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Hysteresis loss is a heat loss caused by the magnetic properties of the armature.

When an armature core is in a magnetic field, the magnetic particles of the core tend to line up with the magnetic field. When the armature core is rotating, its magnetic field keeps changing direction. The continuous movement of the magnetic particles, as they try to align themselves with the magnetic field, produces molecular friction. This, in turn, produces heat. This heat is transmitted to the armature windings. The heat causes armature resistances to increase.

To compensate for hysteresis losses, heat-treated silicon steel laminations are used in most dc generator armatures. After the steel has been formed to the proper shape, the laminations are heated and allowed to cool. This annealing process reduces the hysteresis loss to a low value.

Figure (2-15) : Eddy currents in dc generator armature cores.

Week 5

3.1 Introduction

Motors change electric energy into mechanical energy. Direct current motors and generators are constructed very similarly as explain in the previous chapter. They function almost oppositely at first because a generator creates voltage when conductors cut across the lines of force in a magnetic field, while motors result in torque-- a turning effort of mechanical rotation. Simple motors have a flat coil that carries current that rotates in a magnetic field. The motor acts as a generator since after starting, it produces an opposing current by rotating in a magnetic field, which in turn results in physical motion.

3.2 Constructions and Operation Principle of DC Generator

Motors change electric energy into mechanical energy. Direct current motors and generators are constructed very

similarly described earlier in the previous chapter. They function almost oppositely at first because a generator creates voltage when conductors cut across the lines of force in a magnetic field, while motors result in torque a turning effort of mechanical rotation.

Simple motors have a flat coil that carries current that rotates in a

magnetic field as shown in fig.(3-1). The motor acts as a generator since after starting, it produces an opposing current by rotating in a magnetic field, which in turn results in physical motion.

This is accomplished as a conductor is passed through a magnetic field, then the opposing fields repel each other to cause physical motion. The left hand rule can be used to explain the way a simple motor works fig.(3-2). The pointer finger points in the direction of the magnetic field, the middle finger points in the direction of the

Figure(3-1): Simple motor

Figure(3-2): Left hand rules

DC motor has a rotating armature in the form of an electromagnet. A rotary switch called a commutator reverses the direction of the electric current twice every cycle, to flow through the armature so that the poles of the electromagnet push and pull against the permanent magnets on the outside of the motor. As the poles of the armature electromagnet pass the poles of the permanent magnets, the commutator reverses the polarity of the armature electromagnet. During that instant of switching polarity, inertia keeps the DC motor going in the proper direction. See the diagrams shown in fig.(3-3).

(a) (b) (c)

Figure(3-3) :Diagrams that explains the operation of a DC motor.

a) A simple DC electric motor. When the coil is powered, a magnetic field is generated around the armature. The left side of the armature is pushed away from the left magnet and drawn toward the right, causing rotation.

b) The armature continues to rotate.

c) When the armature becomes horizontally aligned, the commutator reverses the direction of current through the coil, reversing the magnetic field. The process then repeats.

3.2.1 The dc motor torque

When the conductor is bent into a coil, the physical motion performs an up and down cycle. The more bends in a coil, the less pulsating the movement will be. This physical movement is called torque, and can be measured in the equation:

T = k

_t

Ф I

_a

where :

T = Torque in (Newton- meter)

k_t = Constant depending on physical dimension of motor

Ф = Total number of lines of flux entering the armature from one N pole through the loop of wire from the battery. As the loop sides cut the magnetic field, a voltage is induced in them, the same as it was in the loop sides of the dc generator.

