Day 24 Night – DC Machines → Testing & Efficiency

Concept Notes (Deep Explanation + Examples)

🌟 Introduction

In DC machines (both motors and generators), efficiency is one of the most important parameters that tells us how effectively electrical energy is converted into mechanical energy (motor) or vice versa (generator).
Testing methods help determine efficiency, losses, and performance without dismantling the machine.

In simple terms:
🌀 Efficiency = Output Power / Input Power
In DC machines, energy conversion involves electrical, magnetic, and mechanical forms. Losses occur in each stage, reducing the overall efficiency.

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⚡ Types of Losses in DC Machines

1️⃣ Copper Losses (I²R losses)
Occurs in armature, field winding, and brush contact.
Example: In lab, when current flows through field coils, heat develops in the copper wires.
Formula:

2️⃣ Iron or Core Losses
Due to alternating magnetic flux in the armature core.

• Hysteresis loss → due to magnetic reversal

Eddy current loss → due to induced currents in the core

3️⃣ Mechanical Losses
Caused by friction (in bearings) and windage (air resistance).

4️⃣ Stray Losses
Small unaccounted losses (1% of output approximately).

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🔍 Importance of Testing

Direct loading of large DC machines is uneconomical and unsafe. Hence, engineers use indirect testing methods to find efficiency without applying full load.

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🧩 Types of Efficiency Tests

1️⃣ Direct Test (Brake Test)

• Machine loaded directly by a brake drum on the shaft.
• Measure:
  • Input electrical power (V × I)
  • Output mechanical power (2πNT / 60)
• Used for small DC motors (lab experiments).

⚙️ Example:
A DC shunt motor runs at 1000 rpm and delivers 10 N·m torque.
Input power = 1200 W
Output =
Efficiency = latex \times 100 = 87.25%[/latex]

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2️⃣ Indirect Tests

Used for large DC machines to avoid mechanical loading.

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🧮 (A) Swinburne’s Test (No-load Test)

• Simplest and most common test for DC shunt machines.
• Conducted at no-load condition.
• Measures constant losses (iron + mechanical).

Procedure:

1. Run the motor at rated speed without load.
2. Measure voltage (V), current (I₀).
3. Calculate no-load input = .
4. Subtract field copper loss () → constant losses.
5. Predict efficiency at any load.

Formula for efficiency at load current I:

(for generator)
(for motor)

✅ Advantage: Simple, economical, no load needed
❌ Limitation: Cannot include stray load losses

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🧮 (B) Hopkinson’s Test (Back-to-Back Test)

• Conducted on two identical DC machines mechanically coupled.
• One acts as a motor, the other as a generator.
• Very little power from supply; hence, it’s an efficiency test at full load.

Working Principle:

1. Both machines connected back-to-back.
2. Supply given to one machine → runs as motor.
3. This motor drives the other → acts as generator and supplies power back.
4. Only losses are supplied from the mains!

⚙️ Efficiency =

✅ Advantage: Full-load testing with small power
❌ Limitation: Requires two identical machines

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🧮 (C) Field’s Test (for DC Series Machines)

• Conducted on two identical DC series machines.
• One acts as motor, other as generator.
• Similar to Hopkinson’s test but suitable for series type.

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🧮 (D) Retardation Test (for DC Shunt Motor)

Used to determine rotational losses (mechanical + iron).
Machine allowed to run freely after disconnecting supply; rate of fall of speed gives losses.

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⚙️ Example (Efficiency Calculation)

At no-load:
V = 220 V, I₀ = 2 A
Rₐ = 0.5 Ω, I_f = 0.5 A

Constant losses =

At full-load current 10 A:
Armature Cu loss =
Total loss = 340 + 45.125 = 385.125 W
Input =
Efficiency =

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⚙️ Formulas (Plain LaTeX Only)

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🔟 10 MCQs (GATE + ECET Mixed)

1. Swinburne’s test is conducted on
   A) DC Series Motor
   B) DC Shunt Motor
   C) DC Compound Motor
   D) DC Series Generator
2. Hopkinson’s test is also known as
   A) Open-circuit test
   B) Back-to-back test
   C) Retardation test
   D) Brake test
3. Constant losses in DC machine include
   A) Copper losses
   B) Mechanical and iron losses
   C) Eddy and hysteresis losses only
   D) None
4. Retardation test is used to find
   A) Efficiency
   B) Mechanical losses
   C) Stray losses
   D) Armature reaction
5. The efficiency of a DC motor is maximum when
   A) Iron loss = Copper loss
   B) Constant loss = Variable loss
   C) Constant loss = 2 × Variable loss
   D) None
6. In Swinburne’s test, losses due to load current are
   A) Neglected
   B) Measured
   C) Multiplied
   D) Compensated
7. Hopkinson’s test requires
   A) One machine
   B) Two identical machines
   C) Three machines
   D) None
8. The main advantage of Swinburne’s test is
   A) Full load testing
   B) Simple and economical
   C) Very accurate
   D) Requires two machines
9. The output power of a DC motor is given by
   A) VI
   B) 2πNT/60
   C) VI²
   D) I²R
10. Efficiency of a 220 V, 10 A DC machine with total losses 200 W is
    A) 90.9%
    B) 92%
    C) 89%
    D) 95%

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✅ Answer Key

Q.No Answer
1 B
2 B
3 B
4 B
5 B
6 A
7 B
8 B
9 B
10 A

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🧠 MCQ Explanations (Step-by-Step)

1️⃣ Ans: B — Swinburne’s test is used for DC shunt motors at no-load condition.
2️⃣ Ans: B — Hopkinson’s test is called back-to-back as both machines run simultaneously.
3️⃣ Ans: B — Constant losses = Iron + Mechanical losses.
4️⃣ Ans: B — Retardation test helps determine rotational losses.
5️⃣ Ans: B — Efficiency is max when constant loss = variable loss.
6️⃣ Ans: A — In Swinburne’s test, load current losses are not directly measured.
7️⃣ Ans: B — Hopkinson’s test uses two identical machines mechanically coupled.
8️⃣ Ans: B — Economical as it’s done at no-load without full current.
9️⃣ Ans: B — Mechanical output = .
10️⃣ Ans: A — Efficiency = .

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🎯 Motivation / Why Practice Matters (ECET 2026 EEE)

Testing and efficiency questions appear almost every year in ECET EEE!
They test your understanding of losses, machine performance, and lab applications — topics that connect theory to real-world electrical setups.
If you master this, you’ll easily solve 2–3 marks worth of questions in seconds.

⚡ Remember — efficient machines save energy and money.
🧠 Practice more, visualize every test like you’re in the lab.
🔥 “Today’s effort in learning efficiency will multiply your exam efficiency tomorrow!”

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