- Electrical Machines Overview: Electrical machines are devices like transformers, generators, and motors that either convert electricity to mechanical power or vice versa.
- Transformers: A transformer transfers electrical energy between two circuits without changing the frequency, crucial for regulating voltage levels in power distribution.
- Generators: Generators convert mechanical energy to electrical energy, using electromagnetic induction, vital for producing electricity in power plants.
- Motors: Motors convert electrical energy to mechanical energy, essential in applications from household appliances to industrial machines.
- Operational Principles: The operation of these machines relies on electromagnetic principles, where electric currents and magnetic fields interact to produce or transform energy.
Electric machines, also called electrical machines, use electromagnetic fields to transfer or convert electrical energy. A transformer transfers power between electrical circuits. A generator converts mechanical power to electricity, while a motor converts electrical power to mechanical power.
Types of Electrical Machines
Three broad groups covered here are the transformer, generator and motor.
Transformers: A transformer accepts and delivers electrical power, transferring energy through a changing magnetic field without a rotating shaft.
Electrical Generator: A generator accepts mechanical shaft power and delivers electrical power, apart from losses.
Electrical Motor: A motor accepts electrical power and delivers mechanical shaft power, apart from losses.
Electrical machines can also be described as static or rotating.
A conventional transformer is a static electrical machine.
Motors and generators are rotating electrical machines, although linear-machine forms also exist.
Transformer: A conventional transformer operates by mutual induction. Its magnetic core provides a low-reluctance path that links the windings of transformer. An alternating primary current establishes changing core flux, which induces voltage in both windings. The working principle of transformer follows Faraday’s law. In the ideal common-flux model, induced voltage per turn is equal in the primary and secondary, so their voltage ratio equals their turns ratio. Real winding resistance, leakage flux, magnetising current and core loss cause voltage regulation and loss under load.
Transformers can be described as step-up or step-down for a stated direction of power flow. A Step up transformer has a higher secondary-to-primary turns ratio and raises voltage in that direction. A step down transformer has a lower ratio. Frequency remains the same in a conventional transformer, while current changes inversely in the ideal power balance.
By application, common categories include the power transformer, distribution transformer and instrument transformer. These categories describe duties and are not an exhaustive list.
By winding arrangement, examples include an isolated two-winding transformer and an auto transformer with an electrically connected shared winding.
By insulation and cooling construction, common groups include liquid-immersed units and the dry type transformer. The liquid is not always mineral oil, so liquid-immersed is the broader term.
By phase arrangement, a transformer may be a single phase transformer or a three-phase transformer.
A three-phase bank can use one three-phase unit or three single-phase units. Selection depends on rating, transport, redundancy, maintenance and system requirements.
When magnetic flux linking a conductor changes, Faraday’s law says that emf is induced. Moving a conductor relative to a magnetic field is one way to change that flux. Rotating electrical generators work by maintaining this change while mechanical torque supplies the converted power.
Generators are often grouped by terminal output as a DC generator or an AC generator, also called an alternator. Machine construction and excitation provide further classifications.
DC Generator: In a conventional rotating-armature machine, the DC generator armature is on the rotor and the field poles are on the stator. Rotation induces alternating emf in individual armature conductors. The commutator and brushes mechanically rectify the connection so the terminals deliver unidirectional voltage and current.
AC Generator: Large synchronous alternators commonly place the three-phase armature on the stator and the field system on the rotor. The electricity generated in the stator feeds the external circuit. Rotor field excitation may come through slip rings, a brushless exciter or permanent magnets, depending on the machine.
The electric motors discussed below include the commutated DC motor and common AC motor families. Modern classifications also include electronically commutated and reluctance machines.
DC Motor: A brushed DC motor feeds its rotating armature through brushes and commutator segments. Current-carrying conductors in the magnetic field experience force and produce torque; Fleming’s left hand rule is a direction mnemonic for this force. Traditional field connections include the separately excited DC motor, shunt wound DC motor, series wound DC motor and compound wound DC motor.
Two major AC motor families are the Induction Motor and synchronous motor. Both use a rotating stator field, but their rotor excitation and operating speed differ.
Induction Motors: Supply and winding arrangements include the single phase induction motor and three phase induction motor. Rotor construction can use a squirrel cage rotor or a wound rotor.
In an induction motor, the stator produces a rotating magnetic field. Relative speed between that field and the rotor induces rotor current. The rotor field then interacts with the stator field to produce torque. Under motor load the rotor runs below synchronous field speed; this speed difference is slip and is required to sustain induced rotor current.
Synchronous Motor: In a synchronous motor, the stator produces a rotating field and the rotor provides its own magnetic field through a field winding, permanent magnets or reluctance. In steady operation the rotor remains in step with the stator field and turns at synchronous speed.
Other types of electric motors include a servo motor used in a feedback-controlled motion system, the stepper motor and the hystersis motor. Servo describes the control role rather than one electromagnetic construction.
