A
motor-generator (an
M-G set or a
dynamotor for
dynamo-In Engineering:* Dynamo, a magnetic device originally used as an electric generator* Dynamo theory, a theory relating to magnetic fields of celestial bodies* Solar dynamo, the physical process that generates the Sun's magnetic field-Europe:...
-motor) is a device for converting
electrical powerElectricity is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge...
to another form. In some contexts, the other form is
mechanical energyIn physics, mechanical energy describes the sum of potential energy and kinetic energy present in the components of a mechanical system.-Simplifying assumptions:...
; in other contexts, it is a different form of
electricityElectricity is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge...
. The two senses refer to different types of equipment.
The Motor-generator set is used to convert
frequencyFrequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency....
,
voltageVoltage is commonly used as a short name for electrical potential difference. Its corresponding SI unit is the volt...
, and
phaseThe phase of an oscillation or wave is the fraction of a complete cycle corresponding to an offset in the displacement from a specified reference point at time t = 0. Phase is a frequency domain or Fourier transform domain concept, and as such, can be readily understood in terms of simple harmonic...
of power. They may also be used to provide complete line isolation in addition to the various types of power conversion and control.
A
motor-generator (an
M-G set or a
dynamotor for
dynamo-In Engineering:* Dynamo, a magnetic device originally used as an electric generator* Dynamo theory, a theory relating to magnetic fields of celestial bodies* Solar dynamo, the physical process that generates the Sun's magnetic field-Europe:...
-motor) is a device for converting
electrical powerElectricity is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge...
to another form. In some contexts, the other form is
mechanical energyIn physics, mechanical energy describes the sum of potential energy and kinetic energy present in the components of a mechanical system.-Simplifying assumptions:...
; in other contexts, it is a different form of
electricityElectricity is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge...
. The two senses refer to different types of equipment.
The Motor-generator set is used to convert
frequencyFrequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency....
,
voltageVoltage is commonly used as a short name for electrical potential difference. Its corresponding SI unit is the volt...
, and
phaseThe phase of an oscillation or wave is the fraction of a complete cycle corresponding to an offset in the displacement from a specified reference point at time t = 0. Phase is a frequency domain or Fourier transform domain concept, and as such, can be readily understood in terms of simple harmonic...
of power. They may also be used to provide complete line isolation in addition to the various types of power conversion and control. Large motor-generators were widely used to convert industrial amounts of power while smaller motor-generators (such as the one shown in the picture) were used to convert
batteryAn electrical battery is a combination of one or more electrochemical cells, used to convert stored chemical energy into electrical energy. Since the invention of the first Voltaic pile in 1800 by Alessandro Volta, the battery has become a common power source for many household and industrial...
power to higher DC voltages.
By comparison, simple, low-powered consumer electronics devices like
vacuum tubeIn electronics, a vacuum tube, electron tube , thermionic valve, or valve is a device used to amplify, switch, otherwise modify, or create an electrical signal by controlling the movement of electrons in a low-pressure space...
car radios did not use motor-generators. Instead, they would typically use an
inverterAn inverter is an electrical device that converts direct current to alternating current ; the resulting AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits....
circuit consisting of a
vibratorIn early electronics vibrators were used in inverter circuits to provide an alternating current electric power supply from a direct current source....
(a self-exciting relay) and a
transformerA transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core, and thus a varying magnetic field...
to produce the B+ voltages required for the vacuum tubes.
A
motor-generator is physically different from a normal electric motor attached to a separate generator , in that both rotor coils of the motor and the generator are wound around a single rotor, and both coils share the same outer field coils or magnets. Typically the motor coils are driven from a commutator on one end of the shaft, when the generator coils output to another commutator on the other end of the shaft. The entire rotor and shaft assembly is only slightly larger in size than in a normal electric motor, and may not have any exposed drive shafts.
Electrical power handling
In the context of
electric powerElectric power is defined as the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt.When electric current flows in a circuit, it can transfer energy to do mechanical or thermodynamic work...
generation and large fixed electrical power systems, a
motor-generator consists of an
electric motorAn electric motor uses electrical energy to produce mechanical energy, usually through the interaction of magnetic fields and current-carrying conductors. The reverse process, producing electrical energy from mechanical energy, is accomplished by a generator or dynamo. Traction motors used on...
mechanically coupled to an electric generator (or
alternatorAn alternator is an electromechanical device that converts mechanical energy to alternating current electrical energy. Most alternators use a rotating magnetic field but linear alternators are occasionally used...
