The field test is a regenerative/back-to-back test suited to DC series machines, including traction motors, and allows efficiency to be determined under load conditions without wasting full output power.
Formula / Key Relation
Regenerative test principle:
Supply input ≈ combined losses of the two machines
Efficiency is obtained from measured losses and machine output/input.
Detailed Explanation
Concept & ReasoningTraction motors are commonly DC series motors (in the classical syllabus), and ordinary no-load methods are unsuitable because a series motor must not be run at no load. The field test uses two similar mechanically coupled series machines, one operating as a motor and the other as a generator, so power circulates between them and the supply provides mainly the losses. From measured input and losses, efficiency can be determined at substantial load. Hopkinson's test is classically used for shunt machines, while Swinburne's is an indirect no-load test mainly for shunt/compound machines.How to Solve It in the ExamSeries…
Concept & Reasoning
Traction motors are commonly DC series motors (in the classical syllabus), and ordinary no-load methods are unsuitable because a series motor must not be run at no load. The field test uses two similar mechanically coupled series machines, one operating as a motor and the other as a generator, so power circulates between them and the supply provides mainly the losses. From measured input and losses, efficiency can be determined at substantial load. Hopkinson's test is classically used for shunt machines, while Swinburne's is an indirect no-load test mainly for shunt/compound machines.
How to Solve It in the Exam
Series traction motor → think field test/regenerative loading.
Important Exam Point
Match the test method to the DC motor connection type.
Related Revision Path
This question sits in the revision path Electrical Machines → DC Motors → Testing of DC Series Motors .
Quick Trick
Series traction motor → think field test/regenerative loading.
Common Mistake
Do not select Swinburne's test for a series traction motor; no-load operation of a series motor is unsafe.
Exam Tip
Match the test method to the DC motor connection type.
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