science

Shaft voltage

Shaft voltage 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 Shaft voltage rather than just read about it. In short: Shaft voltage occurs in electric motors and generators due to leakage, induction, or capacitive coupling with the windings of the motor. It can occur in motors powered by variable-frequency drives, as often used in heating, ventilation, air conditioning and refrigeration systems.

Key takeaways

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

Reference excerpt

Shaft voltage occurs in electric motors and generators due to leakage, induction, or capacitive coupling with the windings of the motor. It can occur in motors powered by variable-frequency drives, as often used in heating, ventilation, air conditioning and refrigeration systems. DC machines may have leakage current from the armature windings that energizes the shaft. Currents due to shaft voltage causes deterioration of motor bearings, but can be prevented with a grounding brush on the shaft, grounding of the motor frame, insulation of the bearing supports, or shielding. Shaft voltage can be induced by non-symmetrical magnetic fields of the motor (or generator) itself. External sources of shaft voltage include other coupled machines, and electrostatic charging due to rubber belts rubbing on drive pulleys. Every rotor has some degree of capacitive coupling to the motor's electrical windings, but the effective inline capacitor acts as a high-pass filter, so the coupling is often weak at 50–60 Hz line frequency. But many Variable Frequency Drives (VFD) induce significant voltage onto the shaft of the driven motor, because of the kilohertz switching of the insulated gate bipolar transistors (IGBTs), which produce the pulse-width modulation used to control the motor. The presence of high frequency ground currents can cause sparks, arcing and electrical shocks and can damage bearings.

Counter-measures Techniques used to minimise this problem include: insulation, alternate discharge paths, Faraday shield, insulated bearings, ceramic bearings, grounding brush and shaft grounding ring.

Faraday shield

An electrostatic shielded induction motor (ESIM) is one approach to the shaft-voltage problem, as the insulation reduces voltage levels below the dielectric breakdown. This effectively stops bearing degradation and offers one solution to accelerated bearing wear caused by fluting, induced by pulsewidth modulated (PWM) inverters.

Grounding brush Grounding the shaft by installing a grounding brush device on either the non-drive end or drive end of a VFD electric motor provides an alternate low-impedance path from the motor shaft to the motor case. This method channels the current away from the bearings. It significantly reduces shaft voltage and therefore bearing current by not allowing voltage to build up on the rotor.

Shaft grounding ring A shaft grounding ring is installed around the motor shaft and creates a low impedance pathway for current to flow back to the motor frame and to ground. Various styles of rings exist such as those containing microfilaments making direct contact with the shaft or rings that clamp onto the shaft with a carbon brush riding on the ring (not directly on the shaft).

Insulated bearings Insulated bearings eliminate the path to ground through the bearing for current to flow. However, installing insulated bearings does not eliminate the shaft voltage, which will still find the lowest impedance path to ground. This can potentially cause a problem if the path happens to be through the driven load or through some other component.

Shielded cable

High frequency grounding can be significantly improved by installing shielded cable with an extremely low impedance path between the VFD and the motor. One popular cable type is continuous corrugated aluminum sheath cable.

Worked examples

Example 1 — a first encounter with Shaft voltage

Start with the simplest possible case. Write down what Shaft voltage 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 Shaft voltage 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 Shaft voltage 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 Shaft voltage

In research: Shaft voltage 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 Shaft voltage 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: Shaft voltage is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electric motors, Electric power, Electrical safety, so understanding it makes those chapters shorter.
In everyday life: Look for Shaft voltage 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 Shaft voltage in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Shaft voltage 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 Shaft voltage out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Shaft voltage in simple terms?

Why does Shaft voltage 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 Shaft voltage?

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 Shaft voltage.