How to establish speed of a running AC motor? - Page 1
Forum section: electronics / projects

author: nick.sek
author_rank: Regular Contributor
timestamp: November 22, 2012, 03:08:30 am
content: I need to create a digital signal in order to create a means of generating feedback on the speed of an AC motor.I'm making a variable frequency drive, and control is being done by a micro controller; and I'm wondering how I could establish a means of determining the speed and use that signal to render the speed of the system.Part of me is thinking of attaching a encoder connected to the shaft of the AC motor; but I was wondering if anyone has any other ideas for this?Thanks for the help...Nick

author: peter.mitchell
author_rank: Super Contributor
timestamp: November 22, 2012, 04:18:42 am
content: Hall effect sensor is probably what you're looking for.

author: Kremmen
author_rank: Super Contributor
timestamp: November 22, 2012, 08:54:16 am
content: Quote from: nick.sek on November 22, 2012, 03:08:30 amSince you say it is a VFD drive i assume the target motor is a tun of the mill 3 phase induction motor. These do not have any encoders fitted as standard and if you need to do something like this, it will significantly reduce the applicability of your device. The major strong point of an induction motor is that they are for all practical purposes indestructible physically. No "ordinary" user will want to mess with any encoder stuff. If you are thinking of an application like a CNC mill spindle then it is no big deal since the application is complex anyway, but to make this kind of addition mandatory in all cases is risky.You could instead measure the the currents and back EMF voltages in the stator windings and do some DSP calculations to find out the motor speed from this voltage. It will not be trivial but that is how it is usually done in space vector modulation that is part of the field oriented and direct torque control schemes.

author: AlfBaz
author_rank: Super Contributor
timestamp: November 22, 2012, 11:40:25 am
content: I would be very difficult to get precision motor movement and controlled torque using vector control without an encoder. I would rather spend the extra dollars and time installing an encoder than farting around with drive parameters in a futile attempt to get the same accuracy that you get with the addition of an encoder.In my experience (its not extensive, and it was a while ago so I don't know how much better the motor models are) the most accurate system would be vector mode with an encoder, then scalar with an encoder, vector without encoder and finally scalar only mode

author: nick.sek
author_rank: Regular Contributor
timestamp: November 22, 2012, 02:31:54 pm
content: Actually it is just a single phase 3/4 HP motor.I was thinking encoder was the ways to go but I was wondering if anyone had another means of finding out the speed. Does any one has a recommendation of what encoder to use (needs to be tough as it will be exposed to higher speeds and other harsh conditions) ? It will be mounted onto a lthe and will be exposed to speeds of 3000 RPM. Other than that I plan to make it run off a 1:1 belt drive to make isolated from the process.Thanks for all the feedback.Nick

author: nick.sek
author_rank: Regular Contributor
timestamp: November 22, 2012, 02:38:33 pm
content: http://www.phidgets.com/products.php?product_id=3530_0What your opinion on this model here?

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 22, 2012, 02:59:20 pm
content: Quote from: nick.sek on November 22, 2012, 02:31:54 pmlaser/led with strip(s) on shaft orTachometerprobably can do. but hall effect as mitchel said is capable to deliver for your requirement.

author: muvideo
author_rank: Frequent Contributor
timestamp: November 22, 2012, 04:07:35 pm
content: You had many answers, consider also the magnetic resolvers, something like this:http://www.ams.com/eng/Products/Magnetic-Position-Sensors/Magnetic-Rotary-Position-Sensors/AS5161it has 300uS delay, it's 1.5% of turn at 3000 rpm, and you could compensate for it,plenty of resolution. But I've never used it and dont know if it's that easy or where canbe bought.Quote from: nick.sek on November 22, 2012, 02:31:54 pmJust for curiosity, why did you choose a single (split) phase motor?The classic 3 phase squirrel cage motor is a good, clean and very documented machine.You have to use a 3 phase bridge anyway, unless you want to use a capacitor,and you will not use a capacitor, don't you?Fabio.

