Swinburne's Test of DC Machine — Procedure, Efficiency Calculation & Limitations
Swinburne's test is an indirect, no-load method for determining the efficiency of DC shunt and compound machines at any desired load without actually loading the machine. Named after Sir James Swinburne, this test is widely used in laboratories and industry because it requires very little power — only enough to overcome no-load losses.
Principle of Swinburne's Test
The fundamental principle is simple: run the DC machine at no-load as a motor, measure the total input power, subtract the small armature copper loss at no-load, and the remainder equals the constant losses (iron losses + mechanical losses + shunt field copper loss). Once constant losses are known, efficiency at any load can be predicted mathematically.
This works because in shunt and compound machines, the flux remains approximately constant from no-load to full-load. Therefore, iron losses and mechanical losses remain nearly unchanged regardless of loading.
Circuit Diagram & Procedure
The machine is connected as a shunt motor and run at rated voltage and rated speed with no mechanical load.
Procedure:
- Connect the machine as a DC shunt motor with ammeters in the line and field circuit
- Apply rated voltage and allow the motor to reach rated speed (adjust field rheostat if needed)
- Record V, I₀, and I_sh at steady state
- Measure armature resistance Rₐ separately using voltmeter-ammeter method at standstill (correct for temperature rise to 75°C)
Determination of Constant Losses
At no-load, the entire input power is consumed by losses only (output is zero). The losses present at no-load are:
- Iron losses (hysteresis + eddy current)
- Mechanical losses (friction + windage)
- Shunt field copper loss (V × I_sh)
- No-load armature copper loss (very small)
No-load armature current, Iₐ₀ = I₀ − I_sh
No-load armature copper loss = Iₐ₀² × Rₐ
Constant Loss, Pc = V × I₀ − Iₐ₀² × Rₐ
Note: The constant loss Pc includes iron losses + mechanical losses + shunt field copper loss. The shunt field current remains the same at all loads in a shunt machine.
Efficiency When Running as Motor
Let I = line current at the desired load.
Armature current, Iₐ = I − I_sh
Armature copper loss = Iₐ² × Rₐ = (I − I_sh)² × Rₐ
Total losses = (I − I_sh)² × Rₐ + Pc
η_motor = [V×I − {(I − I_sh)² × Rₐ + Pc}] / (V × I)
Efficiency When Running as Generator
Let I = load current delivered by the generator.
Armature current, Iₐ = I + I_sh
Armature copper loss = (I + I_sh)² × Rₐ
Total losses = (I + I_sh)² × Rₐ + Pc
η_gen = (V × I) / [V×I + (I + I_sh)² × Rₐ + Pc]
Numerical Example
Problem: A 220V DC shunt machine has Rₐ = 0.5Ω, R_sh = 110Ω. During Swinburne's test: V = 220V, I₀ = 5A. Find efficiency at 50A load as motor and generator.
Iₐ₀ = 5 − 2 = 3A
Pc = 220×5 − (3²×0.5) = 1100 − 4.5 = 1095.5W
As Motor (I = 50A):
Iₐ = 50 − 2 = 48A
Cu loss = 48² × 0.5 = 1152W
η = (220×50 − 1152 − 1095.5)/(220×50) = 8752.5/11000 = 79.6%
As Generator (I = 50A):
Iₐ = 50 + 2 = 52A
Cu loss = 52² × 0.5 = 1352W
η = (220×50)/(220×50 + 1352 + 1095.5) = 11000/13447.5 = 81.8%
Advantages of Swinburne's Test
- Economical: Only no-load power is required, making it suitable for large machines where full-load testing would waste enormous energy
- Convenient: Efficiency at any desired load can be pre-determined without actually loading the machine
- Dual prediction: Both motor and generator efficiencies can be calculated from a single test
- Non-destructive: Machine is not subjected to mechanical or thermal stress during testing
Disadvantages of Swinburne's Test
- Iron loss variation ignored: At full load, armature reaction distorts the flux distribution, increasing iron losses beyond the no-load value
- Temperature rise not accounted: Rₐ increases with temperature at full load, but Swinburne's test uses cold resistance
- Commutation not tested: Commutation problems (sparking, brush heating) only appear under load and cannot be detected
- Stray load losses excluded: Additional losses due to leakage flux and eddy currents in conductors under load are not captured
Limitations of Swinburne's Test
- Applicable only to shunt and compound machines where flux is approximately constant
- Not suitable for DC series machines — in series machines, flux varies directly with load current, so no-load operation is meaningless (and dangerous — series motors can run away at no-load)
- Cannot determine the actual full-load temperature rise or commutation performance
Frequently Asked Questions
1. Why is Swinburne's test called an indirect method?
Because the machine is never actually loaded. Efficiency is calculated mathematically from no-load loss measurements rather than by directly measuring input and output power under load.
2. Can Swinburne's test be performed on a DC series motor?
No. A DC series motor cannot run safely at no-load — without mechanical load, the speed increases dangerously (runaway condition). Also, flux in a series machine varies with load current, so no-load losses are not representative of loaded conditions.
3. What losses are included in the constant loss Pc?
Constant loss includes iron losses (hysteresis + eddy current), mechanical losses (bearing friction + windage), and shunt field copper loss (V × I_sh). These remain approximately constant from no-load to full-load in shunt machines.
4. Why does Swinburne's test give slightly optimistic efficiency values?
Because it ignores: (a) increased iron losses due to armature reaction at full load, (b) higher Rₐ due to temperature rise, and (c) stray load losses. All these additional losses reduce actual efficiency below the predicted value.
5. What is the difference between Swinburne's test and Hopkinson's test?
Swinburne's test uses one machine at no-load and predicts efficiency mathematically. Hopkinson's (back-to-back) test uses two identical machines mechanically coupled — one as motor, one as generator — and tests under actual full-load conditions while drawing only loss power from the supply.