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MAINTENANCE OF ELECTRIC MOTORS

Inevitably, the maintenance of electric motors will require maintenance personnel to know the

connections of motors. The next few sections will provide a summary of how the internal

connections are made in the 3-PH induction motor.

In the following discussion, we shall assume a 36-coil, four-pole, lap-wound, three-phase motor.

All three-phase motors are wound with a number of coils, usually as many coils as slots.

These coils are so connected as to produce three separate windings called phases, each of

which 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.

The number of coils in each phase is given by the total number of coils in the motor

divided by the number of phases.

o A 36 coil motor will have 36 coils ÷ 3 = 12 coils per phase phases.

All three-phase motors have their phases arranged in either a wye (Y) connection or a delta (∆)

connection.

A wye-connected three-phase motor is one in which the ends of each phase are joined

together.

The beginning of each phase is connected to the line.

Figure 8.3 shows the wye connection.

Because of the pattern formed by the phases in the diagram, this circuit is also called a Y

(wye) connection (actually an inverted Y).

Figure 8.3. Diagram of a star connection. 8.4. Diagram of a delta connection

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A delta connection is one in which the end of each phase is connected to the beginning

of the next phase.

Figure 8.4 shows the end of the A phase connected to the beginning of the B phase. The

end of the B phase is connected to the beginning of the C phase, and the end of the C

phase is connected to the beginning of the A phase.

At each connection, a wire is brought out to the line.

Another way is to connect the end of A to the beginning of C, the end of C to the

beginning of B, and the end of B to the beginning of A.

Poles.

Motors coils are connected to produce a number of poles

The number of coils in each pole is given by, the total number of coils divided by the

number of poles.

o A 36-coil, four-pole motor has 36 coils ÷ 4 poles = 9 coils per pole

o Each pole consists of nine coils

Group.

A group is a definite number of adjacent coils connected in series.

The number of groups in each phase is equal to the number of poles.

In all three-phase motors there are always three groups in each pole, one from each

phase.

Thus, one group is from phase A, another group from phase B, and a third group from

phase C.

Therefore, if a pole has nine coils, there must be three coils in each group.

This section of three coils is often called a pole-phase group or polegroup.

The coils of anyone group are always connected in series.

The total number of groups is equal to the number of poles multiplied by the number of

phases.

In the motor being discussed, 4 poles x 3 phases = 12 groups.

If the number of groups is known, it is easy to determine the number of coils in each group.

Wye Connection. A schematic diagram of a three-phase, four-pole, series-wye (1Y) motor is shown in Figure 8.5a.

It should be noted that:

Each phase has four groups.

Each group under a phase represents the coils under a pole.

Each phase therefore has four poles.

The diagram labels one group of phase A. The coil shown actually represents 3 coils.

The groups of a phase are connected in series with each other.

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Each group represents the coils under a pole.

Each pole has one group of each phase under it.

The latter two points are illustrated in figure 8.5b. This is a circular representation of the same

information with the coils place around the circumference of the motor showing their true

position in the motor.

The wye point indicates that it is a wye-connected motor. The diagram also shows that the

groups in a phase are connected in series. Therefore, the schematic diagram indicates that the

motor is a three-phase, four-pole, series-wye (1Y) connection. The term 1Y indicates that the

groups in each phase are series connected providing only one path for current flow through the

phase winding.

Delta Connection.

The same motor will next be connected as a four-pole, series-delta-connected motor. Figure 8.6a

shows that:

The groups are connected in series.

Because there are four groups in each phase, that it is a four-pole motor.

Because it has no wye point and is connected by joining the end of the A phase to the

beginning of the C phase, and so on, it is delta connected.

This is a three-phase, four-pole, series-delta (1∆) connection.

Because this is a three-phase, four-pole motor, it will have 3 phases x 4 poles = 12 groups

of three coils each.

Each group has three coils connected in series.

The diagrams of Figure 8.6 show that the procedure in connecting either a wye or delta motor is

the same except for the point at which the ends of the phases are connected.

Figure 8.5a. Schematic diagram of a four pole, 8.5b Circular diagram of a four

series star connection. pole, series star connection.

