Archive for the ‘Motor’ Category

Induction Motor Control Theory (1)

December 15, 2008
Induction Motor Design has a major effect on the behaviour and performance of an induction motor. Very often the details or class of design of a motor are not well understood or promoted.

1. Stator design
The stator is the outer body of the motor which houses the driven windings on an iron core. In a single speed three phase motor design, the standard stator has three windings, while a single phase motor typically has two windings. The stator core is made up of a stack of round pre-punched laminations pressed into a frame which may be made of aluminium or cast iron. The laminations are basically round with a round hole inside through which the rotor is positioned. The inner surface of the stator is made up of a number of deep slots or grooves right around the stator. It is into these slots that the windings are positioned. The arrangement of the windings or coils within the stator determines the number of poles that the motor has. A standard bar magnet has two poles, generally known as North and South. Likewise, an electromagnet also has a North and a South pole. As the induction motor Stator is essentially like one or more electromagnets depending on the stator windings, it also has poles in multiples of two. i.e. 2 pole, 4 pole, 6 pole etc. The winding configuration, slot configuration and lamination steel all have an effect on the performance of the motor. The voltage rating of the motor is determined by the number of turns on the stator and the power rating of the motor is determined by the losses which comprise copper loss and iron loss, and the ability of the motor to dissipate the heat generated by these losses. The stator design determines the rated speed of the motor and most of the full load, full speed characteristics.
2. Rotor Design
The Rotor comprises a cylinder made up of round laminations pressed onto the motor shaft, and a number of short-circuited windings.The rotor windings are made up of rotor bars passed through the rotor, from one end to the other, around the surface of the rotor. The bars protrude beyond the rotor and are connected together by a shorting ring at each end. The bars are usually made of aluminium or copper, but sometimes made of brass. The position relative to the surface of the rotor, shape, cross sectional area and material of the bars determine the rotor characteristics. Essentially, the rotor windings exhibit inductance and resistance, and these characteristics can effectively be dependant on the frequency of the current flowing in the rotor. A bar with a large cross sectional area will exhibit a low resistance, while a bar of a small cross sectional area will exhibit a high resistance. Likewise a copper bar will have a low resistance compared to a brass bar of equal proportions.Positioning the bar deeper into the rotor, increases the amount of iron around the bar, and consequently increases the inductance exhibited by the rotor. The impedance of the bar is made up of both resistance and inductance, and so two bars of equal dimensions will exhibit a different A.C. impedance depending on their position relative to the surface of the rotor. A thin bar which is inserted radialy into the rotor, with one edge near the surface of the rotor and the other edge towards the shaft, will effectively change in resistance as the frequency of the current changes. This is because the A.C. impedance of the outer portion of the bar is lower than the inner impedance at high frequencies lifting the effective impedance of the bar relative to the impedance of the bar at low frequencies where the impedance of both edges of the bar will be lower and almost equal. The rotor design determines the starting characteristics.

3. Equivalent Circuit

The induction motor can be treated essentially as a transformer for analysis. The induction motor has stator leakage reactance, stator copper loss elements as series components, and iron loss and magnetising inductance as shunt elements. The rotor circuit likewise has rotor leakage reactance, rotor copper (aluminium) loss and shaft power as series elements. The transformer in the centre of the equivalent circuit can be eliminated by adjusting the values of the rotor components in accordance with the effective turns ratio of the transformer.From the equivalent circuit and a basic knowledge of the operation of the induction motor, it can be seen that the magnetising current component and the iron loss of the motor are voltage dependant, and not load dependant. Additionally, the full voltage starting current of a particular motor is voltage and speed dependant, but not load dependant. The magnetising current varies depending on the design of the motor. For small motors, the magnetising current may be as high as 60%, but for large two pole motors, the magnetising current is more typically 20 – 25%. At the design voltage, the iron is typically near saturation, so the iron loss and magnetising current do not vary linearly with voltage with small increases in voltage resulting in a high increase in magnetising current and iron loss.

Basic Motor Theory (15) – Finish

December 14, 2008

Losses And Efficiency

Friction and Windage
These losses include bearing friction, brush friction, and windage. They are also known as mechanical losses. They are constant at a given speed but vary with changes in speed. Power losses due to friction increase as the square of the speed and those due to windage increase as the cube of the speed.

