Detecting AC Motor Speed
Detecting AC Motor Speed
(OP)
I am interested in detecting motor speed on an AC motor. We do not have the EE expertise so I turn to you in the Eng-Tips forum. We used a meter in the lab to show that there are detectible pulses generated by the motor when we spin the shaft with no power applied. Our question is this; can we take advantage of this to determine the speed of the motor while it is powered and under load?
Ultimately we are interested in being able to determine when the motor or the system needs some sort of attention by sensing the motor speed or some other indicative parameter. Others have proposed using a Hall Effect system, but we are trying to make this as simple as possible.
Thanks in advance!
- - -Updraft
Ultimately we are interested in being able to determine when the motor or the system needs some sort of attention by sensing the motor speed or some other indicative parameter. Others have proposed using a Hall Effect system, but we are trying to make this as simple as possible.
Thanks in advance!
- - -Updraft





RE: Detecting AC Motor Speed
If you do truely need to monitor speed, then you would do better to use some speed sensor at the motor rather than trying to determine it from the motor leads.
RE: Detecting AC Motor Speed
If sensing current is the best way then is there a cheap/easy way to integrate this into an indicator at the control panel? (We are thinking of LEDs rather than meters.)
- - -Updraft
RE: Detecting AC Motor Speed
Current-activated switches with adjustable setpoints are not very expensive anymore.
Here's a wide variety:
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You could also do current transducers and use a transducer to signal 1) Motor spinning, no belt, 2) Motor spinning and loaded, or 3) Motor not spinning (power out).
Let us all know how you decide to go...
Good on ya,
Goober Dave
RE: Detecting AC Motor Speed
Google "broken belt", "load drop" etcetera.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Detecting AC Motor Speed
http://www
RE: Detecting AC Motor Speed
Not sure about sensing the difference between an unloaded belt and a broken belt though, as the motor probably would see very very little difference there -- If sensing a broken belt is key, you'd probably do better to put some kind of a sensor on the belt itself (monitoring the belt rather than the motor).
RE: Detecting AC Motor Speed
Some of our applications are very sensitive to motor speed, thus the comment in my first post. Is there a way, without adding a Hall effect sensor, to sense AC motor speed, preferably by sensing something going on in existing the wires? If we HAVE to add something to detect speed is there anything more cost effective than the Hall effect system?
- - -Updraft
RE: Detecting AC Motor Speed
Do have a look at an Ossanna diagram. It will tell you a lot more about it. There is a discussion on these topics, including a few useful links here: http://www
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Detecting AC Motor Speed
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If we learn from our mistakes I'm getting a great education!
RE: Detecting AC Motor Speed
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RE: Detecting AC Motor Speed
It's cheaper and more reliable. It avoids high voltage.
If you want really nice. Measure the ambient and the motor temp. Calculate the rise.
This will tell you how hard you are pushing the motor's specs. It can tell how much the load is changing with time. It can show bearing issues, etc.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Detecting AC Motor Speed
RE: Detecting AC Motor Speed
Many VFDs allow you to pick power for the 4-20 mA output.
Roy
RE: Detecting AC Motor Speed
- - -Updraft
RE: Detecting AC Motor Speed
- current provides earlier warning.
- using current saves you from having to run an extra set of leads to the motor - may be an advantage if the power leads are accessible.
- a set of temperature contacts (thermistor) is pretty darned cheap compared to a current sensor. If current, what kind of current sensor would it be? CT is non-intrusive but more expensive. Shunt provides simple voltage output, but a little more intrusive and perhaps introduce some new failure modes for both the motor and the sensing circuit.
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Detecting AC Motor Speed
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Detecting AC Motor Speed
Is this to be plugged into 120vac outlet? Where is the remote indication vs the equipment?
