Assignment 10
List the two basic categories of alternators.
1. Rotating armature alternator
2. Rotating field alternator
The amount of EMF produced in an armature is proportional to what three factors?
1. The number of conductors in the armature
2. The strength of the magnetic field
3. The speed of rotation
What are the three factors that determine the strength of an electromagnet?
1. The number of turns and/or the length of the conductor
2. The magnetic strength of the core material
3. The amount of current flow
How is voltage controlled while maintaining the frequency of an alternator?
The field excitation is controlled by a rheostat (a variable resistor)
Is the field excitation current of an alternator AC or DC?
The field excitation current is a DC source
If one phase of voltage to a three-phase motor opens, the motor may slow down, overheat, or possibly stop. What is this condition known as?
Single Phasing
Is excitation current and output voltage directly or inversely proportional in an alternator?
Directly proportional. More excitation current, stronger magnetic field, more output voltage
List three types of prime movers.
1. Steam turbine, water,
2. Diesel engine, Gas engine
3. Wind
When a conductor cuts through magnetic lines of flux, a voltage is induced into the conductor. What is this principle called?
Electromagnetic induction - voltage produced by lines of magnetic flux cutting a conductor
What should the normal electrical phase displacement be between the interconnected stator coils of a squirrel-cage induction motor?
120 Electrical Degrees
What type of 3∅ AC motor has the highest starting torque characteristics?
Wound Rotor Induction Motor
If resistance is removed from the excitation circuit of an alternator, will the magnetic field strength increase or decrease?
If resistance is decreased, there's more excitation current, magnetic field increases
If resistance is added to the excitation circuit of an alternator, will the output voltage increase or decrease?
Output Voltage will Decrease
List the two types of rotor assemblies found in rotating field type alternators.
1. Salient pole
2. Cylindrical
What component is each salient field pole piece bolted to in a salient pole rotor
assembly?
Fabricated steel spider hub (safe guard against centrifugal force) of the rotating
member
Does a rotating field type permanent-magnet alternator require slip rings and/or brushes?
No
What is a major disadvantage to rotating armature type alternators?
Limited to a lower KVA capacities because of the difficulty of transferring high AC generated voltages and currents from the rotating armature to the external load through slip rings and brushed
List four advantages to rotating field type alternators.
1. Armature output (stator component) does not require slip rings or brushes
2. The DC field (rotor) requires lower currents and voltages for the excitation
therefore small slip rings and brushes required
3. Rotating field stator can be physically smaller resulting in mechanical
advantages such as reduced bearing size and reduced vibration
4. Stationary armature windings are more easily and efficiently cooled
What is the mechanical force used to turn an alternator called?
Prime Mover
What is resistance to magnetism called?
Reluctance
What is the minimum number of terminal leads required for connection of a dual-voltage 230/460V squirrel-cage induction motor?
9 Leads
What are the field excitation terminals labeled in a single-phase alternator?
F1 (+) F2 (-)
What would be the lead identification numbers for the ‘B’ phase windings of a 9-lead, dual-voltage wound-rotor induction motor?
The leads are identified T2, T5, T8
What are the two types of magnets that can be used in the rotor construction of a rotating field type alternator?
1. Electromagnet
2. Permanent magnet
What two conditions must be met for a synchroscope dial pointer to be at a standstill and in the 12 o’clock position?
When the dial pointer stops, the frequency of the two alternators are equal. When the dial pointer stops at the 12 o’clock position, the frequency and phase position of the voltages are equal
When a squirrel-cage induction motor is first started, why is the in-rush current so high?
The stator windings have a very small value of resistance. CEMF produced from the rotor magnetic field is not established yet. As the rotor turns current in the stator field will then be reduced.
What type of rotating field structure is used for high speed prime movers such as steam turbine-driven alternators?
Cylindrical rotors are used for high-speed prime movers
When a squirrel-cage induction motor is drawing more current than the nameplate full load current, list two conditions that could be causing the increased current?
Single Phasing or overloaded motor
When is a synchronous motor considered a synchronous condenser?
A synchronous motor is a synchronous condenser where it is operated with no load and used for power correction only.
Shall a synchronous motor be started with DC connected to the rotor?
No. DC excitation can only be connected to the rotor when the motor reaches close to synchronous speed
How can a synchronous motor be made to have a leading power factor?
A leading power factor is produced when the rotor circuit is overexcited
What are the four types of enclosures for AC machines?
