science

Induction disk motor

Induction disk motor is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Induction disk motor rather than just read about it. In short: Induction disk motor is a low-power, low-speed AC motor that is primarily known for its use in analog electrical meters. It is also called a Ferraris disk after the inventor, Galileo Ferraris.

Key takeaways

Induction disk motor belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Induction disk motor to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Induction disk motor from memory before moving on to harder problems.

Reference excerpt

Induction disk motor is a low-power, low-speed AC motor that is primarily known for its use in analog electrical meters. It is also called a Ferraris disk after the inventor, Galileo Ferraris.

Construction The rotor of the motor consists of a conductive disk, through which a changing magnetic flux is applied by one or more split phase electromagnetic coils. Eddy currents are generated in the disk and the reaction between the eddy currents and the flux produces the driving torque. The design of an induction electrical meter uses pairs of coils, one using the current to be measured, one voltage on the terminals. The design of coils makes the magnetic fields in each pair out-of-phase, inducing the torque proportional to the current and voltage. A permanent magnet works as a brake ("retarding element") making the speed proportional to the torque.

Applications Due to its low efficiency, the induction disk motor applications are limited. The motors are best known for their use in the common household induction-type electrical meters. The motors were also used in electromechanical time switches. In the 1920s, the motors briefly found a niche in phonographs, displacing the spring power due to the advantages of silent operation, constant speed, lack of vibration, and reliability. The shaft of a relatively powerful motor (35 watts) was vertical and directly connected to the turntable with no speed reducing gears. The motor had high torque (6 inch-ounces), giving it rapid acceleration while starting. The 78 to 80 revolutions per minute speed was controlled by a fly-ball governor.

The Ferraris disks were used in the electromechanical relays used for power system protection. The accompanying illustration shows a device where the disk rotation triggers the action for a circuit breaker (not shown). One of the three coils is wired in opposition to the other two, but uses twice the turns. This produces a situation in which the forces applied to the disk are roughly balanced and the disk does not rotate, held in place by adjustable force. If the three phase currents become unbalanced (for example, due to an electrical fault), the disk rotates and a protective action is initiated.

References

Sources American Institute of Electrical Engineers (June 1926). "Induction Disk Phonograph Motor". Transactions of the American Institute of Electrical Engineers. American Institute of Electrical Engineers. Retrieved 2025-06-17. McColl, A.E. (1920). "Automatic Protective Devices for Alternating-Current Systems". Journal of the Institution of Electrical Engineers. 58 (293). IEE: 525–554. Retrieved 2025-06-17. Webster, John G. (2003-07-15). "Ac Induction Energy Meters". Electrical Measurement, Signal Processing, and Displays. CRC Press. ISBN 978-0-203-00940-6. Retrieved 2025-06-17.

Illustrations

Induction disk motor: Protective relay checking the balancing of three phases based on the Ferraris disk — Protective relay checking the balancing of three phases based on the Ferraris disk

Worked examples

Example 1 — a first encounter with Induction disk motor

Start with the simplest possible case. Write down what Induction disk motor claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Induction disk motor before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Induction disk motor ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Induction disk motor

In research: Induction disk motor appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Induction disk motor in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Induction disk motor is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electricity stubs, Induction motors, so understanding it makes those chapters shorter.
In everyday life: Look for Induction disk motor outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Induction disk motor in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Induction disk motor means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Induction disk motor out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Induction disk motor in simple terms?

Induction disk motor is a low-power, low-speed AC motor that is primarily known for its use in analog electrical meters. It is also called a Ferraris disk after the inventor, Galileo Ferraris.

Why does Induction disk motor matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Induction disk motor?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Induction disk motor.