Transformers, DC and AC motors, induction and synchronous machines, transmission lines, per-unit system and protection.
40 cards · basic cards · AI-written, checked twice. Edit anything.
- What is the voltage ratio of an ideal transformer?
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Vp/Vs = Np/Ns (voltage ratio equals turns ratio)- What is the no-load current in an ideal transformer?
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Zero (assuming ideal core with no losses)- What causes copper loss in a transformer?
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I squared R losses in primary and secondary windings- What causes iron loss in a transformer?
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Hysteresis and eddy current losses in the core- Under what condition is transformer efficiency maximum?
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When copper loss equals iron loss- What is the impedance ratio Zp/Zs of an ideal transformer?
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(Np/Ns) squared (turns ratio squared)- How does per-unit impedance change when referred through an ideal transformer?
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Per-unit impedance is invariant (unchanged)- What is the primary purpose of transformer cooling?
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To dissipate heat from core and copper losses- Write the back EMF equation for a DC motor
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Eb = K times Phi times omega (K is constant, Phi is flux, omega is angular velocity)- What determines the starting torque of a DC motor?
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T = K times Phi times Ia (proportional to flux and armature current)- Why is starting current very high in a DC motor without a starting resistor?
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Back EMF is zero at startup, so only armature resistance limits current- How does speed change in a DC motor when load decreases?
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Speed increases because armature current decreases, reducing back EMF drop- What happens to speed when field current decreases in a separately excited DC motor?
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Speed increases (reduced flux requires higher speed for same terminal voltage)- Write the torque equation for a DC motor in terms of input power
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T = Pin / omega (input power divided by angular velocity)- What is the synchronous speed formula for an AC induction motor?
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Ns = 120f / p (f in Hz, p = number of poles, result in rpm)
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