A Generator Motor is a device for converting electrical power to another form. Generator Motor 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 Generator Motors were widely used to convert industrial amounts of power while smaller Generator Motors were used to convert battery power to higher DC voltages.
While a Generator Motor 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.
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 such as vacuum tube vehicle radio receivers did not use expensive, noisy and bulky Generator Motors. 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.
In the context of electric power generation and large fixed electrical power systems, a Generator Motor 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.
Generator Motors 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 Generator Motor may contain a large flywheel to improve its ride-through; however, consideration must be taken in this application as the Generator Motor 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 Generator Motor 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 Generator Motor was a vertical type two-bearing machine with oil-bath bearings.
Generator Motor 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 drives and compatible motors have increasingly supplanted traditional MG-driven elevator installations, since AC drives are typically more efficient by 50% or more than DC-powered machinery.
Another use for Generator Motors 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.
Similarly, Generator Motors 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).
On the other hand, 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 some hundreds of kilograms which are intrinsically attached to their own large thermal mass. They also have inherently excellent resistance to electrostatic discharge (ESD).
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