Armature Copper Losses
These are the I2 R losses of the armature circuit, which includes the armature winding, commutator, and brushes. They vary directly with the resistance and as the square of the currents.

Field Copper Losses
These are the I2 R losses of the field circuit which can include the shunt field winding, series field winding, interpole windings and any shunts used in connection with these windings. They vary directly with the resistance and as the square of the currents.

Core Losses
These are the hysteresis and eddy current losses in the armature. With the continual change of direction of flux in the armature iron, an expenditure of energy is required to carry the iron through a complete hysteresis loop. This is the hysteresis loss. Also since the iron is a conductor and revolving in a magnetic field, a voltage will be generated. This, in turn, will result in small circulating currents known as eddy currents. If a solid core were used for the armature, the eddy current losses would be high. They are reduced by using thin laminations, which are insulated from each other. Hysteresis and eddy current losses vary with flux density and speed.

Efficiency
For generations or motors, the efficiency is equal to the output divided by the input. However, in a generator, the input is mechanical while the output is electrical. In a motor the opposite is true, therefore:
Motor Efficiency = (Input – Losses) / Input
Generator Efficiency = Output / (Output + Losses)

Section 3: Horsepower Basics

In 18th century England, coal was feeding the industrial revolution and Thomas Newcomen invented a steam driven engine that was used to pump water from coal mines. It was a Scott however, by the name of James Watt, who in 1769 improved the steam engine making it truly workable and practical. In his attempt to sell his new steam engines, the first question coal mine owners asked was “can your engine out work one of my horses?” Watt didn’t know since he didn’t know how much work a horse could do. To find out, Watt and his partner bought a few average size horses and measured their work. They found that the average horse worked at the rate of 22,000 foot pounds per minute. Watt decided, for some unknown reason, to add 50% to this figure and rate the average horse at 33,000 foot pounds per minute.
What’s important is that there is now a system in place for measuring the rate of doing work. And there is a unit of power, horsepower.
If steam engines had been developed some place else in the world, where the horse was not the beast of burden, we might be rating motors in oxen power or camel power. Today, motors are also rated in Watts output.
hp = lb x fpm / 33,000
hp = ft-lb x rpm / 5,252
kW = hp x 0.7457
hpMetric = hp x 1.0138
Horsepower as defined by Watt, is the same for AC and DC motors, gasoline engines, dog sleds, etc.

Horsepower and Electric Motors
Torque = force x radius = lb x ft = T
Speed = rpm = N
Constant = 5252 = C
HP = T x N / C
Torque and DC Motors
T = k Ia

At overload, torque increases at some rate less than the increase in current due to saturation

D2 L and Torque
258AT = 324 D2 L
259AT = 378 D2 L
With the same frame diameter, the 259AT has 17% more D2 L and thus 17% more and 17% more Torque. Motor torque increases with an increase in iron and copper, combined with current. It can then be said that it takes iron and copper to produce torque and torque makes products. Or to put it another way, what you purchase to make product is TORQUE and that is IRON and COPPER. The rate of doing work is power and HORSEPOWER is a unit of power.

Speed and DC Motors

Shunt wound DC motors
With motor load, temperature and field current held constant, speed is controlled by armature voltage.
E = ((Z / a) x x P x (N / 60) x 10-8 ) + (I Ra + I Rip + I Rb )

The sum of the voltage drop in the armature circuit can be represented as IR
N = (E – IR) / K
Speed example: given motor is design G6219, frame MC3212, 50 hp, 1150 rpm, 500 volt armature, 85 amps full load, 0.432 armature circuit resistance hot, 0.206 armature circuit resistance cold
Edrop = IR = 85 amp x 0.432 = 36.72 volts

500 v arm – 36.72 v drop = 463.28 working volts
Volts per rpm = 463.28 / 1150 rpm = 0.40285
Nbase speed = 1150 rpm = (500 v – 36.72 v) / 0.40285
With 250 v on the armature, there is 213.28 working volts (250 – 36.72)
213.28 / 0.40285 = 529 rpm (not 1/2 speed, 575 rpm)
N = 529 rpm = (250 v – 36.72 v) / 0.40285
N = (E – IR) / K = (E – IR) / 0.40285
K changes with changes in load and temperature
HPMetric = HP x 1.0138
kW = HP x 0.7457

