PowerPoint slides t/a Electrical Principles for the Electrical Trades (Machines) Vol 2 6e by Jim...

PowerPoint slides t/a Electrical Principles for the Electrical Trades (Machines) Vol 2 6e by Jim Jenneson and Bob HarperCopyright © 2012 McGraw-Hill Australia Pty Ltd

7–1

Chapter 7Synchronous machines


7–2

Purpose

• This chapter describes the construction and operating characteristics of synchronous machines.

• It outlines requirements for the parallel operation of alternators.


7–3

Introduction

• A machine designed to be connected to the supply and run at synchronous speed is called a synchronous machine.

• The synchronous generator is also referred to as an alternator.

• The principles of construction and operation for alternators and synchronous motors are similar.


7–4

Three-phase alternator construction

• The three-phase synchronous machine has two main windings:– a three-phase AC winding– another winding that carries DC.

• Typically the rotor has the DC winding and the stator has the AC winding.


7–5

Three-phase alternator construction (continued)

• The advantages of a DC rotor include:– extra winding space for the AC windings– easier to insulate for higher voltages– simple and strong rotor construction– lower voltages and currents in the rotating

windings– high current windings have solid

connections to external circuits– better suited to the higher speeds of

turbine drives.


7–6

• The stator of the three-phase synchronous machine consists of a slotted laminated core into which the stator winding is fitted.

• The stator winding consists of three identical sets of coils set out in a sequential series to form a definite number of magnetic poles. The three separate windings are physically displaced by 120°E.



7–7

• There are two types of alternator rotors:– low speed (salient pole)– high speed (cylindrical).

• The low speed rotor consists of a spider similar to that used in DC machines. The field poles and coils are bolted to this spider. The physical constraints of this construction limit the use of this type of rotor to low speeds.



7–8

• The high speed rotor counteracts centrifugal forces by having a small diameter compared to its length.



7–9

• Low speed prime movers include:– diesel engines– hydroelectric turbines.

• High speed prime movers include:– steam turbines– gas turbines.



7–10

• The relationship between speed, frequency and number of poles is:

f = np/120

Where:n = speed of the rotating magnetic field in rpm

f = supply frequency in Hz

p = number of poles



7–11



7–12

• Cooling of low speed alternators is typically straight forward due to the larger diameter and shorter axial length that provides a large surface area for radiation and the fanning effect of the rotor’s movement.

• The fanning effect can be assisted by the addition of fan blades and larger machines can have ducts within the core to assist cooling.



7–13

• Cooling of high speed alternators is problematic due to the length of the machine.

• Typically these machines are totally enclosed and clean cooling air is forced through the machine.

• The use of hydrogen instead of air improves cooling but increases the complexity of the auxiliary systems and is therefore used only on very large capacity machines.



7–14

• DC excitation of the rotor windings is usually achieved by a DC generator called an exciter.

• The exciter is coupled to the rotor shaft.

• Adjustment of the exciter field allows the strength of the magnetic field of the rotor to be varied.



7–15



7–16

• Some large DC exciters have their own exciter.

• Some alternators use a brushless excitation system.



7–17

• The generated voltage can be calculated using:

Vg = 4.44ФfNkdkp

Where:Vg = generated voltage per phase in VФ = flux per pole in Wbf = frequency in HzN = number of turns per phasekd = constant dependent on winding distributionkp = constant dependent on coil pitch



7–18

• Since the alternator characteristics and frequency are constant the generated voltage is dependent on the strength of the rotor flux.



7–19

• An alternator on load can be analysed by considering the equivalent circuit.



7–20

• The nature of the load (resistive or reactive) will affect the alternator’s output voltage.



7–21

• The voltage output from the alternator on load is therefore dependent on the type and size of the connected load.



7–22

• The voltage regulation of an alternator can be calculated:

VR% = ((VNL – VFL)/VFL) x 100%

Where:VR% = % voltage regulation

VNL = no load voltage in V

VFL = full load voltage in V



7–23



7–24

• An alternator is rated according to:– frequency– voltage– current.

• The output is given in VA since the power factor of the load is beyond the control of the manufacturer.



