13-06-2014, 03:44 PM
Electric Motors and Generators
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Introduction
n this lecture we consider various forms of rotating electrical machines
These can be divided into:
generators – which convert mechanical energy into electrical energy
motors – which convert electrical energy into mechanical energy
Both types operate through the interaction between a magnetic field and a set of windings
A Simple AC Generator
We noted earlier that Faraday’s law dictates that if a coil of N turns experiences a change in magnetic flux, then the induced voltage V is given by
If a coil of area A rotates with respect to a field B, and if at a particular time it is at an angle to the field, then the flux linking the coil is BAcos, and the rate of change of flux is given by
A Simple DC Generator
The alternating signal from the earlier AC generator could be converted to DC using a rectifier
A more efficient approach is to replace the two slip rings with a single split slip ring called a commutator
this is arranged so that connections to the coil are reversed as the voltage from the coil changes polarity
hence the voltage across the brushes is of a single polarity
adding additional coils produces a more constant output
DC Generators or Dynamos
Practical DC generators or dynamos can take a number of forms depending on how the magnetic field is produced
can use a permanent magnet
more often it is generated electrically using field coils
current in the field coils can come from an external supply
this is known as a separately excited generator
but usually the field coils are driven from the generator output
this is called a self-excited generator
often use multiple poles held in place by a steel tube called the stator
AC Generators or Alternators
Alternators do not require commutation
this allows a simpler construction
the field coils are made to rotate while the armature windings are stationary
Note: the armature windings are those that produce the output
thus the large heavy armature windings are in the stator
the lighter field coils are mounted on the rotor and direct current is fed to these by a set of slip rings
DC Motors
When current flows in a conductor it produces a magnetic field about it - as shown in (a) below
when the current-carrying conductor is within an externally generated magnetic field, the fields interact and a force is exerted on the conductor - as in (b)
AC Motors
AC motors can be divided into two main forms:
synchronous motors
induction motors
High-power versions of either type invariably operate from a three-phase supply, but single-phase versions of each are also widely used – particularly in a domestic setting
Electrical Machines – A Summary
Power generation is dominated by AC machines
range from automotive alternators to the synchronous generators used in power stations
efficiency increases with size (up to 98%)
Both DC and AC motors are used
high-power motors are usually AC, three-phase
domestic applications often use single-phase induction motors
DC motors are useful in control applications
Key Points
Electrical machines include both generators and motors
Motors can usually function as generators, and vice versa
Electrical machines can be divided into AC and DC forms
The rotation of a coil in a uniform magnetic field produces a sinusoidal e.m.f. This is the basis of an AC generator
A commutator can be used to produce a DC generator
The magnetic field in an electrical machine is normally produced electrically using field coils
DC motors are often similar in form to DC generators
Some forms of AC generator can also be used as motors
The most widely used form of AC motor is the induction motor