Interview questions & answers
Q1. Explain the process of voltage buildup in a DC shunt generator.
Voltage buildup in a shunt generator occurs due to residual magnetism in the field poles — even with no field current, a small residual flux induces a small EMF in the rotating armature, which drives a small current through the field winding, slightly increasing flux, which increases EMF, which increases field current, in a cumulative process that continues until the field resistance line intersects the OCC. A generator will fail to build up if there is no residual magnetism, the field connections are reversed relative to residual flux, or the field circuit resistance exceeds the critical resistance. In practice, if a generator fails to build up, briefly connecting the field terminals to a battery (flashing the field) restores residual magnetism.
Follow-up: What is the critical field resistance of a DC shunt generator?
Q2. What is the OCC (Open Circuit Characteristic) of a DC generator and what does it show?
The OCC (also called the magnetisation curve or no-load saturation curve) plots the open-circuit EMF against the field current at rated speed — it shows how EMF increases with field current, initially linear (unsaturated) and then flattening (saturated). For a 220 V, 10 kW shunt generator, the OCC reaches 220 V at around 1.5–2 A field current, and 250 V at 3.5–4 A. The OCC at different speeds are geometrically similar (scaled vertically by speed ratio), which is used to predict performance at speeds other than the test speed.
Follow-up: How is the critical resistance determined graphically from the OCC?
Q3. What are the different types of DC generators and their characteristics?
DC generators are classified by how the field winding is connected: separately excited (field from external source, stable, used for precise voltage control), shunt (field in parallel with armature, self-excited, flat voltage regulation, most common), series (field in series with armature, rising voltage with load, used for boosters), and compound (series and shunt field combined, cumulative compound gives stable voltage, differential compound gives falling voltage). A 415 V flat-compound generator maintains terminal voltage within ±5% from no-load to full load — the series winding compensates for armature resistance drop.
Follow-up: Why is a differentially compounded generator rarely used in practice?
Q4. What causes voltage drop in a DC generator under load and how is it compensated?
Terminal voltage drops under load due to three factors: resistive drop in armature winding (Ia×Ra), voltage drop at brushes (about 2 V for carbon brushes regardless of current), and armature reaction that demagnetises the field flux. In a 220 V, 50 A DC shunt generator with Ra = 0.3 Ω, the armature drop alone is 15 V at full load. A compound generator uses a series field winding to increase flux with load, compensating the drops — a flat-compound generator is designed so terminal voltage at full load equals the no-load voltage.
Follow-up: What is the difference between a flat-compound and an over-compound DC generator?
Q5. What is commutation in a DC generator and what problems arise from poor commutation?
Commutation is the reversal of current in armature coils as they pass under the brushes — during the short time a coil is short-circuited by the brush, its current must reverse from +Ia to -Ia. Poor commutation due to high coil inductance causes the current reversal to lag behind the brush movement, producing a voltage spike at the trailing brush edge that causes sparking and arc damage to both the commutator surface and brush. Interpoles (commutating poles) between main poles generate a localised reversing EMF that exactly cancels the reactance voltage and forces ideal linear commutation.
Follow-up: What is the polarity of interpoles in a DC generator relative to the main poles ahead in the direction of rotation?
Q6. How do you determine the internal and external characteristics of a DC shunt generator?
The internal characteristic plots EMF (Eg) versus armature current (Ia) — it falls with Ia due to armature reaction demagnetisation, dropping perhaps 5–8% from no-load to full armature current. The external characteristic plots terminal voltage (Vt) versus load current (IL) — it falls more steeply than the internal characteristic because the additional drop Ia×Ra adds to the armature reaction effect. For a 20 kW, 220 V shunt generator, terminal voltage typically falls from 230 V at no-load to 220 V at full load — the 10 V difference is the designed regulation.
Follow-up: Why does the external characteristic of a shunt generator eventually curve back (terminal voltage drops rapidly) at high loads?
Q7. What is the condition for maximum efficiency of a DC generator?
Maximum efficiency occurs when variable losses (copper losses = Ia²×Ra) equal the constant losses (iron, friction, windage, and shunt field copper loss). For a 15 kW DC shunt generator with constant losses of 600 W and Ra = 0.4 Ω, maximum efficiency occurs at Ia = √(600/0.4) = 38.7 A. Generators supplying variable loads — like emergency diesel sets in hospitals — are designed for maximum efficiency at 75–80% of rated load since they rarely operate at 100% load continuously.
Follow-up: How does the efficiency-load curve of a DC generator differ from that of a transformer?
Q8. What is the significance of the back EMF in a DC generator?
In a DC generator, the term corresponding to back EMF in a motor is the generated EMF (Eg = Vt + Ia×Ra + brush drop) — the armature must generate this EMF to push current against both the terminal voltage and internal resistance drops. For a 220 V, 40 A DC generator with Ra = 0.3 Ω and 2 V brush drop, Eg = 220 + 40×0.3 + 2 = 234 V. The mechanical torque input from the prime mover does work against this generated EMF, converting mechanical power to electrical power — Eg×Ia equals the electromagnetic power converted.
Follow-up: What is the armature torque of a DC generator and how is it related to the prime mover input torque?
Q9. Why is a separately excited DC generator preferred for precise voltage control applications?
In a separately excited generator, the field current is independent of terminal voltage — even if load current fluctuates wildly, field current (and thus flux) remains stable, giving excellent voltage regulation and allowing the voltage to be set to any desired value by an independent field rheostat or electronic controller. A 48 V, 200 A separately excited generator used in a plating line can maintain output voltage within ±0.5% regardless of the plating current variation. Shunt generators cannot achieve this precision because their field current changes as terminal voltage droops under load.
Follow-up: What are the limitations of using a separately excited DC generator for high-power applications?
Q10. What is the Hopkinson test on DC machines and what are its advantages?
The Hopkinson test (back-to-back test) connects two identical DC machines mechanically coupled and electrically in a loop — one runs as a motor driving the other as a generator, with the generator feeding the motor. The external supply only provides the losses (typically 2–5% of rated power), making it very economical for testing large machines. A pair of 50 kW DC machines can be fully tested at rated current and full flux using a 2–3 kW supply, allowing accurate efficiency measurement at various loads without the need for a large load bank or separate drive machine.
Follow-up: What is the main limitation of the Hopkinson test and how are individual machine losses separated?
Common misconceptions
Misconception: A DC generator produces AC inside and DC at the terminals due to the commutator.
Correct: This is actually correct — the commutator mechanically rectifies the AC EMF generated in the rotating armature conductors into unidirectional (DC) current at the brushes.
Misconception: Increasing field resistance in a shunt generator increases terminal voltage.
Correct: Increasing field resistance reduces field current, reduces flux, reduces generated EMF, and therefore reduces terminal voltage.
Misconception: The internal characteristic and the external characteristic of a DC generator are the same.
Correct: The internal characteristic plots Eg vs Ia; the external characteristic plots Vt vs IL — they differ by the armature resistance drop (Ia×Ra) and brush voltage drop.
Misconception: Series field winding in a compound generator is connected in parallel with the armature.
Correct: The series field winding carries full load current and is connected in series with the armature circuit — only the shunt winding is connected in parallel.