Source: http://www.reliance.com/mtr/mtrthrmn.htm

Basic Motor Theory (14)

December 14, 2008

Motor Characteristics

Motor Operation

As previously stated, a conductor moving through a magnetic field due to the motor action also generates a voltage which is in opposition to the applied voltage. This is the back EMF. Then for motor action the voltage equation is:

V = E + IA RA = K1 Flux S + IA RA

where:
V = applied or terminal voltage
E = back EMF
IA = armature current
RA = armature circuit resistance’s
K1 = machine constants
Flux = flux per pole
S = speed

When comparing this equation with the voltage equation of a generator, it can be seen that in a generator the generated voltage is higher than the terminal voltage while in a motor the opposite is true. Therefore, as long as the generated voltage is less than the terminal voltage, a machine operates as a motor and takes power from the electrical side, but when the generated voltage becomes greater than the terminal voltage, the machine becomes a generator, supplies electric power, and requires mechanical energy to keep operating.
The back or counter EMF acts as a control for the amount of current needed for each mechanical load. When the mechanical load is increased, the first effect is a reduction in speed. But a reduction in speed also causes a reduction in back EMF, thus making available an increased voltage for current flow in the armature. Therefore, the current increases which in turn increases the torque. Because of this action, a very slight decrease in speed is sufficient to meet the increased torque demand. Also, the input power is regulated to the amount required for supplying the motor losses and output.
Speed Torque Curves
Speed torque curves for the three forms of excitation are shown in Figure 25. In a shunt excited motor, the change in speed is slight and, therefore, it is considered a constant speed motor. Also, the field flux is nearly constant in a shunt motor and the torque varies almost directly with armature current.
In a series motor the drop in speed with increased torque is much greater. This is due to the fact that the field flux increases with increased current, thus tending to prevent the reduction in back EMF that is being caused by the reduction in speed. The field flux varies in a series motor and the torque varies as the square of the armature current until saturation is reached. Upon reaching saturation, the curve tends to approach the straight line trend of the shunt motor. The no load speed of a series motor is usually too high for safety and, therefore, it should never be operated without sufficient load.
A compound motor has a speed torque characteristic which lies between a shunt and series motor.
Speed Regulation
Speed regulation is the change in speed with the change in load torque, other conditions being constant. A motor has good regulation if the change between the no load speed and full load speed is small.
Percent Speed Regulation = (SNL – SFL) / SFL x 100 A shunt motor has good speed regulation while a series motor has poor speed regulation. For some applications such as cranes or hoists, the series motor has an advantage since it results in the more deliberate movement of heavier loads. Also, the slowing down of the series motor is better for heavy starting loads. However, for many applications the shunt motor is preferred.
Motor Starting
When the armature is not rotating, the back EMF is zero and the total applied voltage is available for sending current through the armature. Since the armature resistance is low, an enormous current would flow if voltage were applied under this condition. Therefore, it is necessary to insert an additional resistance in series with the armature until a satisfactory speed is reached where the back EMF will take over to limit the current input.

Basic Motor Theory (13)

December 14, 2008

Generator Characteristics

No Load Saturation Curve
A typical no load saturation curve is shown in Figure 23. This is similar to the magnetization curve mentioned previously except that it represents the entire magnetic circuit of a machine rather than one particular magnetic material.

Also, it has generator output voltage plotted against field current rather than flux density against magnetizing force. This can be done since generator voltage is directly proportional to the field flux and the number of turns is fixed. There is a different saturation curve for each speed. The lower straight line portion of the curve represents the air gap because the magnetic parts are not saturated. When the magnetic parts start to saturate, the curve bends over until complete saturation is reached. Then the curve becomes a straight line again.

