CHAPTER XXIII

MOTORS

An electric motor is just the reverse of a dynamo; it is _a machine for converting electrical energy into mechanical energy_.

The electrical energy delivered by the dynamo must be obtained from a steam engine, gas engine, or other power; the mechanical energy obtained from the motor comes from the energy of the current flowing through its armature.

=Ques. What is the construction of a motor?=

Ans. It is constructed in the same manner as a dynamo.

Any machine that can be used as a dynamo will, when supplied with electrical power, run as a motor, and conversely, a motor when driven by mechanical power, will supply electrical energy to the circuit connected to it. Dynamos and motors, therefore, are convertible machines, and the differences that are found in practice are largely mechanical; they arise chiefly from the conditions under which the motor must work. Hence, the study of the motor begins with a knowledge of the dynamo, and accordingly the student should understand thoroughly all the fundamental principles of the dynamo, as already given, before proceeding further with the study of the motor.

=Principles of the Motor.=--All the early attempts to introduce motors failed, chiefly because the law of the conservation of energy was not fully recognized. This law states that _energy can neither be created nor destroyed_.

Early experimenters discovered, by placing a galvanometer in a circuit with a motor and battery, that, _when the motor was running, the battery was unable to force through the wires so strong a current as that which flowed when the motor was standing still_. Moreover, the faster the motor ran, the weaker did the current become.

=Ques. Why does less current flow when the motor is running than when standing still?=

Ans. Because the motor, on account of its rotation acts as a dynamo and thus tends to set up in the circuit a _reverse electromotive force_, that is, an electromotive force in opposite direction to the current which is driving the motor.

=Ques. What is the real driving force which causes the armature of a motor to rotate?=

Ans. _The propelling drag_, that is, the drag which the magnetic field exerts upon the armature wires through which the current is flowing, or in the case of deeply toothed cores, upon the protruding teeth.

=The Propelling Drag.=--In fig. 389 is shown the condition which prevails when a conductor carrying no current is placed in a uniform magnetic field. The magnetic lines pass straight from one pole to the other. The field is not distorted whether the conductor be at rest or in motion, so long as there is no flow of current. This represents the condition in the air gap of a motor or dynamo, when no current is flowing in the armature.

=Ques. What happens when a current flows in the conductor of fig. 389.=

Ans. It sets up a magnetic field of its own as shown in fig. 390.

=Ques. What is the effect of this magnetic field?=

Ans. It distorts the original field (fig. 389) in which the conductor lies, making the magnetic lines denser on one side and less dense on the other as in fig. 390.

=Ques. What is the nature of these distorted magnetic lines?=

Ans. They tend to shorten themselves to their original form of straight lines.

=Ques. What effect has this on the conductor?=

Ans. It produces a force on the conductor tending to push it in the direction indicated by the arrow, fig. 390.

The distorted magnetic lines may be regarded as so many rubber bands tending to straighten themselves; The result then is clearly to force the conductor in the direction indicated.

According to Lenz' law, the direction of the current in the armature of a dynamo is such as to oppose the motion producing it. When the armature of a dynamo is rotated, the bending of the lines of force of the main magnetic field due to armature reaction acts as a drag against the motion of the armature. Armature reaction increases with the increase of the armature current. Therefore, the effect of the drag increases with the increase of load and requires an additional expenditure of power to drive the armature.

In a motor, the direction of the actuating current is the reverse of that of the armature current of a dynamo, consequently, the armature reaction which constitutes a drag, acting against rotation of the armature of a dynamo, becomes a pull in the direction of rotation of the armature of a motor and constitutes its real turning effect or _torque_ which is used at the pulley to do mechanical work. The greater the load applied to the motor, the greater will be the amount of current taken from the supply mains, and consequently, the greater the _torque_.

=Ques. What are the essential requirements of construction in a motor?=

Ans. They are: 1, a magnetic field, 2, conductors placed perpendicular to the field, and 3, provision for motion, of the conductors across the field in a direction perpendicular to both themselves and the field.

=The Reverse Electromotive Force.=--When an electric current flows through some portion of a circuit in which there is an electromotive force, the current will there either receive or give up energy, according to whether the electromotive force acts _with_ or _against_ the current.

