alternating current machines 10&11
TRANSCRIPT
L/O 10 & 11 jh 3/05 1
3 Phase Synchronous Machines
Learning 10 & 11
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A cage type induction motor will travel at a speed slightly slower than synchronous speed due to slip.
If there were no slip the motor would travel at synchronous speed…..a cage motor can not deliver torque at synchronous speed.
A SYNCHRONOUS MOTOR has special DC windings which latch onto the rotating magnetic field.
The RMF drags the rotor around at synchronous speed.
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In simple terms….
A motor can generate and a generator can be made to motor.
A synchronous motor and an alternator are one and the same machine.
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DC field
AC output on rotor
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An alternator can motor and under bad circumstances drive the diesel engine like a compressor !
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Synchronous motor rotor
Rotor & amortisseur windings (bars)
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www.panasia.com
Salient poles on rotor
Synchronous Motors
Stator
Pony motor Excitation gear AC/stator windings
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Synchronous motor rotor with DC excitation unit. AC is induced into rotor of exciter and rectified to smooth DC for the DC fields which are embedded in the laminations. 4 pole ??
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Construction details.
• Large diameter rotors for slow moving machines. Often salient pole type.
• Small rotors for faster machines eg; 6,4,2 pole machines
• Stators are essentially the same for all speeds
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Frequency and speed.
f = ------------ Hz.n x p
120
n = RPM
f = frequency in hertz
P = number of poles
120 = 60 x 2
Minutes to seconds
Pairs of poles eg 1 x N & 1 x S
P 251 L/O 10.2
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For an ALTERNATOR.
What do we need to produce an EMF ????
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For an ALTERNATOR.
What do we need to produce an EMF ????
1. Conducting path
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For an ALTERNATOR.
What do we need to produce an EMF ????
1. Conducting path
2. Magnetic field
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For an ALTERNATOR.
What do we need to produce an EMF ????
1. Conducting path
2. Magnetic field
3. Relative motion between the 2
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Consider this equation
Vg = 4.44 x Ø x f x N x kd x kp
This is for explanation only
Not in the exam.
4.44 = 4 x 1.11 (form factor)
Ø = flux per pole
f = frequency (speed of cutting)
N = number of turns in coil
Kd & Kp = winding type and style
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Different shapes of sine waves
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Shows how each conductor/coil contributes to the sine wave within the alternator.
For interest only.
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Voltage regulation.
L/O 11.2 p 254
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No load volts Full load No load
100% load @ .8pf Vert = 20V/div Horiz = 1s/div
Voltage regulator catches load.
750kVA Alternator
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Alt
Alternator
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Alt
Load
Alternator
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Alt
LoadR
XLInternal
Impedance
Alternator
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As an alternator goes on load, the internal losses cause the output voltage to drop.
The powerfactor of the load affects the output voltage.
Jenneson p 254
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Drop to internal losses.
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Voltage regulation
Read carefully the example 11.3 p 254
Important….you need to know this !!
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Frequency .. No load to 100% FLC
Horiz = 1sec/div
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Nameplate data
L/O 10.4 etcNRG turbine set
L/O 10 & 11 jh 3/05 24NRG Turbine set ExciterbearingMain Altnr.
L/O 10 & 11 jh 3/05 25NRG gas turbine
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SlipringsTo take DC to the rotor
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The effects of change in excitation of an alternator.
Read carefully 11.3.4 this is difficult stuff.
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A stand alone alternator is one which delivers power to its own isolated load away from the power grid.
L/O 11.1 p 255
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A stand alone alternator is one which delivers power to its own isolated load away from the power grid.
Paralled machines are either supplying load to a grid system or they could be supplying a common load along with one or more alternators.
L/O 11.1 p 255
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A stand alone alternator is one which delivers power to its own isolated load away from the power grid.
Paralled machines are either supplying load to a grid system or they could be supplying a common load along with one or more alternators.
A machine could be
•“tied to the grid”
•Separate from the grid
•Paralled to one or more machines
L/O 11.1 p 255
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Parallel operation
1. Waveforms the same
2. Same phase sequence
3. Same voltages
4. Voltages in phase
5. Same frequency
These are usually ”user changeable”
Jenneson p 255
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The frequencies must be the same.
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Phase sequence need to be the same
The voltages and sequence need to be identical between the mains and the incoming machine.
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Alternator at Gladstone Base Hospital (one of 3 sets throughout the hospital.)
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3 phase Dunlite machine
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Small 1 phase portable 5kVA unit
750W portable unit
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Synchronous motors
In simple terms….
A DC field in the rotor “locks into” the rotating magnetic field of the stator and the rotor gets dragged around at synchronous speed.
Jenneson p 259 L/O 11Ref. to Ralph’s pp
L/O 10 & 11 jh 3/05 37Dragline machinesThe square grey machines are sync. motors
L/O 10 & 11 jh 3/05 38Transportable sub-station to power dragline.
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