Application of DC Series Motor

A Term paper of Electrical Science-102 On Application of DC Series Motor


The ability to control the speed with great accuracy is an attractive feature of dc motor. There is several method of connecting the stator winding of a dc machine. Especially important are the series and shunt connection. This term indicates respectively that the field winding is connected in series and parallel with armature winding. The speed of rotation can be related to the magnitude of the field current for each connection arrangement. Further the torque can be related to the field and armature current. Hence quiet different characteristics result for each different connection. For example: where a large starting torque is required such as cranes and train, the series motor is used.

Series motor develops high torque at load and has ability to pick up. Its speed increase with the decrease in load. Dc series motor are never started on no load because their speed at no load is dangerously high. Series motor are best suited for electric traction work....

Application of d.c. motors

Some elementary principles of application alone are dealt with here. The focus is on the mechanical equation of dynamics which is reproduced here once again.

Here TM and TL are the motor torque and the load torques respectively which are expressed as functions of !. Under steady state operation d!/dt will be zero. The application of motors mainly looks at three aspects of operation.

1. Starting

2. Speed control

3. Braking

The speed of the machine has to be increased from zero and brought to the operating speed. This is called starting of the motor. The operating speed itself should be varied as per the requirements of the load. This is called speed control. Finally, the running machine has to be brought to rest, by decelerating the same. This is called braking. The torque speed characteristics of the machine is modified to achieve these as it is assumed that the variation in the characteristics of the load is either not feasible or desirable. Hence the methods that are available for modifying the torque speed characteristics and the actual variations in the performance that these methods bring about are of great importance. When more than one method is available for achieving the same objective then other criteria like, initial cost, running cost, efficiency and ease operation are also applied for the evaluation of the methods. Due to the absence of equipment like transformer, d.c. machine operation in general is assumed to be off a constant voltage d.c. supply. The relevant expressions may be written as,

As can be seen, speed is a function of E and _ and T is a function of _ and Ia. Using these equations, the methods for starting , speed control and braking can be discussed.

Starting of d.c. machines

For the machine to start, the torque developed by the motor at zero speed must exceed that demanded by the load. Then TM - TL will be positive so also is d!/dt, and the machine accelerates. The induced emf at starting point is zero as the ! = 0 The armature current with rated applied voltage is given by V/Ra where Ra is armature circuit resistance.

Normally the armature resistance of a d.c. machine is such as to cause 1 to 5 percent drop at full load current. Hence the starting current tends to rise to several times the full load current. The same can be told of the torque if full flux is already established. The machine instantly picks up the speed. As the speed increases the induced emf appears across the terminals opposing the applied voltage. The current drawn from the mains thus decreases, so also the torque. This continues till the load torque and the motor torque are equal to each other. Machine tends to run continuously at this speed as the acceleration is zero at this point of operation.

Series Motor Diagram

The series motor provides high starting torque and is able to move very large shaft loads when it is first energized. From the diagram you can see that the field winding in this motor is wired in series with the armature winding. This is the attribute that gives the series motor its name.
Since the series field winding is connected in series with the armature, it will carry the same amount of current that passes through the armature. For this reason the field is made from heavy-gauge wire that is large enough to carry the load. Since the wire gauge is so large, the winding will have only a few turns of wire. In some larger DC motors, the field winding is made from copper bar stock rather than the conventional round wire used for power distribution. The square or rectangular shape of the copper bar stock makes it fit more easily around the field pole pieces. It can also radiate more easily the heat that has built up in the winding due to the large amount of current being carried.

The amount of current that passes through the winding determines the amount of torque the motor shaft can produce. Since the series field is made of large conductors, it can carry large amounts of current and produce large torques. For example, the starter motor that is used to start an automobile's engine is a series motor and it may draw up to 500 A when it is turning the engine's crankshaft on a cold morning. Series motors used to power hoists or cranes may draw currents of thousands of amperes during operation.
The series motor can safely handle large currents since the motor does not operate for an extended period. In most applications the motor will operate for only a few seconds while this large current is present. Think about how long the starter motor on the automobile must operate to get the engine to start. This period is similar to that of industrial series motors.

Series Motor Operation

Operation of the series motor is easy to understand. The field winding is connected in series with the armature winding. This means that power will be applied to one end of the series field winding and to one end of the armature winding (connected at the brush).

When voltage is applied, current begins to flow from negative power supply terminals through the series winding and armature winding. The armature is not rotating when voltage is first applied, and the only resistance in this circuit will be provided by the large conductors used in the armature and field windings. Since these conductors are so large, they will have a small amount of resistance. This causes the motor to draw a large amount of current from the power supply. When the large current begins to flow through the field and armature windings, it causes a strong magnetic field to be built. Since the current is so large, it will cause the coils to reach saturation, which will produce the strongest magnetic field possible.

