The DC compound motor is a combination of the series motor and the shunt motor. It has a series field winding that is connected in series with the armature and a shunt field that is in parallel with the armature. The combination of series and shunt winding allows the motor to have the torque characteristics of the series motor and the regulated speed characteristics of the shunt motor. Figure 12-17 shows a diagram of the compound motor. Several versions of the compound motor are also shown in this diagram.
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Cumulative Compound Motors
Figure 12-17a shows a diagram of the cumulative compound motor. It is so called because the shunt field is connected so that its coils are aiding the magnetic fields of the series field and armature. The shunt winding can be wired as a long shunt or as a short shunt. Figure 12-17a and Fig. 12-17b show the motor connected as a short shunt where the shunt field is connected in parallel with only the armature. Figure 12-17c shows the motor connected as a long shunt where the shunt field is connected in parallel with both the series field, interpoles, and the armature.
Figure 12-17a also shows the short shunt motor as a cumulative compound motor, which means the polarity of the shunt field matches the polarity of the armature. You can see in this figure that the top of the shunt field is positive polarity and that it is connected to the positive terminal of the armature. In Fig. 12-17b you can see that the shunt field has been reversed so that the negative terminal of the shunt field is now connected to the positive terminal of the armature. This type of motor is called a differential compound because the polarities of the shunt field and the armature are opposite.
The cumulative compound motor is one of the most common DC motors because it provides high starting torque and good speed regulation at high speeds. Since the shunt field is wired with similar polarity in parallel with the magnetic field aiding the series field and armature field, it is called cumulative. When the motor is connected this way, it can start even with a large load and then operate smoothly when the load varies slightly.
FIGURE 12-17 (a) Diagram of a cumulative compound motor, (b) Diagram of a differential compound motor, (c) Diagram of an interpole compound motor.
You should recall that the shunt motor can provide smooth operation at full speed, but it cannot start with a large load attached, and the series motor can start with a heavy load, but its speed cannot be controlled. The cumulative compound motor takes the best characteristics of both the series motor and shunt motor, which makes it acceptable for most applications.
Differential Compound Motors
Differential compound motors use the same motor and windings as the cumulative compound motor, but they are connected in a slightly different manner to provide slightly different operating speed and torque characteristics. Figure 12-17b shows the diagram for a differential compound motor with the shunt field connected so its polarity is reversed to the polarity of the armature. Since the shunt field is still connected in parallel with only the armature, it is considered a short shunt.
In this diagram you should notice that Fl and F2 are connected in reverse polarity to the armature. In the differential compound motor the shunt field is connected so that its magnetic field opposes the magnetic fields in the armature and series field. When the shunt field's polarity is reversed like this, its field will oppose the other fields and the characteristics of the shunt motor are not as pronounced in this motor. This means that the motor will tend to overspeed when the load is reduced just like a series motor. Its speed will also drop more than the cumulative compound motor when the load increases at full rpm. These two characteristics make the differential motor less desirable than the cumulative motor for most applications.
Compound Interpole Motors
The compound interpole motor is built slightly differently from the cumulative and differential compound motors. This motor has interpoles connected in series with the armature (Fig. 12-17c). The interpoles are connected in series between the armature and series winding. It is physically located beside the series coil in the stator. It is made of wire that is the same gauge as the series winding and it is connected so that its polarity is the same as the series winding pole it is mounted behind. Remember that these motors may have any number of poles to make the field stronger.
The interpole prevents the armature and brushes from arcing due to the buildup of magnetic forces. These forces are created from counter EMF called armature reaction. They are so effective that normally all DC compound motors that are larger than 1/2 hp will utilize them. Since the brushes do not arc, they will last longer and the armature will not need to be cut down as often. The interpoles also allow the armature to draw heavier currents and carry larger shaft loads.
When the interpoles are connected, they must be tested carefully to determine their polarity so that it can be matched with the main pole. If the polarity of the interpoles does not match the main pole it is mounted behind, it will cause the motor to overheat and may damage the series winding.
Reversing the Rotation
Each of the compound motors shown in Fig. 12-17 can be reversed by changing the polarity of the armature winding. If the motor has interpoles, the polarity of the interpole must be changed when the armature's polarity is changed. Since the interpole is connected in series with the armature, it should be reversed when the armature is reversed. The interpoles are not shown in the diagram to keep it simplified. The armature winding is always marked as A1 and A2 and these terminals should be connected to the contacts of the reversing motor starter.
Controlling the Speed
The speed of a compound motor can be changed very easily by adjusting the amount of voltage applied to it. In fact, it can be generalized that prior to the late 1970s, any industrial application that required a motor to have a constant speed would be handled by an AC motor, and any application that required the load to be driven at variable speeds would automatically be handled by a DC motor. This statement was true because the speed of a DC motor was easier to change than an AC motor. Since the advent of solid-state components and microprocessor controls, this condition is no longer true. In fact, today a solid-state AC variable-frequency motor drive can vary the speed of an AC motor as easily as that of DC motors. This brings about a condition where you must understand methods of controlling the speed of both AC and DC motors. Information about AC motor speed control is provided in Chapter 11 (see the related links).
Figure 12-18 shows the characteristic curves of the speed versus armature current for the compound motors. From this diagram you can see that the speed of a differential compound motor increases slightly when the motor is drawing the armature highest current. The increase in speed occurs because the extra current in the differential winding causes the magnetic field in the motor to weaken slightly because the magnetic field in the differential winding opposes the magnetic in series field. As you learned earlier in the speed control of shunt motors, the speed of the motor will increase if the magnetic field is weakened.
FIGURE 12-18 (a) Characteristic curve of armature current versus speed for the differential compound motor and cumulative compound motor, (b) Composite of the characteristic curves for all of the DC motors.
Figure 12-18 also shows the characteristic curve for the cumulative compound motor. This curve shows that the speed of the cumulative compound motor decreases slightly because the field is increased, which slows the motor because the magnetic field in the shunt winding aids the magnetic field of the series field.
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Reader Comments | Submit a comment »
Thanks for the useful information on compound motors..
- Jan 18, 2011
Excellent ! Well written
- firstname.lastname@example.org - Jul 11, 2010
interpole is in all type of dc motor?
this is very good information to learn
- mani, ge. email@example.com - Apr 11, 2010
A dc compound motor ,110V 25A draws a steady high current of 100A.What is the causes and remedy
- Compound Motor, NETCO. firstname.lastname@example.org - Feb 28, 2007
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