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For a wye connection, the ends of each phase are connected together for a wye point

For a delta connection, the ends of each phase are connected to the beginning of another

phase.

Parallel Connections.

Many three-phase motors are designed so that each phase has two circuits or two paths for the

current to travel. These are called two-circuit, or two-parallel, connections. The schematic

diagram of a two-parallel wye (2Y) connection is given in Figures 8.7a and Figure 8.7b shows a

circular diagram of the same motor.

The parallel connection of the groups in each phase provides two paths for the current to

follow.

There are four groups in each phase, and this forms a four-pole motor.

Figure 8.8 shows a 2-delta connection.

8.6a Schematic diagram of a four pole, 8.6b Circular diagram of a four

series delta connection pole, series delta connection

Figure 8.7. A four-pole, two- parallel (2Y) connection

(a) Schematic diagram (b) Circular diagram.

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Connecting Three-phase Motors for Two Voltages.

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

either of two voltages. The purpose in making motors for two voltages is to enable the same

motor to be used in localities that have different power-line voltages.

To accommodate this, the pole group leads from each phase is connected (soldered) to insulated

flexible wires and brought of the motor as required for specific external connections. Usually the

leads external to the motor are connected to provide a series connection for the higher voltage

and a two parallel connection for the lower voltage.

Figure 8.9 shows four coils of a phase winding. Each coil is capable of withstanding 115 V. If

the coil are connected in series, the motor may be used on a 460-volt, ac power supply. Each coil

receives 115 volts. If the four coils are connected in two parallel to a 230-volt line, as shown in

Figure 3-72, each coil still will receive 115 volts.

Figure 8.8. A four-pole, two-parallel delta (2∆) connection.

Figure 8.9 Four coils of a phase winding.

Figure 8.10 Four coils of a phase connected 2-parallel

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Figure 8.11 shows a four-parallel connection for a 115-volt operation of the motor. Each coil still

receives 115 volts. Regardless of the line voltage, the coil voltage is the same.

This principle of voltage dividing between the coils is applied to a three-phase, four-pole, motor

in Figures 8.5 to 8.8 above. If the higher voltage is required, the coils are connected in series-

connected as in figures 8.5 and 8.6. If it is used on a low voltage line, it will be connected for

two parallel, as shown in Figure 8.7 and 8.8.

Practically all three- phase, dual-voltage motors have nine leads brought out of the motor from

the winding. These are marked T1 through T9, so that they may be connected externally for either

of two voltages.

Connecting a Two-Voltage (Dual-Voltage) Wye Motor.

The motor may be connected for low voltage to give one or two star points as shown in figure

8.12. Both diagrams are correct.

Figure 8.11 Four coil connected 4-parallel.

Figure 8.12. Alternate ways of connecting the wye point in a two wye motor.

(a) Conning to give one star point. (b) Connecting using two star points

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The standard terminal markings for wye-connected motors are shown in Figure 8.13. There are

four circuits in this motor

three circuits of two terminals

one circuit of three terminals.

This information will be used later for testing.

Figure 8.13. Markings and connections for wye connected, dual voltage motors.

Figure 8.14 The spiral method of finding the proper numbers for a

nine-lead, one and two wye schematic.

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The motor has three coil group ends internally connected to give one star point. The other ends

of the groups are brought out as terminals for external connections. An easy way to determine

the numbering system of a wye schematic is the spiral method as shown in Figure 8.14.

Starting at T I, draw a line through T 2 and T 3.

Then drop down to the next lead of the A phase, T 4, and go through T 5 and T 6.

Continue on to the third lead of the A phase and complete the spiral from T7 through T8

and T9

Each phase has a two-section winding

These sections may be connected in series for the higher voltage and in parallel for the

lower voltage.

To connect for the high voltage, connect groups in series, as shown in Figure 8.15. Use the

following procedure:

Connect leads T6 and T9, connect leads T 4 and T 7; connect leads T 5 and T 8.

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

To connect this same motor for the low voltage, the groups are connected in two parallel, as

shown in Figure 8.16. Use the following procedure:

Connect lead T 7 to T 1 and to line lead L1;

Connect lead T 8 to T 2 and to line lead L2;

Connect lead T 3 to T 9 and line lead L3;

Connect T 4, T 5, and T 6 together to form an external wye.

