Electrical Sciences DC Generators
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The voltage that is delivered across the load
Provides the energy conversion in a DC machine.
The mechanical conversion from AC to DC at the brushes of a DC machine
Has a field winding in parallel with the generator armature and a field winding in series with the generator armature
Power lost as heat in the windings; caused by the flow of current through the coils of the DC armature or DC field.
Induced voltage that acts counter to applied voltage
Counter-electromotive force (CEMF)
If the two fields of a compound generator are wound so that their magnetic fields aid one another
If the two fields of a compound generator are wound so that their flux fields oppose one another
Circulating currents within the iron core of the armature. A loss of energy
Direct current winding through the field winding flowing through electromagnet conductors to produce a magnetic field. In order for a DC generator to operate properly, the magnetic field must always be in the same direction.
Field excitation current
The heat produced by this friction from rotation of magnetic domains in the iron of the armature.
Magnetic hysteresis loss
Caused by bearing friction, brush friction on the commutator, or air friction (windage), which is caused by the air turbulence due to armature rotation.
Provides the rotating element in a DC machine.
Field excitation current that comes directly from the output of the generator
Field excitation current that comes from a separate DC source external to the generator
Separately excited generator
The field winding of a DC generator is connected in series with the armature
The field winding of a generator is connected in parallel with the generator armature
Part of a motor or generator that is stationary. In DC machines, the purpose of the stator is to provide the magnetic field.
The voltage that can be measured at the output of the generator
The voltage that can be measured at the output of the generator.
Define counter-electromotive force (CEMF) as it applies to a DC machine
This induced voltage opposes the applied voltage; it counteracts some of the applied voltage, which reduces the current flow through the armature.
Describe the effects of commutation in a DC generator
Commutation is the positioning of the DC generator brushes so that the commutator segments change brushes at the same time the armature current changes direction. More simply stated, commutation is the mechanical conversion from AC to DC at the brushes of a DC machine
- State the purpose of the following components of a DC machine:
- The purpose of the armature is to provide the energy conversion in a DC machine
- The purpose of the rotor is to provide the rotating element in a DC machine
- The stator is the part of a motor or generator that is stationary
- The purpose of the field in a DC machine is to provide a magnetic field for producing either a voltage or a torque.
List the three conditions necessary to induce a voltage into a conductor
- A conductor
- A magnetic field
- Relative motion between the two
Given the direction of the magnetic field and motion of the conductor, determine the direction of the current induced into a conductor using the left-hand rule for generators
Describe how terminal voltage of a DC generator is adjusted
- DC generator output voltage is dependent on three factors
- The number of conductor loops in series in the armature, armature speed, and magnetic field strength.
The strength of the magnetic field, however, can be changed quite easily by varying the current through the field winding. This is the most widely used method for regulating the output voltage of a DC generator
State the basis behind each of the four DC generator ratings
- Voltage: Voltage rating of a machine is based on the insulation type and design of the machine.
- Current: The current rating is based on the size of the conductor and the amount of heat that can be dissipated in the generator.
- Power: The power rating is based on the mechanical limitations of the device that is used to turn the generator and on the thermal limits of conductors, bearings, and other components of the generator.
- Speed: Speed rating, at the upper limit, is determined by the speed at which
List the four internal losses found in a DC generator
- Copper losses
- Eddy-current losses
- Hysteresis losses
- Mechanical losses
Describe the differences in construction between a shunt-wound and a series-wound DC generator with respect to the relationship between the field and the armature
When the field winding of a generator is connected in parallel with the generator armature, the generator is called a shunt-wound generator
When the field winding of a DC generator is connected in series with the armature, the generator is called a series-wound generator
Describe the relationship between the shunt and series fields for cumulatively-compounded and differentially-compounded DC generators
The compound generator has a field winding in parallel with the generator armature (the same as a shunt-wound generator) and a field winding in series with the generator armature (the same as a series-wound generator)
If the two fields are wound so that their flux fields oppose one another, the generator is said to be differentially-compounded
If the two fields of a compound generator are wound so that their magnetic fields aid one another, the generator is said to be cumulatively-compounded.
Describe the voltage vs. load current characteristics for a flat-compounded, over-compounded, and under-compounded DC generator.
In practical compounded generators, the change in output voltage from no-load to full-load is less than 5 percent. A generator with this characteristic is said to be flat-compounded
For some applications, the series winding is wound so that it overcompensates for a change in the shunt field. The output gradually rises with increasing load current over the normal operating range of the machine. This type of generator is called an over-compounded generator.
The series winding can also be wound so that it under compensates for the change in shunt field strength. The output voltage decreases gradually with an increase in load current. This type of generator is called an under-compounded generator.
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