Producing Back EMF

The strength of these magnetic fields provides the armature shafts with the greatest amount of torque possible. The large torque causes the armature to begin to spin with the maximum amount of power. When the armature begins to rotate, it begins to produce voltage. This concept is difficult for some students to understand since the armature is part of the motor at this time.

The basic theories of magnetism that anytime a magnetic field passes a coil of wire, a current will be produced. The stronger the magnetic field is or the faster the coil passes the flux lines, the more current will be generated. When the armature begins to rotate, it will produce a voltage that is of opposite polarity to that of the power supply. This voltage is called back voltage, back EMF (electromotive force), or counter EMF. The overall effect of this voltage is that it will be subtracted from the supply voltage so that the motor windings will see a smaller voltage potential.
When Ohm's law is applied to this circuit, that when the voltage is slightly reduced, the current will also be reduced slightly. This means that the series motor will see less current as its speed is increased. The reduced current will mean that the motor will continue to lose torque as the motor speed increases. Since the load is moving when the armature begins to pick up speed, the application will require less torque to keep the load moving. This works to the motor's advantage by automatically reducing the motor current as soon as the load begins to move. It also allows the motor to operate with less heat buildup.
This condition can cause problems if the series motor ever loses its load. The load could be lost when a shaft breaks or if a drive pin is sheared. When this occurs, the load current is allowed to fall to a minimum, which reduces the amount of back EMF that the armature is producing. Since the armature is not producing a sufficient amount of back EMF and the load is no longer causing a drag on the shaft, the armature will begin to rotate faster and faster. It will continue to increase rotational speed until it is operating at a very high speed. When the armature is operating at high speed, the heavy armature windings will be pulled out of their slots by centrifugal force. When the windings are pulled loose, they will catch on a field winding pole piece and the motor will be severely damaged. This condition is called runaway and you can see why a DC series motor must have some type of runaway protection. A centrifugal switch can be connected to the motor to de-energize the motor starter coil if the rpm exceeds the set amount. Other sensors can be used to de-energize the circuit if the motor's current drops while full voltage is applied to the motor. The most important part to remember about a series motor is that it is difficult to control its speed by external means because its rpm is determined by the size of its load. (In some smaller series motors, the speed can be controlled by placing a rheostat in series with the supply voltage to provide some amount of change in resistance to control the voltage to the motor.)
The relationship between series motor speed and armature current. From this curve you can see that when current is low (at the top left), the motor speed is maximum, and when current increases, the motor speed slows down (bottom right). You can also see from this curve that a DC motor will run away if the load current is reduced to zero. (It should be noted that in larger series machines used in industry, the amount of friction losses will limit the highest speed somewhat.)

Reversing the Rotation

The direction of rotation of a series motor can be changed by changing the polarity of either the armature or field winding. It is important to remember that if you simply changed the polarity of the applied voltage, you would be changing the polarity of both field and armature windings and the motor's rotation would remain the same.

Since only one of the windings needs to be reversed, the armature winding is typically used because its terminals are readily accessible at the brush rigging. Remember that the armature receives its current through the brushes, so that if their polarity is changed, the armature's polarity will also be changed. A reversing motor starter is used to change wiring to cause the direction of the motor's rotation to change by changing the polarity of the armature windings. Figure 12-12 shows a DC series motor that is connected to a re-versing motor starter. In this diagram the armature's terminals are marked Al and A2 and the field terminals are marked Sl and S2.

When the forward motor starter is energized, the top contact identified as F closes so the Al terminal is connected to the positive terminal of the power supply and the bottom F contact closes and connects terminals A2 and Sl. Terminal S2 is connected to the negative terminal of the power supply. When the reverse motor starter is energized, terminals Al and A2 are reversed. A2 is now connected to the positive terminal. Notice that S2 remains connected to the negative terminal of the power supply terminal. This ensures that only the armature's polarity has been changed and the motor will begin to rotate in the opposite direction.
You will also notice the normally closed (NC) set of R contacts connected in series with the forward push button, and the NC set of F contacts connected in series with the reverse push button. These contacts provide an interlock that prevents the motor from being changed from forward to reverse direction without stopping the motor. The circuit can be explained as follows: when the forward push button is depressed, current will flow from the stop push button through the NC R interlock contacts, and through the forward push button to the forward motor starter (FMS) coil. When the FMS coil is energized, it will open its NC contacts that are connected in series with the reverse push button. This means that if someone depresses the reverse push button, current could not flow to the reverse motor starter (RMS) coil. If the person depressing the push buttons wants to reverse the direction of the rotation of the motor, he or she will need to depress the stop push button first to de-energize the FMS coil, which will allow the NC F contacts to return to their NC position. You can see that when the RMS coil is energized, its NC R contacts that are connected in series with the forward push button will open and prevent the current flow to the FMS coil if the forward push button is depressed. You will see a number of other ways to control the FMS and RMS starter in later discussions and in the chapter on motor controls.

