ADVERTISEMENTS:After reading this article you will learn about:- 1. Induction Motors in Mines 2. Principle of the Induction Motor in Mines 3. Induction Effect in Rotor 4. Starting of Induction Motor 5. Starting Equipments for Induction Motors 6.

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Slipring Induction Motors 7. Synchronous Motors used in Mines 8.

Insulation Resistance of a Induction Motor.Contents:. Induction Motors in Mines. Principle of the Induction Motor in Mines. Induction Effect in Rotor. Starting of Induction Motor. Starting Equipments for Induction Motors.

Slipring Induction Motors. Synchronous Motors used in Mines.

Insulation Resistance of a Induction Motor1. Induction Motors in Mines. ADVERTISEMENTS:In mines, induction motors are mostly used in a flameproof enclosure. Besides the enclosure, performance of the induction motors is the same as that of the other motors, as per the particular design.

We know from our experience and knowledge that, among the induction motors, the squirrel cage-types are the most simple of all electric motors.Induction Motors consists of two parts only. One is the stator, a stationary winding which is connected to the supply, and the other is a rotor-a rotating winding which rotates within the stator and drives the load.The squirrel cage motors can be designed to operate from single or three phase supplies. A three phase Induction motor will start under load as soon as the supply is switched on. Starters are used only if it is necessary to reduce the starting current.Because of their simplicity, squirrel cage motors are widely used in mines and also in other industries. They are used underground to drive drills, coal cutters; loaders, conveyors and haulages, and they may also be found to be used extensively in pumps, auxiliary fans and small compressors. ADVERTISEMENTS:The stator consists of a hollow cylinder built up of lamination of soft iron.

The interior of the cylinder is slotted to receive the conductors of a three phase winding. The conductors of the winding are insulated from each other and the whole insulation of the stator is properly impregnated with varnish or resin of special electrical grade to prevent the ingress, of moisture and dirt and any other foreign particles.The core and coil is worked in a steel or cast iron yoke. Fig 11.1(a) shows a sketch of a stator.The Fig 11.1(b) shows a sketch of a squirrel cage rotor. The rotor consists of a cylindrical cage of copper bars or aluminium bars (cast in case of small motors) and short circuited by copper or brass ring at each end, giving it the shape of a cage.

This is why the induction motors are also called squirrel cage motors as they look like a squirrel’s cage. ADVERTISEMENTS:Alter­natively, the whole cage may be cast in one piece from aluminium alloy. The cage is set in a cylindrical core, built up of soft iron laminations, which is keyed to a shaft, already machined properly.

The rotor is supported by bearings at each end of the shaft.It is matched to the stator so that there is a very small air gap of few thousandths of an inch (generally varying from.015 to.028 in each side) between the surface of the rotor and the inner surface of the stator.A small but uniform air gap is most essential for the efficient operation of the induction motor, as a whole. In fact importance of air gap is so great that if it is not properly machined, the whole motor changes its characteristics and performance.2. Principle of the Induction Motor in Mines. ADVERTISEMENTS:In common with all other electric motors a cage motor creates a mechanical power through the motor principle as described by the reaction of current carrying conductors in the rotor with a magnetic field. The defining feature of an induction motor is that the currents in the rotor conductors are induced by the same field as that with which they react.The performance and operation of an induction motor depends on the possibility of producing a magnetic field which rotates, whilst the windings which produce it, remain stationary.Such a field can only be produced by a winding connected to an alternating current supply whereas, if a direct current is applied to a winding to produce an electro-magnetic field, the position of the field in space is determined entirely by the position of the winding. The field can be made to rotate only by turning the windings themselves.We can design the stator of an induction motor to produce a rotating field of two, four, six or any even number of poles, and then the design of the winding will depend upon the number of poles required. Each phase of the supply is connected to a winding in the stator.

ADVERTISEMENTS:The windings are designed so that each gives the required number of poles and the windings are interconnected either in star or delta. In the star formation, the three ends of the windings not connected to the supply are connected together.The windings in each phase are arranged so that, in each half cycle of their phase, one half of the winding produce north poles whilst the other half produce south poles. The polarity of every winding reverses at every half-cycle.The windings are equally spaced around the stator in order of phases.

