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Questions and Answers
What is the purpose of conducting OC and SC tests on an alternator?
What is the purpose of conducting OC and SC tests on an alternator?
In the EMF method, how is the synchronous impedance Zs calculated?
In the EMF method, how is the synchronous impedance Zs calculated?
What is the consequence of calculating voltage regulation using the optimistic method?
What is the consequence of calculating voltage regulation using the optimistic method?
Why is the EMF method considered a 'pessimistic method' for calculating regulation?
Why is the EMF method considered a 'pessimistic method' for calculating regulation?
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What is ignored in the MMF method when calculating the voltage drop in the machine?
What is ignored in the MMF method when calculating the voltage drop in the machine?
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Which sequence of actions is part of the Open Circuit Characteristic (O.C.C) procedure?
Which sequence of actions is part of the Open Circuit Characteristic (O.C.C) procedure?
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What does the formula for percentage regulation in an alternator represent?
What does the formula for percentage regulation in an alternator represent?
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What does Eo represent in the context of the generated voltage?
What does Eo represent in the context of the generated voltage?
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When calculating Eo, what should be noted about the phase relationship of the armature current and field mmf?
When calculating Eo, what should be noted about the phase relationship of the armature current and field mmf?
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What is the recommended method for adjusting field rheostats during the O.C.C?
What is the recommended method for adjusting field rheostats during the O.C.C?
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Study Notes
Objectives of Testing
- Conduct Open Circuit (OC) and Short Circuit (SC) tests to determine Open Circuit Characteristics (OCC) and Short Circuit Characteristics (SCC).
- Pre-determine full load regulation under various power factor conditions: unity power factor (upf), zero lag, zero lead, 0.6 lag, and 0.6 lead.
Machine Specifications
-
Alternator:
- Rating: 3.75 KVA
- Voltage: 420 V
- Current: 5.2 A
- Speed: 1500 rpm
- Field Voltage: 220 V
- Field Current: 1.4 A
-
DC Motor:
- Rating: 3 KW
- Voltage: 220 V
- Current: 13 A
- Speed: 1440 rpm
Required Apparatus
- Ammeters:
- MI: 0-10 A
- MC: 0-1.5 A, 0-5 A
- Voltmeters:
- MI: 0-300 V, 0-15 V
- Rheostats:
- 7002, 1.1 A (2 Nos)
- 3702, 1.5 A
- 1002, 5 A
Principles of Regulation
- Regulation formula:
- % Regulation = ((Eo - V) / Eo) × 100, where Eo is open circuit voltage and V is rated terminal voltage.
EMF Method
- Calculate synchronous impedance (Zs) using values from OCC and SCC.
- Synchronous impedance (Zs) formula: Zs = V / I, where V is terminal voltage and I is armature current.
- Involves inefficiencies:
- Assumes synchronous impedance is constant, which is affected by saturation.
- Tends to yield higher regulation values, known as the "pessimistic method."
MMF Method
- Considers voltage drop due solely to armature reaction, ignoring leakage reactance and resistance.
- Relevant phasors include:
- Armature mmf in phase with armature current (Ia).
- Field mmf (F) and induced EMF (Eo).
- The regulation calculation:
- Regulation = ((Eo - V) / V) × 100% with adjustments for field currents based on lag or lead.
- Drawbacks:
- Tends to underestimate regulation due to using voltage in the saturated region of SCC, termed the "optimistic method."
Testing Procedure
-
Open Circuit Characteristics (O.C.C):
- Open TPDT switch, start the DC motor, adjust field to synchronous speed.
- Switch on DC supply to the alternator field, initially set resistance to maximum.
- Gradually increase excitation, noting generated voltages at various field currents (up to 2A).
- Subsequently reduce excitation to achieve minimum generated voltage.
-
Short Circuit Characteristics (S.C.C):
- Set TPDT to short circuit alternator armature.
- Increase excitation until rated current is reached, then note corresponding field current.
- Return to initial conditions and open TPDT switch.
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Description
Test your understanding of alternator regulation by experimenting with EMF and MMF methods. This quiz covers concepts related to open-circuit and short-circuit tests, focused on pre-determining full load regulation based on various power factors. Sharpen your knowledge on the associated apparatus and parameters.
