For a DC machines laboratory following type of DC supply will be suitable

Question

Examoogle · Accepted Answer

Induction motor DC generator set

Explanation: An Induction Motor-DC Generator (M-G) set is the most suitable source for a DC machines laboratory because it provides an isolated, variable DC output that is galvanically separated from the AC mains and can be easily controlled for testing.

Working Principle: The Induction Motor converts electrical energy from the AC supply into mechanical energy to drive the shaft of a DC generator. The DC generator, acting as an electromechanical energy converter, produces a DC voltage at its terminals. The magnitude of this output can be precisely varied by adjusting the field current of the generator, allowing for flexible testing of various DC machine characteristics without interference from supply fluctuations.

Key Formulas: $E_g = \frac{P\phi NZ}{60A}$ — Induced EMF in DC generator; $P_{out} = V_t I_a$ — Electrical power output

Key Points: Provides complete galvanic isolation from the AC power grid.; Easily controllable output voltage by varying generator field excitation.; Acts as a buffer, preventing sudden load changes in the lab from affecting the main grid stability.; Allows for regenerative testing arrangements in advanced machine labs.

Advantages: Stable and clean DC output; Easy control over terminal voltage; Good regulation

Disadvantages: High initial cost compared to rectifiers; Requires maintenance due to rotating parts; Lower overall efficiency due to double conversion

Applications: University and industrial testing laboratories; Variable speed drive research; DC machine performance testing

Additional Info: Mercury arc rectifiers are obsolete due to efficiency issues and environmental concerns regarding mercury vapor.. Rotary converters were historically used but are less flexible than M-G sets for laboratory testing.. Synchronous M-G sets are also used, but induction motor sets are preferred due to the simplicity of starting the induction motor.

Answer: C is correct — The Induction Motor-DC Generator set is the preferred choice as it provides an isolated, adjustable, and stable DC supply essential for laboratory machine testing.

Exam Tip: Always remember that in lab settings, isolation and ease of voltage control are prioritized over the higher efficiency provided by static rectifiers.

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