Figure 23. No Load Saturation Curve


Figure 23.1 No Load Saturation Curve


Figure 23.2 No Load Saturation Curve

Generator Build Up
Generator build up usually refers to the gradual rise in voltage at the armature terminals when the machine is self-excited and operated at normal speed. This is illustrated in Figure 25 by referring to the field resistance line which shows how the field current varies as field voltage is varied. The slope of this line is the field resistance at a constant temperature. The voltage rise starts with the residual magnetism of the field iron. This provides a small voltage output E1 that is fed back to the field as 1. 1 increases the flux providing a slightly larger voltage, E2 . E2 causes 2 to flow. This process continues until the machine starts to saturate and stops at the point where the field resistance line intersects the saturation curve. If the speed of the machine is reduced so that the saturation curve becomes tangent to field resistance curve, the voltage will not build up. This is known as the critical speed. Also, at any given speed, if the field resistance is increased by addition of external resistance, a critical resistance can be reached.


Figure 25. DC Motor Curves

Voltage Output The voltage equation has been expressed as:
E = K1 S.

However, this is the generated voltage and part of it must be used to overcome the IR drops in the machine, which are caused by the resistance’s of the armature, series field, interpoles, brushes, etc. If these resistance’s are combined together and called armature resistance, then the voltage output at the generator terminals can be expressed as:
V = E – Ia Ra – K1 Flux S – Ia Ra

where:
E = generated voltage
Ia = armature current
Ra = armature circuit resistance
K1 = machine constants
Flux = flux per pole
S = speed.

External Characteristics
The curve showing the relationship between output voltage and output current is known as the external characteristic. Shown in Figure 24 are the external characteristic curves for generators with various types of excitation. If a generator, which is separately excited, is driven at constant speed and has a fixed field current, the output voltage will decrease with increased load current as shown. This decrease is due to the armature resistance and armature reaction effects. If the field flux remained constant, the generated voltage would tend to remain constant and the output voltage would be equal to the generated voltage minus the IR drop of the armature circuit. However, the demagnetizing component of armature reactions tends to decrease the flux, thus adding an additional factor, which decreases the output voltage.


Figure 24. DC Generator Curves

In a shunt excited generator, it can be seen that the output voltage decreases faster than with separate excitation. This is due to the fact that, since the output voltage is reduced because of the armature reaction effect and armature IR drop, the field voltage is also reduced which further reduces the flux. It can also be seen that beyond a certain critical value, the shunt generator shows a reversal in trend of current values with decreasing voltages. This point of maximum current output is known as the breakdown point. At the short circuit condition, the only flux available to produce current is the residual magnetism of the armature.
To build up the voltage on a series generator, the external circuit must be connected and its resistance reduced to a comparatively low value. Since the armature is in series with the field, load current must be flowing to obtain flux in the field. As the voltage and current rise the load resistance may be increased to its normal value. As the external characteristic curve shows, the voltage output starts at zero, reaches a peak, and then falls back to zero.
The combination of a shunt field and a series field gives the best external characteristic as illustrated in Figure 24. The voltage drop, which occurs in the shunt machine, is compensated for by the voltage rise, which occurs in the series machine. The addition of a sufficient number of series turns offsets the armature IR drop and armature reaction effect, resulting in a flat-compound generator, which has a nearly constant voltage. If more series turns are added, the voltage may rise with load and the machine is known as an over-compound generator.

Voltage Regulation
Voltage Regulation is the change in terminal voltage with the change in load current at constant speed. A generator has good regulation if the change in voltage between no load and full load is small. If the change is large, the regulation is poor. Expressed in equation form:
Percent Voltage Regulation = (ENL – EFL ) / EFL x 100 or for some compound machines, Percent Voltage Regulation = (EFL – ENL ) / EFL x 100
Figure 24 shows that the regulation of a separately excited machine is better than that of a shunt machine. However, the best regulation is obtained with a compound machine. The series machine has practically no regulation at all and, therefore, has little practical application.

Basic Motor Theory (12)

December 14, 2008
Commutation

The maximum voltage from an armature winding can be obtained when the brushes are in contact with those conductors, which are midway between the poles. This will result in the greatest possible number of conductors cutting the magnetic lines in one direction between a positive and negative brush. This brush position is known as the no load neutral position of the brushes. The current in a given armature coil reverses in direction as the coil sides move from one pole to another of opposite polarity, whereas the function of the commutator is to keep the current unidirectional. This reversal of current is known as commutation. The commutator acts as a switch to keep the current flowing in one direction.