Figure 8.15 A two-voltage star motor with groups

connected in series for high voltage.

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Connecting a Two-Voltage Delta Motor.

Refer to Figure 8.17 for the standard terminal markings of a dual-voltage, delta-connected motor.

Note that a dual-voltage, delta-connected motor has three circuits of three terminals each.

Figure 8.16 A two-voltage star motor with groups

connected in parallel for low voltage

Figure 8.17. Standard markings and connections of a delta

connected dual voltage motor.

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Figure 8.18 shows how the numbering is done with a delta schematic.

Figure 8.19 shows a schematic diagram for both high and low-voltage connections.

For a high-voltage operation:

Connect lead T 4 to T 7; connect lead T 5 to T 8 connect lead T 6 to T 9;

Connect leads T1, T 2, and T 3 to L1, L2, and L3, respectively.

For a low-voltage operation:

Connect leads T1, T7, and T6 to the line lead L1;

Connect leads T2, T4, and T8 to line lead L2;

Connect leads T3, T5, and T 9 to line lead L3.

Figure 8.18 The spiral method of finding the proper numbers for

a nine-lead, one and two delta schematic

(a) (b)

Figure 8.19 A two-voltage delta connection with groups (a) in series for

high-voltage operation. (b) Parallel for low-voltage.

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Wye-Delta, Dual-Voltage.

Some motors are designed so that they may be connected in delta for low voltage and in wye for

high voltage. The voltage ratio between high and low should be √3 to 1. Figure 8.20 shows the

terminal markings for this type of motor. Note that six leads are brought out of the motor, two

from each phase.

Figure 8.20

This connection is also used for starting large motors or, with smaller motors, when reduced

torque on starting is needed.

The motor is started with a controller that connects the windings as wye for starting.

It is then switched to delta for running.

There is less inrush current when the motor starts on the wye connection.

When there is less current, there is also less torque.

The motor has full power when the controller connects its windings as delta for running.

Any delta- connected motor can be converted to a wye-delta.

The end of each phase is disconnected from its respective line; a lead is put on it; and it is

brought out of the motor.

The voltage ratio between wye and delta is √3, or 1.73.

This means that if a wye-connected motor is mistakenly connected delta, the windings

would receive 1.73 times as much voltage as they are designed for.

The motor would draw excessive amperes and soon bum out.

If a delta-connected motor were mistakenly connected wye, the windings would receive

voltage applied divided by 1.73, or times 0.58.

A fifty-eight percent reduction in voltage would cause the motor to have much less power

than its rating and the amperes at no load would be very low.

Voltage L1 L2 L3 Tie Together

High T1 T2 T3 T4T5T6

Low T1 T6 T2T4 T3T4

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Connection of multi-speed motors.

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Identifying the nine leads 3-PH, dual-voltage wye connected motors.

The following equipment is used for this test:

An ac voltmeter. Scale up to 460 volts.

A source of three-phase current-208, 220, or 230 volts.

A circuit tester, a test lamp or buzzer, and a battery.

Testing for the Four Circuits

If there is any doubt about the condition of the winding, the circuits should be tested for

shorts and grounds.

Test the nine leads for complete circuits using the buzzer, lamp, or other circuit tester.

o If there are four circuits-three of two leads and one of three leads-this motor must

be wye connected.

o If the test shows three circuits of three leads each, this will be a delta-connected

motor.

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Note that the circuits will be

o T 7, T 8, T 9 -the internal wye;

o T 1-T 4; T 2-T 5; and T 3-T 6.

Ball Bearings

Ball bearings have excellent characteristics for their use in electric motors:

A wide selection of seals and protective shields.

All angles in smaller motors.

Can be permanently greased.

Special grease for high temperatures.

Allow manufacturers to use smaller air gap.

Ease of installation (no reaming).

Fewer items required in inventory because of their standard size.

Withstand high speeds very well.

The components of a ball bearing are:

The outer race.

The balls.

The spacing strap.

The inner race.

The shield, seal, or a combination of both.

The lubrication.