This is illustrated in fig. 395, which represents a circuit in which there is a dynamo and a motor. Each is rotating clockwise, and accordingly, each generates an electromotive force tending upward from the lower to the upper brush. In both cases the upper brush is positive. In the dynamo, however, where energy is being supplied to the circuit, the electromotive force is in the same direction as the current, and in the motor, where work is being done, the electromotive force is in the reverse direction to that of the dynamo.

=Ques. Describe similar conditions which prevail in the operation of a dynamo.=

Ans. When no current is being generated by the dynamo, little power is required to drive it, but when the external circuit is closed and current is forced through it against more or less resistance, work is being done, hence more power is required. In other words, there is an opposition to the mechanical force applied at the pulley which is proportional to the electric power delivered by the dynamo. An opposing reaction or _reverse force_ then is set up in a dynamo when it does work.

=Ques. In the operation of a motor what is the nature of the reverse electromotive force?=

Ans. It is proportional to the velocity of rotation, the strength of the magnets, and to the number and arrangement of the wires on the armature, that is, the reverse voltage depends on the _rate_ at which the lines of force are cut.

In the diagrams:

The pump corresponds to the dynamo. The high level pipe " " " positive conductor. The low level pipe " " " negative conductor. The valve " " " switch. The water motor " " " electric motor. The water pressure (called head) " " " electric pressure (called voltage). The flow in gallons per minute " " " amperes. The size of pipe " " " size of conductor. The foot pounds " " " watts.

The greater the difference between the height of the two pipes the higher the pressure, and the greater the difference between the pressures of the two conductors the higher the voltage. The larger the diameter of the pipes the less resistance is offered to the flow of water, and the larger the diameter of the conductors the less resistance is offered to the flow of electricity. The more water required by the water wheel, the more power is required to drive the pump. The more electricity required by the motor the more power is required to drive the generator.

=Ques. Describe an experiment which shows the existence of a reverse electromotive force in a motor.=

Ans. The apparatus required consists of a small motor, battery, and ammeter. They should be connected in one circuit and the deflection of the ammeter observed when the armature is held stationary, and when it rotates with various loads.

In an experiment of this kind made on a motor with separately excited magnets, the following figures were obtained:

Revolutions per minute 0 50 100 160 180 195 Amperes 20 16.2 12.2 7.8 6.1 5.1

Apparently, if the motor had been helped on to run at 261½ revolutions per minute, the current would have been reduced to zero. In the last result obtained, the current of 5.1 amperes was absorbed in driving the armature against its own friction at the speed of 195 revolutions per minute.

=Ques. Explain the action of the current supplied to a motor for its operation.=

Ans. The motor current passing through the field magnets polarizes them and establishes a magnetic field, and entering the armature, polarizes its core in such a way that the positive pole of the core is away from the negative pole of the magnetic field, and the negative pole is away from the positive pole of the magnetic field. The magnetic repulsions and attractions thus created cause the armature to rotate in a position of magnetic equilibrium or so as to bring its positive and negative poles opposite the negative and positive poles respectively of the magnetic field. It is evident that unless suitable means were provided to reverse the polarity of the armature core at the instant it reached the position of the magnetic equilibrium, the armature would not rotate any further. The construction is such that the polarity of the armature core, or the direction of the current in the armature coils is reversed at the proper instant automatically by the commutator, thus giving continuous rotation.

=Direction of Rotation of Motors.=--In the case of either a motor, or a dynamo used as a motor, the direction in which the armature will rotate is easily found by the left hand rule, as illustrated in fig. 411, when the polarity of the field magnets and the direction of currents through the armature are known.

=Ques. How may the rotation of a motor be reversed?=

Ans. By reversing either the current through the fields, or the current through the armature.

=Ques. What will happen if both currents be reversed?=

Ans. The motor will run in the same direction as before.

=Ques. What is the effect of supplying current to a series dynamo?=

Ans. It will run in a direction opposite to its motion as a dynamo.

=Ques. What is the result of reversing the direction of current at the terminals of a series motor?=

Ans. It will not change its direction of rotation, since the current still flows through the armature in the same direction as through the field.

