Transcript Synchronous Motors

EC010504(EE) Electric
Drives & Control
Dr. Unnikrishnan P.C.
Professor, EEE
Module II
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Transformer √
Three Phase Induction Motor √
Single Phase Induction Motor √
Alternator
Synchronous Motor
Synchronous Generator-Alternator
Synchronous Motors
• Constant-speed machine
• Propulsion for “Queen Elizabeth 2”
– 44 MW
– 10 kV
– 60 Hz
– 50 pole
– 144 r/min
Queen Elizabeth 2is an ocean liner built for the Cunard
Line which was operated by Cunard as both
a transatlantic liner and a cruise ship from 1969 to
2008. She was designed for the transatlantic service
from her home port of Southampton, UK, to New York
Synchronous Motors ………
• Construction
– Stator identical to that of a three-phase induction
motor – now called the “armature”
– Energize from a three-phase supply and develop
the rotating magnetic field
– Rotor has a DC voltage applied (excitation)
– Rotor could be a permanent-magnet type
Synchronous Motors (continued)
• Operation
– Magnetic field of the rotor “locks” with the
rotating magnetic field – rotor turns at
synchronous speed
Stator Construction
• Stator is identical to
the induction motor
• Laminated low
silicon steel rings
joined together
• Slots insulated with
Mylar
• Example of 36 slot stator
with 3 coil conductors per slot,
12 slots per phase
Stator Construction
Rotor Construction
Two Types of Rotor
• Salient Pole
• Cylindrical
Rotor Construction
Salient-Pole Rotor with brushless excitation
Rotor Construction
Operation as a Synchronous Generator
Equations
Synchronous Speed
𝑁𝑠 =
120 𝑓
𝑃
𝑅𝑃𝑀
where f = supply frequency required and P = Poles
Induced EMF in an alternator
𝐸𝑅𝑀𝑆 = 2.22  𝑓 𝑧 = 4.44  𝑓 𝑇𝑝ℎ Volts
where  = Flux per pole set up by rotor current
Z = Conductor in series per phase
𝑧
𝑇𝑝ℎ = 𝑇𝑢𝑟𝑛𝑠 𝑝𝑒𝑟 𝑝ℎ𝑎𝑠𝑒 =
2
Synchronous Impedance
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E = Induced Emf per phase
V = Terminal voltage per phase
𝐼𝑎 = Armature current per phase
𝑅𝑎 = Armature resistance per phase
𝑋𝑙 = Armature leakage reactance per phase
𝑋𝑎 = Reactance per phase representing Armature
reaction
Induced emf per phase 𝑬 = 𝑽 + 𝑰𝒂 𝑹𝒂 +j𝑰𝒂 𝑿𝒍 +𝒋𝑰𝒂 𝑿𝒂
𝑬 = 𝑽 + 𝑰𝒂 𝑹𝒂 +j𝑰𝒂 (𝑿𝒍 +𝑿𝒂 ) = 𝑽 + 𝑰𝒂 (𝑹𝒂 +j𝑿𝒔 ) = 𝑽 + 𝑰𝒂 𝒁𝒔
𝑿𝒔 = (𝑿𝒍 +𝑿𝒂 ) is called synchronous reactance
𝒁𝒔 = (𝑹𝒂 +j𝑿𝒔 ) is called synchronous Impedance
Phasor Diagram of an Alternator at Lagging pf
load
BE=VCos∅
𝑂𝐷 = 𝑉𝑆𝑖𝑛∅
E = (𝑉 𝑐𝑜𝑠∅ + 𝐼𝑎 𝑅𝑎 )2 +(𝑉𝑠𝑖𝑛∅ + 𝐼𝑎 𝑋𝑠 )2
−−−− −𝐹𝑜𝑟 𝐿𝑎𝑔𝑔𝑖𝑛𝑔 𝑝𝑓
E = (𝑉 𝑐𝑜𝑠∅ + 𝐼𝑎 𝑅𝑎 )2 +(𝑉𝑠𝑖𝑛∅ − 𝐼𝑎 𝑋𝑠 )2
−−−− −𝐹𝑜𝑟 𝐿𝑒𝑎𝑑𝑖𝑛𝑔 𝑝𝑓
Voltage Regulation
A convenient way to compare the voltage behaviour of
two generators is by their voltage regulation (VR).