Windings produce a north pole during the positive half cycle of their phase. A typical layout of windings is shown diagrammatically in Fig. 11.2(a).However Fig. 11.2(b) shows how a two pole rotating field is produced by stator having six windings. Because of the relation between the alternating cycles in the three phases, current strength will reach a peak in successive windings round the stator.Then the pole of aggregate field will at one moment be at winding 1A (north) and IB (south), then they will be at winding 3B (north) and winding IB (north), and 1A (south) and so on. The effect of connecting a three phase supply to a stator having six windings is to produce a two pole magnetic field which completes one revolution for every cycle of the supply.Speed of Field Rotation:For a two pole field to complete one revolution, every winding in the stator must have a north polarity once and a south polarity once. A two pole field rotates once per cycle, because each winding changes polarity once in the course of a cycle.For a four pole field to complete one revolution, every winding must have each polarity twice.

For a six pole field one revolution requires the windings to have each polarity three times, and so on.Now as we see that the windings change polarity only once per cycle, it follows that the more poles there are, the slower will be the rotation of the field and the speed of the rotor. For example, when connected to a 50 c/s. Supply, a two pole field rotates at 3000 rpm., a four pole field at 1500 rpm, a six pole field at 1000 rpm and an eighth pole field at 750 rpm.The speed of this field rotation is called the synchronous speed, and this can be described in terms of the formula;The field can be made to rotate in either a clockwise or an anti-clockwise direction. In fact, to reverse the direction of rotation, it is merely necessary to reverse the order of any two phases. Thus for example, if phase connections are 1-2-3 and produces a clockwise rotation, then anticlockwise rotation will be produced by connections 3-2-1, 2-1-3 or 1-3-2.

Induction Effect in Rotor:When the stator winding is connected to the stator, the rotating magnetic field sweeps across the conductors of the rotor. These conductors are, therefore, in a changing magnetic field. Each conductor has an e.m.f.

Induced in it, and as all the rotor conductors are shorted, and thus interconnected by the end rings, currents are able to circulate.The effect is exactly the same as if the fields were stationary and the rotor conductors were turned in the direction opposite to that in which the stator field rotates.The direction of current-flow in the rotor conductors can therefore be found by applying Fleming’s Right Hand Rule for generators. Fig 11.3 illustrates clearly to explain induction of current and its effect causing force and eventually the rotation of the rotor. ADVERTISEMENTS:Due to the principle of induction, the currents are induced to flow in the rotor conductors, the motor principle comes into operation, and a force is exerted on each conductor. Earthbound beginnings ana. By applying Flemings Left Hand Rule for motors, it can be seen that, in any conductor, the motor force operates in the opposite direction to that in which the conductor must move to induce the motivating current.In an induction motor, the force acting on each conductor tends to move it in the same direction as that in which the rotating stator field cuts across it.

This phenomenon is explained in Fig. The forces acting on the conductors summed together produce a torque which turns the rotor in the direction of field rotation, and hence the rotor keeps rotating as long as the stator winding are connected to a healthy supply.The torque produced by a motor depends upon the strength of current flowing in the rotor. Heavy currents react with the rotating field to produce a large torque; and, as per the same principle, light currents produce only a small torque.The strength of the current induced in the rotor depends, in turn, on the rate at which the rotating field is sweeping across the conductors, i.e.

Upon the relative motion between rotor and field, which is called slip.In fact a large amount of slip results in heavy induced current, but if the rotor approaches synchronous speed, induced currents are reduced and torque falls off. The rotor can never reach synchronous speed, because at this speed, there is no relative motion between rotor and field, and no torque would be provided.The amount of slip and, therefore, the speed of the motor are directly related to the torque required to drive the load. In a four-pole machine running in a 50 c/s. Supply system and developing say 50 horse-power, the speed of the stator field would be 1500 rpm.Now when running on full load, the speed of the motor would be between 1450 and 1470 rpm., depending upon the efficiency of the motor. However if the load were reduced, the motor would speed up slightly, and at no load, the motor would run just under 1500 rpm., say at about 1490 to 1495 r.p.m.The speed of the motor, therefore, depends primarily upon the synchronous speed of the stator field, and is modified slightly by the load driven.

