Motor Design

ELECTRIC MOTOR DESIGN

AC MOTORS

Most of your work with motors, at shore stations especially, will be with a-c motors. Dc motors have certain advantages but a-c power is more widely used and a-c motors are less expensive and on the whole, more reliable. For example, sparking at the brushes of a dc motor can be very dangerous if there is explosive gas or dust in the surrounding air. On most a-c motors, brushes and commutators are not used and little maintenance is required. They are suited to constant speed applications and are designed to operate at a different number of phases and voltages. A-c motors are designed in various sixes, shapes, and types such as the induction, series, and synchronous, but as a Construction Electrician in the U. S. Navy, you will be concerned primarily with the induction motors. This type of motor includes, among others, the split-phase, capacitor, repulsion-induction, and the polyphase motors.

SPLIT-PHASE MOTORS

A split-phase motor is usually of fractional horsepower. It is used to operate such devices as small pumps, oil burners, and washing machines. It has four main parts. These are the rotor, the stator, the end plates (or end bells, as they are sometimes called), and a centrifugal switch The rotor consists of three parts. One of these parts is the core which is made up of sheets of sheet steel called laminations. Another part is a shaft on which these laminations are pressed. The third part is a squirrel-cage winding consisting of copper bars which are placed in slots in the iron core and connected to each other by means of copper rings located on both ends of the core. In some motors the rotor has a one-piece cast aluminum winding. The stator of a split-phase motor consists of a laminated iron core with semiclosed slots, a steel frame into which the core is pressed, and two windings of insulated copper wire that are placed into the slots and are called the running and starting windings. End bells, which are fastened to the motor frame by means of bolts or screws, serve to keep the rotor in perfect alignment. These end bells are equipped with bores or wells in the center, and are fitted with either sleeve or ball bearings to support the weight of the rotor and thus permit it to rotate without rubbing on the stator.

The centrifugal switch is located inside the motor on one of the end bells. It is used to disconnect the starting winding after the rotor has reached a predetermined speed, usually 75 percent of the full load speed. The action of the centrifugal switch is as follows: the contacts on the stationary part of the switch (the stationary part is mounted on the end bell) are closed when the motor is not in motion and make contact with the starting winding. When the motor is energized and reaches approximately 75 percent of full load speed, the rotating part of the switch (mounted on the rotor) is forced by centrifugal force against the stationary arm, thereby breaking the contact and disconnecting the starting winding from the circuit. The motor is then operating on the running winding as an induction motor. Figure shows the two major parts of a centrifugal switch.

Figure - Two major parts of a centrifugal switch.

The direction of rotation of a split-phase motor may be reversed by reversing the connections leading to the starting winding. This action can usually be done on the terminal block in the motor. Figure shows a diagram of the connections of a split-phase motor.

Troubleshooting and Repair Motors require occasional repairs, but many of these can be eliminated by following a preventive maintenance schedule. Preventive maintenance, in simple terms, means taking care of the trouble before it happens. For example, oiling, greasing, cleaning, keeping the area around the equipment clean, and seeing that the equipment has the proper protective fuses and overload protection are preventive maintenance steps that eliminate costly repairs.

To analyze motor troubles in a split-phase motor, the first check is for proper voltage at the terminal block. If you have the proper voltage, check the end bells for cracks and for alignment. The bolts or screws may be loose and the ends may be out of line. The next check is for a ground With the motor disconnected, check the connections from the terminal block to the frame with an ohmmeter or megger. If you find a ground in this test, remove the end bell with the terminal block and centrifugal switch and separate the starting winding and running winding and make another ground check on each of these windings. In many cases you will find the ground in the loops where the wires are carried from one slot to the next one. This situation can sometimes be repaired without removing the winding. In some cases, the ground may be in the centrifugal switch due to grease that has accumulated from over-greasing. If the first test does not show a ground in the motor, check to see that the rotor revolves freely. If the rotor turns freely connect the motor to the source of power

Figure.- Diagram of the connections of a split-phase motor.

and again check to see that the rotor turns freely when energized. If the rotor turns freely with no voltage applied, but locks when it is applied, you will know that the bearings are worn enough to allow the iron in the rotor to make contact with the iron in the pole pieces. If the trouble is a short, either the fuse will blow or the winding will smoke when the motor is connected to the line. In either event the motor will have to be disassembled A burned winding is easily recognizable by its smell and the burned appearance. The only remedy is to replace the winding. If the starting winding is burned, it can usually be replaced without disturbing the running winding, but check closely to be sure that the running winding is not damaged. In making a check for a shorted coil, the proper procedure is to use an ohmmeter to check the resistance in the coil that you suspect to be bad. Then check this reading against a reading from a coil that is known to be good.