DC Series Motor Used as a Universal Motor

The series motor is used in a wide variety of power tools such as electric hand drills, saws, and power screwdrivers. In most of these cases, the power source for the motor is AC voltage. The DC series motor will operate on AC voltage. If the motor is used in a hand drill that needs variable-speed control, a field rheostat or other type of current control is used to control the speed of the motor. In some newer tools, the current control uses solid-state components to control the speed of the motor. You will notice that the motors used for these types of power tools have brushes and a commutator, and these are the main parts of the motor to wear out. You can use the same theory of operation provided for the DC motor to troubleshoot these types of motors.

Graph Between Time and Current

Speed control of DC motors

In the case of speed control, armature voltage control and flux control methods are available. The voltage control can be from a variable voltage source like Ward-Leonard arrangement or by the use of series armature resistance. Unlike the starting conditions the series resistance has to be in the circuit throughout in the case of speed control. That means considerable energy is lost in these resistors. Further these resistors must be adequately cooled for continuous operation. The variable voltage source on the other hand gives the Motor the voltage just needed by it and the losses in the control gear is a minimum. This method is commonly used when the speed ratio required is large, as also the power rating.

Field control or flux control is also used for speed control purposes. Normally field weakening is used. This causes operation at higher speeds than the nominal speed. Strengthening the field has little scope for speed control as the machines are already in a state of saturation and large field is needed for small increase in the flux. Even though flux weakening gives higher speeds of operation it reduces the torque produced by the machine for a given armature current and hence the power delivered does not increase at any armature current. The machine is said to be in constant power mode under field weakening mode of control. Above the nominal speed of operation, constant flux mode with increased applied voltage can be used; but this is never done as the stress on the commutator insulation increases.

Thus operation below nominal speed is done by voltage control. Above the nominal speed field weakening is adopted. For weakening the field, series resistances are used for shunt as well as compound motors. In the case of series motors however field weakening is done by the use of 'diverters' . Diverters are resistances that are connected in parallel to the series winding to reduce the field current without affecting the armature current.

Braking the DC motors

When a motor is switched off it 'coasts' to rest under the action of frictional forces. Braking is employed when rapid stopping is required. In many cases mechanical braking is adopted. The electric braking may be done for various reasons such as those mentioned below:

1. To augment the brake power of the mechanical brakes.

2. To save the life of the mechanical brakes.

3. To regenerate the electrical power and improve the energy efficiency.

4. In the case of emergencies to step the machine instantly.

5. To improve the through put in many production process by reducing the stopping time.

In many cases electric braking makes more brake power available to the braking process where mechanical brakes are applied. This reduces the wear and tear of the mechanical brakes and reduces the frequency of the replacement of these parts. By recovering the mechanical energy stored in the rotating parts and pumping it into the supply lines the overall energy efficiency is improved. This is called regeneration. Where the safety of the personnel or the equipment is at stake the machine may be required to stop instantly.

Extremely large brake power is needed under those conditions. Electric braking can help in these situations also. In processes where frequent starting and stopping is involved process time requirement can be reduced if braking time is reduced. The reduction of the process time improves the throughput.

Application of DC motors and generators

It is seen from the earlier sections that the d.c. machine is capable of having variety of torque-speed characteristics depending on the circuit conditions. The need for generating these characteristics will be clear only when they are seen along with the characteristics of the loads that they operate with. Even though a detailed treatment of motor load systems is outside the scope here, it may be useful to look into the typical torque-speed characteristics of some of the common loads.

Loads are broadly divided into,

(a) Passive loads

(b) Active loads

They may be unidirectional in operation or work in either direction (Reversible loads).

Passive loads absorb the mechanical energy developed by the motors while active loads are capable of working as both sinks and sources for mechanical energy. The direction of rotation may be taken to be clockwise/counter clockwise rotation. Normally the direction in which the load operates most of the time, is taken as the positive direction of rotation. Any torque which accelerates the motor load system in the positive direction of rotation is termed as a positive toque. With this rotation torques of motors, generators or loads can be represented graphically on a four quadrantal diagram. The torque being taken as an independent variable, is represented along the x-axis. Y-axis represents the speed. While the frictional torque is not a function of speed, the cutting toque is proportional to the speed and the fan torque varies as the square of the speed. These can only absorb mechanical power and hence are represented in quadrantal II for positive direction of rotation. Similar loads produce characteristics in quadrant IV for negative direction of rotation.

The torque is constant and depends on the difference in the weight of the case and the counter weight, and the radius of the drum. The characteristics of the load exists in all the four quadrants and is capable of delivering as well as absorbing mechanical power. Hence it is called as an active load. The governing equation when the motor and a load are connected together is

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