There is no satisfactory and proven successful means of controlling or varying the speed of a simple induction motor, so that, for all practical purposes, it is a constant speed motor.For this reason, the induction motor has become so popular, as most of the drive needs constant speed. Modern industrial civilization should thank scientist tensely for his invention of induction motor in 1885.4. Starting of Induction Motor:A cage induction motor will start up under load if it is switched directly on to fuller supply voltage. The method of starting is known as direct-on-line (D.O.L.) switching or starting. At the moment of starting, slip (and therefore the induced rotor current) is at its greatest, so that the motor draws a heavy current from the supply until it approaches normal running speed.A cage motor may take from five to six times its normal full load current.All the smaller cage motors used in a mine, such as those in face equipment, are started by direct line switching.

To accommodate the starting current, all the protective devices in the motor circuit are so designed that they will not trip out during the starting period.During the period when the motor is starting and running up to speed, the heavy current taken reduces the power available to the other machines sharing the distribution lines. For this reason the rotors of many underground motors are designed to limit the initial surge of current as much as possible.One method of limiting starting current is to provide the rotor with a double, or even a triple cage. Current can also be limited by careful design of the cage bars.Fig 11.5 shows a sketch of a Double Cage Rotor, and Fig. 11.6 illustrates Sections Rotor Bars generally used in Double Cage Rotors. In-fact, double cage rotor is constructed with a high resistance cage set into the surface of the core, and a low resistance copper cage set well into the core.At the moment of starting, when the rotor is stationary, the frequency of the e.m.f. Induced in the cage bars, which depends upon the difference between the rotor and rotating field speeds, is about 50 c/s. The supply frequency.At this frequency, the copper cage being surrounded by iron has a very high inductive reactance which prevents heavy current from flowing in it.

The current induced in the outer cage is sufficient to allow the motor to start with a high torque (up to twice the normal load torque), but the resistance of the cage limits the starting current.As the motor gathers speed, the difference between the rotor and rotating field speeds is greatly reduced, and the frequency of the induced e.m.f. Becomes much lower. The reactance of the copper cage is therefore very much less, the currents induced in it are consequently stronger (although the induced e.m.f. Becomes much smaller) and the cage takes over the main duty of producing torque.There is also triple cage rotor, which has three separate cages. It starts on a very high resistance cage, and a second intermediate cage takes over before the main running cage finally comes into full operation. There is however another type of rotor with a single cage which operate in a manner very similar to a double cage rotor. It has bars with specially designed cross sections as shown in Fig.

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11.6 showing two possible shapes.A large part of each bar is set deep in the core, and this part has a high reactance on starting. Current flows only in the small sections near the surface which offer a high resistance to heavy currents. The motor therefore starts with a high torque and moderate starting current.As the motor gathers speed, the reactance of the deep set parts of the bars decreases, so that current can flow freely through the whole of each bar. The cage then acts as a low resistance cage.Let us discuss in brief the expressions of starting torque (T s) and starting current (I s), as per the equivalent diagram as shown in Fig.

These expressions are given as they will be helpful to the electrical engineers in understanding the performance and problems of induction motors.If P 1 = Power Input, V 1= Input voltage to stator, and I 1, = input current to stator, and cos φ 1 is the power factor, thenPower input per phaseOut of this the I 1 2R, is dissipated in the stator windings, and the loss (-E 1) I 1 heats the core, due to hysteresis and eddy currents. Here R 1 = Stator Resistance, and E 1 = Stator induced e.m.f. Per phase.Therefore P 1 may be expressed in the following way:The angle between the vectors (-E 1) and (-)I 2 is (as shown in fig.

11.7(b), showing vector diagram of an induction motor) that between E 2 and I 2 in the rotor, shown as φ 2. Since (-E 1) is the voltage component associated with the mutual flux, and (-I 2) is the current component equivalent to the rotor current, then (-E 1,) (-I 2) Cos φ 2must be the power delivered by transformer action to the rotor, i.e.,This can be explained as out of the power delivered to the rotor, the fraction s is used in the rotor itself and lost in the rotor as heat. Now the remaining (1-s)P 2, does not appear in the vector diagram among the rotor quantities.In fact, it is converted into mechanical power, and developed at the rotor shaft, which can therefore be expressed as:P m = (l-s)P 2 (and this includes friction and wind-age power). The whole thing can be expressed as:That is, the rotor power will always be divided in this ratio.