An open circuit can be caused by a break in a wire in the winding, or by the centrifugal switch not closing properly when the motor is at a standstill. Too much end play in the rotor shaft may cause the rotating part of the centrifugal switch to stop at a point where it allows the contacts on the stationary part of the switch to stand open. Should the rotor have more than 1/ 64-inch end play, place fiber washers on the shaft to line the rotor up properly. If the motor windings are severely damaged, the motor must be sent to a motor shop for repairs. The repairs will usually be done in a shop operated by Public Works or the motor may be sent outside the base to a civilian operated motor shop. For this reason only the basic principles of the winding procedure will be covered.

Figure.- The pitch of a coil.

Repair of a split-phase motor with a damaged winding consists of several operations: taking the winding data, stripping the old windings, insulating the slots, winding the coils and placing them in the slots, connecting the windings, testing, varnishing and baking the winding.

Before the motor is taken apart, the end plates should be marked with a center punch so that they may be reassembled properly. One punch mark should be put on the front end plate and a corresponding mark made on the frame. Two marks should be made on the opposite end plate and also on the frame at that point.

Taking the winding data is one of the most important parts of the operation This action consists of obtaining and recording information concerning the old winding; namely, the number of poles, the pitch of the coil (the number of slots that each coil spans), the number of turns in each coil, the size of the wire in each winding, the type of connection (series or parallel), the type of winding, and slot insulation. See figure.

This data is taken while removing the old winding from the motor frame. One coil should be cut at a place where the number of turns may be counted. The size of the wire and other data is then entered on the data sheet.

SPLIT PHASE MOTOR DATA

Figure.- Split-phase motor data sheet.

Clean the old insulation and varnish from the slots before installing the new slot insulators. This cleaning is usually done with a torch. The slot insulators are formed from one of several types of material available for this purpose. The best procedure is to reinsulate the slots with the same type and size insulation that was used in the original winding.

The coils are then wound according to the data sheet and replaced in the slots in the same position as the windings that were removed. The starting windings are ALWAYS placed 90 electrical degrees out of phase with the running windings.

When all the connections between the poles of the windings have been completed and tested and the leads attached, the stator should be placed in a baking oven at a temperature of about 250° F and baked for three hours to remove any trace of moisture. Heating the windings also helps the varnish to penetrate the coils.

The stator is then dipped in a good grade of insulating varnish, allowed to drip for about an hour and then placed in the oven and baked for several hours. When the stator is removed from the oven, the inner surface of the core of the stator should be scraped to remove the varnish so that the rotor will have sufficient space to rotate freely.

Control for a Split-Phase Motor The control switch for a split-phase motor is usually a simple OFF and ON switch if the motor is equipped with an overload device. If the motor does not have this overload device, the switch will be of a type illustrated in figure 7-33. This type of switch has two push buttons; one to start and one to stop the motor. It uses interchangeable thermal overload relay heaters for protection of various size motors. In some cases, a 30- ampere safety switch with the proper size fuse may be used.

CAPACITOR MOTORS The capacitor motor is similar to the split-phase motor, but an additional unit, called a capacitor, is connected in series with the starting winding, These motors may be of capacitor-start or the capacitor-run type. The capacitor is usually installed on top of the motor; but it may be mounted on the end of the motor

Figure - Starting switch for a single-phase motor.

frame, or inside the motor housing, or remote from the motor. Acapacitor acts essentially as a storage unit. All capacitors have this quality and all are electrically the same. The only difference is in the construction The type of capacitor usually used in fractional-horsepower motors is the paper capacitor. This type of capacitor has strips of metal foil separated by an insulator, usually waxed paper. The strips are rolled or folded into a compact unit which is placed in a metal container either rectangular or cylindrical in shape. 'Iwo terminals are provided for connections.

