motor–generator

In the context of electric power generation and large fixed electrical power systems, a motor–generator consists of an electric motor mechanically coupled to an electric generator (or alternator). 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 torque; this provides electrical isolation and some buffering of the power between the two electrical systems.

One use is to eliminate spikes and variations in "dirty power" (power conditioning) or to provide phase matching between different electrical systems.

An MG set used to provide a variable three phase AC voltage for an Electron Beam Welding Machine high voltage power supply.

Motor–generators may be used for various conversions including:

• Alternating current (AC) to direct current (DC)
• DC to AC[3]
• DC at one voltage to DC at another voltage. (Also called a dynamotor, short for dynamo-motor)
• Creating or balancing a three-wire DC system.
• AC at one frequency to AC at another harmonically-related frequency
• AC at a fixed voltage to AC of a variable voltage
• AC single-phase to AC three-phase

Before solid state AC voltage regulation was available or cost effective, motor generator sets were used to provide a variable AC voltage. The DC voltage to the generators armature would be varied manually or electronically to control the output voltage. When used in this fashion, the MG set is equivalent to an isolated variable transformer.

An Alexanderson alternator is a motor-driven, high-frequency alternator which provides radio frequency power. In the early days of radio communication, the high frequency carrier wave had to be produced mechanically using an alternator with many poles driven at high speeds. Alexanderson alternators produced RF up to 600 kHz, with large units capable of 500 kW power output. 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 1920s.

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 output's electric current can be very clean (noise free) and will be able to ride-through brief blackouts and switching transients at the input to the M–G set. This may enable, for example, the flawless cut-over from mains power to AC power provided by a diesel generator set.

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 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 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,[4] or where high isolation from transient surge voltages is required.

Motor–generator sets have been replaced by semiconductor devices for some purposes. In the past, a popular use for MG sets was in elevators. Since accurate speed control of the hoisting machine was required, the impracticality of varying the frequency to a high power AC motor meant that the use of an MG set with a DC hoist motor was a near industry-standard solution. Modern AC variable-frequency drive electronics with compatible motors have increasingly supplanted traditional MG-driven elevator installations, since electronic AC drives are typically more efficient by 50% or more than MG DC-powered machinery.[5]

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.[6] 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.[7]

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 arc lamps 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).

In principle, any electrical generator can also serve as an electric motor, or vice versa. In hybrid vehicles and other lightweight power systems, a "motor–generator" is a single electric machine that can be used as an electric motor or a generator, converting between electrical power and mechanical power.

From the 2014 season, Formula 1 racing cars will have two of what are described as 'motor-generator units' (MGU)[8] This makes the cars more fuel-efficient by harvesting energy from the turbocharger and under braking. However these are not motor-generators as described here, but are more like dynamotors, single units which can act as either a generator or a motor. They can be used to provide an additional 160 BHP to the wheels to aid acceleration and overtaking, or can be used to spin the turbo to increase boost pressure faster, thereby reducing turbo lag.