However, the fast rate of change in direction of the current in any given coil induces an appreciable voltage in that coil which tends to keep the current flowing in the original direction. Therefore, the current reversal is delayed causing an accelerated rate of change near the end of the commutation period. This results in an arc if the reversal is not completed before the brush breaks contact with the coil involved. Any arcing is detrimental to the operation of the machine and must be counteracted.

Armature Reaction
Since the armature conductors carry current they set up a magnetic field which distorts or opposes the main field. This is called armature reaction and is a function of the amount of load present. Figure 21 shows the MMF and flux wave shapes due to the armature reaction only; and Figure 22 shows the combined effect of both. It can be seen that armature reaction causes the flux to shift, thus tending to saturate one pole tip. If this effect is appreciable, it can be detrimental to the satisfactory performance of the machine. If severe enough, it may result in a flashover, which is the progressive arcing over successive bars until the arc extends from positive to negative brush, thus short circuiting the machine terminals.


Figure 20. MMF and Flux Wave Shape due to Main Field only


Figure 21. MMF and Flux Wave Shape due to Armature Reaction only

Figure 22. Flux Wave Shape, combined effect

Brush Shifting
One method of reducing the arcing due to non-linear commutation is to shift the brushes away from the geometrical neutral position. Then commutation will occur when the applicable coil is under the influence of a weak magnetic field that will generate a voltage in the coil, which opposes the induced voltage due to current change. Therefore, this new voltage will assist rather than hinder the current reversal. In a generator, it is necessary to shift the brushes forward in the direction of rotation for good commutation. This is true because the current flow through the conductors is in the same direction as the voltage and, it commutation is delayed until the coil sides are under the next pole, it will be assisted by the current reversing voltage. In a motor, it is necessary to shift the brushes against the direction of rotation because current flow is in opposition to the induced voltage. The amount of shift necessary depends on the load so a given shift will not be satisfactory for all loads. One effect of shifting brushes is that a demagnetization component of armature reaction is introduced. In other words, when the brushes are shifted, the armature reaction will not only distort the main field flux but it will also directly oppose the main field. This will result in a reduction of the field flux. Another effect is that if the brushes are shifted far enough, it is possible to reduce the number of effective turns because there will be voltages in opposition to each other between two brushes.
In generators the demagnetization component of armature reaction would be detrimental because there will be a decrease in generated voltage with increase in load. However, in a motor, the effect would be beneficial because the speed would tend to remain constant.

Interpoles
Another method to combat the induced voltage caused by current reversal is the use of interpoles. The interpoles are located at the geometric neutral points midway between the main poles and provide reversing magnetic field of proper strength and polarity. They eliminate the need for brush shifting and, because of this, the demagnetization effect of armature reaction is eliminated. The interpole must have sufficient strength to overcome the armature reaction and provide a reversing field, therefore, it is connected in series with the armature winding. When the armature current is increased in the same proportion. In a generator, the interpole must have the same polarity as the next pole in the direction of rotation while in a motor the interpole must have the same polarity as the last pole.

Basic Motor Theory (11)

December 14, 2008

Field Windings

The field windings provide the excitation necessary to set up the magnetic fields in the machine. There are various types of field windings that can be used in the generator or motor circuit. In addition to the following field winding types, permanent magnet fields are used on some smaller DC products. See Figure 19 for winding types.


Shunt Wound – DC Operation
Typical Speed – Torque Curve
Shunt wound motors, with the armature shunted across the field, offer relatively flat speed-torque characteristics. Combined with inherently controlled no-load speed, this provides good speed regulation over wide load ranges. While the starting torque is comparatively lower than the other DC winding types, shunt wound motors offer simplified control for reversing service.

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Compound Wound – DC Operation
Typical Speed – Torque Curve
Compound wound (stabilized shunt) motors utilize a field winding in series with the armature in addition to the shunt field to obtain a compromise in performance between a series and shunt type motor. This type offers a combination of good starting torque and speed stability. Standard compounding is about 12%. Heavier compounding of up to 40 to 50% can be supplied for special high starting torque applications, such as hoists and cranes.

