Stepper Motor Basics

A stepper motor is an electromechanical device which converts electrical pulses into discrete mechanical movements. The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence. The motors rotation has several direct relationships to these applied input pulses. The sequence of the applied pulses is directly related to the direction of motor shafts rotation.
The speed of the motor shafts rotation is directly related to the frequency of the input pulses and the length of rotation is directly related to the number of input pulses applied.

- Stepper Motor Advantages and Disadvantages

Advantages:
1. The rotation angle of the motor is proportional to the input pulse.
2. The motor has full torque at standstill (if the windings are energized)
3. Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3 – 5% of a
step and this error is non cumulative from one step to the next.
4. Excellent response to starting/stopping/reversing.
5. Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is simply dependant
on the life of the bearing.
6. The motors response to digital input pulses provides open-loop control, making the motor simpler and less
costly to control.
7. It is possible to achieve very low speed synchronous rotation with a load that is directly coupled to the shaft.
8. A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input
pulses.

Disadvantages
1. Resonances can occur if not properly controlled.
2. Not easy to operate at extremely high speeds.
Open Loop Operation
One of the most significant advantages of a stepper motor is its ability to be accurately controlled in an open loop system. Open loop control means no feedback information about position is needed. This type of control eliminates the need for expensive sensing and feedback devices such as optical encoders. Your position is known simply by keeping track of the input step pulses.

- Stepper Motor Types

There are three basic stepper motor types. They are :
• Variable-reluctance
• Permanent-magnet
• Hybrid
- Variable-reluctance (VR)
This type of stepper motor has been around for a long time. It is probably the easiest to understand from a structural point of view. Figure 1 shows a cross section of a typical V.R. stepper motor. This type of motor consists of a soft iron multi-toothed rotor and a wound stator. When the stator windings are energized with DC current the poles become magnetized. Rotation occurs when the rotor teeth are attracted to the energized stator poles.
- Permanent Magnet (PM)
Often referred to as a “tin can” or “canstock” motor the permanent magnet step motor is a low cost and low resolution type motor with typical step angles of 7.5° to 15°. (48 – 24 steps/revolution) PM motors as the name implies have permanent magnets added to the motor structure. The rotor no longer has teeth as with the VR motor.
Instead the rotor is magnetized with alternating north and south poles situated in a straight line parallel to the rotor shaft. These magnetized rotor poles provide an increased magnetic flux intensity and because of this the PM motor exhibits improved torque characteristics when compared with the VR type.
- Hybrid (HB)
The hybrid stepper motor is more expensive than the PM stepper motor but provides better performance with respect to step resolution, torque and speed. Typical step angles for the HB stepper motor range from 3.6° to 0.9° (100 – 400 steps per revolution). The hybrid stepper motor combines the best features of both the PM and VR type stepper motors. The rotor is multi-toothed like the VR motor and contains an axially magnetized concentric magnet around its shaft. The teeth on the rotor provide an even better path which helps guide the magnetic flux to preferred locations in the airgap. This further increases the detent, holding and dynamic torque characteristics of the motor
when compared with both the VR and PM types.
The two most commonly used types of stepper motors are the permanent magnet and the hybrid types. If a designer is not sure which type will best fit his applications requirements he should first evaluate the PM type as it is normally several times less expensive. If not then the hybrid motor may be the right choice.
There also excist some special stepper motor designs. One is the disc magnet motor. Here the rotor is designed sa a disc with rare earth magnets, See fig. 4. This motor type has some advantages such as very low inertia and a optimized magnetic flow path with no coupling between the two stator windings. These qualities are essential in some applications.
Size and Power In addition to being classified by their step angle stepper motors are also classified according to frame sizes which correspond to the diameter of the body of the motor. For instance a size 11 stepper motor has a body diameter of approximately 1.1 inches. Likewise a size 23 stepper motor has a body diameter of 2.3 inches (58 mm), etc. The body length may however, vary from motor to motor within the same frame size classification. As a general rule the available torque output from a motor of a particular frame size will increase with increased body length.
Power levels for IC-driven stepper motors typically range from below a watt for very small motors up to 10 – 20 watts for larger motors. The maximum power dissipation level or thermal limits of the motor are seldom clearly stated in the motor manufacturers data. To determine this we must apply the relationship P =V · I. For example, a size 23 step motor may be rated at 6V and 1A per phase. Therefore, with two phases energized the motor has a rated power dissipa-tion of 12 watts. It is normal practice to rate a stepper motor at the power dissipation level where the motor case rises 65°C above the ambient in still air. Therefore, if the motor can be mounted to a heatsink it is often possible to increase the allowable power dissipation level. This is important as the motor is designed to be and should be used at its maximum power dissipation ,to be efficient from a size/output power/cost point of view.

