An electrical lab for power electronics experiments is built around equipment that can source, switch, measure and safely dissipate electrical power at levels far above those used in a basic electronics lab. The electrical experiment equipment in this lab ranges from a Variac and a regulated DC supply to thyristor trainer kits, rheostat loads and isolated measuring instruments. Each item exists because a converter, chopper or inverter experiment makes a specific demand on the bench.
Power electronics is the study of converting electrical power from one form to another using semiconductor switches such as diodes, SCRs, TRIACs, MOSFETs and IGBTs. Rectifiers convert AC to DC, choppers convert one DC level to another, inverters convert DC to AC, and AC voltage controllers vary the RMS value of an AC supply. Every one of these conversions can be demonstrated on a bench, but only if the lab electrical equipment around the circuit is chosen correctly.
This guide covers what each category of equipment does, how the instruments should be selected, a worked numerical example that links meter choice to converter output, which experiments use which equipment, the safety rules that apply, and how the whole setup relates to Indian university labs and competitive exams.
A general electrical lab, built around circuit theorems, transformers and machines, works with sinusoidal waveforms and steady readings. A basic electronics lab works with small signals at a few volts. A power electronics lab sits between the two and adds a difficulty of its own: the waveforms are chopped, the voltages are mains-level, and the circuits are switched at speeds that make a casual measurement misleading.
Three consequences follow from this. First, the supply must be variable and, where possible, isolated from the mains. Second, the load must be able to absorb real power without overheating. Third, the measuring instruments must be able to read non-sinusoidal waveforms and must not create a short circuit through their own ground connection.
A laboratory text on power electronics experiments states that completely benign power electronics experiments are not possible, and that damaged components or costly repairs follow when basic safety rules are ignored. That is the right mindset for equipment selection: every item on the list either supplies power, controls it, absorbs it, measures it or protects the person at the bench.
Readers who want the foundation first can start with NvisTech's guide to what equipment an electronics engineering lab needs, which covers the small-signal instruments that this lab builds on.
The source is where most beginners go wrong, because a mains socket is not an acceptable supply for a converter experiment. Three types of sources cover the usual syllabus.
A Variac, also called an autotransformer or variable transformer, lets the AC input be raised slowly from zero to the required value. Starting an experiment at zero and winding the voltage up is the standard procedure, because a wiring error then shows up as an abnormal current at low voltage rather than as a failed device at full voltage. One caution matters here: a Variac changes the voltage but does not provide isolation, because its output is still electrically connected to the mains.
An isolation transformer, with a 1:1 ratio, provides the isolation that the Variac lacks. It separates the circuit under test from the earthed mains, so that touching one conductor does not complete a path to ground through the body. A common arrangement is to feed the Variac from the mains and pass its output through the isolation transformer before it reaches the circuit. Some commercial power electronics trainers also offer an optional isolation transformer for the oscilloscope, which the instruments section below explains.
A regulated DC power supply, usually 0 to 30 V with an adjustable current limit, covers the DC side of the syllabus: gate and base drive for device characteristic experiments, the input of a chopper, or a battery-like source for a small inverter. The current limit works as a safety feature as much as a specification, since it caps the fault current when a student wires a short.
Three-phase experiments, such as a three-phase fully controlled bridge rectifier, need a three-phase supply with a three-phase autotransformer or a transformer bank. Many Indian lab inventories list this as a separate three-phase module rather than something students build from loose components.
Most Indian engineering college labs run power electronics experiments on modular trainer kits rather than loose components. The equipment inventory published by NIT Hamirpur's power electronics lab is a representative example. It lists modules for SCR, MOSFET, IGBT and TRIAC static characteristics, single-phase and three-phase half and fully controlled bridge converters, forced commutation circuits, a Jones chopper, a UJT firing module, a push-pull inverter, a modified McMurray-Bedford full bridge inverter, AC phase control using SCR, TRIAC and DIAC, and a PWM inverter control module.
Reading that list as a set of categories is more useful than memorizing it. A device characteristic module holds the power device on a board with its gate drive circuit, bias supplies and meter terminals, so that the V-I curve can be plotted safely. A firing circuit module generates the gate pulses that trigger an SCR at a chosen delay angle, using R, RC or UJT triggering. A converter module combines the power devices, the firing circuit and the output terminals for a controlled rectifier, chopper, inverter or AC voltage controller. A commutation module demonstrates how an SCR is turned off in circuits where the supply does not do it naturally.
For the inverter experiments in particular, NvisTech's NV6001 Inverter Trainer is built so that the basics of inverter operation can be understood step by step, and its NV6003 Power Supply Trainer covers the transformer, rectifier, filter and regulator stages that appear in the first few power supply experiments.
More advanced setups move beyond fixed trainers. IIT Madras describes a DC-DC converter analysis setup in its power electronics laboratory that uses a DSP controller for PWM, with adjustable resistors, inductors and capacitors on flexible power circuit boards, so that buck, boost, buck-boost, forward, flyback and interleaved boost topologies can be built on one platform. The same lab also lists a three-phase inverter setup with a rheostat load, a variable inductor load and a Variac.