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Series Wound – DC Operation
Typical Speed – Torque Curve
Series wound motors have the armature connected in series with the field. While it offers very high starting torque and good torque output per ampere, the series motor has poor speed regulation. Speed of DC series motors is generally limited to 5000 rpm and below. Series motors should be avoided in applications where they are likely to lose there load because of their tendency to “run away” under no-load conditions. These are generally used on crane and hoist applications.

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Permanent Magnet – DC Operation
Typical Speed – Torque Curve
Permanent magnet motors have no wound field and a conventional wound armature with commutator and brushes. This motor has excellent starting torques, with speed regulation not as good as compound motors. However, the speed regulation can be improved with various designs, with corresponding lower rated torques for a given frame. Because of permanent field, motor losses are less with better operating efficiencies. These motors can be dynamically braked and reversed at some low armature voltage (10%) but should not be plug reversed with full armature voltage. Reversing current can be no higher than the locked armature current.

Figure 19. Field Windings

Separately Excited Winding
When the field is connected to an external power source, it is a separately excited field.

Straight Shunt Winding
This winding is connected in parallel with the armature. Shunt windings usually consists of a large number of turns of small size wire. This is a good winding for reversing applications since it provides the same amount of torque in both directions. The torque/ current curve is non-linear above full load. Shunt wound motors often have a rising speed characteristic with increased load.

Series Winding
This winding is connected in series with the armature. A series winding usually consists of a small number of turns of large size wire. With this winding, the motor can produce high starting and overload torque. This design is not used for applications with light loads or no load conditions.

Compound Winding
This winding consists of a shunt winding and a series winding. This is also known as compound excitation. The series winding can be designed as a starting series only or as a start and run series.
Stabilized Shunt Winding
Like the compound winding, this winding consists of a shunt winding and a series winding. The series or stabilizing winding has a fewer number of turns than the series winding in a compound wound machine. A stabilizing winding is used to assures a speed droop with overload. It also adds to the torque in one direction of operation and subtracts from torque in the reverse direction of operation and in regeneration.

Shunt Compensated Winding
Shunt compensated motors have a shunt winding and a pole face series winding made up of large conductors placed in slots in the face of the main field poles. The direction of current in the compensating windings is the opposite of the current in the armature conductors passing under the poles. The flux produced by the compensating windings neutralizes the flux of the armature conductors passing under the poles so that distortion of air gap flux is minimized. Shunt compensated motors maintain constant or set speed well at all loads, no load through overload. Unlike the stabilized shunt winding, the pole face winding adds to torque in both the forward and reverse direction of rotation. Shunt compensated windings, due to cost and difficulty of construction, are provided on large motors only, usually 840 frames and larger.

Basic Motor Theory (10)

December 14, 2008
Armature Windings
Gramme Ring Winding
The old Gramme Ring type winding, now obsolete, is shown in Figure 9 and its equivalent circuit in Figure 10. It can be seen that there are an equal number of voltage-generating conductors on each side of the armature and the conductor voltages are additive from bottom to top on each side. There are two paths between the positive and negative brushes and the voltage per path is the generated voltage of the machine. Each path provides half of the current output.


Figure 9. Two Pole Gramme Ring Winding


Figure 10. Equivalent Circuit, Two Pole Gramme Ring Winding

Drum Winding
The Drum type winding is made of coils, one of which is illustrated in Figure 11. The straight portions of the coil are the parts rotating through the magnetic field in which the voltage is induced. Therefore, each single coil has two conductors. This has the advantage over the Gramme Ring winding where only one side of each coil is used as an active conductor. There are two classes of drum windings depending upon how the coils are connected to the commutator.


Figure 11. Drum Type Winding Coil

Lap Winding
When the end connections of the coils are brought to adjacent bars as shown in Figure 12, a lap or parallel winding is formed. In this type winding, there are as many paths through the armature as there are poles on the machine. Therefore, to obtain full use of this type winding, there must be as many brushes as there are poles, alternate brushes being positive and negative. Any winding can be illustrated in one of two forms, the circular form or the development form. A simplex lap winding is shown in Figure 13 (circular form) and Figure 14 (development form.) In this particular circular form, the flux cutting portions of the conductors are shown as straight lines radiating from the center and are numbered for convenience in connecting them to the commutator which is in the center of the diagram. The outermost connecting lines represent the end connections on the back of the armature and the inner connecting lines represent the connections on the front or commutator end of the armature. The development form of winding represents the armature as if it were split open and rolled out flat. It is somewhat simpler to understand but the continuity of the winding is broken. The lap winding is best suited for low voltage, high current ratings because of the number of parallel paths.