- When to Use a Stepper Motor

A stepper motor can be a good choice whenever controlled movement is required. They can be used to advantage in applications where you need to control rotation angle, speed, position and synchronism. Because of the inherent advantages listed previously, stepper motors have found their place in many different applications.
Some of these include printers, plotters, scanners, highend office equipment, hard disk drives, fax machines and many more.

STARTING THE SYNCHRONOUS MOTOR

Synchronous motors are started using several reduced voltage methods. The most common is starting across the line with full ac voltage to the armature windings. The squirrel cage windings begin the task of accelerating the motor from zero speed. As the motor speed increases, the discharge resistor provides the torque required for the motor to reach synchronous speed. When the synchronous speed is reached, the starting resistor is switched out of the field circuit and excitation can be applied to lock the stator and field poles into synchronism.
It is important to properly time the application of excitation to the main field. The purpose of the dc excitation system is to apply current to the field winding, creating a rotating electromagnet field that couples the rotor field to the rotating ac field in the armature winding when the motor is operating at synchronous speed. When dc excitation is applied to the motor field, the position of the rotor with respect to the stator magnetic field determines the reaction of the rotor.
If the N and S rotor and stator poles are aligned, such that the magnetic flux lines flow easily from the rotor through theairgap, the rotor flux will lock in step with the stator flux and the motor will become synchronous.
If the rotor poles are 180 electrical degrees out of phase with the stator poles, but motor acceleration is decreasing the angle of displacement, it is likely that accelerating torque plus magnetic attraction will combine to draw the rotor rapidly into pole alignment with the stator. Synchronizing additionally depends on the slip frequency between rotor and stator. Synchronizing torque from the magnetic linkage of rotor and stator must be sufficient to accelerate the rotor to keep it locked in step.
Another approach to motor startup is switching out the starting resistor and applying excitation based upon time after the motor ac supply power is applied. Here, a dc contactor closes applying excitation to the field after a fixed time. This approach can be used if the acceleration time of the motor is known and the motor is able to reach nearly synchronous speed without excitation.
In some applications, a speed signal is used to apply excitation when the motor has accelerated
to 92 - 95% of rated speed. The precise timing for switching out the starting resistor and applying dc to the main field is monitored by electronics on the rotating field.
The most straightforward approach to synchronous motor starting is to monitor the frequency of the voltage across the field starting resistor, as the motor nears synchronous speed, the slip frequency approaches zero across the resistor. At a specific slip frequency and rotor angle, dc is applied to the main field and the starting resistor is switched out to provide a very smooth transition from “starting” to synchronous operation. Application of the field can be most reliably performed using solid state devices instead of mechanical breakers or contactors. Most brushless excitation systems include such a scheme; similar devices can be used with brush-type systems to perform the same function.

CONSTRUCTION OF A SYNCHRONOUS MOTOR

Synchronous motors, like synchronous generators, consist of a fixed stator and a field that rotates concentric with the stator. The stator contains armature windings that are electrically connected to the ac supply system while the rotor contains a field winding that is electrically connected to a source of excitation (dc). Since the primary purpose of the field winding is to create a rotating magnetic field, the field winding is wound around “poles” attached to the rotor in a configuration that produces magnetic north and south poles that are 180 electrical degrees apart. During starting of the motor, the field winding is not effectively coupled with the armature windings in the stator, and no net torque is produced in the field when ac power is connected to the stator winding. To produce starting torque, a supplementary winding is provided on the rotor that effectively couples electromagnetically with the armature windings. This winding is a “squirrel cage” arrangement of bars placed across each poleface that are electrically shorted at each end.
The squirrel cage winding on the rotor is formally known as the damper or ammortiseur winding. When ac power is connected to the stator, current is induced in the squirrel cage winding. This results in a net torque that is applied to the rotor. This squirrel cage winding is also used in the induction motor to produce motor torque during starting and running operation. Torque is produced by the electromagnetic interaction of stator and rotor only when a slip speed exists. Thus, the induction motor speed increases from zero until just below synchronous speed as torque decreases with increasing rotor speed.