1. Open-type
2. Weather-proof
3. Drip-proof
4. Totally enclosed type
As the load increases on a squirrel-cage induction motor, does both power factor and torque improve or decline?
Motors perform best under load, with an increase in load power factor and torque will improve
How is hysteresis minimized in an AC machine?
Hysteresis is minimized by use of silicon-steel alloys for the construction of the rotating machine
What detrimental condition causes reduced voltage and increased current flow to the stator windings of a squirrel-cage induction motor?
Single Phasing
If the number of poles in the stator of a synchronous motor is reduced, does the speed of the motor increase or decrease?
Increase
What two factors determine the synchronous speed of a synchronous motor?
1. Number of Poles
2. Frequency
In reference to the left-hand rule for conductors, what do the fingers and thumb represent?
Thumb - Points to the direction of current flow
Fingers - Direction of magnetic field around the conductor
In reference to the left-hand rule for electromagnets, what do the fingers and thumb represent?
Thumb - Points to the direction of the north pole
Fingers - Direction of current flow are the coil
List five different types of rotor assemblies unique to various types of AC machines.
1. Squirrel-cage rotor
2. Wound rotor
3. Salient pole rotor
4. Cylindrical rotor
5. Armature rotor
What is the formula for speed regulation?
% Speed regulation =
NL speed - FL speed / FL speed x 100
What does efficiency of a motor compare?
Motor efficiency compares the machines output to the input
Is a field winding in a squirrel-cage induction motor considered an electrical load or an electrical source?
The field windings are considered an electrical load because power is consumed while performing its function
What type of winding consists of bare copper bars embedded in the cores of the salient field poles?
Amortisseur winding is a squirrel-cage winding on the rotor of a synchronous motor used for starting purposes only
List two disadvantages to ‘V’ type rotor bar design in squirrel-cage induction motors.
1. Lowest starting torque per ampere of starting current
2. Produces the best torque at running speeds
Are the rotor’s field windings connected in parallel or series for a cylindrical rotor type alternator?
The field windings are connected in series for alternate polarity and connected to two slip rings mounted on the shaft
How many terminal leads are connected to slip rings on wound-rotor induction motor?
Three slip rings are connected to three leads from a wound-rotor motor
What are the terminal leads connected to the slip rings on a wound-rotor motor labeled?
Terminal lead marks are M1, M2, M3
What type of alternator is used for main electrical distribution generation?
Rotating field alternator
What are the brush leads labeled in a rotating field type alternator?
The brush leads are labeled F1, F2
Are the two windings of each phase in a 230/460V rated squirrel-cage induction motor connected in series or parallel for operation at 240 volts?
Parallel
What instrument is used to indicate phase position and frequency when paralleling alternators?
Synchro scope
What rotor bar design for a squirrel-cage induction motor has the highest starting torque and lowest starting current?
Type A
What are the minimum number of stator leads required for a 3-phase, dual-voltage, single-configuration machine?
9 Leads
What are the minimum number of stator leads required for a 3-phase, single-voltage, dual-configuration machine?
6 Leads
What are the minimum number of stator leads required for a 3-phase, single-voltage, single-configuration machine?
3 Leads
What are the minimum number of stator leads required for a 3-phase, dual-voltage, dual- configuration machine?
12 Leads
How can the speed of a wound-rotor induction motor be controlled?
Secondary resistance control (adjustable-speed drive)
List five advantages of a squirrel-cage induction motor.
1. Self Starting
2. The rotor uses embedded rotor bars
3. No commutator or slip rings to service
4. No brushes to replace or maintain
5. As the load increases, both the power factor and torque improves
Terminal marking _______ is the positive (+) side of the DC field winding and _______ is the negative (-) side of the DC field winding in an AC alternator.
Terminal marking F1 is the positive (+) side of the DC field winding and F2 is the negative (-) side of the DC field winding in an AC alternator.
Are the number of poles you calculated in the previous question, the total number of poles in the stator or the number of poles per phase?
Poles per Phase
Calculate how many salient field poles per phase the generators at McArthur have when each unit rotates at 85.7 RPM
84 poles
Salient pole type rotor assembly components
1. Shaft
2. Slip rings
3. Field windings
4. Pole pieces
Cylindrical type rotor assembly components
1. Shaft
2. Slip rings
3. DC field windings
4. Laminated iron core