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Detecting AC Motor Speed
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Detecting AC Motor Speed
Communication between the sensing and the indication should not be a burden either. We were thinking of wireless (Zigbee, bluetooth, others) or through the power line (power line modem) rather than running separate communication wire. Most of our application would be through retrofit so not having to run a separate signal wire is a huge benefit.
As a matter of fact, the retrofit is a huge driver. We want to sense operating conditions that need attention. We want to do this on existing equipment so it should be something simple to attach at/near the motor and at/near the switch. It should utilize as much existing hardware (motor, wiring, switches) as possible. If it is cost effective to replace an existing switch with the new indicating switch that would be okay, but we want to avoid having to change or crack open a motor or running new wires. Opening the wiring box on a motor or attaching something to the motor or splicing it into the wiring is acceptable.
This is ultimately for optimizing service/operation of motorized systems.
I hope that helps give a clearer picture of what we are trying to do.
- - -Updraft
RE: Detecting AC Motor Speed
Siemens 3UG4641 Ires monitor
Under the description is a javascript link that says "Illustration for current/cos φ monitoring" and if you click that, it pops up a chart that explains the differences very nicely.
Disclaimer: I work for Siemens, but this is not an advertisement. I know this produce very well, but there may be others like it, I just don't know.
RE: Detecting AC Motor Speed
You can monitor speed of the motor based on the pulsations you noticed, but not while it is running. Those pulsations are caused by the rotor bars passing through some residual (or applied) magnetism in the stator windings. There are what are called "standstill" monitors which do that to determine, for safety reasons, whether a disconnected motor is still spinning. Some DC injection brakes also use that method to detect when to turn off the DC injection, but they monitor the unused winding in that case.
RE: Detecting AC Motor Speed
How does this power monitoring deal with a load like a conveyor, or a mixer, or an elevator, or refrigeration? These loads wander all over the place. They can vary by 70%! These variations don't generally matter even if they are substantially over the motor's rating, just as long as the average value keeps the motor below its designed temperature rise. How does this current monitoring show that the motor has thrown its fan? How does it show that the cooling ports or fins are completely clogged shut? Only the case temperature is going to warn you of this.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Detecting AC Motor Speed
RE: Detecting AC Motor Speed
Keith Cress
kcress - http://www.flaminsystems.com
RE: Detecting AC Motor Speed
Tell us your problem. If you need help with a problem, tell us the problem. You have decided that knowing the motor speed may solve your problem. Yes we can help you determine the motor speed, but this may not be the best solution to your problem, and may not even solve your problem.
The speed of an induction motor varies a little when the load increases or decreases, but not much. Slipping belts or load loss is a serious safety issue in grain elevators. It is checked by a speed switch on the driven equipment, not on the motor. There are other issues that you may wish to monitor, but each should be evaluated and solved independently.
Bill
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"Why not the best?"
Jimmy Carter
RE: Detecting AC Motor Speed
If a greater accuracy is required, then an external speed sensor would likely be required, eg., to determine if a belt has broken or if the system is simply unloaded, or to distinguish between an overload and a jam.
RE: Detecting AC Motor Speed
Several very large grain elevators destroyed with loss of life bear testimony to this. Elevators now use speed monitoring on the driven machinery, configured as slip detectors, to alarm belt problems.
Bill
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"Why not the best?"
Jimmy Carter
RE: Detecting AC Motor Speed
I think that given the small size of some of these motors (1/4HP), sensing the current alone would be impractical. I've noticed several times that the basic current on a small motor appears to FALL as load is applied...there is also a very small difference between unloaded and full load currents. It would be simple to check these conditions in your applications I guess, just put a current clamp on your lead, connect to a DMM and take readings in the various conditions...
Cheers,
Mort
RE: Detecting AC Motor Speed
Maybe something from www.loadcontrols.com would work for you.
RE: Detecting AC Motor Speed
Keith Cress
kcress - http://www.flaminsystems.com
RE: Detecting AC Motor Speed
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compare this to an encoder on the motor, obviously do the math to get the gear changes.
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