The synchronous motor then can be started like an induction motor, with the torque on the rotor dependent on the difference between the rotor speed and the frequency of the power being applied to the stator winding. The torque supplied by the squirrel cage is at a maximum when ac power is first supplied to the stator winding, decreases as the rotor accelerates, and approaches zero as the rotor approaches synchronous speed. The absolute value of the accelerating torque is a function of the resistance of the bars in the squirrel cage:
• Higher resistance bars produce higher starting torque (and hotter squirrel cage
windings)
• Lower resistance bars produce lower starting torque with less heat generation.
If the starting torque produced by the squirrel cage winding is not adequate to roll the rotor, the rotor is said to be “locked” and ac power must be quickly removed from the stator windings to avoid overheating both the armature and the squirrel cage windings.
The stator winding is connected to the ac supply system at startup, and the bars of the squirrel cage winding on the rotor produces an accelerating torque on the rotor. The field winding is connected to a field discharge resistor during startup, and no external excitation is applied to the field.

As the rotor accelerates, the field winding is coupled to the stator field via the armature winding. AC current is induced into the field based upon the difference between the frequency of the applied ac voltage to the motor and the frequency associated with the instantaneous speed of the rotor. If the field winding were open circuited during startup, dangerously high voltages could be induced in the field winding, hence a starting resistor is used to limit the voltage seen by the field winding during startup while dissipating the energy induced in the field. The resistance of the starting resistor also affects the synchronizing torque available as the rotor approaches rated speed:
• Synchronizing torque increases with the resistance of the field discharge resistor.
A low resistance produces lower synchronizing torque.
• Insulation limits of the field winding limit the resistor, because field voltage increases
with increasing values of discharge resistor.
Sizing the starting resistor is an “art” that balances starting torque and allowable field voltage during startup. In some applications, the load is removed from the synchronous motor during startup and applied only after the motor has reached stable operation at synchronous speed, in order to reduce torque required to start.

Synchronous Motor Characteristics

Synchronous Motors are three-phase AC motors which run at synchronous speed, without slip.
(In an induction motor the rotor must have some “slip”. The rotor speed must be less than, or lag
behind, that of the rotating stator flux in order for current to be induced into the rotor. If an induction
motor rotor were to achieve synchronous speed, no lines of force would cut through the rotor,
so no current would be induced in the rotor and no torque would be developed.)
Synchronous motors have the following characteristics:
· A three-phase stator similar to that of an induction motor. Medium voltage stators are often used.
· A wound rotor (rotating field) which has the same number of poles as the stator, and is supplied by an external
source of direct current (DC). Both brush-type and brushless exciters are used to supply the DC field current to
the rotor. The rotor current establishes a north/south magnetic pole relationship in the rotor poles enabling the
rotor to “lock-in-step” with the rotating stator flux.
· Starts as an induction motor. The synchronous motor rotor also has a squirrel-cage winding, known as an
Amortisseur winding, which produces torque for motor starting.
· Synchronous motors will run at synchronous speed in accordance with the formula:

Synchronous RPM =(120 x Frequency)\(Number of Poles)

Example: the speed of a 24 -Pole Synchronous Motor operating at 60 Hz would be:
120 x 60 / 24 = 7200 / 24 = 300 RPM
Synchronous Motor Operation
· The squirrel-cage Amortisseur winding in the rotor produces Starting Torque and Accelerating Torque to
bring the synchronous motor up to speed.
· When the motor speed reaches approximately 97% of nameplate RPM, the DC field current is applied to the
rotor producing Pull-in Torque and the rotor will pull-in -step and “synchronize” with the rotating flux field in
the stator. The motor will run at synchronous speed and produce Synchronous Torque.
· After synchronization, the Pull-out Torque cannot be exceeded or the motor will pull out-of-step. Occasionally,
if the overload is momentary, the motor will “slip-a-pole” and resynchronize. Pull-out protection must be
provided otherwise the motor will run as an induction motor drawing high current with the possibility of
severe motor damage.
Advantages of Synchronous Motors
The initial cost of a synchronous motor is more than that of a conventional AC induction motor due to the
expense of the wound rotor and synchronizing circuitry. These initial costs are often off-set by:
· Precise speed regulation makes the synchronous motor an ideal choice for certain industrial processes and as a
prime mover for generators.
· Synchronous motors have speed / torque characteristics which are ideally suited for direct drive of large horsepower,
low-rpm loads such as reciprocating compressors.
· Synchronous motors operate at an improved power factor, thereby improving overall system power factor and
eliminating or reducing utility power factor penalties. An improved power factor also reduces the system voltage
drop and the voltage drop at the motor terminals.

