CWTMini50HF

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.

The double pulse test is widely used to test the parameters of power MOSFETs and power IGBT’s. The circuit replicates the type of inductive load that is typical in power converter design. The test can be used to measure the semiconductor energy loss during switch on/off as well as the reverse recovery time of the diode.

The CWTMini50HF is the perfect choice for measuring the current flowing in modern power devices including SiC semiconductors. The 50MHz bandwidth is sufficient to measure a rise time of 12ns without distortion and the measurement delay is predictable and repeatable, which enables the user to correctly de-skew the current measurement and calculate energy loss.

The coil contains no magnetic materials and has a small cross section which minimises the inductance inserted in the primary circuit which could negatively affect the maximum achievable switching speed of the device being tested.

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.

Variable Frequency/Speed Drives (VFD / VSD) used to control AC motors can produce high frequency voltages that may ultimately appear on the machine shaft. These voltages are the result of capacitive coupling of the VSD PWM voltage to the motor shaft. The voltages on the shaft can be sufficient to cause arcing currents to flow through the motor bearings to ground. 

These discharging currents can cause heating and even melting of the surface of the bearing raceways. The damage caused by bearing currents can lead to premature failure of the motor drive as well as costly maintenance and down time. 

PEM have a range of probes for engineers to monitor these currents, directly measuring the bearing current, which ultimately can be used to evaluate the effectiveness of steps taken to mitigate against bearing currents.

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.

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.