How to measure the speed of 3 phase induction motor using magnetometer or hall effect sensor??

Thread Starter

Shivam Paliwal

Joined Feb 4, 2022
1
We are working on speed measurement in a device, which will measure the RPM of the 3-phase induction motor just by mounting it on the motor as shown in the below image.
rpm measurement device mounted on motor









Our Requirements-
  1. The sensor should measure the speed of 2 HP to 1000 HP 3-phase induction motor.
  2. The sensor will be mounted on the housing of the 3 phase Induction motor
  3. Motor speed varies from 400 rpm to 3600 rpm.
  4. FYI-(We are not focusing on any BLDC motor application)
Few Questions-
  1. Can anyone suggest a sensor that will be suitable for all the above requirements?
  2. Can anyone share any document which specifically focuses on RPM measurement of 3 phase induction motor using hall effect sensor or magnetometer Sensor if it is mounted on the housing of the Induction motor?
 

LowQCab

Joined Nov 6, 2012
2,041
Three-Phase-Induction-Motors normally run with "zero-slip", at any time,
in other words, if the Motor is slipping, and not running exactly at it's Name-Plate-RPM,
then it's about to smoke some windings.

Therefore, the speed of the Motor is an absolute function of the Frequency of the Input Voltage.

An Electronic-Speed-Controller "may" let the Motor slip by a
small percentage when in "Soft-Start-Mode".
Some Electronic-Speed-Controllers will display the
actual Motor-RPM after entering the appropriate multiplier.

There are many unpredictable Magnetic-Fields around a running Motor,
it's very unlikely that anyone can make a "universal", "One-Size-Fits-All", "RPM-Detector"
that just arbitrarily "sticks" to the side of a Motor's Housing.
However, there are plenty of readily available Optical-Tachometers which will give
an instant RPM reading when aimed at the Motor's Shaft.
.
.
.
 

LesJones

Joined Jan 8, 2017
3,705
Hi LowQCab, I don't agree with your statement " Three-Phase-Induction-Motors normally run with "zero-slip", " They need slip to produce current in the shorting bars in the rotor. If you look at the specifications of three phase motors you will see that that they do not run at synchronous speed. So for an example a two motor would be rated at about 2800 RPM on 50 hz (Synchronous speed would be 3000 rpm.

Les.
 

Reloadron

Joined Jan 15, 2015
6,584
Depending on the motor face there is all sorts of speed sensors like Optical with a focus on the motor shaft. I used optical quite a bit placing a few small pieces of reflective tape on the coupling and mounted the sensor on motor face plate, Count the pulses and do the math. A Google of tachometer sensors should get you started. I like optical but there is likely a half dozen other types (Optical Sensors, Proximity Sensors, Infrared Sensors, Magnetic Sensors) . Paint the coupling black and add the reflective tape strips was what I generally did. Monarch Instruments markets a T-5 tape along with optical sensors.

You start with choosing a speed sensor type configured to mount on your motor.

Ron
 

strantor

Joined Oct 3, 2010
5,967
Three-Phase-Induction-Motors normally run with "zero-slip", at any time,
in other words, if the Motor is slipping, and not running exactly at it's Name-Plate-RPM,
then it's about to smoke some windings.

Therefore, the speed of the Motor is an absolute function of the Frequency of the Input Voltage.
Yeah sorry, I think you've got synchronous and asynchronous motors swapped in your head. @LesJones is right, and your description is also right, but not for an induction motor; your description applies to a synchronous motor like a permanent magnet notor.
it's very unlikely that anyone can make a "universal", "One-Size-Fits-All", "RPM-Detector"
that just arbitrarily "sticks" to the side of a Motor's Housing.
However, there are plenty of readily available Optical-Tachometers which will give
an instant RPM reading when aimed at the Motor's Shaft.
I agree with you on this though. I think the only way without interfacing with the shaft (only the housing) is to measure vibrations. As I learned recently in my attempts at hacky DIY rotor dynamic balancing, vibrations don't always happen in the same period of the rotor's revolution.
 

Marley

Joined Apr 4, 2016
475
All induction motors slip. The slip induces currents into the "squirrel cage" in the rotor. The magnetic field created by these currents create the output torque. As you increase the load, the slip only slightly increases. The current drawn from the supply increases - but again only slightly. The power factor (phase angle of the current) changes much more. If the motor is overloaded the current drawn increases a lot. This fact can be used to trigger overload relays to protect the motor.

Interesting fact: If you externally drive the motor above the synchronous speed (while connected to an AC supply) by a similar amount of slip, current will be driven into the supply. It becomes a generator (alternator).

A simple way to measure the rpm (any motor) is to fit a small magnet to a rotating part and mount a hall-effect sensor nearby. You can make something but easier to buy something like this.
 

Reloadron

Joined Jan 15, 2015
6,584
  1. The sensor should measure the speed of 2 HP to 1000 HP 3-phase induction motor.
  2. The sensor will be mounted on the housing of the 3 phase Induction motor
  3. Motor speed varies from 400 rpm to 3600 rpm.
  4. FYI-(We are not focusing on any BLDC motor application)
With that in mind I should have added the motor is likely driven using a VFD (Variable Frequency Drive). Typical AC motors running at 60 Hz would be as follows:

Common AC motor units are constructed with either two or four poles. ... Two pole AC motors operating at 60 Hz will always run at approximately 3600 rpm, and four pole AC motors will have speeds around 1800 rpm. Speed = 120 x frequency (Hz) / poles of the motor. Example 120 x 60 hertz / 4 poles = 1,800 rpm.
Using the above example a 4 pole motor running at 60 Hz should be running at 1800 RPM but if we look at the motor nameplate data it's typical to see numbers like 1750 or 1760 RPM. Even under no load the motor will have dome of the mentioned pole slips. This is where if we really want to accurately measure the true RPM a good tachometer system becomes our new best friend. :) Anyway even knowing the motor's power frequency be it from a VFD really is not going to tell us the true shaft rotational speed. How we measure the RPM is often a matter of what fits and works under any outside environmental conditions.

Ron
 

Reloadron

Joined Jan 15, 2015
6,584
A simple way to measure the rpm (any motor) is to fit a small magnet to a rotating part and mount a hall-effect sensor nearby.
That works well as long as the added magnet doesn't upset the balance of the motor's output shaft. I used plenty of magnetic pickup systems and they did fine.

Ron
 

MaxHeadRoom

Joined Jul 18, 2013
25,198
A very simple method is a white paint mark on the shaft and use a small retro-reflector device, an old member here developed such a method for RPM control of a HS router spindles for CNC work.
The example can be seen on the SuperPID controller site.
I have used one, they operate Extremely well!
I see post#4 by Ron covered it already!
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
25,198
We are working on speed measurement in a device, which will measure the RPM of the 3-phase induction motor just by mounting it on the motor as shown in the below image.
I am not sure I see how a non-invasive sensor could work in that position,?
Directly beneath the sensor would be the rotating field at the supply frequency, the rotor frequency would be behind this supply frequency.
 
Top