This induced voltage causes current to flow in the loop. this current direction opposite to that of the battery current. Since this generator-action voltage is opposite that of the battery, it is called "Back emf." (The letters emf stand for electromotive force, which is another name for voltage.) The two currents are flowing in opposite directions. This proves that the battery voltage and the back emf are opposite in polarity. At the beginning of this discussion, we disregarded armature current while explaining how back emf was generated. Then, we showed that normal armature current flowed opposite to the current created by the back emf. We talked about two opposite currents that flow at the same time. However, this is a bit oversimplified, as you may already

large as the applied voltage, and because they are of opposite polarity as we have seen, the back emf effectively cancels part of the armature voltage. The single current that flows is armature current, but it is greatly reduced because of the counter emf. In a dc motor, there is always a counter emf developed. This counter emf cannot be equal to or greater than the applied battery voltage; if it were, the motor would not run. The back emf is always a little less. However, the back emf opposes the applied voltage enough to keep the armature current from the battery to a fairly low value. If there were no such thing as back emf, much more current would flow through the armature, and the motor would run much faster. However, there is no way to avoid the back emf.

3.3 Types and characteristics of DC Motors

There are three basic types of dc motors:

(1) Series motors heavy wire, are connected in series with the armature winding. Both a diagrammatic and a schematic illustration of a series motor is shown in fig.(3-4). The same current flowing through the field winding also flows through the armature winding. Any increase in current, therefore, strengthens the magnetism of both the field and the armature.

Figure(3-4) :Series DC motor

Because of the low resistance in the windings, the series motor is able to draw a large current in starting. This starting current, in passing through both the field and

armature windings, produces a high starting torque, which is the series motor's principal advantage.

The speed of a series motor is dependent upon the load. Any change in load is accompanied by a substantial change in speed. A series motor will run at high speed when it has a light load and at low speed with a heavy load. If the load is removed entirely, the motor may operate at such a high speed that the armature will fly apart. If high starting torque is needed under heavy load conditions, series motors have many applications. Series motors are often used in aircraft as engine starters and for raising and lowering landing gears, cowl flaps, and wing flaps.

3.3.2 Shunt DC Motor

In the shunt motor the field winding is connected in parallel or in shunt with the armature winding. See fig.(3-5), The resistance in the field winding is high. Since the field winding is connected directly across the power supply, the current through the field is constant.

The field current does not vary with motor speed, as in the series motor and, therefore, the torque of the shunt motor will vary only with the current through the armature. The torque developed at starting is less than that developed by a series motor of equal size.

Figure(3-5) :Shunt DC motor

The speed of the shunt motor varies very little with changes in load. When all load is removed, it assumes a speed slightly higher than the loaded speed. This motor is particularly suitable for use when constant speed is desired and when high starting torque is not needed.

3.3.3 Compound DC Motor

The compound motor is a combination of the series and shunt motors. There are two windings in the field: a shunt winding and a series winding. A schematic of a compound motor is shown in fig.(3-6).

The shunt winding is composed of many turns of fine wire and is connected in parallel with the armature winding. The series winding consists of a few turns of large wire and is connected in series with the armature winding. The starting torque is higher than in the shunt motor but lower than in the series motor. Variation of speed with load is less than in a series wound motor but greater than in a shunt motor. The compound motor is used whenever the combined characteristics of the series and shunt motors are desired.

Figure(3-6) :Compound DC motor

Like the compound generator, the compound motor has both series and shunt field windings. The series winding may either aid the shunt wind (cumulative compound) or oppose the shunt winding (differential compound).

The starting and load characteristics of the cumulative compound motor are somewhere between those of the series and those of the shunt motor.

Because of the series field, the cumulative compound motor has a higher starting torque than a shunt motor.

Cumulative compound motors are used in driving machines which are subject to sudden changes in load. They are also used where a high starting torque is desired, but a series motor cannot be used easily.

In the differential compound motor, an increase in load creates an increase in current and a decrease in total flux in this type of motor. These two tend to offset each other and the result is a practically constant speed.

However, since an increase in load tends to decrease the field strength, the speed characteristic becomes unstable. Rarely is this type of motor used in aircraft systems.

A graph of the variation in speed with

changes of load of the various types of dc motors is shown in fig.(3-7).

Figure(3-7) : Composite of the characteristic curves for all of the DC motors.

Week 6

3.4 Motor Nameplate

Motor nameplates are provided by virtually all manufacturers to allow users to accurately identify the operating and dimensional characteristics of their motors years after installation.

