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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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Under normal condition, how much faster can a machine run than its rated speed?
10 %
20 %
12 %
8 %
10 %
Quick Summary: Under normal operating conditions, rotating machines (such as motors and generators) are designed to withstand brief excursions in speed. Industry standards generally specify that machines should be capable of operating safely at 10% above their rated speed without sustaining structural damage.
Under normal operating conditions, rotating machines (such as motors and generators) are designed to withstand brief excursions in speed. Industry standards generally specify that machines should be capable of operating safely at 10% above their rated speed without sustaining structural damage.
Nmax=1.10×Nrated — Relation between rated and maximum allowable speed
Fcentrifugal=mrω2 — Centrifugal force related to rotational speed
The 10% margin is provided to account for transient disturbances in the power system, such as frequency fluctuations or momentary loss of load. This buffer ensures that centrifugal forces do not exceed the mechanical stress limits (hoop stress) of the rotor assembly, which typically scales with the square of the angular velocity, ω2.
The 10% limit is a mechanical safety standard defined for electrical machines.
Exceeding this limit can cause rotor winding displacement or damage to bearing lubrication.
This provision is crucial for protection against accidental load rejection in generator sets.
Prevents structural damage during transient speed increases.
Provides a safety margin for frequency regulation errors.
Increased wear and tear on bearings if sustained for long periods.
Risk of resonant vibration if machine critical speeds are nearby.
AC induction motors
Synchronous generators
DC shunt and series motors
This 10% rule is standard in many IEEE and IEC machine testing specifications.
Option B (20%) is generally considered unsafe for long-term mechanical integrity of high-speed rotors.
A is correct — Electrical machines are standardized to safely handle a speed increase of up to 10% beyond their rated speed without mechanical failure.
Always relate mechanical speed limits to the frequency of the supply system, as Ns=P120f, meaning a 10% rise in frequency leads directly to a 10% rise in synchronous speed.