In fact the torque is directly proportional to rotor power input, P 2; and which itself is proportional to stator input, considering the stator losses to be small. Therefore motor input is directly proportional to the torque for a given main flux and stator voltage.5. Starting Equipments for Induction Motors:Starting equipments are required mainly to reduce the starting current of the motors. And this is done with the help of external control equipment. These methods are star-delta starting, and autotransformer starting.These are used sometimes with heavier motors such as those used for driving heavy duty pumps etc. In such motors if direct supply is used to start the motor, due to heavy starting current, the power supply would be disrupted. Star-Delta Starting:A machine designed for star-delta starting (unlike a machine designed for direct line starting or auto-transformer starting) will have the two ends of each phase brought out of separate terminals, giving a total of six terminals for the stator field.

A switch is then connected into the circuit, as shown in Fig. 11.8 so that the stator field connection can be altered by changing the position of the switch.The system operates in this way – the equipment is started up with the stator connected in star; when the machine has reached full speed the switch is changed over, so that the stator windings are connected in delta, and the machine runs throughout its normal operation with delta connection.For any given field winding used the current when the phases are connected in star is less (by) than the current used when the phases are connected in delta. With star connection, the phase to phase voltage is applied to two phase windings in series whereas, with delta connection, full voltage is applied across one phase winding only.Starting current therefore is about twice full load current. Star delta starting also reduces starting torque, to some extent, but it may not be possible to start the motor on full load.During starting as the winding is connected temporarily in star, the phase voltage is reduced to= 0.58 of normal and the motor behaves as if the auto-transformer were employed with a ratio of 0.58. The starting current per phase is I S = 0.58I Sc, the line current is (0.58) 2 x I = 0.33I Sc.

The starting torque is one third of short circuit valueThis method of starting is cheap and effective, so long as the starting torque is not required to exceed about 50 per cent of full load torque. It can be used for machine tools, pumps etc. Stator Resistance Starting: (SRS):As we know from the principles of induction motors, that the output and torque for a given slip varies as the square of the applied voltage. Therefore, any reduction in the voltage applied means the simultaneous reduction of the starting torque.And this principle is followed in the stator resistance starting method by connecting three phase external resistance units in series with the stator terminal. Fig 11.8(a) shows the simple circuit for this type of starting.When the stator input voltage is reduced (by adjusting the external stator resistance unit) from its normal value, say, to the fraction x, the no load and short-circuit currents will be changed in almost the same proportion. But the main flux which, over the range of normal loads, is roughly constant is determined by the applied voltage and will reduce substantially in proportion to the reduced voltage.The magnetizing current will similarly be reduced, so long as the magnetic circuit is not highly saturated.

Moreover, the core losses are proportional roughly to the square of the flux density, and consequently, of the voltage; the active component of the no-load current will be reduced in proportion to the voltage fall.Whereas the short circuit is given by the quotient of applied voltage and short-circuit impedance, there will be a close approximation to a linear function of the supplied voltage. Therefore, if the starting current is reduced by a fraction say, x, of normal value, the starting torque will also reduce by x 2 of its normal value. Auto-Transformer Starter:Starting current can also be reduced by connecting two auto transformers in ‘V’ across the three phases of the stator winding as shown in Fig. The auto-transformers have the effect of reducing the voltage applied to the stator winding, so that the initial current taken by the motor is reduced.When the machine approaches full speed, the auto-transformers are switched out, so that the full supply voltage is then applied to the stator.

Here also, starting torque, to some extent, is reduced. 11.9 shows that the auto-transformer is used to reduce the phase voltage to the fraction x of normal value.

Then the motor current at starting is I s = xl sc, and the starting torque T s = X 2T scThis is exactly the same as the case of putting resistance in stator circuit to reduce the voltage. But in this method the advantage is that the voltage is reduced by transformer, not by the resistance.6. Slipring Induction Motors:Slipring induction motors operate on the same induction principle as the squirrel cage motors.