GENERATOR COMPONENTS AND EXCITATION SYSTEM

Introduction

In a gas turbine set the rotating turbine drives the compressor and accessory equipment with available excess energy to produce shaft power which drives the generator. Approximately one half of the work produced is available to produce electricity; the other half is used to drive the compressor. The function of the generator is to convert mechanical energy of the turbine into electrical energy.

The generator is connected via a breaker and a generator transformer to the high voltage switchgear. During start-up of the gas turbine set, the generator is used as a motor to accelerate the gas turbine and the compressor. Therefore a static frequency converter is connected to the medium voltage switchgear. Its output is connected to the stator winding of the generator for feeding a rotating current of increasing frequency. For generation of a rotating magnetic field a static excitation system feeds the rotor of the generator of a direct current (DC). In order to couple the rotating magnetic field of the stator and the physically rotating rotor, the rotor is excited by DC. The thus resulting rotor field and stator field exert forces/torques upon each other coupling the “current rotation” in the stator to the rotation of the rotor.

THE ROTOR

The rotor is made from a single steel forging. The steel is vacuum-degassed to minimize the possibility of hydrogen-initiated cracking. Reheating and quenching also hardens the forging. Stress-relieving heat treatment is done following rough machining. Ultrasonic examination is performed at various stages of the rotor. The generator counter torque increases to 4 to 5 times the full-load torque when a short circuit occurs at the generator terminals. The rotor and turbine-end coupling must be able to withstand this peak torque.
ROTOR WINDING

Each winding turn is assembled separately in half-turns or in more pieces. The joints are at the centers of the end turns or at the corners. They are brazed together after assembling each turn,

to form a series-connected coil. The coils are made of high-conductivity copper with a small amount of silver to improve the creep properties. The gas exits through radially aligned slots.

Slot liners of molded glass fiber insulate the coils. These separators of glass fiber are used between each turn. They insulate against almost 10 V between adjacent turns.

The end rings and end discs are separated from the end windings by thick layers of insulation. Insulation blocks are placed in the spaces between the end windings to ensure the coils do not distort. The winding slots are cut in diametrically opposite pairs. They are equally pitched over two-thirds of the rotor periphery, leaving the pole faces without winding slots. This results in a difference between the stiffness in the two perpendicular axes. This difference leads to vibration at twice the speed. Equalizing slots are cut in the pole faces to prevent this problem from occurring. The slots are wider and shallower than the winding slots. They are filled with steel blocks to restore the magnetic properties. The blocks contain holes to allow the ventilating gas to flow.

The average winding temperature should not exceed 115°C. The hydrogen enters the rotor from both ends under the end windings and emerges radially from the wedges. Flexible leads made of thin copper strips are connected to the ends of the winding. These leads are placed in two shallow slots in the shaft. Wedges retain them. The leads are connected to radial copper studs, which are connected to D-shaped copper bars placed in the shaft bore. Hydrogen seals are provided on the radial studs. The D-leads are connected to the slip rings by radial connection bolts.
ROTOR END RINGS

The end rings are used to restrain the rotor end windings from flying out under centrifugal forces. These rings have traditionally been made from nonmagnetic austenitic steel, typically 18 percent Mn, 4 percent Cr. A ring is machined from a single forging. It is shrunk-fit at the end of the rotor body. The material of the end rings was proven to be susceptible to stress corrosion cracking. A protective finish is given to all the surfaces except the shrink-fit to ensure that hydrogen, water vapor, and so forth do not contact the metal.

The rings should be removed during long maintenance outage (every 8 to 10 years) and inspected for detailed surface cracking using a fluorescent dye. Ultrasonic scanning is not sufficient due to the coarse grain structure. A recent development has proven that austenitic steel containing 18 percent Mn and 18 percent Cr is immune to stress-corrosion cracking. New machines use this alloy. It is also used for replacement rings. This eliminates the need for periodic inspection. It is important to mention that a fracture of an end ring can result in serious damage to the machine and at least a few months’ outage. It is highly recommended to replace the traditional material with the new material.
Stator Casing

The stator core and core frame are mounted inside the casing. The casing must withstand the load and fault torques. It must also provide a pressure-tight enclosure for the hydrogen. Annular rings and axial members are mounted inside the casings to strengthen them and allow the hydrogen to flow. The end shields are made of thick circular steel plates. They are reinforced by ribs to withstand the casing pressure with minimal axial deflection. The stationary components of the shaft seal are housed in the end shields. The outboard bearing is also housed inside the end shields in some designs. The sealing of the end shield and casing joints must be leak free against hydrogen pressure.