The capacitor-start motor has, like the split-phase motor, a centrifugal switch which opens the starting winding when the rotor has reached the predetermined speed, while the capacitor-run motor does not have the centrifugal switch and the starting winding stays in the circuit at all times. Figure 7-34 shows a capacitor-start motor winding circuit. The capacitor motor provides a higher starting torque with a lower starting current than the split-phase motor.

Figure - Capacitor start motor winding circuit. TROUBLESHOOTING AND REPAIR

The procedure for troubleshooting and repair for the capacitor motor is the same as for the split-phase motor except for the capacitor. Capacitors are rated in microfarads and are made in various ratings, according to the size and type. Acapacitor may be defective due to moisture, overheating or other conditions. In such a case it must be replaced with another one of the same value of capacity. To test a capacitor, remove the motor leads from the capacitor and connect the capacitor in series with a 10-amp fuse across a 110-volt line. If the fuse burns out, the capacitor is short-circuited and must be replaced. If the fuse does not burn out, leave the capacitor connected to the line for a few seconds to build up a charge. Do not touch the terminals after the charging process as serious injury may result from the stored charge. Short the terminals with an insulated handle screw driver. A strong spark should show if the capacitor is good. If no spark or a weak spark results, the capacitor must be replaced.

The procedure for rewinding a capacitor motor is the same as for the split-phase motor except for the capacitor.

UNIVERSAL MOTORS

A universal motor is one that operates on either single-phase ac or dc power. These motors are normally made in sizes ranging from 1/ 200 to 1/ 3 horsepower.

You can get them in larger sizes for special conditions. The fractional horsepower sizes are used on vacuum cleaners, sewing machines, food mixers, and power hand tools. The salient-pole type is the most popular type of universal motor. The salient-pole type consists of a stator with two concentrated field windings, a wound rotor, a commutator, and brushes. The stator and rotor windings in this motor are connected in series with the power source. There are two carbon brushes that remain on the commutator at all times. These two brushes are used to connect the rotor windings in series with the field windings and the power source (fig. 7-35). The universal motor does not operate at a constant speed. The motor runs as fast as the load permits; i. e., low speed with a heavy load and high speed with a light load. Universal motors have the highest horsepower-to-weight ratio of all the types of electric motors.

The operation of a universal motor is much like a series dc motor. Since the field winding and armature are connected in series, both the field winding and armature winding are energized when voltage is applied to the motor. Both windings produce magnetic fields which react to each other and cause the armature to rotate. The reaction between magnetic fields is caused by either ac or dc power.

SHADED-POLE MOTORS

The shaded-pole motor is a single-phase induction motor that uses its own method to produce starting torque. Instead of a separate winding like the split-phase and capacitor motors, the shaded-pole motor's start winding consists of a copper band across one tip of

Figure - Universal motor schematic.

each stator pole. This copper band delays the magnetic field through that portion of the pole. When ac power is applied, the main pole reaches its polarity before the shaded portion of the pole. This action causes the shaded poles to be out of phase with the main poles and a weak rotating magnetic field is produced. Because of the low-starting torque, it isn't feasible to build motors of this type larger than 1/ 20 horsepower. They are used with small fans, timers, and various light-load control devices. Remember, all single-phase induction motors have some auxiliary means to provide the motor with starting torque. The method used for this starting torque depends on the application of the motor.

CONNECTING THREE-PHASE MOTORS

Connecting a three-phase motor is a simple operation. All three-phase motors are wound with a number of coils, with a 2-to-1 ratio of slots to coils.

These coils are connected to produce three separate windings called phases, and each must have the same number of coils. The number of coils in each phase must be one-third the total number of coils in the stator. Therefore, if a three-phase motor has 36 coils, each phase will have 12 coils. These phases are usually called Phase A, Phase B, and Phase C. All three-phase motors have their phases arranged in either a wye connection or a delta connection.

WYE CONNECTION

A wye-connected three-phase motor is one in which the ends of each phase are joined together paralleling the windings. The beginning of each phase is connected to the line. Figure 7-42 shows the wye connection.

DELTA CONNECTION

A delta connection is one in which the end of each phase is connected in series with the next phase. Figure 7-43 shows the end of Phase A connected to the beginning of Phase B. The end of Phase B is connected to the beginning of Phase C, and the end of Phase C is connected to the beginning of Phase A. At each connection, a wire is brought out to the line

Figure.- Star, or wye, connection.