They, however, differ from squirrel cage motors in the form of rotor employed and in the method of starting. Unlike cage motors, the speed of the slipring motor can be controlled.Generally slipring motors are used for heavy duty, such as driving large compressors, and main haulages, where high power and a close control of starting current are essential. Even in main winder motors slipring motors are used.The stators of the slipring motors are same as those of squirrel cage motors, but the rotor of a slipring motor consists of a three phase winding formed out of copper conductors, and set into a laminated soft iron core.The conductors and the windings are insulated from one another and from the core, and the whole insulation is impregnated with special varnish of electrical grade. One end of each phase winding is connected to a star point within the rotor, the other ends of the windings are brought out to three slipping’s mounted on the rotor shaft.The rotor sliprings are connected to three terminals through three sets of brushes. A starter unit, connected to the terminals, completes the rotor circuit externally.The starter unit consists of three variable resistances connected in star.

It is connected to the three slipring terminals so that each phase of the rotor winding has variable resistance in series with it, as shown in Fig. 11.10.The resistance of the rotor circuit can therefore, be varied by an external control. To start the motor, the resistances are set at their highest value. When the supply to the stator winding is switched on, the motor starts slowly with a high torque and relatively low stator current.The resistances are progressively reduced, thereby permitting the motor to speed up, until the three terminals are, in effect, short circuited and the motor runs at full speed. A slipring motor can be made to run below its maximum speed by leaving parts of the external resistances in series with the rotor windings.The actual speed of the motor will depend upon the load it is driving and the amount of resistance left in circuit. Control over a considerable range of speeds is possible by this method, but care should be taken regarding torque speed characteristics of the motor, otherwise the motor might be damaged. Short-Circuit Gear:A motor which is intended to run continuously at one speed, such as a motor driving a compressor, is sometimes fitted with a mechanism for short circuiting the sliprings, so that the rotor circuit can be completed within the machine.

The brushes may be raised at the same time, so that brush wear is reduced to a minimum.If a machine is fitted with a short circuiting switch, the starter is connected to the rotor only during the actual period of starting as shown in Fig 11.10. When the motor has been run up to speed, the short circuiting switch operated, usually by means of a handle on the side of the slipring enclosure, and the motor then runs as an internally connected machine. Power Factor:All the squirrel cage and slipring induction motors run at a lagging power factor.

Induction motors running on full load usually have power factors between 0.8 and 0.9 depending upon the design of the machine. If a motor drives less than its full load, the power factor deteriorates, below half load it may fall to as low as 0.5 or sometime even lower.7.

Synchronous Motors used in Mines:Like an induction motor, a synchronous motor also consists of a stator with a rotor running within it. The stator, like that of an induction motor, is wound so that, when connected to a three phase alternating current supply, a rotating field is produced. The speed of rotation depends upon the frequency of the supply and the number of poles in the field.The rotor, however, unlike that of an induction motor, has an excitation winding which is energised by a direct current supply. The supply is fed to it by brushes bearing on two sliprings, and the rotor is wound so that a steady polarized field, having the same number of poles as the stator field, is produced.Now when the stator field is energised by a three phase alternating current supply and the rotor is energised by a direct current supply, each pole of the rotor is attracted to an opposite pole of the rotating field.The poles of the rotor, therefore, follow the corresponding rotating poles, so that the rotor rotates at the same speed as the stator field, i.e. It rotates at synchronous speed and therefore this motor is called synchronous motor.

The speed of this type of motor is however invariable. Starting:A synchronous motor, as such, cannot start on its own because it produced no starting torque. Torque in-fact is produced only when the rotor poles are following the poles of the rotating field, so that; before the motor can drive its load, the rotor must already be running at approximately synchronous speed. In order to start a synchronous motor, some method must be employed to run it up to speed before energizing the rotor.Various methods have been used to run synchronous motors up to speed on starting.