Figure 12. Lap Winding connected to commutator bars


Figure 13. Simplex Lap Winding, Circular Form


Figure 14. Simplex Lap Winding, Development Form

Wave Winding
When the end connections of the coils are spread apart as shown in Figure 15 a wave or series winding is formed. In a wave winding there are only two paths regardless of the number of poles. Therefore, this type winding requires only two brushes but can use as many brushes as poles. The simplex wave winding in Figure 16 (circular) and Figure 17 (development) shows that the connections to the armature do not lap back toward the coil but progress forward. The coil voltages are cumulative but it is necessary to travel several times around the armature and to traverse half the total winding in order to trace the path between the positive and negative brush. The wave winding is best suited for high voltage low current ratings since it has only two paths.


Figure 15. Wave Winding connected to commutator bars


Figure 16. Simplex Wave Winding, Circular Form


Figure 17. Simplex Wave Winding, Development Form

Slots and Coils
The number and size of slots depend upon the generator or motor requirements. The slot has to be large enough to hold the correct number of conductors but at the same time, the tooth has to be large enough to pass the necessary magnetic flux. Normally, in a simple winding, there are as many coils as there are slots. This means that each slot contains two coil sides, one side of each coil being at the top of a slot and the other at the bottom of a slot. Each coil may consist of one or more turns depending on the applied or generated voltage of the unit. A typical arrangement of coil sides and slots is shown in Figure 18. Solid lines represent the front end connections to the commutator and dotted lines represent the back end connections.

Slot Pitch
Slot pitch refers to the number of slots spanned by each coil. For example, in Figure 18, the top of coil in slot 1 has its bottom in slot 4, therefore, the slot pitch is 1-4 or 3. Since the top of the coil is directly under the north pole and the bottom is directly under the south pole, the winding is known as a full pitch winding. In many cases, for various reasons, the pitch is reduced to less than full pitch. For example, if the coils in Figure 6 spanned 2 slots instead of three, the winding would become a two-thirds pitch winding.


Figure 18. Coil Sides in Armature Slots

Basic Motor Theory (9)

December 14, 2008
DC General Construction
A typical DC generator or motor usually consists of: An armature core, an air gap, poles, and a yoke which form the magnetic circuit; an armature winding, a field winding, brushes and a commutator which form the electric circuit; and a frame, end bells, bearings, brush supports and a shaft which provide the mechanical support. See figure 8.


Figure 8. Four Pole DC Motor

Armature Core or Stack
The armature stack is made up thin magnetic steel laminations stamped from sheet steel with a blanking die. Slots are punched in the lamination with a slot die. Sometimes these two operations are done as one. The laminations are welded, riveted, bolted or bonded together.
Armature Winding
The armature winding is the winding, which fits in the armature slots and is eventually connected to the commutator. It either generates or receives the voltage depending on whether the unit is a generator or motor. The armature winding usually consists of copper wire, either round or rectangular and is insulated from the armature stack.
Field Poles
The pole cores can be made from solid steel castings or from laminations. At the air gap, the pole usually fans out into what is known as a pole head or pole shoe. This is done to reduce the reluctance of the air gap. Normally the field coils are formed and placed on the pole cores and then the whole assembly is mounted to the yoke.
Field Coils
The field coils are those windings, which are located on the poles and set up the magnetic fields in the machine. They also usually consist of copper wire are insulated from the poles. The field coils may be either shunt windings (in parallel with the armature winding) or series windings (in series with the armature winding) or a combination of both.
Yoke
The yoke is a circular steel ring, which supports the field, poles mechanically and provides the necessary magnetic path between the pole. The yoke can be solid or laminated. In many DC machines, the yoke also serves as the frame.
Commutator
The commutator is the mechanical rectifier, which changes the AC voltage of the rotating conductors to DC voltage. It consists of a number of segments normally equal to the number of slots. The segments or commutator bars are made of silver bearing copper and are separated from each other by mica insulation.
Brushes and Brush Holders
Brushes conduct the current from the commutator to the external circuit. There are many types of brushes. A brush holder is usually a metal box that is rectangular in shape. The brush holder has a spring that holds the brush in contact with the commutator. Each brush usually has a flexible copper shunt or pigtail, which extends to the lead wires. Often, the entire brush assembly is insulated from the frame and is made movable as a unit about the commutator to allow for adjustment.
Interpoles
Interpoles are similar to the main field poles and located on the yoke between the main field poles. They have windings in series with the armature winding. Interpoles have the function of reducing the armature reaction effect in the commutating zone. They eliminate the need to shift the brush assembly.
Frame, End Bells, Shaft, and Bearings
The frame and end bells are usually steel, aluminum or magnesium castings used to enclose and support the basic machine parts. The armature is mounted on a steel shaft, which is supported between two bearings. The bearings are either sleeve, ball or roller type. They are normally lubricated by grease or oil.
Back End, Front End
The load end of the motor is the Back End. The opposite load end, most often the commutator end, is the Front End of the motor.