VOLTAGES Most small-and medium-sized three-phase motors are made so that they can be connected for two voltages.

The purpose in making dual-voltage motors is to enable the same motor to be used in facilities with different service voltages. shows four coils which, if connected in series, may be used on a 460-volt ac power

Figure.- Four 115-volt coil connected in series to produce 460 volts

Figure- Four 115-volt coil connected in parallel for 230 volts;

each coil still receives only 115 volts. supply. Each coil receives 115 volts. If the four coils were connected in two parallel sets of coils to a 230-volt line, as shown in figure 7-45, each coil would still receive 115 volts. So, regardless of the line voltage, the coil voltage is the same. This is the principle used in all dual-voltage machines. Therefore, if four leads are brought out of a single-phase motor designed for 460/ 230 or 230/ 115-volt operation, the motor can be readily connected for either voltage.

Dual-Voltage Wye Motor When you are connecting a dual-voltage wye motor, remember practically all three-phase dual-

Figure .- Terminal markings and connection for a wye-connected dual-voltage motor.

voltage motors have nine leads brought out of the motor from the winding. These are marked TI through T9, so that they may be connected externally for either of the two voltages. These are standard terminal markings and are shown in figure 7-46 for wye-connected motors.

HIGH VOLTAGE.- to connect for high voltage, you should connect groups in series, as shown in figure 7-47. Use the following procedure:

1. Connect T6 and T9; twist and wire nut.

2. Connect leads T4 and T7; twist and wire nut.

3. Connect T5 and T8; twist and wire nut.

4. Connect leads TI, T2, and T3 to the three-phase line.

MOTOR MAINTENANCE, TESTING, AND REPAIR

An electric motor must be checked, maintained, and repaired just like any other piece of mechanical equipment. With proper servicing, a motor will last longer and give more efficient service. Included in maintenance services are cleaning, lubrication, ventilation, and testing.

Cleaning Inspect motors internally and externally for foreign materials, such as dust, dirt, corrosion, and paint. Open-frame motors may be blown out with compressed air. You should not apply too many coats of paint to motors. A thick coat of paint will interfere with heat dissipation.

CAUTION Air pressure used for cleaning should not exceed 25 psi nozzle pressure. Excessive pressure can damage the insulation on the windings.

Wipe all excess dirt, grease, and oil from the surfaces of a motor with a cloth moistened with an approved solvent.

WARNING Do not use flammable or toxic solvents when cleaning motors. Solvents may cause injury to personnel or damage to equipment.

Lubrication Lubrication should be done according to the manufacturer's instructions. Improper lubrication causes motor bearings to overheat and eventually causes bearing failure. Check a motor for signs of grease and oil-seal failure. If an inside seal fails, the lubricant can get into the motor windings and deteriorate the insulation. This condition also allows dust to adhere to the windings and restricts air circulation, then the motor windings heat and burn out. Inadequate lubrication causes the bearings to wear excessively and, eventually, to seize. When lubricating a motor, refer to the manufacturer's manual to determine the correct type of lubricant to use. Some motors have bearings lubricated with oil, while others require grease. Many motor bearings are lubricated and sealed at the factory and usually last the life of the bearing.

Ventilation Check the running temperature of all motors. If the motor temperature is hotter than specified on the nameplate, you must find the problem. The normal procedure for diagnosing motor overheats is to check the motor for restricted ventilation. Inspect the area around the motor for any obstructions which could hamper free air circulation. If air circulation is not hampered in any way and the motor continues to run hot, reduce the load on the motor or use a motor with more power capability.

Testing The proper testing of a motor should be done in a logical sequence. Proper testing can prevent unnecessary labor and parts. Testing motors is generally classed under two major methods: visual tests and operational tests.

VISUAL TESTS.- A visual test can discover a great deal about the condition of a motor and the possible causes of trouble. Read the nameplate data and be sure that the motor connections are correct for the supplied voltage.

Look at the windings to see if the insulation has overheated (or has been overheating). You can tell when the insulation is burned by the odor within the motor. If you aren't sure of the condition of the windings, test them with a megger to determine if the windings have been damaged beyond use. Connect the leads of the megger to each set of windings.

CAUTION Disconnect the motor leads from each other to ensure reading only one winding at a time. If the winding is good, you will get a reading of continuity. If the winding indicates a large amount of resistance, it is damaged and must be replaced.

