Motor-generator

Motor-generator

A motor-generator (an M-G set or a dynamotor for dynamo-motor) is a device for converting electrical power to another form. In some contexts, the other form is mechanical energy; in other contexts, it is a different form of electricity. The two senses refer to different types of equipment.

The M-G set is used to convert frequency, voltage, and phase 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 battery power to higher DC voltages.

By comparison, simple, low-powered consumer electronics devices like vacuum tube car radios did not use motor-generators. Instead, they would typically use an inverter circuit consisting of a vibrator (a self-exciting relay) and a transformer 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.

Conversion to/from mechanical energy

In the context of hybrid vehicles and other lightweight power systems, a motor-generator is used to describe a single power transducer that can be used as either an electric motor or a generator, converting between electrical power and mechanical power. In principle, any electrical generator 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 Drive is an example of this newer usage.

Electrical power handling

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 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, tokamak fusion devices impose very large peak loads, but relatively low average loads, on the electrical grid. The DIII-D and Princeton Large Torus (PLT) tokamaks used a large flywheel 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 brake on the flywheel.

Conversions

Motor-generators may be used for various conversions including:

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 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 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 discharge (ESD).

See also

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