We've put together some details on frequently asked questions, please contact us for futher information.
Do I always need an integrator with a coil?
A coil can be used without an integrator but the direct output voltage from a coil is frequency-dependent and can have a large component of RF noise superimposed. The integrator removes the RF noise and converts the coil output into a voltage waveform which accurately reproduces the current waveform, in both amplitude and phase, over a wide frequency range. If you plan to use a coil on its own we will provide an individual calibration for that coil, expressed as a mutual inductance or output/kA at 50Hz.
How do I order / specify a replacement or duplicate coil or integrator?
Rocoil keeps comprehensive records of all the goods we despatch. All goods have a unique serial number from which they can be identified. If you supply us with the serial number, details of a previous order, or details about the goods then we can supply the goods again. If you are not sure what to order then contact us and we can help you decide what you need.
I am not sure what coil or integrator to order?
If you give us information about your application we can advise which product will best meet your needs or develop a coil specially for the purpose. We specialise in the production of custom designs to suit specific applications. If we think your application is not suitable for a Rogowski coil we will tell you.
Do you make custom designed coils that are not on your product list?
We specialise in the production of custom designs to suit specific applications. We are very customer focussed and have a lot of experience working on research projects with our customers and making specialist coils specifically to meet a particular need.
What is an interchangeable coil / integrator?
Individual coils have slightly different outputs, even if they are of the same design, and for best accuracy coils and integrators have to be calibrated together. Rocoil have devised a system where coils are calibrated during manufacture so that any coil will work with any integrator that has been designed to work with the same interchangeable system. We have a number of interchangeable systems, (designated 2500R, 820R, 430R, 270R), which aim to provide the best compromise between frequency response and the effects of temperature. The advantage of an interchangeable system is that a coil or integrator can be replaced without the need to re-calibrate the whole system. This includes coils of different length or integrators with a different specification from the original.
What is the lowest frequency I can measure?
It is not possible to design a system that responds down to DC. The low-frequency response of a Rogowski-coil system is determined by the design of the integrator. Generally, integrators designed for low-currents cannot also be designed for very low frequencies As an indication, an integrator with a 100A/Volt range would have a -3dB point below 0.3Hz, an integrator with a 10A/Volt range would have a -3dB point below 0.9Hz. For an integrator with multiple ranges the low-frequency performance is determined by the most sensitive range. Low-frequency performance is closely related to the 'droop' effect.
What coil is best for my application?
Applications vary over a wide range. We make compact coils where space is limited, very long coils for large conductors, high-precision coils for accurate measurements. If you tell us what you need the coil/integrator for then we can advise you which product will best meet your needs or, if possible, develop a coil specially for your needs. We specialise in the production of custom designs to suit specific applications.
Can I measure a generator shaft current going through bearings?
This application usually needs a long coil with an integrator calibrated for low currents (e.g.2A/Volt). We have provided systems for this application.
Can I have a very long coil?
We can wind coils of length up to 10m or more. This includes one-off requirements.
What is meant by Droop?
This is a problem that occurs mainly when measuring asymmetrical transients. At the end of the transient, when the current being measured is zero, the output of the Rogowski coil system is non zero. This is a consequence of the low-frequency limitations of the integrator. For a given transient the size of the droop effect can be calculated accurately. For a digitally-recorded waveform there is a correction procedure that enables the droop to be removed.
More detail can be found in the article 'Dealing with Droop'
Does the conductor have to be central in the coil?
For best accuracy the coil should be used centred on the conductor. However all coils are tested to determine their performance off-centre. Flexible coils can be moved off-centre by at least 0.5 of the coil radius before the output changes by 1%. Also flexible coils do not have to be mounted in a circular form. Rigid coils can be moved off-centre by 0.5 of the coil radius before the output changes by 0.1%.
Can we talk directly to technical staff?
Yes, by phone on +44 (0) 1423 547946 or by e-mail at firstname.lastname@example.org.
Can you repair goods that are old?
Yes. All goods have a one year warranty, where we will repair or replace. Our goods are designed to be repairable. We are able to fix integrators that are more than 25 years old.
Is insulation needed between the coil and the integrator?
For flexible coils - We claim 600V - Category IV. However, because the coil is flexible and the insulation is soft, it could become damaged by contact with a sharp edge, for example, or an accidental overheating of the conductor. We therefore recommend, where possible, for the user to apply extra insulation especially in safety-critical applications. N.B. The coil should not be installed on a conductor whilst it is live.
For rigid coils - the outer layer of the coils is insulated by a number of layers of tape. This is mainly for the protection of the coil. Whilst this probably provides a degree of electrical protection, we are not prepared to make any claims in this respect. Where dangerous voltages are involved we recommend that users provide electrical insulation appropriate to the situation either by air spacing or solid insulation, or that the insulation is tested with the coil in position. N.B. A dangerous voltage is one that would harm a person who made contact or would damage instrumentation attached to the coil.
