RPM sensor motor connected to VFD
RPM sensor motor connected to VFD
(OP)
When is a RPM sensor needed on a motor that is connected to a VFD? VFDs can estimate the RPMs of a motor but when is this not accurate enough?
Thanks!!
Thanks!!
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS Come Join Us!Are you an
Engineering professional? Join Eng-Tips Forums!
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail. Posting GuidelinesJobs |
RPM sensor motor connected to VFD
|
RPM sensor motor connected to VFDRPM sensor motor connected to VFD(OP)
When is a RPM sensor needed on a motor that is connected to a VFD? VFDs can estimate the RPMs of a motor but when is this not accurate enough?
Thanks!! Red Flag SubmittedThank you for helping keep Eng-Tips Forums free from inappropriate posts. Reply To This ThreadPosting in the Eng-Tips forums is a member-only feature.Click Here to join Eng-Tips and talk with other members! |
ResourcesWhat is rapid injection molding? For engineers working with tight product design timelines, rapid injection molding can be a critical tool for prototyping and testing functional models. Download Now
The world has changed considerably since the 1980s, when CAD first started displacing drafting tables. Download Now
Prototyping has always been a critical part of product development. Download Now
As the cloud is increasingly adopted for product development, questions remain as to just how cloud software tools compare to on-premise solutions. Download Now
|
RE: RPM sensor motor connected to VFD
Anyhow, a paper machine drive almost certainly cannou work without a speed transducer. Today's specifications say at least 0.05 and usually 0.01 percent speed accuracy. That is something you cannot have without a speed transducer, often an incremental encoder with around 1024 PPR.
A test bench for motors used to run without speed transducers and you could have something like +/- 0.5 percent accuracy from the speed estimator in a VFD. But that is not enough any more. Efficiency needs to measured with at least 0.1 percent accuracy and since the torqu transducer is difficult to get any better, the speed measurement needs to be as good as it can be.
Generally, if you need better than around 0.5 percent accuracy over torque, speed and temperature range, you need a speed transducer.
Some built-in estimators/observers can do better than that if calibrated continuoiusly. But that is usually only in special situations.
The SKF Sensor Bearing is a good alternative to a "real" encoder. It is built into the bearing and is thus very well protected. It is also low cost and can achieve high accuracy if reciprocal speed measurement is used.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: RPM sensor motor connected to VFD
RE: RPM sensor motor connected to VFD
First, speed error is generally due to changes in torque demand. In an induction motor, this error is mostly slip. So the question becomes, how well does the drive compensate for torque induced slip speed changes. With a good vector drive, this can get down in the range of one-tenth of motor slip without an encoder. If you need better than that, an encoder is required. Note here that the error is a result of torque changes. If your torque doesn't change, you won't have much speed error to start with.
Second, in some applications, especially those involving web products and tension control, cumulative error is just as important as actual error. For example, even if you are very accurate with actual error, if it is all negative or all positive, eventually you are going to have too much or too little tension. No encoderless system will assure non-cumulative error. For that you need an encoder.
Third, speed reference error is often overlooked. That is error either in the speed signal going into the drive or error in the drive translating the input command into an actual output speed. Usually, the majority of this error is due to the analog input terminal analog-to-digital conversion. A 10 bit resolution A/D input will not be nearly as accurate as a 14 bit resolution input. This is a matter of purchasing a drive with the input resolution adequate for the intended purpose.
RE: RPM sensor motor connected to VFD
RE: RPM sensor motor connected to VFD
Actually, again, even the best encoder and speed control system won't work with some materials. A couple of perfect drives with a sheet running between them behave like a perfect integrator - it integrates over time. And if time is long enough, the sheet will either drop or tear (if not resilient).
That's why you find devices like dancing rollers, triangulation sensors or sometimes even ultra-sound position sensors to control hang or force sensors to control force in many sections in a paper machine or rolling mill.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: RPM sensor motor connected to VFD
I have seen a few systems with persistent positive error when the motor nameplate data was in error, especially full load speed, but that is most uncommon.
Cumulative error definitely happens in sensorless systems and there is no way to really control it or make it cycle around the zero error point.