EE1F24- AC synchronous machine characteristics/windings Flashcards
(24 cards)
Why do we use stationary ac windings and rotating poles?
Only require 2 split rings as opposed to 4, these two only have to handle the smaller excitation current.
- Extra space allows to makes possible to use more insulation and therefore enable operating voltages up to 33kv
-Increased robustness allows higher speeds therefore greater outputs for given size
What are the two categories of Synchronous machines?
-Those with salient or projecting poles
-Those with cylindrical rotors
When are each of the two categories of synchronous machines used?
Salient poles-
What is a salient pole rotor?
Has projecting poles mounted onto a large diameter rotor core, each pole has its own field winding and sticks out like a spoke on a wheel.
What are the features of a salient pole rotor?
-Large Diameter short axial length
-clearly visible often laminated poles
-Air gap is non-uniform, larger between poles smaller underneath
-Each pole has field winding round wound it
-Good for slow, torque-rich operation
Why are salient pole rotors used at slow speeds?
-Large mechanical size
-Easier to cool at slow speeds
What is a cylindrical/non-salient rotor?
Smooth, solid steel cylinder with slots machined along surface to hold to field windings.
What are the features of a cylindrical rotor?
-Small diameter long axial length
-Uniform air gap all around
-Designed for high mechanical strength
-Essential for fast high power density machines
Why are cylindrical rotors used at high speeds?
-high speed creates enormous centrifugal force
-Stronger and better balanced
-Uniform air gap facilitates high-speed magnetic performance and prevents vibration
What are the two types of stator windings?
-Single-layer winding
-Double-layer winding
What is the main challenge of single-layer windings?
Main challenge is to arrange the end connections in a manner so that they do not obstruct one another.
What is armature reaction?
When load current flows in the stator winding it creates its own magnetic field which interacts with the rotor field, this is called the armature reaction and effects the net magnetic field.
What are the effects of armature reaction at lagging and leading power factor?
Lagging- armature field opposes the rotor field leading to reduced terminal voltage
Leading- armature field aids rotor field leading to increased terminal voltage.
What are the principles of the three-phase synchronous motor?
-Operates in in synchronism with stators rotating magnetic field
-Needs external means to start as torque is not produced at standstill
-Once up to speed rotor locked into sync with field, no slip
-Direction of torque depends on the interaction between stator field and rotor field
What are the advatanges of a synchronous motor?
-Constant speed of operation
-Power factor control is possible by varying excitation
-Suitable for power factor correction (over excited = capacitor like behaviour)
What are the disadvantages of a synchronous motor?
-Not self-starting
-Requires complex control for synchronisation
-Poor response to overloads can lead to be pulled out of synchronisation
What is the equation for voltage regulation?
Voltage reg= (E-V/V)x100
E=no-load/open-circuit voltage
V=full load terminal voltage
What are the conditions required for synchronization of generators?
-Same terminal voltage
-Same frequency
-Same phase angle
-Same phase sequence
What are the effects of varying driving torque for synchronous generators?
-effects real power sharing
-Can vary the distribution of load between the generators operating in parallel
What effect can varying excitation current cause in synchronous generators?
Varying the excitation current can be used to control the terminal voltage.
What can the benefit of armature reaction be?
When motoring stator currents oppose the generated e.m.f creating a torque which helps to keep the motor in sync.
What is the equation for the voltage induced in each stator phase winding?
E=4.44(RMP/60)NBmaxA
RPM-revs/min
N- effective number of stator turns/phase
Bmax- Maximum magnetic flux density in Tesla
A- Effective area of the rotor magnetic poles
What controls air gap magnetic flux density in a generator?
Controlled by rotor field current, Flux distribution assumed sinusodial.