One method is to build a small separate induction motor, called a pony motor on the main shaft, but this method is now rarely used. Most synchronous motors in use at collieries have a winding incorporated in the main rotor, so that it can be run up as induction motor, using the main field.The three types of synchronous motor most commonly in use at collieries, are the synchronous induction motor, the auto-synchronous and the cage synchronous motors. In fact these are distinguished by the methods of their starting. Synchronous Induction Motor:One type of synchronous induction motor has a rotor with two windings. One winding is the excitation winding which is connected to the direct current supply via two sliprings. The other winding is a three phase induction winding connected to starting resistances via three further sliprings. The motor therefore has five sliprings as shown in Fig.

11.11 (a).The motor is started as a slipring induction motor, using starting resistances. When the motor has run up to approximately synchronous speed, the direct current exciter supply is switched on and the induction winding is open circuited.In another type of synchronous induction motors, the rotor has a three phase winding with three sliprings. The motor is started like a slipring machine using 7 starting resistances. As the motor approaches synchronous speed, the direct current exciter supply is switched on and the induction winding is open circuited.With some motors, only two sliprings are used by the excitor supply, one phase of the rotor winding being inoperative. Alternatively, in other motors, all three sliprings are used, two phases of the windings being in parallel and third in series as shown in Fig. 11.11(b).Auto Synchronous Motor:An auto synchronous motor is similar to a synchronous induction motor, except that it is designed to eliminate the need for switching as the motor approaches normal running speed. The rotor winding is permanently connected to the excitor through the slipring and brushes.The motor starts as an induction motor, with the rotor circuit completed through the d.c.

As the motor gathers speed, direct current flows in the rotor winding in addition to the induced alternating current. When the rotor reaches synchronous speed, no currents are induced in the rotor, since there is no relative motion between the field and the rotor. Cage Synchronous Motor:The rotor of this type has only the exciter winding brought out to sliprings, but there is also a form of cage embedded in the rotor core. The motor is started as a cage motor. When the motor approaches synchronous speed, the direct current supply is switched on.When the motor is running, the cage acts as a damper winding and prevents any “hunting” i.e. Slight variations in motor speed which can cause vibrations.

Auto-transformer starting is usually employed, but some machines of this type are started by direct on line switches. Excitation Circuit:The excitation current for the rotor is usually obtained from a small exciter generator mounted on the same shaft as the rotor, and forming an integral part of the machine. The only external supply required, therefore, is the normal main supply.A control unit is provided, which enables the current flowing in the rotor winding to be varied. For any given load, a certain minimum excitation current is required. The torque which the motor is capable of producing depends upon the strength of the rotor field. If this field is too weak, it will not develop sufficient torque to drive the load and, as a result, stalling occurs. Power Factor:At minimum excitation, the motor runs at a low lagging power factor, between 0.6 and 0.8, depending upon the load and the design of the machine.

If the excitation current is increased above the minimum necessary to drive the load, the speed and torque remain constant, but the power factor improves.At a certain value of excitation current, unity power factor is achieved. If the excitation current is still further increased, a leading power factor develops, and from thereon, the leading power becomes lower as excitation current is increased. By heavy over-excitation, a synchronous motor can run with a leading power factor as low as 0.6 or less. Uses:Because of their difficult starting characteristics and the fact that their speed is invariable, synchronous motors are used only where a continuous drive at constant speed is required.At collieries, synchronous motors are commonly used for driving the main winder, main ventilation fan and for driving heavy duty compressors. Because of their ability to run at a leading power factor, these motors offer a method of power factor correction for the colliery electrical system.8.

Insulation Resistance of a Induction Motor:Inspection and maintenance of alternating currents at regular intervals is most essential if a mine has to run smoothly. The operation of the regular routine service is given below. However, not all these operations can be performed in-bye or coalface, that is, inside the mine, and for this reason motors used underground at the coalface or in the gate are brought to the surface periodically for a thorough overhaul.The maintenance schedule for each individual motor giving the frequencies of inspection and the checks which must be made on each occasion must be prepared by the colliery electrical engineer considering the importance, and performance of each machine.

And this must be strictly followed by the management as well as by the electricians and operators and the engineers. Inspection of Insulation Resistance:In case of squirrel cage induction motor, the insulation of the stator winding, and in case of slipring induction motor, the insulation resistance of the rotor and also of the slipring, is to be inspected from time to time.

This interval should be set by the colliery electrical engineer considering the operational surrounding and performance of the motors.