3.4.1 Definition Nameplate

The nameplate is usually a metal plate, secured by a pair of screws or rivets, and is generally located on the side of the motor. (Expert maintenance technicians will tell you that the nameplate is always located on the side of the motor where the nameplate is most difficult to read!)

The following cryptic information will usually be stamped into the nameplate (stamping is used because it doesn't wear off as ink tends to do. Unfortunately, the lack of contrast can make it difficult to read. Sometimes, a little bit of dirty oil or grease applied to the nameplate and then wiped "smooth" puts the dark substance into the indentations of the stamped letters and allows for easier reading.).

3.4.2 Nameplate Terms

12) CW (Clockwise Rotation) or CCW (Counter-Clockwise Rotation)

1) Motor Manufacturer

This is the trade name of the company which manufactured the motor. If you are lucky, the company's home city, and perhaps even an address and/or telephone number will be on the nameplate.

2) Mod. (Model), Tp. (Type), or Cat. (Catalog)

Some companies distinguish between a Model number and a Type number. (I don't know why). In any event, this is the key number that you need if you want to contact the manufacturer.

3) Ser. (Serial Number)

Serial numbers are important because they often contain "date codes". This is information which helps the manufacturer determine when the motor was manufactured. Since many motors have multiple revisions through their lifecycle as the manufacturing process (hopefully) improves, this helps determine which set of drawings to use and lets the technical people at the manufacturer help you quicker and more accurately.

4) HP (Horsepower) or KW (kilowatts)

If you are using an American made motor or an older English or Canadian motor, it will probably be rated in Horsepower. European and Asian motors are usually rated in kilowatts -- unless they have been designed for export to the American market.

Rule to remember: 1 HP = 3/4 KW (more precisely 746 watts).

Second rule to remember: Volts x Amps = Watts.

5) RPM (Revolutions per Minute)

The number of times each minute that the shaft turns on its axis. This is rated at the Hertz listed. Typical values are 1750, 1450, 3450, etc. If more than one speed is listed, this indicates a multi-speed motor. Note that AC inverter drives can change the speed of a motor from its rated speed.

6) V (Volts)

The operating voltage of the motor. DC motors will have numbers such as 24, 48, 90, 180, or other voltage, and will usually say "VDC".

7) ARM. (Armature)

This is the maximum voltage which can be applied to the armature of a DC motor. Typical values are 90 or 180 VDC. An amperage will often be listed.

8) FLD. (Field)

This is the voltage which should be applied to the field of a DC motor. Typical values are 100, 150, 200 VDC. An amperage will often be listed.

9) A (Amps)

The amount of current consumed by the motor.

10) Fr (Frame)

The physical dimensional standard to which the motor adheres. This is critical when it is necessary to locate a mechanical replacement for an old motor. NEMA motor frames have been established for decades to allow motors from various manufacturers to replace each other. For example, a foot-mount NEMA 56 frame motor has the same mounting dimensions no matter which manufacturer has built it.

NEMA refers to the National Electrical Manufacturers Association. NEMA is part of the IEC. The IEC is the International Electrotechnical Commission. Although the IEC includes Japan and the United States of America among its members, the IEC is essentially a European Community standards association. IEC standards are heavily influenced by VDE - the German electrical standards association.

11) Enc. (Enclosure)

This is the degree of protection offered by the enclosure. Common terms are TEFC, TEBC, TENV, ODP, TEAO, etc.

TEFC

A TEFC enclosure on a motor means "Totally Enclosed, Fan Cooled". This motor is probably the most commonly used motor in ordinary industrial environments. It costs only a few dollars more than the open motor, yet offers good protection against common hazards.

The motor is constructed with a small fan on the rear shaft of the motor, usually covered by a housing. This fan draws air over the motor fins, removing excess heat and cooling the motor.