Can I measure direct currents?
You cannot measure true DC directly with a Rogowski coil. However we find that different people mean different things by 'DC' so this is what you can do: Measure the DC offsets on AC transients; Measure uni-directional current pulses as well as oscillatory pulses; Measure the AC part of waveforms that contain a DC component such as a full-wave rectified current or a transformer inrush current; Measure the ripple superimposed on a DC current.; Measure very low frequencies (less than 0.1 Hz). It is also possible to measure a DC current by measuring the AC currents feeding the rectification system that supplies the DC. This is actually quite an accurate method. We have provided a system for measuring alternator rotor currents that worked this way and believe this technique has also been used for monitoring currents in an aluminium electrolysis plant.
Can I use a Rogowski coil to replace a current transformer?
You cannot use a Rogowski coil as a direct replacement for a current transformer. However we have designed an integrator which incorporates a power amplifier giving a 1Amp output which can be used in place of a 1A output current transformer.
Is the measurement accuracy affected by temperature?
The main effects of temperature are to change the resistance of the winding (which reduces the output) and thermal expansion of the coil former (which increases the output). For rigid coils, with careful design, these effects can be made to cancel each other giving a change in coil output that changes by a few ppm over a 40°C temperature range. Flexible coils cannot be treated the same way. The temperature coefficient depends on the length of the coil and the input impedance of the integrator. For example, for a 1m long coil the temperature coefficient with a 270R input impedance would be -0.05%/°C and with a 2500R input impedance it would be -0.007%°C. Unfortunately there is a conflict between designing coils to have a low temperature coefficient and having a wide frequency response.
What is the high-frequency limit?
The high-frequency limit is mainly determined by the properties of the coil. It depends on the type of coil, the size of the coil and the length of the output lead. As a very rough guide for rigid coils the -3dB point is in the range 40 - 78 kHz. For flexible coils the range is up to about 300kHz but will be much lower for a long coil with a long output lead. For any combination of coil length and lead length we can calculate the frequency response. We can also supply special 'low-output' coils which have a frequency response greater than 1MHz.
What is the largest current I can measure?
We have supplied coils for measuring currents greater than 1MA. We do not have the facilities to generate very large currents but because a Rogowski coil measuring system is 'linear' we can reliably calibrate systems intended for large currents using a much lower current.
What are Rogowski coils?
It is impossible to cover the whole subject of Rogowski coils in one message. However, the basic system consists of an 'air-cored' coil coupled to an electronic integrator. The output of the coil is proportional to the rate of change of the current. The output of the integrator is a voltage waveform which accurately reproduces the current waveform. This includes complex waveforms with a high harmonic content. The integrator output can be used with any electronic metering system such as a multimeter, oscilloscope, transient recorder or spectrum analyzer.
Rogowski coils are useful in a wide range of situations where conventional current transformers are not suitable such as where there is limited access, very large currents, transient currents or very large conductors. Rocoil can measure currents ranging from less than 1Amp to more than 1 million Amps.
Rocoil supply coils and integrators separately. Coil prices vary depending on the size and type of coil. We have a very wide range of integrators to suit different applications. We SPECIALISE in the production of custom designs to suit specific applications.
What is the commodity code/HS code for your goods?
90 303 307 00
What are the export control conditions for your goods?
Our goods are not listed on the US Commerce Control List, nor in Annex 1 of EC regulation 1334/2000
How much do your products cost?
Our prices vary depending on, for example, the length of the coil, the length of the output lead, the type of connectors, the range required for integrators… Please contact us stating what you need and we will send you a quote. If you are unsure what you need then we can discuss your requirements and advise you which product will best meet your needs.
What is the delivery time for your goods?
Most goods are despatched within 3-5 weeks of acknowledging your order. If you need your goods more quickly, please ask and we will try our best.
Can I have a discount?
Orders of 10+ = 10% discount per unit; orders of 100+ = 15% discount per unit. If you place large orders on an ongoing basis so that the cumulative total ordered is significant we may also offer you a further discount.
Where are your goods manufactured?
We are a UK company and all our goods are manufactured in-house. We do not resell products manufactured by other suppliers.
Are you associated with Dent instruments?
NO - we do not produce any goods for Dent instruments, and are not associated with them in any way. We do not endorse ANY of their products.
What paperwork do you send with shipments?
Depending on where shipments are being delivered we provide a:
waybill; commercial invoice; despatch note/packing list.
What is your VAT number?
Our VAT number is GB 613 0785 59.
What is your EORI number?
Our EORI number is GB 613 0785 59000.