). The motor runs on the electrical input current while the generator creates the electrical output current, with power flowing between the two machines as a mechanical
torqueTorque, also called moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
; this provides electrical isolation and some buffering of the power between the two electrical systems. One use of this type of motor-generator is to eliminate spikes and variations in "dirty power" or to provide phase matching between different electrical system; another is to buffer extreme loads on the power system. For example,
tokamakA tokamak is a machine producing a toroidal magnetic field for confining a plasma which is characterized by azimuthal symmetry and the use of a plasma-borne electric current to generate the helical component of the magnetic field necessary for stable equilibrium...
fusion devices impose very large peak loads, but relatively low average loads, on the electrical grid. The
DIII-DDIII-D is the name of a tokamak machine developed in the 1980s by General Atomics in San Diego, USA, as part of the ongoing effort to achieve magnetically confined fusion. DIII-D pioneered new technology including the use of beams of neutral particles to penetrate the confinement field of the...
and
Princeton Large Torus (PLT)Princeton Plasma Physics Laboratory is a United States Department of Energy national laboratory for plasma physics and nuclear fusion science located on Princeton University's Forrestal Campus in Plainsboro Township, New Jersey. Its primary mission is research into and development of fusion as an...
tokamaks used a large
flywheelA flywheel is a mechanical device with a significant moment of inertia used as a storage device for rotational energy. Flywheels resist changes in their rotational speed, which helps steady the rotation of the shaft when a fluctuating torque is exerted on it by its power source such as a...
on multiple motor-generator rigs to level the load imposed on the electrical system: the motor side slowly accelerated a large flywheel to store energy, which was consumed rapidly during a fusion experiment as the generator side acted as a
brakeA brake is a device for applying a force against the friction of the road, slowing or stopping the motion of a machine or vehicle, or alternatively a device to restrain it from starting to move again. The kinetic energy lost by the moving part is usually translated to heat by friction...
on the flywheel. Similarly, the next generation U.S. Navy aircraft carrier Electromagnetic Aircraft Launch System (EMALS) will use a flywheel motor-generator rig to supply power instantaneously for aircraft launches at greater than the ship's installed generator capacity.
Conversions
Motor-generators may be used for various conversions including:
- Alternating current
In alternating current the movement of electric charge periodically reverses direction. An electric charge would for instance move forward, then backward, then forward, then backward, over and over again...
(AC) to direct currentDirect current is the undirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also be through...
(DC)
- DC to AC
- DC at one voltage
Voltage is commonly used as a short name for electrical potential difference. Its corresponding SI unit is the volt...
to DC at another voltage
- AC at one frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency....
to AC at another harmonicIn acoustics and telecommunication, a harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency. For example, if the fundamental frequency is...
ally-related frequency
High-frequency machines
An
Alexanderson alternatorAn Alexanderson alternator is a rotating machine invented by Ernst Alexanderson for the generation of high frequency alternating current up to 100 kHz, for the purpose of radio communication...
is a motor driven high frequency alternator which provides
radio frequencyRadio frequency is a frequency, or rate of oscillation, of electromagnetic radiation within the range of about 3 Hz to 300 GHz. This range corresponds to the frequency of alternating current electrical signals used to produce and detect radio waves...
power. In the early days of radio communication, the high frequency
carrier waveIn telecommunications, a carrier wave, or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually of much higher frequency than the input signal....
had to be produced mechanically using an
AlternatorAn alternator is an electromechanical device that converts mechanical energy to alternating current electrical energy. Most alternators use a rotating magnetic field but linear alternators are occasionally used...
with many poles driven at high speeds. The Alexanderson alternator produced upward of 100 kHz with power outputs upward of 200 kW. While electromechanical converters were regularly used for long wave transmissions in the first three decades of the 20th century, electronic techniques were required at higher frequencies. The Alexanderson alternator was largely replaced by the vacuum tube oscillator in the 1920's.
Motor-generators used to increase Ride-Through
Motor-generators have even been used where the input and output currents are essentially the same. In this case, the mechanical inertia of the M-G set is used to filter out transients in the input power. The resulting output power can be (electrically) very clean (noise free) and will be able to
ride-through brief blackouts and switching transients on the input power. This may enable, for example, the flawless cut-over from mains power to AC power provided by a
dieselA diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber during the final stage of compression...
generator set.