The enclosure is "Totally Enclosed". This ordinarily means that the motor is dust tight, and has a moderate water seal as well. Notice that TEFC motors are not secure against high pressure water. For these applications, consider the "wash down"

motor, which is usually designed to withstand regular washing, such as found in a food processing facility. In addition, the TEFC motor is not "Explosion-proof", nor is it capable of operation in "Hazardous Environments".

TEBC

A TEBC enclosure on a motor means "Totally Enclosed, Blower Cooled". TEBC motors are most commonly used for variable speed motors combined with variable speed drives of some sort. Sometimes these motors are rated as "Inverter duty" or "Vector duty". They are considerably more expensive than similarly rated TEFC motors. The motor is constructed with a dust tight, moderately sealed enclosure which rejects a degree of water. A constant speed blower pulls air over the motor fins to keep the motor cool at all operating speeds. Notice that this motor is not suitable for used in "washdown" or

"Hazardous" environments.

TENV

A TENV enclosure on a motor means "Totally Enclosed, Not Ventilated". TENV motors are used in a wide variety of smaller horsepower variable speed applications. It is particularly effective in environments where a fan would regularly clog with dust or lint. The motor is constructed with a dust-tight, moderately sealed enclosure which rejects a degree of water. The motor radiates its entire excess heat through the body of the motor: Hence, the TENV motor has extra metal and extra fins to allow radiation of this heat. The TENV motor is commonly built with special high temperature insulation, since the motor is designed to run hot. As such, care should be taken to avoid human contact with the body of the motor, as well as contact between inflammable objects and the motor. Notice that this motor is not suitable for use in "washdown" or "Hazardous" environments.

ODP

An ODP enclosure on a motor means "Open, Drip Proof". ODP motors are relatively inexpensive motors used in normal applications. The construction of an ODP motor consists of a sheet metal enclosure with vent stamped to allow good air flow. The vents are designed in such a way that water dripping on the motor will not normally flow into the motor. A fan is mounted on the motor's rear shaft to pull air through the motor to keep the motor cool. The ODP motor is relatively inexpensive, but care should be taken not to use the motor in applications where the TEFC motor is required.

TEAO

A TEAO enclosure on a motor means "Totally Enclosed, Air Over". TEAO motors are designed to be used solely in the airstream of the fan or blower which they are driving. As such, they are very low cost, but of limited application. TEAO motors are constructed with a dust-tight cover and an aerodynamic body. They rely upon the strong air flow of the fan or blower which they are driving to cool them. TEAO motors are not suitable for use in

"Hazardous" environments.

NEMA Enclosure Standard 250

NEMA enclosure standards represent an enclosure's ability to protect against the external environment. The following represent brief summaries of the NEMA standard. some examples of NEMA Enclosure Standard 250

1- Type 1

Intended for indoor use primarily to provide a degree of protection against (hand) contact with the enclosed equipment. Sometimes known as a "finger-tight"

enclosure. This is the least costly enclosure, but is suitable only for clean, dry environments.

2- Type 2

Intended for indoor use primarily to provide a degree of protection against limited amounts of falling dirt and water.

3- Type 3

Intended for outdoor use primarily to provide a degree of protection against windblown dust, rain, and sleet; undamaged by ice which forms on the enclosure.

4- Type 3R

Intended for outdoor use primarily to provide a degree of protection against falling rain and sleet; undamaged by ice which forms on the enclosure. This is the most common outdoors enclosure.

12) CW (Clockwise Rotation) or CCW (Counter-Clockwise Rotation)

When facing the motor from the shaft end, this is the direction of rotation of the motor (if the motor is unidirectional).

3.5 Power Losses and Efficiency

Losses occur when electrical energy is converted to mechanical energy (in the motor), or mechanical energy is converted to electrical energy (in the generator). For the machine to be efficient, these losses must be kept to a minimum. Some losses are electrical, others are mechanical. Electrical losses are classified as copper losses and

Losses occur when electrical energy is converted to mechanical energy (in the motor), or mechanical energy is converted to electrical energy (in the generator). For the machine to be efficient, these losses must be kept to a minimum. Some losses are electrical, others are mechanical. Electrical losses are classified as copper losses and

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