CWT

The CWT range has been used in a wide variety of EMC applications. It has a wide-bandwidth from sub Hz into the MHz range and configurable current ranges from less than 1A to >100kA.

Two examples demonstrate the versatility:

• The CWT has been used as the reference measurement in EMC magnetic impulse immunity testing where the DUT has to continue operation in the presence of a local impulse for several kA
• The CWT has also been used in many EMC traction applications where a measurement of mA level currents at frequencies > 1kHz need to be resolved in the presence of large 16.6 or 50/60Hz currents.

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, whereas 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 a PEM Rogowski current probe.

Pulsed power applications are very diverse but it are essentially the process of accumulating energy over a relatively long period of time, e.g. in capacitor or battery banks, or large inductors and releasing it instantly, thus increasing the instantaneous power, often producing peak currents in the range 100kA to >1MA.

PEM’s wide-band Rogowski coils have been used in the following pulsed power applications:

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

Rogowski current probes do not measure the DC component of a current. However unlike a current transformer or a fluxgate 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.

In industrial induction heating applications a current sensor is often used to monitor both current amplitude and zero-crossing detection, typically providing the feedback signal to a voltage-fed medium frequency inverter. Measured currents range from 500A to 10,000A and the frequency varies from a few hundred Hz to 10’skHz. The phase error of the measurement needs to be tightly controlled for accurate zero crossing detection. The RCTi has a very small and highly predictable phase error over this frequency range and fits the application perfectly due to its easy installation, high reliability and small size.

Additionally Rogowski probes are useful for diagnostic work in developing and maintaining induction heating coils and power converters, where fast switching frequencies, or hf resonance demand higher frequency performance than the RCTi.

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

The CWT range is non-intrusive injecting only a few pH into the primary circuit, it has a high frequency bandwidth of up to 50MHz,  a predictable known measurement delay, versions to mitigate interference from fast voltage transients, and comes in a variety of thin and clip-around coils with different isolation voltages, making it easy to insert into even difficult to reach parts of the circuit.

With its wide-bandwidth, high peak current ratings and high di/dt capability, the CWT is a useful tool for monitoring lightning strikes in large structures such as wind turbines. These strikes are typically high frequency pulses of several hundred kiloamps e.g. 8/20µs or 10/350µs waveforms. However it is just as important to be able to measure high energy ground to cloud strikes of a few hundred amps and up to 1 second in duration. Therefore any current sensor needs a very wide-bandwidth from less than 0.1Hz up in to the MHz range to successfully measure lightning.

As well as our CWT and CWTHF units PEM has also supplied custom coils up to a 20m circumference for monitoring lightning in massive structures such as wind-turbines or rocket launch towers.