author: Bloch
author_rank: Unknown
timestamp: November 22, 2012, 07:25:01 pm
content: Quote from: nick.sek on November 22, 2012, 02:31:54 pmIf you instead use a 3 phase motor. You will have so much more control over the motor!I want to mention it is possible to know the speed without a mechanical sensor.If you make a field-oriented control (FOC) or vector control drive >http://en.wikipedia.org/wiki/Field-oriented_control

author: Bloch
author_rank: Unknown
timestamp: November 22, 2012, 07:36:52 pm
content: Quote from: nick.sek on November 22, 2012, 03:08:30 amhttp://www.kuebler.com/english2/prod-sen-inkremental-3700.htmlAnd if it is just for a short test then use a Heat-Shrink Tubing to connect them.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 22, 2012, 11:22:07 pm
content: My friend Megan Cornner has designed a sensorless 3 phase motor drive using a dsPIC for a model airplane. The back EMF sensing was done using voltage dividers from each phase to ground. But she was using brushless DC motors, where floating phases nicely work as generators. Doing the same with an induction motor is a lot more difficult.One common application of a sensorless induction motor drive is for a common refrigeration or air conditioning compressor. In that case, the motor is sealed inside the compressor assembly so it is not possible to directly sense the shaft speed. One approach I have seen is to take the FFT of the motor current, then look for a spike a little below the fundamental. Another is to sample the motor current at the fundamental frequency and look for a beat signal of a few Hz, which gives the slip speed. Both of those rely on a load that varies with shaft angle, which most compressors do.

author: hlavac
author_rank: Frequent Contributor
timestamp: November 23, 2012, 07:59:20 pm
content: Paint a white spot on the shaft and place a reflective IR sensor likeCNY70next to itCount pulses per second...

author: ee851
author_rank: Regular Contributor
timestamp: November 23, 2012, 08:50:13 pm
content: Help me grasp how this CNY70 device works. I understand the transistor output is continuous until the IR light beam continously emitted by the "emitter" is interrupted once-per-shaft-revolution by a reflector on the shaft that reflects the IR light beam away from the detector, thus breaking the phototransistor's output ("detector"). Is this correct ?How do you position the beam so that the IR beam shines continuously on the spinning shaft? This is the part that isn't clear to me. It looks like the "detector" and the "emitter" are in the same IC package. So how does the IR light beam break the optic path between the two components if they're both in the same package?

author: G7PSK
author_rank: Super Contributor
timestamp: November 23, 2012, 09:50:23 pm
content: The IR is reflected back to the device hence with a reflector fitted to the shaft you get one pulse per rev.

author: ptricks
author_rank: Frequent Contributor
timestamp: November 23, 2012, 11:58:40 pm
content: You can use the IR trick and position the sensor a few mm away from the shaft. By painting a reflective line on the shaft you will get a pulse every revolution. There are some great reflective paints out there, rest of shaft area paint black. You can use really bright visible light and a photo resistor as well if there is something like a gear to reflect the light off of, similar to how you set timing in an auto motor using a timing light.

author: FJV
author_rank: Regular Contributor
timestamp: November 24, 2012, 12:27:57 am
content: Drive a small DC motor with the shaft, using the DC motor as a generator.The voltage the dc motor generates is a measure for the speed. (higher speed gives higher voltage)May not work super accurately at low speeds, or with low quality generating DC motors.I also saw a method on the internet where a stepper motor was used as an encoder.The pulses were generated by the inductance of the magnet on the coils.It would only work above a certain speed though.http://www.4qdtec.com/stpen.html

author: free_electron
author_rank: Super Contributor
timestamp: November 24, 2012, 12:43:43 am
content: AM i the only one thinking this is an absurd problem ?You make a variable frequency drive for a three phase motor. so YOU control the waveforms. the rotor will follow the waveform you apply. so you know the RPM ...the only thing you need to look for is axle jamming. that is done by lookgin at the back emf of the unpowered node.

author: robrenz
author_rank: Super Contributor
timestamp: November 24, 2012, 02:05:11 am
content: It only follows the frequency exactly at zero torque. As soon as it has to generate any torque there will be slip.

author: hlavac
author_rank: Frequent Contributor
timestamp: November 24, 2012, 02:39:49 am
content: Quote from: ee851 on November 23, 2012, 08:50:13 pmLED shines at 45 degrees angle and light gets reflected off the surface positioned close to it (3mm or so) into the phototransistor in the other half of the device. Look into the datasheet for an illustration.

author: Kremmen
author_rank: Super Contributor
timestamp: November 24, 2012, 08:19:01 am
content: Quote from: robrenz on November 24, 2012, 02:05:11 amAn induction motor will always slip in practice, because only that way the torque producing rotor induction can take place. This is not the issue however. Vector modulation - more precisely a control scheme called Direct Field Oriented control will establish the rotor speed by observing key signals such as stator voltages and currents. One of the results of this observation is the slip frequency. The slip induced error can accounted for in the speed controller so that the true shaft rotation matches the commanded value. The math is a bit involved so i guess it is not really an option to the OP but this is the principle anyway. As usual, the net provides more info, this for example. See p. 54 onwards:https://etd.lib.metu.edu.tr/upload/12604965/index.pdf

author: Bloch
author_rank: Unknown
timestamp: November 24, 2012, 08:59:40 am
content: Quote from: free_electron on November 24, 2012, 12:43:43 amYou will know the RPM - slip (higher load/temp = higher slip)Some times it is not important ie a drill or a sawOther times in the industry it can be very important. ie Two machines which are connected together. or the teacher say soBut if the RPM/Position is important then they will often use Servo Motors instead.

