Guide 05 Naval electrical power
What Is a Motor-Generator Set?
A motor-generator, or MG, set is an electric motor mechanically coupled to a generator, used to convert electrical power from one form to another: one frequency to another, AC to DC, or one voltage to another. Its rotating mass also isolates sensitive loads from a noisy supply and rides through brief blackouts and switching transients.
- Topic
- Naval electrical power
- Checked
- 23 September 2026
- Sources
- 3
- Questions
- 5
01
The basic idea
An MG set may be two distinct machines, a motor and a generator, coupled on a common shaft, or a single combined unit called a dynamotor, in which motor and generator windings share a single rotor and field coils. Either way, the motor draws power from the supply at one frequency, voltage or form, and the generator delivers it in another, with no electrical connection between the two sides beyond the shaft itself.
Because the load is connected to the generator rather than directly to the supply, the arrangement isolates that load from spikes and variations in a dirty supply. The set's own mechanical inertia filters out transients in the input power, so equipment fed from an MG set can ride through brief blackouts and switching disturbances that would otherwise reach it directly.
02
A rotary frequency converter is one application
A rotary frequency converter is a motor-generator set built specifically for frequency conversion: a motor running from the ship's or shore's 50 or 60 Hz supply drives a generator wound to deliver a different frequency, commonly 400 Hz for naval radar, weapon, communication and navigation equipment, on a common shaft or through a coupling. The same isolation and ride-through properties that make MG sets useful in general are exactly what make the rotary version attractive for supplying equipment that is sensitive to a noisy or interrupted input.
03
Load levelling with a flywheel
Beyond simple conversion, some installations add a large flywheel to a motor-generator rig specifically to level the load the connected equipment imposes on the wider electrical system, smoothing out sharp swings in demand rather than letting them reach the generators directly. Tokamak fusion devices and aircraft carriers are both examples of large, sharply fluctuating electrical loads managed this way, drawing down stored energy in the flywheel during a demand spike rather than pulling it straight from the supply.
04
Why solid state equipment has not fully replaced them
Semiconductor devices, static frequency converters and switch mode rectifiers among them, have replaced motor-generator sets for many of the jobs MG sets once did, particularly where size, weight and maintenance matter more than tolerance of a rough supply. MG sets remain a common solution wherever electrical isolation, harmonic cancellation or straightforward frequency conversion under a demanding supply is what the job actually needs, which is why rotary converters still sit alongside static ones on many warships rather than having been displaced entirely.
Asked next
Questions people also ask
What Is a Motor-Generator Set?
What does a motor-generator set actually convert?
It converts electrical power from one form to another: between frequencies, such as 50 or 60 Hz to 400 Hz, between AC and DC, or between voltages. The motor draws power in one form from the supply and the generator, mechanically coupled to it, delivers power in the other form to the load, with no direct electrical connection between the two sides.
How does a motor-generator set isolate a load from a noisy power supply?
The load connects to the generator, not directly to the incoming supply, so spikes and variations in dirty input power do not pass straight through. The set's own mechanical inertia also filters out transients in the input, letting equipment fed from the generator ride through brief blackouts and switching disturbances that would otherwise reach it.
Is a rotary frequency converter the same thing as a motor-generator set?
Yes. A rotary frequency converter is a motor-generator set built for frequency conversion specifically: a motor running from the input supply drives a generator wound for a different output frequency, commonly 400 Hz for naval equipment, on a common shaft. It shares the same isolation and ride-through properties as any other motor-generator arrangement.
Why do aircraft carriers use flywheels with motor-generator sets?
A large flywheel added to a motor-generator rig levels the load that sharply fluctuating equipment imposes on the wider electrical system, drawing down stored rotational energy during a demand spike rather than pulling it straight from the generators. Aircraft carriers and tokamak fusion devices are both examples of large, sharply variable loads managed this way.
Have motor-generator sets been replaced by solid state equipment?
For many purposes, yes: semiconductor devices such as static frequency converters and switch mode rectifiers have taken over jobs MG sets once did, especially where size, weight and maintenance matter more than tolerance of a rough supply. MG sets remain a common choice where electrical isolation, harmonic cancellation or straightforward frequency conversion under a demanding supply is the actual requirement.
Checked against
Sources
- 01Motor-generator, Wikipedia
- 02Frequency changer, Wikipedia
- 03SOLAS Chapter II-1, electrical installations on merchant ships (warships are exempt from SOLAS), International Maritime Organization
This guide explains how the system works in general, as the documents above describe it, and was last checked on 23 September 2026. Rules and thresholds change: the governing document is the authority, not this page.
On this site
Where this meets the work
- The DQA(N) type approved rotary frequency converter
- Special electrical equipment work
- The marine division
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