Clamp Meter
This is an instrument that measures current by taking the conductor between its jaws without opening the circuit. Current-transformer types that measure AC only cannot read direct current; a model with a Hall effect sensor is required for DC measurement. Only a single core must pass through the jaws for the measurement.
The instrument measures the magnetic field around the conductor. If both the outgoing and the returning core are taken into the jaws, the fields cancel each other and the instrument reads zero. This is the most common usage error.
- DC measurement: on Hall effect types, zeroing is done with the jaws closed before measuring; DC readings taken without zeroing are offset.
- Low current: the resolution of clamp meters is limited at low currents. It is generally not suitable for measuring a parasitic drain of a few hundred milliamperes; there a series-connected ammeter is needed.
- Position: the conductor must be in the middle of the jaws and perpendicular; reading at the edge causes deviation.
- Advantage: the measurement can be made without opening the circuit at all, while energised, and within seconds.
Driver supply current, motor running current and battery charging current are quickly verified with this instrument. A continuous current measured above the label value points to a mechanical load, a voltage drop or a wrong setting.
Context in which this term is used: DC motor and driver fault diagnosis matrix