The motor-generator set may contain a large
flywheelA flywheel is a mechanical device with a significant moment of inertia used as a storage device for rotational energy. Flywheels resist changes in their rotational speed, which helps steady the rotation of the shaft when a fluctuating torque is exerted on it by its power source such as a...
to improve its ride-through; however, consideration must be taken in this application as the motor-generator will require a large amount of current on re-closure, if prior to the pull-out torque is achieved, resulting in a shut down. The in-rush current during re-closure will depend on many factors, however. As an example, a 250 kVA motor generator operating at 300 ampere of full load current will require 1550 ampere of in-rush current during a re-closure after 5 seconds. This example used a fixed mounted flywheel sized to result in a 1/2 Hz per second
slew rateIn electronics, the slew rate represents the maximum rate of change of a signal at any point in a circuit.Limitations in slew rate capability can give rise to non linear effects in electronic amplifiers...
. The motor-generator was a vertical type 2 bearing machine with oil bath bearings.
The motor-generator today
Motor-generators have been replaced by semiconductor devices for some purposes. In industrial settings where harmonic cancellation, frequency conversion, or line isolation is needed, MG sets remain a popular solution.
A useful feature of the motor-generator is that they can handle large short-term overloads better than semiconductor devices of the same average load rating. Consider that the thermally current-limited components of a large semiconductor inverter are solid-state switches massing a few grams with a thermal time constant to their heat sinks of likely more than 100 ms, whereas the thermally current limited components of an MG are copper windings massing some hundreds of kilos which are intrinsically attached to their own large thermal mass. They also have inherently excellent resistance to
electrostatic dischargeElectrostatic discharge is the sudden and momentary electric current that flows between two objects at different electrical potentials caused by direct contact or induced by an electrostatic field...
(ESD).
Modern use of the term motor-generator
In the context of hybrid vehicles and other lightweight power systems, a
motor-generator can be used to describe a single power
transducerA transducer is a device, electrical, electronic, electro-mechanical, electromagnetic, photonic, or photovoltaic, that converts one type of energy or physical attribute to another for various purposes including measurement or information transfer .There are two kinds of transducers...
that can be used as either an
electric motorAn electric motor uses electrical energy to produce mechanical energy, usually through the interaction of magnetic fields and current-carrying conductors. The reverse process, producing electrical energy from mechanical energy, is accomplished by a generator or dynamo. Traction motors used on...
or a
generatorIn electricity generation, an electrical generator is a device that converts mechanical energy to electrical energy. The reverse conversion of electrical energy into mechanical energy is done by a motor; motors and generators have many similarities...
, converting between electrical power and
mechanical powerIn physics, mechanical energy describes the sum of potential energy and kinetic energy present in the components of a mechanical system.-Simplifying assumptions:...
. In principle, any
electrical generatorIn electricity generation, an electrical generator is a device that converts mechanical energy to electrical energy. The reverse conversion of electrical energy into mechanical energy is done by a motor; motors and generators have many similarities...
can also serve as an electric motor, or vice versa. A device that is specifically designed for use in either mode may be called a "motor-generator"; the literature distributed by Toyota to describe the
Hybrid Synergy DriveHybrid Synergy Drive, is a set of hybrid car technologies developed by Toyota and used in the company's Prius, Highlander Hybrid, Camry Hybrid, Lexus RX 400h/RX 450h, Lexus GS 450h, Lexus LS 600h/LS 600hL, and Lexus HS 250h automobiles. It is also used in the Nissan Altima Hybrid...
is an example of this newer usage.
See also
- Rotary converter
A rotary converter is a type of electrical machine which acts as a mechanical rectifier or inverter. It was used to convert AC to DC or DC to AC power before the advent of chemical or solid state power rectification...
- Frequency converter
- Cascade converter
A Cascade Converter is a type of motor-generator which was patented in 1902 by J. L. la Cour and O. S. Bragstad.It consists of an induction motor driving a dynamo through a shaft...
- Booster (electric power)
A Booster was a motor-generator set used for voltage regulation in direct current electrical power circuits. The development of alternating current and solid-state devices has rendered it obsolete...
- 3 phase loads
- Harry Ward Leonard
Harry Ward Leonard was an electrical engineer and inventor whose 30-year career spanned the late 19th century and the early 20th century. He is best known for his invention, the Ward Leonard motor control system...