carries both a series field and a shunt field. In a cumulative compound motor, the mmf of the two fields add. The shunt field is always stronger than the series field.

Fig. 5.12 shows the connection and schematic diagrams of a compound motor. When the motor runs at no-load, the armature current / in the series winding is low and the mmf of the series field is negligible. However, the shunt field is fully excited by current Ix and so the motor behaves like a shunt machine: it does not tend to run away at no-load.

As the load increases, the mmf of the series field increases but the mmf of the shunt field remains constant. The total mmf (and the resulting flux per pole) is therefore greater under load than at no-load. The motor speed falls with increasing load and the speed drop from no-load to full-load is generally between 10 percent and 30 percent.

Figure 5.12 a. Connection diagram of a dc compound motor.
b. Schematic diagram of the motor.

Figure 5.13 Typical speed versus torque characteristics of various dc motors.

If the series field is connected so that it opposes the shunt field, we obtain a differential compound motor. In such a motor, the total mmf decreases with increasing load. The speed rises as the load increases, and this may lead to instability. The differential compound motor has very few applications.

Fig. 5.13 shows the typical torque-speed curves of shunt, compound and series motors on a per-unit basis. Fig. 5.14 shows a typical application of dc motors in a steel mill.

Reversing the direction of rotation

To reverse the direction of rotation of a dc motor, we must reverse either (1) the armature connections or (2) both the shunt and series field connections. The interpoles are considered to form part of the armature. The change in connections is shown in Fig. 5.15.

Figure 5.14 Hot strip finishing mill composed of 6 stands, each driven by a 2500 kW dc motor. The wide steel strip is delivered to the runout table (left foreground) driven by 161 dc motors, each rated 3 kW. (Courtesy of General Electric)

[+] Enlarge Image
Figure 5.15
a. Original connections of a compound motor.
b. Reversing the armature connections to reverse the direction of rotation.
c. Reversing the field connections to reverse the direction of rotation.

Starting a shunt motor

If we apply full voltage to a stationary shunt motor, the starting current in the armature will be very high and we run the risk of

a. Burning out the armature;
b. Damaging the commutator and brushes, due to heavy sparking;
c. Overloading the feeder;
d. Snapping off the shaft due to mechanical shock;
e. Damaging the driven equipment because of the sudden mechanical hammerblow.

All dc motors must, therefore, be provided with a means to limit the starting current to reasonable values, usually between 1.5 and twice full-load current. One solution is to connect a rheostat in series with the armature. The resistance is gradually reduced as the motor accelerates and is eventually eliminated entirely, when the machine has attained full speed.

Today, electronic methods are often used to limit the starting current and to provide speed control.

Face-plate starter

Fig. 5.16 shows the schematic diagram of a manual face-plate starter for a shunt motor. Bare copper contacts are connected to current-limiting resistors R1, R2, R3, and R4. Conducting arm 1 sweeps across the contacts when it is pulled to the right by means of insulated handle 2. In the position shown, the arm touches dead copper contact M and the motor circuit is open. As we draw the handle to the right, the conducting arm first touches fixed contact N.

The supply voltage Es immediately causes full field current Ix to flow, but the armature current / is limited by the four resistors in the starter box. The motor begins to turn and, as the cemf Eo builds up, the armature current gradually falls. When the motor speed ceases to rise any more, the arm is pulled to the next contact, thereby removing resistor R1 from the armature circuit. The current immediately jumps to a higher value and the motor quickly accelerates to the next higher speed. When the speed again levels off, we move to the next contact, and so forth, until the arm finally touches the last contact. The arm is magnetically held in this position by a small electromagnet 4, which is in series with the shunt field.

Figure 5.16 Manual face-plate starter for a shunt motor.

If the supply voltage is suddenly interrupted, or if the field excitation should accidentally be cut, the electromagnet releases the arm, allowing it to return to its dead position, under the pull of spnng 3. This safety feature prevents the motor from restarting unexpectedly when the supply voltage is reestablished.

Stopping a motor

One is inclined to believe that stopping a dc motor is a simple, almost trivial, operation. Unfortunately, this is not always true. When a large dc motor is coupled to a heavy inertia load, it may take an hour or more for the system to come to a halt. For many reasons such a lengthy deceleration time is often unacceptable and, under these circumstances, we must apply a braking torque to ensure a rapid stop. One way to brake the motor is by simple mechanical friction, in the same way we stop a car. A more elegant method consists of circulating a reverse current in the armature, so as to brake the motor electrically. Two methods are employed to create such an electromechanical brake (1) dynamic braking and (2) plugging.

Dynamic braking

Consider a shunt motor whose field is directly connected to a source Es, and whose armature is connected to the same source by means of a double-throw switch The switch connects the armature to either the line or to an external resistor R (Fig. 5.17).

When the motor is running normally, the direction of the armature current I1 and the polarity of the cemf Eo are as shown in Fig. 5.17a. Neglecting the armature IR drop, Eo is equal to Es

If we suddenly open the switch (Fig 5.17b), the motor continues to turn, but its speed will gradually drop due to friction and windage losses. On the other hand, because the shunt field is still excited, induced voltage Eo continues to exist, falling at the same rate as the speed In essence, the motor is now a generator whose armature is on open-circuit.

Let us close the switch on the second set of contacts so that the armature is suddenly conneted to the external resistor (Fig. 5.17c). Voltage Eo will immediately produce an armature current I2. However, this current flows in the opposite direction to the original current /1 It follows that a reverse torque is developed whose magnitude depends upon I2. The reverse torque brings the machine to a rapid, but very smooth stop.

Figure 5.17a Armature connected to a dc source Es.

Figure 5.17b Armature on open circuit generating a voltage Eo.

Figure 5.17c Dynamic braking.

In practice, resistor R is chosen so that the initial braking current is about twice the rated motor current. The initial braking torque is then twice the normal torque of the motor.

As the motor slows down, the gradual decrease in Eo produces a corresponding decrease in I2. Consequently, the braking torque becomes smaller and smaller, finally becoming zero when the armature ceases to turn. The speed drops quickly at first and then more slowly, as the armature comes to a halt. The speed decreases exponentially, somewhat like the voltage across a discharging capacitor. Consequently, the speed decreases by half in equal intervals of time To. To illustrate the usefulness of dynamic braking. Fig. 5.18 compares the speed-time curves for a motor equipped with dynamic braking and one that simply coasts to a stop.

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Related Links:
DC Motor Calculations, part 1
DC Motor Calculations, part 2
DC Motor Calculations, part 4
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