Motor-Generator (M-G) set is basically a combination of a motor and generator, although it is physically different from an ordinary electric motor that is attached to a separate generator. It has many practical functions that generally involve converting voltage, frequency and phase of power. However, this post will focus on its power quality applications, as it is a mature technology used for isolating sensitive and critical loads from voltage sags and interruptions. Also, it is available in various sizes and configurations to suit different purposes.
Design and Construction
A motor-generator set has both the rotor coils of the motor and generator wound around a single rotor. In addition, both coils share the same outer magnets or field coils. Usually, the motor coils are driven from a commutator on one end of the shaft, while the generator coils output to another commutator on the other end of the shaft. Generally, the entire shaft and rotor assembly is only slightly larger in size than in a normal electric motor, and may not have any exposed drive shafts.
|Motor-Generator (M-G) Set|
Another kind of motor-generator utilizes a special synchronous generator called a written-pole motor, which produces a constant 60-Hz frequency as the machine slows. It is able to supply a constant output by continually changing the polarity of the rotor’s field poles. Therefore, each revolution can have a different number of poles than the previous one. As long as the rotor is spinning at speeds between 3150 and 3600 revolutions per minute (rpm), constant output is maintained. The inertia of the flywheel permits the generator rotor to keep rotating at speeds above 3150 rpm once power shuts off. Normally, the rotor weight creates enough inertia to keep it spinning fast enough to produce 60 Hz for 15 s under full load.
A motor supplied by the incoming line drives a generator, which powers the load. The M-G set may contain a flywheel on the same shaft to provide greater inertia, and subsequently improve ride-through time. When the line is subjected to voltage sags and interruptions, the inertia of the machines and the flywheels maintains the power supply for several seconds. This set-up may also be employed to isolate sensitive and critical loads from other power quality problems such as switching transients and harmonic distortion. Motor-generator sets also have inherently superb resistance to electrostatic discharge (ESD).
In addition, motor-generators have even been employed where the input and output currents are basically the same. Consequently, the mechanical inertia of the M-G set will filter out transients in the input power. The output's electric current can become noise-free and shall be able to ride-through transients and interruptions at the input to the motor-generator set. This may facilitate the smooth cut-over from mains power to AC power provided by a diesel generator set.
Motor-generator sets have been replaced by semiconductor devices for some purposes. However, they are still being preferred in industrial settings where harmonics removal, line isolation and/or frequency conversion is required.
Moreover, the motor-generator can deal with large short-term overloads better than modern semiconductor devices of the same average load rating. This is because the thermally current-limited components of M-G set are copper windings weighing hundreds of kilograms, which are intrinsically attached to their own large thermal mass. In contrast, large semiconductor inverters have solid-state switches that have a few grams of mass with a thermal time constant to their heat sinks of likely more than 100 milliseconds only.
Motor-Generator sets have disadvantages for some types of loads:
- There are losses associated with the machines, although they are not significantly larger than other technologies.
- The frequency and voltage drop during interruptions as the machine slows. This may not work well with some loads.
- Noise and maintenance may be issues with some installations.
Reference:Dugan, R., McGranaghan, M., Santoso, S., and Beaty, H.W. (2004). Electrical Power Systems Quality (2nd ed.).