AC ammeter for measuring VDF output current
AC ammeter for measuring VDF output current
(OP)
Hi everybody;
There are several 400V, 3Ph, induction motors equipped with Variable Speed Drive (VFD) mounted in LV MCC room. The output frequency in normal operation of the motors will be about 20Hz~50Hz. I want to buy/finalize Ammeter to be mounted in Local Control Station (LCS) of the motors. As the ammeter will measure the output current of the VFD's, I need to have good accuracy in 20Hz~50Hz frequency range. Could you possibly explain how can i achieve this.
Thanks
There are several 400V, 3Ph, induction motors equipped with Variable Speed Drive (VFD) mounted in LV MCC room. The output frequency in normal operation of the motors will be about 20Hz~50Hz. I want to buy/finalize Ammeter to be mounted in Local Control Station (LCS) of the motors. As the ammeter will measure the output current of the VFD's, I need to have good accuracy in 20Hz~50Hz frequency range. Could you possibly explain how can i achieve this.
Thanks






RE: AC ammeter for measuring VDF output current
RE: AC ammeter for measuring VDF output current
If there is one VFD for each motor, the best way is to use an analog output on the VFD and use that signal (you can usually have 0-10 V or 4-20 mA) for the instruments on the local Control station. The built-in measurement works very well, has a high accuracy and works across all the frequency range, down to 0 Hz.
If you prefer to measure with separate transducers, you can use ordinary current transformers. They work quite well down to 20 Hz, but you should probably use CT:s that are designed for around twice the burden you would choose for a 50 Hz application. This is to avoid possible saturation at the lower frequency range.
You could also use a LEM (Hall Effect) or similar module. They work all the way down to DC.
Rogowski coils may be used but they are better at higher frequenies and you have to pay attention to the lower band-width limit.
A shunt resistor will not work so well, mostly because there is a lot of EMI that is difficult to handle even with the best isolation amplifiers.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: AC ammeter for measuring VDF output current
But, as Gunnar also said, if you have multiple motors behind one VFD and you want accurate individual current values, that's a different scenario and his advice is sound (as per usual). If you are not adept at integrating something like an LEM or Rogowski Coil, you may want to buy them as "current transducers" rather than raw components. So that's an advantage if just using CTs since it's likely you are already familiar with them.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: AC ammeter for measuring VDF output current
RE: AC ammeter for measuring VDF output current
Keith Cress
kcress - http://www.flaminsystems.com
RE: AC ammeter for measuring VDF output current
Kindly note, there is one motor on each VFD.
As per project specifications, Local Control Station (LCS) will have Start/Stop push buttons, Local/Remote selector switch and an AC ammeter without any power supply. One multi-core cable from LCS to LV MCC has been considered.
I think for current indication from VFD output, a separate cable and also power supply is needed.
So it is preferred to achieve good accuracy by use of a Current Transformer and AC ammeter.
RE: AC ammeter for measuring VDF output current
There are loop instruments that use 4 to 20mA as the signal and you can get LCD meters that use the 0~4mA to power themselves. So separate cables, use the VFD's analog output in 4-20mA mode and use a handful of electronic LCD meters.
Keith Cress
kcress - http://www.flaminsystems.com
RE: AC ammeter for measuring VDF output current
Ebay 2 Loop powered meters
Keith Cress
kcress - http://www.flaminsystems.com
RE: AC ammeter for measuring VDF output current
2) Does your ASD not have an integral display that indicates current?
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)
RE: AC ammeter for measuring VDF output current
You can't get decent accuracy using a CT on a VFD output, and even if you could get a suitable CT the VFD output isn't a sinusoid so the moving coil meter will mis-read.
The control cable isn't really suited to a 4-20mA signal, although if you are using 24V DC control you might be OK. 110V AC or 230V AC control sharing a multicore with instrument signals is very bad practice.
Your choices seem to be either run in a suitable instrument or comms cable, or request a deviation to the spec to omit the field ammeter. Why do you want a field ammeter anyway? Motor current is a lousy indicator of what is happening in the process, and the Mk. I eyeball can determine whether or not the motor is running.
RE: AC ammeter for measuring VDF output current
Otherwise, get an analog meter that uses a 4-20mA input with the scale you want and drive it with the analog output from the VFD. You'd just need to run a shielded 2-wire conductor to the local control panel for the 4-20mA.
RE: AC ammeter for measuring VDF output current
RE: AC ammeter for measuring VDF output current
It's one of those "Just because you can doesn't mean you should" scenarios.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: AC ammeter for measuring VDF output current
A CT works, but will probably saturate at low frequencies. That's why I recommended a CT with about twice the design burden. The OP may not have a possibility, or budget, to put an extra cable from VFD to operator panel. That would otherwise be the simplest, cheaoest and most accurate solution.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: AC ammeter for measuring VDF output current
RE: AC ammeter for measuring VDF output current
http://www.nktechnologies.com/ac-current-transduce...
Then take the 4-20mA output and run it to a loop powered meter like what Keith said, or even just get an analog meter with a 20mA input and have a custom scale made that says 0-100% current or the actual current range based on the motor. Digital meters tend to be more rugged however. I've used these little CUB4 meters from Red Lian, they are small, NEMA 4X (watertight) and very rugged.
http://www.redlion.net/product/cub4-loop-powered-p...
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: AC ammeter for measuring VDF output current
RE: AC ammeter for measuring VDF output current
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: AC ammeter for measuring VDF output current
There is conflict between contract obligations, budget limitation and technical requirements.
So i will request a deviation to the specification, to omit CT and the field ammeter or extra cost for high accurate local metering via VFD output.
RE: AC ammeter for measuring VDF output current
Keith Cress
kcress - http://www.flaminsystems.com
RE: AC ammeter for measuring VDF output current
Why not use the analog output in the drive to drive a remote ammeter? I assume that your drive does have a spare A/O channel, or two, on it. Most modern drives have several A/O channels that can be programmed to provide a whole bunch of data (ie speed, current, power, voltage etc.
I don't understand why anyone would bother with a CT for this application?
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)