Now connect one lead from the megger to the frame of the motor. Connect the other lead of the megger to each lead of the motor, one at a time. A low-resistance reading means insulation breakdown or a short to the motor frame, and replacement of the winding is necessary.

Inspect the commutator for solder thrown from the risers, and for loose, burned, high, and flat bars. Also test for high mica. Notice the surface film on both the commutators and slip rings and the general condition of the brushes. Check the air gap on large motors for any indication of bearing wear or misalignment. For large motors, take an air gap measurement at one reference point on the rotor or armature; then rotate the rotor or armature and measure four points on the stator or field frame to the same reference point. The air gap measurement should be within plus or minus 5 percent at any of these points.

Check the condition and operation of the starting rheostat in dc motors and the starting and control equipment used with ac motors. Also check the terminal connections on all of the control equipment to ensure they are correct and secure. Make sure the proper voltage is at the terminal lead of the motor.

If the visual tests have not revealed the trouble, you should perform some operational tests on the motor.

OPERATIONAL TESTS.- Perform a heat run test, observing the manufacturer's recommendations for that particular motor.

CAUTION Do not attempt to operate a series dc motor without a load.

If the temperature of the motor in normal operation does not exceed the maximum recommended by the manufacturer, the motor is operating satisfactorily.

Always refer to the manufacturer's manual for definite specifications for the motor you are inspecting.

WARNING Be sure the master switch is in the off position before connecting or disconnecting any motor lead connections. Because of their effect on insulating materials, high temperatures shorten the operating life of electric motors. When the windings or the bearings of a motor, not specifically designed for high temperature service, get hotter than 90 degrees centigrade, investigate the operating conditions and relieve the temperature conditions by cooling or relocating the motor. A gradually rising temperature in a motor warrants a shutdown and thorough examination of the unit. The nameplate on the motor usually specifies its normal running temperature in degrees centigrade. Check the current draw of the motor against the data on the nameplate. Excess current causes heating and, in time, will destroy the windings.

Check the motor for proper speed. A speed above or below that indicated on the nameplate signifies a malfunction in the unit. When a motor's operation is sluggish, check the line voltage to the motor. If you find the voltage low, apply the proper value and continue checking to determine if the motor is overloaded. If it is, reduce the load or replace the motor with one of a larger horsepower. There are other conditions which could make motor operation sluggish. You may find that the brushes have shifted off NEUTRAL, and you must reset them. You may also find that the armature or rotor is dragging on the stator or field poles. To correct this situation, you may need new bearings. Afield pole may be loose, causing it to drag on the armature or rotor.

Other conditions which could cause a motor to be sluggish are shorted field-winding circuits, shorted armature windings, and surface leaks across the commutator segments. After finding the fault in the motor, you may have to replace it. When you replace it, be sure to install a motor of the same size.

CAUTION Be sure to de-energize the motor circuit before disconnecting the unit.

While the motor is running, look for any sparking at the brushes. Many faulty conditions contribute to sparking brushes at the commutator. The two major causes are a faulty armature and malfunctioning brushes. Some of the faults that could develop in an armature include the following: rough commutators, bent armature shafts, and short circuits in the armature windings. Brushes may malfunction because they are off NEUTRAL, they bind in the brush holders, they are wound beyond recommended limits, or they intermittently fail to contact the commutator because of insufficient brush spring tension. Whenever a motor is arcing at the brushes, it is advisable to disassemble it, locate the problem, and make the necessary repairs, There are many causes of motor noise. Listen and feel for any unusual noises. You should first check the motor-mounting bolts for looseness and the alignment of the motor with the driven equipment. If the motor is secure and properly aligned, continue your inspection. Check the motor's balance. Also inspect the motor for loose rotor bars or a bent shaft. If any of these conditions exist, you will have to replace the rotor or armature. Sometimes the centrifugal switch rattles or rubs the interior of the motor housing. Align the switch and tighten the mounting bolts. If the switch has excessive wear, replace it. Check all motor accessories for looseness and tighten as needed. Check the drive pulley and the condition of the belts. Loose pulleys rattle and will damage belts. You will hear a distinct slap when the belt has been damaged.

Motor Repair After you have performed visual and operational tests on a motor and isolated the problem, you may have to disassemble the motor to make the repairs. You should know the procedures and precautions for motor repair.

Reference :
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