7–25



7–26

Parallel operation of alternators

• To deliver supply to varying loads at a constant voltage and optimum efficiency it is often necessary to connect multiple alternators in parallel.


7–27

Parallel operation of alternators (continued)

• The requirements for parallel connection of alternators are:– identical output waveforms– the same phase sequence– the same alternator and supply voltages– the alternator and supply voltages in

phase– identical alternator and supply

frequencies.


7–28

• Synchronising is the process of ensuring that the alternator and supply voltages are in phase.

• Smaller alternators may be synchronised using incandescent lamps, but a more exact method involves the use of a synchroscope.



7–29

• The three dark method of synchronising alternators requires the lamps to all become bright and dark simultaneously.



7–30

• If the lamps dim in sequence it means that the phase sequence of the alternator is reversed.

• The rate of lamp flicker indicates the difference in frequencies of the alternator and the supply. When the flickering stops the frequencies are equal.

• When the lamps are dark the alternators can be connected in parallel.



7–31

• The disadvantage of this method is that the voltage across the lamps may not be exactly zero. For smaller alternators self adjustment will occur but for larger alternators this voltage difference could cause considerable damage.

• Greater accuracy can be obtained by following the three dark method with the two bright, one dark method.



7–32



7–33

• With the lamps reconnected in this manner the lamps will go dark and bright in sequence. The order of brightness is an indication of the relative speeds of the alternators.

• Synchronism occurs when the right-hand side lamp is dark and the other two are of equal brilliance.



7–34

• A synchroscope is an instrument that indicates the phase relationships and relative speed for an incoming alternator.

• It has a two-phase stator connected to the incoming alternator. The rotor is connected to the supply. Any difference between the frequencies causes a pointer connected to the rotor to rotate. The direction and speed of rotation indicates the relative speeds of the two alternators.



7–35

• When the pointer is stationary and pointing towards the 12 o’clock position the two machines are in phase and can be paralleled.



7–36

• Automatic synchronisation is achieved using microprocessor control.

• The microprocessor can be used to control start-up and connection to the supply for back-up power supplies. During operation of the alternator the microprocessor can monitor operating conditions and make adjustments as required.



7–37

• An alternator may operate:– on its own– in parallel with an alternator of the same

size– in parallel with a distribution grid.



7–38

• The operation of an alternator that is operating on its own is controlled by the governor and the field current regulator.

• With changes in load the governor adjusts the power of the prime mover to maintain the frequency. At the same time, the voltage regulator will adjust the field current to maintain a constant output voltage.



7–39

• When an alternator is connected in parallel with another of the same size, adjustments to the governor and the field current regulator of one alternator will affect the performance of the other alternator.

• To adjust the load sharing, the governor set point on one alternator must be increased while the governor set point of the other is.



7–40

• To adjust the system frequency the set points on both alternators must be adjusted up or down at the same time.

• Adjustment of the field current to both alternators at the same time will adjust the system voltage. Adjustment of the field current to one alternator only will change the power factor of the load supplied by each alternator.



7–41

• When an alternator is connected to a large transmission system the system controls the voltage and frequency of the alternator.

• To adjust the power supplied to the grid by the alternator, the set point on the governor of the alternator is altered. Similarly, the alternator’s field current regulator controls the power factor of the load taken by that alternator.



7–42

• Hunting in an alternator is the variation of its speed about the set value. This occurs due to small variations in the speed of the prime mover or governor adjustments due to load changes.

• Hunting causes small voltage variations and the production of harmonics that distort the waveform. It can also cause circulating currents to flow between parallel connected alternators, resulting in increased mechanical oscillations and electrical losses.



7–43

• Hunting can be minimised by the use of quite heavy flywheels and amortisseur windings.



7–44

Standby power supplies

• Standby power supplies are generally intended to provide mains power at a specified voltage and frequency.

• Uninterruptible power supplies (UPS) provide standby power where no interruption to a power supply can be tolerated.


7–45

Standby power supplies (continued)


7–46

• The UPS system maintains the supply from the battery bank via an inverter. More critical loads usually have an engine driven alternator on standby to ensure that the battery bank remains fully charged.

