# How can RPM be measured in 3 phase induction motor?

I was thinking measuring magnetc flux from embedded magnets on the shaft via hall effect sensors but I am not sure. What would be the easiest way to measure speed of the rotor? I am talking about mechanical speed so frequency of the phase currents is not the way to do it.

• A tachometer aimed at the shaft. – Harry Svensson Jul 19 '17 at 5:00
• So how did you solve it? – Harry Svensson Jul 30 '17 at 0:24
• @Harry Didn`t currently. But i decided to go with magnetic sensors. it works retty accurately now i just need to embed those onto shaft and build a mechanism to keep them around the shaftvery closely and with 120 degree phase difference – Alper91 Aug 2 '17 at 4:45

The turbine industry magnetises one of the shaft fixing nuts and detects the changing field ...

• Note that you only have one pulse per revolution. To give an accurate result trigger wheels have many teeth. – Jeroen3 Jul 19 '17 at 14:12
• @Jeroen3 ok, have you found any toothed wheels that will do 180,000 rpm and not affect the balance? – Solar Mike Jul 19 '17 at 14:50
• You don't really need one with those speeds... Jet turbines, obviously... – Jeroen3 Jul 19 '17 at 16:59
• Actually I was referring to exhaust driven turbochargers... and since complete revolutions are needed, not partial for positioning purposes, then the accuracy is sufficient. Also as the rotating mass is very small not adding mass is also a factor. – Solar Mike Jul 19 '17 at 17:02

An induction motor is asynchronous and it is therefore hard to measure the rotational speed based on flux. Because the magnetic flux on the stator does not match the speed or the rotor. It has slip.

Motor drivers are able to do it by pulsing short burst of DC on the stator windings whilst analyzing what comes back. This only works when it's not driving. This method is used for flying start.

Perhaps you can measure it based on the harmonics measured by the flux sensor if you mount it on the rotor. But this is inconvenient since you'd need slip rings to get the signal out the electronics on the rotor.

The reliable industry method is to use a induction pickup on a trigger wheel.

Depending on the accuracy needed, there are plenty of available technologies that already exist; tachometer, encoder, resolver etc. They all involve external components attached to the motor shaft.

In the VFD world, "encoderless" speed detection for the purpose of achieving Sensorless Vector Control is done with highly sensitive current detection systems that monitor and filter the stator current flow signals looking for the anomalies that represent the rotor bars passing through the stator magnetic fields and count them to determine the actual rotor speed. You cannot use that for absolute position (i.e. when stopped), but with enough processing power in the drive, it can be highly accurate, as in .001%

I once wrapped a "several turn" coil around a medium sized 3-phase induction motor and low pass filtered the output to get slip frequency i.e. I measured the flux produced by the rotor due to the asynchronous speed of the rotor compared to the fixed stator frequency.

You can calculate speed based on slip and input frequency if you know the number of poles the motor has.

I was at college and it was part of an assignment and it was in the late 1970s so I don't know if this technique will work with more modern machines but it did back then.

• I think your method deteriorates quickly with lower quality power. – Jeroen3 Jul 19 '17 at 14:10
• @Jeroen3 You can build pretty good low pass filters these days - there are enough stages in one small chip (Linear tech switched cap filter, 8 th order low pass) to basically ignore power quality problems such as high harmonics but if you have any evidence then please do tell. – Andy aka Jul 19 '17 at 14:33

The only way is to use an encoder mounted on a rotor shaft.

The induction motor has a slip, therefore measuring the stator flux won't give you any valuable information. Further, the stator flux frequency is equal to the grid frequency, so you don't even need hall sensor to get the flux frequency, you can measure directly the line frequency.