=Ques. What is the behavior of a shunt dynamo when used as a motor?=

Ans. Its direction of rotation remains unchanged.

=Ques. Why is this?=

Ans. Because if the connections be such that the current supplied will flow through the armature in the same direction as when the machine is used as a dynamo, the current through the field will be reversed, since the field windings are in parallel with the brushes.

=Armature Reaction in Motors.=--In the operation of a motor the reaction between the armature and field magnets distorts the field in a similar manner as in the operation of a dynamo. A current supplied from an outside source magnetizes the armature of a motor and transforms it into an electromagnet, whose poles would lie nearly at right angles to the line joining the pole pieces, were it not for the fact that _negative_ lead must be given to the brushes.

Negative lead is the amount of backward advance of the brushes against the direction of the rotation of the armature, measured in degrees from the neutral plane.

If the brushes be given positive lead, that is, placed in advance of the neutral plane _in the direction of rotation_, the cross magnetizing force is converted into one that tends to increase that of the field magnet, while if they be given negative lead, it tends to demagnetize the field magnet.

Since with positive lead the armature polarity strengthens that of the field magnet, it is possible, disregarding sparking, to operate a motor without any other means being taken to magnetize the field magnets, because the armature will induce a pole in the field magnet and then attract itself towards this induced pole.

=Ques. What effect has the cross magnetizing force on the field?=

Ans. It tends to shift the field around in a direction opposite to that of the rotation.

=Ques. What are the conditions of minimum sparking?=

Ans. The same conditions must obtain as in a dynamo, that is, the current in the coil undergoing commutation must be brought to rest and started again in the opposite direction. This involves that while the coil is short circuited by the brush, it should be passing through a field that tends to reverse the direction of the current. Since the coil is already in such a field, the act of commutation must take place before it passes out of this field. Accordingly, a negative lead must be given the brushes.

=Method of Starting a Motor.=--Although motors and dynamos are practically similar in general construction and either one of them will act as the other when suitably traversed by an electric current, there are certain differences between the connections and accessories of a machine operated as generator and one employed as a motor. For instance, when a machine is operated as a dynamo, it is first driven up to speed until it has excited itself to the right pressure, and then it is connected to the circuit; but when a machine is used as a motor it will not start until it has been connected to the circuit, and this must not be done until the proper precautions have been taken to ensure that the current, which will pass through it when so connected, will not be excessive and thereby result in serious injury to the motor. For this reason a rheostat or variable resistance, commonly called a starting box is usually inserted in the armature circuit of a motor to prevent an undue rush of current before the motor attains its speed, and subsequently the speed is regulated by the cutting in or out of the circuit of certain extra resistances which constitute the controller used on a series motor requiring variable torque at variable speed, as in the case of elevator or electric traction service.

=Classes of Motor.=--Motors are classified in the same manner as dynamos. The fields may be either bipolar or multi-polar, and with respect to the type of armature winding employed, motors are classed as:

1. Series wound; 2. Shunt wound; 3. Compound wound.

=Series Motors.=--A series motor is one in which the field magnet coils, consisting of a few turns of thick wire, are connected in series with the armature so that the whole current supplied to the motor passes through the field coils as well as the armature. Fig. 416 is a diagram of a series motor showing the connections and rheostat.

=Ques. What are the characteristics of a series motor?=

Ans. The field strength increases with the current, since the latter flows through the magnet coils. If the motor be run on a constant voltage circuit, with light load, it will run at a very high speed; again, if the motor be loaded heavily, the speed will be much less than before.

=Ques. For what kinds of service are series motors unsuited?=

Ans. Series motors should not be employed where the load may be entirely removed because they would attain a dangerous speed. They should not be used for driving by means of belts, because a sudden release of the load due to a mishap to the belt would cause the motor to "run away."

Very small series motors may be used with belts since their comparatively large frictional resistance represents an appreciable load, restraining the motor from reaching a dangerous speed.

=Ques. For what service are series motors adapted?=

Ans. For gear drive.

In the case of a sudden release of the load the gears provide some load on account of the frictional resistance of the gear teeth.