• % 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑅𝑒𝑔𝑢𝑙𝑎𝑡𝑖𝑜𝑛 =
𝐸−𝑉
𝑉
x 100
Pre-Determination of Voltage Regulation
1. EMF Method or Synchronous Impedance
Method
2. MMF Method
EMF Method or Synchronous
Impedance Method
Circuit diagram for open circuit and short circuit test on alternator
EMF Method ……
• Open circuit characteristics (OCC) (𝑉𝑜𝑐 )𝑝ℎ 𝑉𝑠 𝐼𝑓
• Short Circuit Characteristics (SCC) (𝐼𝑎 )𝑠𝑐 𝑉𝑠 𝐼𝑓
• The Armature resistance per phase 𝑅𝑎
From the Equivalent Circuit:
𝑍𝑠 =
𝑋𝑠 =
Equivalent circuit on short circuit
𝐸𝑝ℎ
𝐼𝑎 𝑠𝑐
=
(𝑉𝑜𝑐 )𝑝ℎ
(𝐼𝑎 )𝑠𝑐 𝑓𝑜𝑟 𝑠𝑎𝑚𝑒 𝐼
𝑓
(𝑍𝑠 )2 − (𝑅𝑎 )2

𝑝ℎ
OCC & SCC of an Alternator
EMF Method ……
No load induced e.m.f. per phase, Eph can be
determined from the equation
𝐸𝑝ℎ =
(𝑉 𝑐𝑜𝑠∅ + 𝐼𝑎 𝑅𝑎 )2 +(𝑉𝑠𝑖𝑛∅  𝐼𝑎 𝑋𝑠 )2
where
Vph = Phase value of rated voltage
Ia = Phase value of current depending on the load condition
cosΦ = p.f. of load
% 𝑅𝑒𝑔𝑢𝑙𝑎𝑡𝑖𝑜𝑛 =
𝐸𝑝ℎ − 𝑉𝑝ℎ
𝑉𝑝ℎ
x 100
EMF Method Advantages & Limitations
• Advantage: synchronous impedance Zs for
any load condition can be calculated. Hence
regulation of the alternator at any load
condition and load power factor can be
determined.
• Limitation: This method gives large values of
synchronous reactance. This leads to high
values of % regulation than the actual. Hence
this method is called Pessimistic method.
MMF Method or Ampere Turn Method
• The effect of armature leakage reactance by an equivalent
additional m.m.f so that this m.m.f may be combined with
the armature reaction m.m.f.
• An alternator requires m.m.f. which is product of field
current and turns of field winding-two components
1. An m.m.f. necessary to induce the rated terminal
voltage on open circuit.
2. An m.m.f. equal and opposite to that of armature
reaction m.m.f.
• The number of turns in the field winding is not known
normally, so the m.m.f. is calculated in terms of the field
current itself.
MMF Method ………….
• The field m.m.f. required to induce the rated terminal
voltage on open circuit can be obtained from o.c.c. This is
denoted as 𝐹𝑂 .
• In s.c. test, field m.m.f. is necessary to overcome drop
across armature resistance and leakage reactance and
also to overcome effect of armature reaction
• But drop across armature resistance and leakage
reactance is very small and can be neglected.
• So in s.c. test, field m.m.f. circulates full load current to
balance the armature reaction effect.
• Ampere-turns required to circulate full load current can
be obtained from s.c.c. Denoted as 𝐹𝐴𝑅 .
MMF Method ………….
MMF Method ………….
• At full load, the total field m.m.f. is the vector sum
of its two components 𝐹𝑂 and 𝐹𝐴𝑅 denoted by 𝐹𝑅
• This depends on the power factor of the load which
alternator is resultant field m.m.f. is denoted as FR.
Let us consider the various power factors and the
resultant 𝐹𝑅 .
MMF Method ………….
Let us consider the various power factors and
the resultant 𝐹𝑅 .
Zero lagging p.f. :
The armature reaction is completely demagnetizing. Hence the resultant 𝐹𝑅 is the
algebraic sum of 𝐹𝑂 and 𝐹𝐴𝑅
MMF Method ………….
Zero leading p.f. :
The armature reaction is completely
magnetizing. Hence the resultant 𝐹𝑅 is the
algebraic difference of 𝐹𝑂 and 𝐹𝐴𝑅
MMF Method ………….
Unity p.f. :
The armature reaction is completely crossmagnetizing (Distorting). Resultant 𝐹𝑅 is the
vector sum of 𝐹𝑂 and 𝐹𝐴𝑅
MMF Method ………….
Lagging p.f. :
The component 𝐹𝑂 is at right angles to 𝑉𝑝ℎ while 𝐹𝐴𝑅 is in
phase with the current (𝐼𝑎 )𝑝ℎ . 𝐹𝑅 is the vector sum of
𝐹𝑂 and 𝐹𝐴𝑅
(𝐹𝑅 )2 = 𝐹𝑂 + 𝐹𝐴𝑅 𝑠𝑖𝑛∅
2
+ 𝐹𝐴𝑅 𝑐𝑜𝑠∅
2
MMF Method ………….
Leading p.f. :
The component 𝐹𝑂 is at right angles to 𝑉𝑝ℎ while 𝐹𝐴𝑅 is in
phase with the current (𝐼𝑎 )𝑝ℎ . 𝐹𝑅 is the vector sum of
𝐹𝑂 and 𝐹𝐴𝑅
(𝐹𝑅 )2 = 𝐹𝑂 − 𝐹𝐴𝑅 𝑠𝑖𝑛∅
2
+ 𝐹𝐴𝑅 𝑐𝑜𝑠∅
2
MMF Method ………….
Once 𝐹𝑅 is known, obtain corresponding voltage which is
induced e.m.f. 𝐸𝑝ℎ , required to get rated terminal voltage
𝑉𝑝ℎ is obtained from o.c.c.
Operation as a Synchronous Motor