Wattmeter (Dynamometer) & Power Measurement
Test Instrument
Measures real power by multiplying voltage and current inside one instrument: a dynamometer movement whose torque tracks V×I×cosφ. Home of the two-wattmeter method and half the numericals in the Measurements paper.
What it looks like
Wattmeter (Dynamometer) & Power Measurement
A typical test instrument — exact shape, colour and markings vary between manufacturers.
Types & variants
Key specs
Current coil rating
AThe fixed coil in series with the load: 5/10/20 A ranges. Overload burns it — always start on the highest range.
Pressure coil rating
VThe moving coil across the load: 150/300/600 V ranges via multipliers.
Power factor class
UPF / LPFLPF wattmeters give full deflection at cosφ = 0.2 — mandatory for transformer open-circuit tests where PF is tiny and a UPF meter barely moves.
Multiplier constant
×Reading × range factor = watts; getting this wrong is the classic lab error.
Connection error
—Pressure coil on supply side vs load side each mis-measures slightly (coil consumption) — a standard viva question with a compensation-winding answer.
Markings
Terminals marked M L (current coil: Mains–Load) and C V (pressure coil: Common–Voltage), with ± polarity dots; ranges printed by each terminal pair. Reversed deflection in the two-wattmeter method means that meter's reading is negative.
Standard values
Lab standards: 5/10 A, 150/300/600 V dynamometer wattmeters, UPF and LPF versions, class 0.5. Digital: single- and three-phase power analyzers reading W, VA, VAR, PF, THD.
How to choose
1) Routine loads: UPF dynamometer or digital meter. 2) Transformer OC test: LPF type, or the reading is unreadably small. 3) Three-phase: two-wattmeter method needs two matched meters (total P = W₁+W₂, and tanφ = √3(W₁−W₂)/(W₁+W₂) — the formula to memorise). 4) Modern work: a power analyzer adds PF and harmonics for SMPS/VFD loads that fool analog meters.
Pinout & package
Four (or five) terminals: current coil in series (M→L), pressure coil across (C→V). Portable wooden/bakelite lab cases with range selectors; digital units use the same wiring or clamp CTs.
Example circuits
- Single-phase load: wattmeter + voltmeter + ammeter, deriving PF = W/(V×I)
- Two-wattmeter method on a 3-phase induction motor, watching W₂ go negative below PF 0.5
- Transformer OC test with LPF wattmeter reading iron loss
- Blondel's theorem in practice: n−1 wattmeters for n wires
Common failures
Burnt current coil from overload (meter reads zero forever), bent pointer, insulation breakdown between coils, and in labs mostly wiring errors: swapped M/L, missing ± link, wrong multiplier factor applied.
How to test
Known resistive load check: a 500 W heater on 230 V should read ~500 W and match V×I. Verify each range's multiplier with the same load. Compare against a digital power meter for calibration drift.
Substitutes
Digital power analyzers replace dynamometers everywhere except the syllabus itself. For rough audits, a clamp power meter or plug-in energy monitor. The energy meter (kWh) integrates what the wattmeter reads instantaneously — related, not interchangeable.