=Ques. What advantage is obtained with series motors with respect to the connections?=

Ans. A single wire only proceeds from the rheostat to the motor, so that, with the return wire, only two wires are required.

=Ques. For what service are series motors specially adapted?=

Ans. Series motors are used principally for electric railways, trolleys, and electric vehicles, and similar purposes where an attendant is always at hand to regulate or control the speed. They are also used on series arc light circuits in which the current is of constant strength. Very small motors are generally provided with series windings.

=Shunt Motors.=--A shunt motor may be defined as one in which the field coils are wound with many turns of comparatively fine wire, connected in parallel with the brushes. The current then is offered two paths: one through the armature, and one through the field coils.

=Ques. What may be said with respect to the speed of a shunt motor?=

Ans. It is practically constant with varying loads.

The variation of speed ranges from 1/10 to 5 per cent., except in the case of small motors, in which the variation may be much greater.

=Ques. How should a shunt motor be started?=

Ans. To properly start the machine, the field coils must be fully excited.

It is, therefore, necessary to switch the magnet coils immediately on to the voltage of supply, while a variable resistance must be provided for the armature circuit. To get both connections at the same time, rheostats for shunt motors are arranged as shown in fig. 418.

=Influence of Brush Position on Speed.=--In the case of a shunt motor supplied with current at constant pressure, the speed is a minimum when the brushes are in the neutral plane, and the effect of giving the brushes either positive or negative lead is to increase the speed, especially with little or no load.

=Ques. Why does the speed increase?=

Ans. When the brushes are shifted from the neutral plane, the reverse voltage between the brushes is decreased, speed remaining unchanged. Accordingly, the pressure in the supply mains forces an increased current through the armature thus producing an increased armature pull which causes the speed to increase until the reverse voltage reaches a value sufficiently large to reduce the current to the value required to supply the necessary driving torque.

=Compound Motors.=--This type of motor has to a certain extent, the merits of the series motor without its disadvantages, and is adapted to a variety of service. If the current flow in the same direction through both of the field windings, then the effect of the series coil strengthens that of the shunt coil; this strengthening is greater, the larger the armature current.

=Ques. Mention some characteristics of the compound motor.=

Ans. Since it is a combination of the shunt and series types, it partakes of the properties of both. The series winding gives it strong torque at starting (though not as strong as in the series motor), while the presence of the shunt winding prevents excessive speed. The speed is practically constant under all loads within the capacity of the machine.

=Ques. Describe the connections for starting a compound motor at a distance.=

Ans. Control at a distance can be effected with only two wires, just as in the case of a series motor. In the diagram fig. 426, the current coming from the rheostat enters the series coil at F, and leaves it at E, thence it flows to the armature brush B, through armature to brush A, and from here back to the dynamo. The shunt winding, which is connected across the brushes, gets a very small voltage at starting and is accordingly very ineffective. The motor then starts as a series motor. The starting effect is smaller than in a series motor because of the fewer turns in the series winding, most of the available space being occupied by the shunt coils.

=Power of a Motor.=--The word "power" is defined as _the rate at which work is done_, and is expressed as the quotient of the work divided by the time in which it is done, thus:

work power = ------ time

The difference between power and work should be clearly understood.

_Work is the overcoming of resistance through a certain distance._ It is measured by the product of the resistance into the space through which it is overcome, thus:

work = distance × space

For instance, in lifting a body from the earth against the attraction of gravity, the resistance is the weight of the body, and the space, the height to which the body is raised, the product of the two being the work done.

The unit of work is the _foot pound_, which is _the amount of work done in overcoming a pressure or weight equal to one pound through one foot of space_.

The unit of power is the _horse power_ which is equal to 33,000 _foot pounds of work per minute, that is_:

foot pounds per minute horse power = ------------------------ 33,000

The unit of power was established by James Watt as the power of a strong London draught horse to do work during a short interval, and used by him to measure the power of his steam engines.

In order to measure the mechanical power of a motor, it is necessary to first determine the following three factors upon which the power developed depends:

1. Pull of the armature, in pounds;

2. Distance in feet at which the pull acts from the center of the shaft;

3. Revolutions per minute.

EXAMPLE.--If the armature pull of a motor having a two foot pulley, be such that a weight of 500 lbs. attached to the rim, is just balanced, and the speed be 1,000 revolutions per minute, what is the horse power?