author: muvideo
author_rank: Frequent Contributor
timestamp: November 24, 2012, 12:24:59 pm
content: Quote from: Bloch on November 24, 2012, 08:59:40 am...that are (not always but often) guess what... 3phase induction motorssure they are equipped with resolvers or very good encoders, and magnetics are built withthe target application in mind.Fabio.

author: ee851
author_rank: Regular Contributor
timestamp: November 24, 2012, 04:24:42 pm
content: Thank you, hlavac. I looked at Fig. 1 in the datasheet. Now I understand. 0.3mm doesn't seem like much distance between the moving part and the nonmoving top of the CNY70, but I guess it's enough if everything is held secure from moving.

author: filip
author_rank: Newbie
timestamp: January 04, 2013, 10:01:02 pm
content: I'm no expert on VFDs, but I have done some reading on the subject and AFAIK you need much better resolution than a whole revolution (one bright spot on the shaft).I remember reading about recommended numbers in a range of around 1000 steps per revolution.This seems to me to be due to the fact that vector control methods generally calculate a desired vector every 50 or so microseconds and they need an exact shaft position for that (50 microseconds at 3000rpm is less than 1 deg).You could experiment with 10 sensors and a 10 bit gray code printout on some kind of a disc - I am collecting protective transparent CDs from CD/DVD cake boxes for experiments - it would be really hard to come up with a reliable mechanical design for that though.

author: Kremmen
author_rank: Super Contributor
timestamp: January 05, 2013, 09:36:53 am
content: There seems to be a bit of confusion here. Let's try to rectify that.Firstly, let's keep VFD (variable freq drives) and servo drives separate, since they are two different creatures. A VFD drive usually drives an induction motor and it may very well employ vector control, in fact that is the norm these days.An AC servo drive drives a permanent magnet synchronous motor, misleadingly often called a BLDC or "brushless DC" motor, even though there is nothing DC about the drive scheme.A motor tends to be called BLDC if the internal magnetic paths are optimized for trapezoidal excitation voltages, and a PM synchronous motor if the voltages are sinusoidal.Now, a VFD drive usually has no sensor feedback from the induction motor it is driving. Nor does it really need any. The rotor does not have a fixed position in relation to the stator mag field due to slip. In an induction motor slip is unavoidable and necessary phenomenon for torque generation. An unaugmented VFD drive may approximate the shaft angular velocity by employing various observers one typical being the MRAS or Model Reference Adaptive System. Here 2 estimators evaluate the rotor flux vector, first evaluating the stator or voltage equation and the second, the rotor or current equation. Only the rotor equation contains the observed quantity omega. Next a difference of the estimator outputs is calculated and fed into a PI controller. That in turn produces a value for the observed quantity omega, which is the fed back to estimator 2. The control loop minimizes the difference between the 2 estimates, at the same time providing a prediction for the observed quantity omega, i.e. shaft speed. When properly implemented, this can give quite good results but never one rivaling a true servo.An AC permanent magnet motor drive may or may not have sensor feedback. Simple brushless applications such as fans, maybe washing machines etc, make do with a similar system as outlined above. Starting such a motor is based on "guesswork" - the drive gives the motor a kick and observes the result. Once the motor is turning, after certain speed it achieves what is called autocommutation where the observer can correctly phase the coil excitation voltages.The simplest sensor feedback is based on identifying the current rotor nominal position sector. This is usually accomplished by 3 hall sensors that together indicate the shaft position within a 60 degree sector. This is sufficient to correctly phase the excitation voltages from standstill, and the motor will start turning correctly right from the start. This is the "normal" method of sensing shaft position for larger non-servo AC synchronous motors.A true servo drive, whether VFD, AC or DC or whatever, always has sensor feedback for the primary controlled quantities. Those are usually position, velocity and force (torque). Position and velocity are calculated from a shaft encoder or similar while torque is calculated from motor current. The precision of the sensors and calculations is dictated by the demands of the application. The shaft position sensor may or may not replace the hall sensors, depending on its nature. An absolute encoder such as some resolvers or Gray disk may do so, but the more common incremental encoder can not.