Probable Causes for 2 X Line Frequency in Motors

December 13, 2008

Can you please tell me the probable causes for 2 X line frequency in motors.
We had a problem of high vibration in one of our motors. Main contributor to vibration is 2 x line freq.. When we opened motor we found all things okay.
Can you tell me reasons for 2 x line frequency.


Answer :
The number one reason for any 2FL (twice line frequency) signature is electrical. Now, the type of electrical problem will be the question:
1) Stator eccentricity, shorted laminations, loose iron and a loose stator core will cause a high 2FL frequency, normally without sidebands.
This will also occur with unbalanced voltage incoming voltage.
2) Some 3600 RPM, and to a limited extent, some 1800 RPM, low voltage concentric-wound motors will show 2FL just because of the placement of the coils in relation to the stator core and rotor. This signature is more pronounced when the motor is connected for Delta.
3)Pole pass frequency (or twice slip frequency) sidebands around running speed and the 2FL peak indicate eccentric rotor conditions (usually dynamic eccentricity).
4) Broken rotor bars will show as multiples of running speed with pole pass frequency sidebands and may also show as rotor bar pass frequency
(RBPF) harmonics. 2FL sidebands around RBPF harmonics indicate looseness in the rotor bars.
5) Loose connections will appear as sidebands around a 2FL peak as 1/3 FL peaks with harmonic sidebands.
The good news is that motor current signature analysis is designed to quickly detect these issues using the motor current instead of mechanical vibration (the two technologies complement each other tremendously).

Phase To Phase

December 13, 2008

Question: Phase To Phase
We have a compressor motor at 480v 217fla.
L1=135A
L2=126A
L3=144A
Is it a problem having this much difference phase to phase on L2/L3?

Answer:
The likely mechanisms of a stator winding fault are either a turn-to-turn or phase-to-phase short, or an insulation to ground fault. A turn-to-turn short is identified as a shorting of one or more windings in a coil. This can develop into a very low impedance loop of wire, which acts as a shorted secondary of a current transformer. This results in excessive current flow through this shorted loop, creating intense heat and possible insulation damage. Due to the nature of the low voltage random wound design a shorted turn could occur with much higher impedance, allowing the motor to run for extended periods of time before eventually burning up the coil with the high currents. As a result it is not unexpected to find low voltage motors still running with bad stator windings. Form wound coils however, do not exhibit high turn impedances and will therefore burn up quickly following the presence of a turn-to-turn short. A phase-to-phase short is identified as a shorting of one or more phases to another phase. This fault can be quite damaging due to the possibility of very large voltage potential existing between phases at the location of the short. In your case we have an inductive imbalance of 6.67% which is not considered abnormal for a 480v motor. This imbalance may be caused by the rotor by design or as a result of a rotor anomaly. Performing a Rotor Influence Check will allow a quick root cause analysis of the 6.67% imbalance identifying either the rotor or stator as the cause. Once you have identified the source you can better focus your analysis and trending.


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