Consider a three-phase squirrel cage...

Question 1 of 270705eccefa4bacbc34ba4d4831a

Consider a three-phase squirrel cage induction motor rated at 450 VAC and 60 Hz. What would happen to the motor if the line frequency dropped from the normally supplied 60 Hz to 55 Hz and the voltage remained normal at 450 VAC.

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Question 1 of 27070

🔍 Key Concepts

• Synchronous speed of an induction motor and how it depends on line frequency • Relationship between slip, load, and actual rotor speed in a squirrel cage motor • What happens to torque and current if voltage stays the same but frequency drops slightly

💭 Think About

• First, think about how the motor’s theoretical no‑load speed changes if frequency goes from 60 Hz to 55 Hz while the number of poles stays the same. • Then consider what the rotor actually does under the same mechanical load when synchronous speed changes: does it draw more or less current, and does that change cause tripping or just a change in speed? • Ask yourself if a modest 5 Hz decrease is likely to cause severe mechanical problems (like excessive vibration) or protective trips, or primarily affect speed and slip.

✅ Before You Answer

• Write the synchronous speed formula and plug in both 60 Hz and 55 Hz to see the direction of speed change. • Check whether a small decrease in frequency, with voltage unchanged, is normally within the motor’s design tolerance or if it immediately causes a trip condition. • Decide which effect is most direct and certain: a change in speed, a change in current, or a sudden fault condition.