The outer race provides a track for the balls to carry the load and to retain them as they

go over the top.

The balls carry the load.

The spacing strap keeps the balls spaced evenly.

The inner race contains the shaft and moves in an electric motor.

The shield keeps the grease in place and will resist some contaminants.

The seal is designed to keep out most contaminants.

A bearing can have shields, seals, or a combination of both. Figure shows these

components.

The lubrication is usually grease.

Grease is available in low, normal, and high temperature ratings.

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Ball bearings have an estimated life based on their running time, load weight, and rpm. Their life

can be shortened by many conditions, among which are:

heat,

vibration,

impact-type loads,

contaminants,

misaligmttent,

overloading,

carrying electrical current,

too little or too much grease.

Bearings should be changed when they become noisy or loose. The shaft should have no up-

down movement and should have very restricted end movement. Some shops change the

bearings on all motors that are repaired, with few exceptions.

The ideal way to remove a ball bearing is to pull evenly on only the inner race with a bearing

puller or press. But most motors are constructed so that this is not possible, and so the only place

to grip the bearing is the outer race. Care must be taken to keep the bearings clean. The thrust

washers and space washers must be reassembled in the order in which they were originally

placed.

The replacement bearing should be selected in accordance with the motor's operating

conditions.

It may be necessary to choose a sealed bearing because of dust or dirty conditions.

If the ambient temperature is high, the choice is a bearing with high-temperature grease.

In the case of high temperature, a fit-free bearing can be used.

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o The fit-free bearing is designed with extra tolerance for heat expansion; an

electric motor produces uneven heating.

o The extra tolerance of the fit-free bearing may allow the shaft to slip in the inner

race, or it may allow the outer race to slip in the end bell. In this case, a special

liquid plastic is used. The plastic cures to a semihard state and will not allow the

components to slip. The semihard plastic will absorb the expansion with little

pressure on the bearing, and it is available at electric motor parts dealers.

The shaft transfers heat to the bearing and the bearing to the end bell.

The end bell has more cooling ability and will not expand at the same rate.

This puts a lot of pressure on the outer race of the bearing and results in early failure.

But the fit-free bearing should handle this expansion problem.

The shaft and end bell must be inspected for wear caused by the bearing's slipping. If there is

wear, the worn area must be rebuilt exactly to the original size, for if there is misalignment, the

replacement bearing will overheat.

After removing the old bearing, cleaning and inspecting the parts, and selecting the new bearing,

the new bearing can be installed. Several methods are used to do this:

1. Heating the bearing with oil.

2. Heating the bearing dry.

3. Pressing on the bearing with special bearing tubes.

4. Hammering the bearing into place with special bearing tubes and a lead hammer.

Heating the bearing in clean oil will uniformly expand the whole bearing. Once expanded, the

bearing is slipped nonstop into place on the shaft. Once the bearing has stopped moving on the

shaft, it will shrink to its original size. If it is not in place, it will have to be pressed or hammered

into place. Handling the bearing in hot oil can be hazardous, and so use caution.

Dry heating the bearing works much the same as with oil.

The bearing is slipped nonstop into place after it is expanded.

The bearing is pressed onto the shaft by applying pressure on the inner race only.

A piece of pipe that has an inside diameter slightly larger than the inside of the bearing's inner

race can be used for this.

The pressure must be applied uniformly to the bearing's inner race. Also, care must be taken to

align the bearing properly with the shaft.

A lead hammer and pipe or tube can be used to drive the bearing into place.

The pipe or tube will distribute the blow evenly on the inner race of the bearing.

Make sure the bearing is aligned properly with the shaft.

The soft lead will reduce the shock of the blow, and the weight of the lead will move the bearing

on the shaft. However, a hard blow from a steel hammer can damage a bearing.

The outer race should never be used to drive the bearing onto the shaft.

Lubrication is used to reduce the friction between the components of a ball bearing.

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A sleeve bearing must have a film of oil between the shaft and the bearing, or else the bearing

will wear very rapidly.

Grease is used in a ball bearing to reduce the heat produced by the friction between the balls and

the rest of the components.

In ball-bearing electric motors that are built to allow for greasing, the end bells have two

passages. The grease is forced into one passage, pushing the old grease out the other passage.