Here, the distance that the pull acts from the center of the shaft is one foot, hence for each revolution the resistance of 500 pounds is overcome through a distance equal to the circumference of the pulley or

π × diameter = 3.1416 × 2 = 6.2832 feet.

The _work done_ in one minute is expressed by the following equation:

{ work } {weight} {circumference} {revolutions} { per } = { in } × { of pulley } × { per } = foot pounds {minute} { lbs. } { in feet } { minute }

= 500 × 6.2832 × 1,000 = 3,141,600.

Hence, the power developed is

3,141,600 ÷ 33,000 = 95.2 horse power.

=Ques. What is "brake" horse power?=

Ans. The net horse power developed by a machine at its shaft or pulley; so called because a form of brake is applied to the pulley to determine the power.

=Ques. Describe the apparatus used in making a brake test.=

Ans. Tests of this kind are usually made with a Prony brake as shown in fig. 428. It consists of a band of rope or strip iron--the latter is the arrangement shown--to which are fastened a number of wooden blocks, several carrying shoulders to prevent the contrivance from slipping off the wheel rim. The brake band is drawn tight, as shown, so that the blocks press against the surface all around. The brake thus formed is restrained from revolving with the pulley by two arms attached near the top and bottom centers of the wheels, and joined at the opposite ends to form a lever which bears upon an ordinary platform scale, a suitable leg or block being arranged to keep its end level with the center of the shaft. By this arrangement the amount of friction between the brake band and the revolving wheel is weighed upon the scales. Since the brake fits tightly enough to be carried around by the wheel, but for the arms bearing upon the scale, the amount of frictional power exerted by the wheel in turning free within the blocks may be transmitted and measured, just as would be the case were a machinery load attached, instead of a friction brake.

=Ques. Why must the point of contact of the brake with the scales be level with the center of the shaft?=

Ans. In order to determine the force acting at right angles to the line joining the point of contact and center of the shaft.

=Ques. What is the distance between the center of the shaft and point of contact with the scales called?=

Ans. The lever arm.

=Ques. What three quantities must be determined in a test in order to calculate the brake horse power?=

Ans. The lever arm, the force exerted on the scales, and the revolutions per minute.

=Ques. How is brake horse power calculated?=

Ans. From the following formula:

B. H. P. = 2 π L N W ----------- 33,000

in which

B. H. P. = brake horse power; L = lever arm, _in feet_; N = number of revolutions per minute; W = force _in pounds_ at end of lever arm as measured by scales.

EXAMPLE--In making a brake test on a motor, the lever arm of the brake is 3 ft., and the reading of the scales is 30 lbs. When the motor is running 1,000 revolutions per minute, what is the brake horse power?

Substituting the given values in the formula,

2 π × 3 × 1,000 × 30 B. H. P. = --------------------- = 17.1 33,000

Now, if the voltmeter and ammeter readings be 220 and 65 respectively, what is the efficiency of the motor at this load?

The amount of power absorbed by the motor, or in other words, the _input_ is

220 × 65 E. H. P. = -------- = 19.16 746

and since the output is 17.1 horse power,

output brake horse power 17.1 efficiency = ------ = ---------------------- = ----- = 89%. input electrical horse power 19.16

=Speed of a Motor.=--The normal speed at which any motor will run is such that the sum of the reverse electromotive force and the drop in the armature will be exactly equal to the electromotive force applied at the brushes. The drop in the armature is the difference between the applied voltage and the reverse voltage.

=Mutual Relations of Motor Torque and Speed.=--The character of the work to be done not only determines the condition of the motor torque and speed required, but also the suitability of a particular type of motor for a given service. There are three general classes of work performed by motors, and these require the following conditions of torque and speed:

1. Constant torque at variable speed;

Suitable for driving cranes, hoists, and elevators, etc., where the load is constant and has to be moved at varying rates of speed.

2. Variable torque at constant speed;

Suitable for driving line shafting in machine shops, which must run at constant speed regardless of variations of torque due to variations in the number of machines in operation at a time, or the character of work being performed.

