Overview

The CWT is a state-of-the art wide-bandwidth ac current probe with typical bandwidths in the range < 1Hz to 16MHz.

The CWT is ideal for power electronic development work because it combines an easy-to-use thin, flexible, clip-around coil with an ability to accurately replicate fast switching current waveforms be they sinusoidal, quasi sinusoidal or pulsed.

Standards and Approvals

• CE marked
• Designed to comply with IEC61010-1
• PEM Ltd is an ISO9001:2015 registered company
• Patented technology

Performance

Features

• Traceable calibration (certificate included)
• Thin, clip-around, Rogowski coil for easy insertion into circuits with restricted access
• Non-intrusive: Loads the circuit under test by only a few pH’s
• No magnetic materials ensures excellent linearity < 0.05% reading
• Galvanically isolated measurement

Applications

The CWT has been supplied to major electrical engineering companies and leading research institutions around the world for the past 20 years. This versatile instrument has been used for a  wide variety of applications, some examples include:

Semiconductor Switching Waveforms

The CWT can be used for a wide variety of power semi-conductor measurements such as switching loss in power semiconductors or checking that in-circuit inductance is not over stressing an IGBT or MOSFET.

The CWT is non-intrusive injecting only a few pH into the primary circuit, it has a high frequency bandwidth of greater than 10MHz, has a predictable known measurement delay, and is thin and clip-around making it easy to insert into even difficult to reach parts of the circuit.

50μs, 3kA peak with a 100ns falling edge
CWT30 with 300mm coil and an 800MHz (3dB) co-axial shunt

Large AC Currents in Generators, Motor  Drives and Power Supplies

Unlike other forms of current measurement the size of the sense coil (the Rogowski coil) is independent of the size of the measured current. Thus for large currents the size of the Rogowski coil can be chosen to suit the conductor diameter, where as probes based on magnetic materials such as hall effect become increasingly bulky and expensive at larger current ratings. Over a 1000A range there really is no better alternative to the PEM’s Rogowski current transducer.

Pulsed Power Applications

PEM’s CWT Rogowski coils have been used:

• to monitor current in the kicker electro-magnets in a large particle accelerator.
• in laser power supplies
• in a ‘Superconducting Fault Current Limiter’ (Cryogenic Application)

High Frequency Sinusoidal Applications – such as Induction Heating

The CWT is capable of measuring large sinusoidal currents at high frequency with good accuracy and a predictable phase response. This is important for applications such as induction heating where it is important to control the zero crossing or for power measurements at high frequency.

As an example the CWT15 (rated for 3000A peak) is capable of measuring continuous sinusoidal currents of up 1000A at over 100kHz.

PEM have supplied custom designed versions of the CWT for even higher frequency applications.

Lightning Strike Measurement

With its wide-bandwidth, high peak current ratings and high di/dt capability (40kA/µs or larger for custom designs), the CWT is  a useful tool for monitoring lightning strikes. These strikes are typically  specified as 8/20µs or 10/350µs waveforms, so the current  transducer needs both high frequency performance in the MHz range but also a negligible  droop characteristic i.e. a low frequency bandwidth of less than 0.1Hz.

As well as our standard CWT units PEM has also supplied custom designed coils up to 20m circumference for monitoring lightning strikes in massive structures such as wind-turbines.

Measuring Small AC Currents in the Presence of Large DC Currents (e.g. capacitor ripple)

Rogowski current transducers do not measure the DC component of a current. However unlike a CT or hall effect device they contain no magnetic materials so are unaffected by the DC current. Therefore it is possible to use a small, flexible, Rogowski coil to measure a small AC current in the presence of a large DC current whereas a sensor based on a magnetic principle would be expensive and bulky to prevent saturation effects. One such common application is measuring the ripple current in capacitors where the main current is DC or slow time varying.

Other applications include:

• Power converter development and diagnostics
• Measuring hf harmonics (from traction to bearing currents)
• rf currents