This is done, if possible, while the motor is running. The bearing cavity should be one-third to

one-half full when filled properly.

If too much grease is left in the bearing cavity, the bearing will chum the grease and cause it to

overheat. Overheating the grease will cause it to separate and break down. When overgreased,

the excess grease sometimes is forced into the inside of the motor and will create many problems

there.

Common Troubles and Repairs

The symptoms encountered in defective three-phase motors are given below. Under each

symptom are listed the possible troubles. The number in parentheses after each trouble indicates

the correspondingly numbered remedy.

1. If a three-phase motor fails to start, the trouble may be

a. Burned-out fuse (1).

b. Worn bearings (2).

c. Overload (3).

d. Open phase (4).

e. Shorted coil or group (5).

f. Open rotor bars (6).

g. Wrong internal connections (8).

h. Frozen bearing (9).

i. Defective controller (10).

j. Grounded winding (11).

2. If a three-phase motor does not run properly, the trouble may be

a. Burned-out fuse (1).

b. Worn bearings (2).

c. Shorted coil (5).

d. Reversed phase (12).

e. Open phase (4).

f. Open parallel connection (13).

g. Grounded winding (11).

h. Open rotor bars (6).

i. Incorrect voltage (7).

3. If the motor runs slowly, the trouble may be

a. Shorted coil or group (5).

b. Reversed coils or groups (8).

c. Worn bearings (2).

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d. Overload (3).

e. Wrong connection (reversed phase) (12).

f. Loose rotor bars (6).

4. If the motor becomes excessively hot, the trouble may be

a. Overload (3).

b. Worn bearings (2) or tight bearing (9).

c. Shorted coil or group (5).

d. Motor running on single phase (4).

e. Loose rotor bars (6).

1. Burned-out Fuse. Remove fuses and test them. To test fuses without removing them from the holder, a

voltmeter must be used. If a test light designed for 230 volts is mistakenly used on 460 volts,

it will blowout and may trigger a severe electrical explosion. If the fuse is open, there will be

a line voltage read across it.

If the fuse bums out while a three-phase motor is in operation, the motor will continue to

operate as a single-phase motor. This means that only part of the winding is carrying the

entire load. If the motor continues to operate in this manner, even for a short time, the

winding will become very hot and bum out. Further, the motor will be noisy in operation and

may not pull the load. To find the trouble, stop the motor and try to start it again. A three-

phase motor will not start with a burned-out fuse. To remedy this condition, locate and

replace the defective fuse.

If the motor is a parallel-connected wye, current will be induced in the open phase and cause

the winding to bum out quickly. This should be prevented if possible.

2. Worn Bearings. If a bearing is worn, the rotor will ride on the stator and cause noisy operation. When the

bearings are so worn that the rotor rests firmly on the core of the stator, rotation is

impossible. To check a small motor for this condition, try moving the shaft up and down,

Motion in this manner indicates a worn bearing. Remove and inspect the rotor for smooth,

worn spots. These indicate that the rotor has been rubbing on the stator. The only remedy is

to replace the bearings.

On a large open motor, the check for worn bearings is made with a feeler gauge. The air

space between the rotor and the stator must be the same at all points. If it is not, the bearing

must be replaced.

3. Overload. To determine whether a three-phase motor is overloaded, remove the belt or load from the

motor and turn the shaft of the load by hand. Usually a broken part or dirty mechanism will

prevent the shaft from moving freely.

Another method is to use an ammeter on each line wire. A higher current reading than on the

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nameplate may indicate an overload.

Many shops and motor repairpersons use a snap-around volt ammeter and ohmmeter to test

the current in the main line leads feeding the motor. The current in each lead should be the

same and approximately the same as the nameplate reading. An excessive reading in one

phase indicates a shorted phase.

This instrument can be used on all motors from split-phase through three-phase and can be

used to test voltage, resistance, and current. It can be used to test unmarked leads on split-

phase motors by using the ohmmeter and also to test voltage across components in motors

and starters.

4. Open Phase. If an open occurs while the motor is running, it will continue to run but will have less power.

An open circuit may occur in a coil or group connection. The motor will continue to run if a

phase opens while the motor is in operation but will not start if at a standstill. The conditions

are similar to those of a blown fuse.