3. Variable torque at variable speed.

Suitable for electric railway work. For example: when a car is started, the torque is at its maximum value and the speed zero, but as the car gains headway, the torque decreases and the speed increases.

=Speed Regulation of Motors.=--The speed of motors connected to constant voltage circuits is usually regulated by the two following methods:

1. By inserting resistances in the armature circuit of a shunt wound motor;

2. By varying the strength of the field of a series motor.

The first method is sufficiently explained under fig. 418 and the second method is illustrated in fig. 430. The controller switch S is so arranged that a greater or lesser number of field coils can be inserted in the field circuit. When the switch arm is on point 1, the motor current will flow through all the field windings, and the strength of the field will be at its maximum. When the switch arm is moved so as to successively occupy positions 2, 3, and 4, thus cutting out of circuit a greater and greater number of field coils the strength of the field will be gradually decreased until practically all of the motor current is led or wired through the armature. Under these conditions, when the field of a motor is at its maximum strength, the motor torque will be at a maximum for any given strength of current, and the reverse electromotive force will also be at a maximum for any given speed, therefore, when the field strength is increased the speed will decrease and _vice versa_.

=Ques. What results are obtained by this method of regulation?=

Ans. The speed of a series motor may be nearly doubled, that is, if the lowest permissible speed of the motor be 250 revolutions per minute it can be readily increased to 500 revolutions per minute by changing the field coil connections from series to parallel. It is on this account, as much as on their powerful starting torque, that series motors have been until recently almost exclusively employed for electric traction purposes.

=Series Parallel Controller.=--When two motors are used in electric railway work, their armatures are connected in series with each other and an extra resistance which prevents the passage of an excessive current through the armature before the motor starts. As the speed of the car increases, the extra resistance is gradually cut out of circuit and the field winding connections changed from series to parallel by means of a series parallel controller, which finally connects each motor directly across the supply mains, or between the trolley line and the track or ground return.

=Efficiency of a Motor.=--The commercial efficiency of a motor is the ratio of the output to the input. As a rule, the power developed by a motor increases as the reverse voltage generated by it decreases, until this voltage equals one half of the voltage applied at the brushes. After this point is reached, the power developed by the motor decreases with the decrease of the reverse voltage. Therefore, a motor performs the largest amount of work when its reverse voltage is equal to one half the impressed voltage.

The efficiency of a motor as just stated is the ratio of the output to the input; this is equivalent to saying that the efficiency of a motor is equal to the brake horse power divided by the electrical horse power.

The electrical horse power is easily obtained by multiplying the readings taken from volt meter and ammeter, which gives the watts, and dividing the product by 746, the number of watts per horse power. That is:

volts × amperes watts Electrical horse power = --------------- = ----- 746 746

=Interpole Motors.=--An interpole motor has in addition to the main poles, a series of interpoles, placed between the main poles. The object of these poles is to provide an auxiliary flux or "commutating" field at the point where the armature coils are short circuited by the brush.

=Ques. What is the object of the commutating field produced by the interpoles?=

Ans. Its object is to assist commutation, that is, to help reverse the current in each coil while short circuited by the brush, and thus reduce sparking.

=Ques. What is the nature of the commutating field?=

Ans. The excitation of the interpoles being produced by series turns, the field will vary with the load, and will, if once adjusted to give good commutation at any one load, keep the same proportion for any other load, provided the iron parts of the circuit be not too highly saturated.

=Ques. State briefly how sparking is reduced or prevented by the action of the interpoles.=

Ans. Sparking is due to self induction in the coil undergoing commutation, which impedes the proper reversal of the current. The action of the interpoles corrects this in that they set up a field in a direction that causes a reversal of the current in the coil while it is short circuited. Thus, the coil at the instant it leaves the brush, is not an idle coil, but has a current flowing in it in the right direction to prevent sparking.

=Ques. Mention some of the claims made for interpole motors.=

Ans. Constant or adjustable speed, and momentary overloads without sparking; constant brush position; operation at adjustable speeds on standard supply circuits of 110, 220, and 500 volts; constant speed with variable load; reversal without changing the position of the brushes.