• Care must be taken to ensure that time ratings for UPS devices are not exceeded.



7–47

• Engine-driven alternators should be selected based on a number of factors including:– purchase price– type of prime mover– starting methods– load sizes and alternator capacities– operation of alternators.



7–48

• In general as the size of the generating set increases the cost per kVA decreases.

• The type of prime mover chosen will depend on:– efficiency– type of service– initial cost– the cost and availability of fuel.



7–49

• Starting methods are governed by the intended use of the generating unit. The quicker the changeover the more expensive the starting method.

• The generating unit must have the electrical capacity and engine power to maintain both the output voltage and the frequency during changes in the connected load.



7–50

• The operation of most standby and back-up alternators is beyond the operator’s control.

• The engine governor controls the speed and hence the supply frequency.

• The automatic voltage regulator controls the output voltage once the frequency is established.



7–51



7–52

Three-phase synchronous motors

• A three-phase synchronous motor has no starting torque. It must be brought up to speed by some other means, the rotor then follows the rotating magnetic field at synchronous speed.

• There is no slip for a synchronous motor, the load causes a torque angle between the rotor and the rotating magnetic field. If the load becomes too great the rotor pulls out of synchronism and stops.


7–53

Three-phase synchronous motors (continued)


7–54

• The stator of the three-phase synchronous motor is the same as that of an alternator or three-phase induction motor.

• When energised the stator produces a rotating magnetic field that rotates at synchronous speed.



7–55

• The rotor of a three-phase synchronous motor is similar to that of an alternator, but it usually has salient poles.

• When energised with DC it produces magnetic poles, which are attracted to those produced in the stator.



7–56

• A synchronous motor works on the principle of magnetic attraction between the rotating stator field and the rotor field.

• The rotor must be brought up to speed by some other means so that the two fields can magnetically lock together and the rotor will then rotate at synchronous speed.



7–57

• The movement of the rotor field with respect to the stator windings induces a voltage in each phase winding.

• The phase relationship between the induced and applied voltages is dependent on the relative positions of each rotor and stator pole, which in turn is dependent on the load applied to the motor.



7–58

• For fixed excitation, any increase in the load on a synchronous motor will cause an increase in the line current, at a lower power factor.



7–59

• For a constant load, variations in field excitation cause corresponding variations in the induced voltage in the stator windings.

• A reduction in the DC field excitation causes an increase in line current and a lagging power factor.

• An increase in the DC field excitation causes an increase in line current and a leading power factor.



7–60

• The stator current is a minimum at unity power factor.



7–61

• Care should be taken when adjusting the excitation of a synchronous motor. Both over-excitation and under-excitation can cause the synchronous motor to become unstable.

• Under-excitation can cause the motor to drop out of synchronism.

• Over-excitation can cause the line current and mechanical load to exceed the machine’s ratings.



7–62

• Changes in load can cause hunting of the rotor speed about the synchronous speed. This hunting causes an undesirable fluctuation in the line current to the motor.

• Hunting can be controlled by the inclusion of an amortisseur winding in the rotor pole faces. The copper bars of the amortisseur winding have an induced magnetic field that opposes the surging effect.



7–63

• The shorting-out bars of the amortisseur winding are often extended around the rotor, resulting in a squirrel cage type rotor winding about the salient poles.

• This squirrel cage winding can assist with starting by allowing the motor to be started as an induction motor.



7–64

• During starting the DC winding on the rotor is short-circuited and a reduced voltage is applied to the stator.

• The motor accelerates to a speed just below synchronous speed at which time the short is removed from the rotor and DC is applied. The stator voltage is increased to the full value. The rotor and stator fields lock together and the motor operates as a synchronous motor.



7–65

• Another method of starting synchronous motors is to use an auxiliary motor to accelerate the rotor towards synchronous speed.

• This method is expensive, particularly if high starting torques are required.



7–66

• Synchronous motors are used for:– power factor correction– voltage control– low speed drives– rock and ore crushing heads.



7–67

Single-phase synchronous motors

• Single-phase synchronous motors are used for applications with low torque requirements that require a constant operating speed.

• Two common types of single-phase synchronous motors are:– reluctance motors– hysteresis motors.