BRUSHLESS EXCITATION SYSTEMS

Most modern gas turbines use brushless excitation systems. The rotating diodes are arranged as a three-phase bridge. The bridge arm consists of two diodes in series. If one of them fails (due to a short circuit), the second diode will continue to operate. Thus, the bridge continues to operate normally. If both diodes fail in the same arm, the fault is detected by a monitoring circuit, which trips the machine. Essential measurements, such as ground-fault indication, field current, and voltage, are taken by telemetry or instrument slip rings.
The Rotating Armature Main Exciter

Brushless machines require less maintenance than conventional ones. They also do not have sliding or rubbing electrical contacts that cause sparking and carbon dust. The main exciter is a three-phase rotating armature AC generator. The DC field is in the stator, and the AC winding is on the rotor. The exciter armature is made of low-loss steel laminations. The laminations are shrunk onto a shaft forged from annealed-carbon steel. Cooling air enters axial slots along the rotor body. The rotor conductors are made of braided strips in parallel. They are radially transposed to reduce eddy current losses. It is mounted on the outboard of the main AC exciter. The three-phase AC power is supplied from the main exciter to the silicon diode rectifier by axial conductors taken along the surface of the shaft. A steel retaining ring contains the components of the rectifier against centrifugal forces. The retaining ring is shrunk on the outside of the hub.

THE VOLTAGE REGULATOR
Background

Early voltage regulators used mechanical components. They had a large deadband, long response time, and required regulator maintenance. Modern AVRs use integrated circuits or digital microprocessor techniques.
System Description

The main function of the AVR is to maintain constant generator terminal voltage while the load conditions are changing. A dual-channel AVR with manual backup is normally used. The reliability of this design is high because the loss of one channel does not affect operational performance. The faulty channel can be repaired during operation.
The Regulator

The AVR is a closed-loop controller. It compares a signal proportional to the terminal voltage of the generator with a steady voltage reference. The difference (error) is used to control the exciter output.

When the load changes, the error increases. The channel A AVR applies a proportional-integral-derivative (PID) algorithm to the error and provides a corrective signal. This signal is amplified by the channel A converter. It is then sent to vary a field resistance. The excitation current will change, and the terminal voltage will change accordingly. It is critical to have a fast, stable response from the AVR. Special signal conditioning networks are introduced in the PID control to prevent instability. Accurate tuning (selection of PID coefficients) of the voltage response is achieved by having adjustable time constants. The AVR receives the generator terminal voltage signal through its own interposing voltage transformer. The voltage signal is rectified and filtered before comparing it with the reference voltage.

Neutral Grounding

The neutral ends of the three stator winding phases are connected together outside the casing. The star point is connected to ground through a neutral grounding device. It is designed to limit the fault current upon a ground fault in the stator winding. The neutral grounding device consists of a single-phase transformer. Its primary is connected between the generator star point and ground. Its secondary is connected to a resistor. This arrangement is chosen because the apparent impedance of the resistor appears on the primary side. This creates a very high impedance that limits the fault current to 15 A.

The End

Introduction of Electrical Energy

Electrical Energy is considered the backbone of modern civilization. Many applications could not be practically realized without this type of energy. Though electricity is often utilized after being transformed into other useful forms of energy, still it represents a clean, efficient and easily controlled way of transmitting energy from one place to another.
Electrical energy can be produced by different methods namely;
- Chemical reaction as in dry cells, batteries ……………..etc.
- Thermal energy as in thermo-couples.
- Solar energy as in solar cells.
- Mechanical energy as in Dynamos.
Most of electrical energy used in practice is being produced by the last method, since a reasonable amount of energy could be generated in less space at a moderate cost and in a rather efficient means. Electrical machine by definition is a bidirectional electro-mechanical energy converter, i.e. it can change mechanical energy into electrical one and hence acts as a GENERATOR or vice versa and thereby acts as a MOTOR.