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A motor–generator (an MG set) is a device for converting electricity to another form. Motor–generator sets are used to convert frequency, voltage, or phase of power. They may also be used to isolate electrical loads from the electrical power supply line. 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 battery power to higher DC voltages.
While a motor–generator set may consist of distinct motor and generator machines coupled together, a single unit dynamotor (for dynamo–motor) has the motor coils and the generator coils wound around a single rotor; both the motor and generator therefore share the same outer field coils or magnets. Radio Amateur's Handbook, 1976, pub. ARRL, p331–332 Typically the motor coils are driven from a commutator on one end of the shaft, while the generator coils provide output to another commutator on the other end of the shaft. The entire rotor and shaft assembly is smaller, lighter, and cheaper than a pair of machines, and does not require exposed drive shafts.
Low-powered consumer devices built before 1933, such as vacuum tube vehicle radio receivers, did not use expensive, noisy and bulky motor–generators. Instead, they used an inverter circuit consisting of a vibrator (a self-exciting relay) and a transformer to produce the higher voltages required for the vacuum tubes from the vehicle's 6 or 12 V battery.
One use is to eliminate spikes and variations in "dirty power" (power conditioning) or to provide phase matching between different electrical systems.
The motor–generator set may contain a large flywheel to improve its ride-through; however, consideration must be taken in this application as the motor–generator will lose speed and may draw a large current when power returns or the circuit-breaker is re-closed. If the speed loss is excessive (the power outage is too long), the re-closure current will trip the protection circuit-breakers, 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 Hz per second slew rate. The motor–generator was a vertical type two-bearing machine with oil-bath bearings.
Motors and generators may be coupled by a non-conductive shaft in facilities that need to closely control electromagnetic radiation, Physical Security Standard for Construction of Sensitive Compartmented Information Facilities , DIANE Publishing, 1994 , page 27 or where high isolation from transient surge voltages is required.
Another use for MG sets was in the southern region of British Rail. They were used to convert the 600 V DC – 850 V DC line supply voltage from the third rail into 70 V DC to power the controls of the EMU stock in use. These have since been replaced with solid state converters on new rolling stock. Thursday, 11 March 2021 MG locomotives have also commonly been used for long distance rail Diesel locomotive transmission throughout the world, due to reliability and wear issues with mechanical and fluid transmissions, but this is generally being replaced with smaller engines with conventional or MG transmission on each carriage. Long distance electric locomotives with high voltage overhead power supply used MG transmission, but this is generally being replaced with distributed motor drive on each carriage with electronic power control and conversion.
Similarly, MG sets were used in the PCC streetcar to produce a 36VDC output from the 600VDC traction supply. The low voltage output charges the streetcar's batteries and supplies current for control and auxiliary equipment (including headlights, gong ringers, door motors and electromagnetic track brakes).
Motor–generator sets were often used to provide the high-current D.C. power for carbon in large movie projectors in the 1950-60's era, before the carbon electrode arc light was replaced with modern xenon arc lamp projection systems (starting in 1963 in the U.S.).
In industrial settings where harmonic cancellation, frequency conversion, or line isolation is needed, MG sets remain a popular solution. A useful feature of motor–generators 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 sometimes hundreds of kilograms which are intrinsically attached to their own large thermal mass. They also have inherently excellent resistance to electrostatic discharge (ESD).
From the 2014 season, Formula 1 racing cars will have two of what are described as 'Motor Generator Units' (MGUs), one each on the drive train and the turbocharger. These are motor/generators rather than motor–generators, single units which can act as either a motor or a generator. They make the cars more fuel-efficient by harvesting energy from regenerative braking and from the turbocharger, and can also be used to provide up to 160 BHP extra to the wheels to aid acceleration and overtaking, or to spin up the turbo to increase boost pressure faster, thereby reducing turbo lag.
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