5. Shorted Coil or Group. Shorted coils will cause noisy operation and also smoke. After locating such defective coils

by means of the eye or balance test, the motor should then be rewound.

When the insulation on the wire fails, the individual turns become shorted and cause the coil

to become extremely hot and burn out. Other coils may then burn out, with the result that an

entire group or phase will become defective.

6. Open Rotor Bars. Open rotor bars will cause a motor to lose power. One sign of open bars is when a motor is

connected to the right voltage at no load, it has a very low amp reading. A light load will pull

down the speed, and at full load the motor will run below the nameplate speed. This high

amount of slip will cause the motor to overheat because of the high current. Open or cracked

rotor bars are hard to locate visually in a cast-aluminum rotor.

Some special-duty motors or large motors have brass or copper bars. It is possible for these

bars to be open or loose in the end rings. Loose bars are repaired by soldering or welding

them to the end rings. There must be a good electrical connection between the bars and the

end rings. Broken bars must be replaced. The bars usually break because of a loose fit in the

rotor slots. The bars will move and vibrate when the motor starts and runs, causing them to

crack and break.

7. Incorrect Voltage. Some T-frame motors are designed for a definite voltage. Thus a motor designed for 208

volts will overheat when operated on 250 volts, and a motor designed for 250 volts will not

have enough power if operated on 208 volts. If the motor is rated 208-220-440 volts on the

nameplate, it will operate well on a range of voltages. Voltage problems become more

serious when the motors are loaded to their rated horsepower. If there are problems with a

motor designed for the wrong voltage, it should be replaced with one of the right voltage.

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8. Wrong Internal Connections.

A good method of determining whether or not a polyphase motor is connected properly is to

remove the rotor and place a large ball bearing in the stator. The switch is then closed to

supply current to the winding. If the internal connections are correct, the ball bearing will

rotate around the core of the stator. If the connections are incorrect, the ball bearing will

remain stationary.

For medium- and large-sized motors, reduced voltage should be used; otherwise, a fuse may

blow.

9. Frozen Bearing.

If oil is not supplied to the part of the shaft that rotates in the bearing, the shaft will become

so hot that it will expand sufficiently to prevent movement in the bearing. This is called

a frozen bearing. In the process of expansion, the bearing may weld itself to the shaft and

make rotation impossible.

To repair, try to remove the end plates. The end plate that cannot be removed easily contains

the bad bearings. Remove the end plate and armature as a unit; hold the armature in a

stationary position, and turn the end plate back and forth. If it is impossible to move the end

plate, loosen the setscrew that holds the bearing in the housing, and try to remove the

armature and bearing as a unit. Be careful to keep the oil ring free from the bearing while this

is being done. The bearing can then be removed by tapping it with a hammer. The shaft will

probably have to be turned down on a lathe to a new size and a new bearing made. If ball

bearings are used, replace with new ones.

10. Defective Controller.

If the contacts on the controller do not make good contact, the motor will fail to start.

11. Grounded Winding.

This will produce a shock when the motor is touched. If the winding is grounded in more

than one place, a short circuit will occur which will burn out the winding and perhaps blow a

fuse. Test for a grounded winding with test lamp and repair by rewinding or by replacing the

defective if coil. It

12. Reversed Phase. This will cause a motor to run more slowly than the rated speed and produce an electrical

hum indicative of wrong connections. Check the connections and reconnect them according

to plan.

13. Open Parallel Connection. This fault will produce a noisy hum and will prevent the motor from pulling full load. Check

for complete parallel circuits.

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How to Recognize a Connection.

Read and record the nameplate data. This will usually tell you if the motor is wound and

connected for single or two speed, single voltage or dual voltage, and sometimes wye or delta.

The speed is always recorded on the nameplate..

If the motor is connected for two voltages (dual voltage), nine leads are brought out and these

may be connected in series or in parallel and as wye or delta. If the motor is a two-speed motor,

only six leads may be brought out. Thus, if the schematic diagram of the above motors is

mentally pictured, little trouble should be encountered in determining the connection.