7–68

Single-phase synchronous motors (continued)

• The stator winding of the reluctance motor is similar to that of the split-phase or capacitor-start motor.

• The squirrel cage rotor is assembled from laminations that are cut to form definite salient poles.

• The number of stator poles is not necessarily equal to the number of rotor poles.


7–69



7–70

• The motor is started as an induction motor. The starting winding is disconnected at 75% of synchronous speed.

• The salient rotor poles are magnetised by the stator poles and they become locked together. The rotor is attracted to the stator pole that is fully magnetised and inertia carries the rotor past when this pole’s magnetism is decreasing.



7–71

• The rotor is then attracted by the next stator pole and the rotation continues.

• In this way each rotor pole travels through the space of two stator poles per cycle of supply frequency.



7–72

• If the number of rotor poles is a multiple of the number of stator poles, the motor will operate at a constant speed that is a submultiple of synchronous speed.

• This is called a subsynchronous reluctance motor.



7–73

• The rotor of a hysteresis motor is constructed from a specially hardened steel cylinder.

• The rotor is supported on a non-magnetic arbor and has substantial hysteresis losses.



7–74

• The hysteresis opposes any change in magnetic polarities of the rotor once they are established.

• The rotor poles lock into the stator poles of the opposite polarities and hence the rotor rotates at synchronous speed.

• The lack of starting torque is overcome by the use of the shaded-pole principle.



7–75

• Single-phase synchronous motors are used for:– radio communications installations– recording devices– electric clocks– synchronous servo systems– aircraft instrumentation.



7–76

Quick quiz

1. What is an alternator?

2. What are two different types of rotors that are used in synchronous machines?

3. List 3 methods for synchronising alternators.

4. Name 2 standby power supplies.

5. List 2 single-phase synchronous motors.


7–77

Quick quiz—answers

1. A synchronous machine that is operating as a generator

2. Salient pole and cylindrical

3. Three dark lamps, two bright lamps and one dark lamp, and using a synchroscope

4. Uninterruptible power supplies (UPS) and engine driven alternators

5. Reluctance motors and hysteresis motors


7–78

Summary

• A synchronous machine rotates at a speed that is dependent on the line frequency and the number of poles.

• Typically the stator has the three-phase winding and the rotor has DC applied to it.

• Low speed machines have salient pole rotors.

• High speed machines have cylindrical rotors.


7–79

Summary (continued)

• Cooling of synchronous machines may be achieved using air or hydrogen blown through the machine.

• DC excitation is usually provided by a DC generator coupled to an alternator’s shaft.

• Alternator ratings are given in VA.


7–80

Summary (continued)

• The frequency of an alternator is controlled by the governor.

• The voltage of an alternator is controlled by the voltage regulator, which adjusts the field excitation.


7–81

Summary (continued)

• The requirements for parallel connection of alternators are:– identical output waveforms– the same phase sequence– the same alternator and supply voltages– the alternator and supply voltages in

phase– identical alternator and supply

frequencies.


7–82

Summary (continued)

• Synchronising is the process of ensuring that the alternator and supply voltages are in phase.

• Alternators may be synchronised using:– three dark lamps– two bright, one dark lamp– a synchroscope.


7–83

Summary (continued)

• When alternators are connected in parallel the governor set points and field excitations will affect the load sharing, frequency, supply voltage and power factor of the supply system.

• Hunting is the variation of speed about the synchronous speed.

• Amortisseur windings and heavy flywheels are methods used to minimise hunting.


7–84

Summary (continued)

• Synchronous motors have no starting torque. They must be brought up to speed so that the rotor magnetic field can lock in with the stator rotating magnetic field.

• Excessive loads cause the magnetic link to be broken and the rotor to cease rotating.

• Varying the field excitation will affect the power factor of the motor.


7–85

Summary (continued)

• Synchronous motors are usually started by one of two methods:– use of an auxiliary motor– connecting the motor as an induction

motor during starting.


7–86

Summary (continued)

• There are two main types of single-phase synchronous motors:– reluctance motor– hysteresis motor.

• Single-phase synchronous motors are inefficient and have low torque. They are used where a constant speed is necessary.

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