How vfd detect speed in open loop control

bodoo23

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Join Date
Jan 2018
Location
Athens
Posts
57
I don't understand how vfd detects speed when we are running in open loop control (sensorless).
For me only possible way;
during auto tuning it calculates resistance and inductance values of motor which means it knows equivalent circuit of motor. while running, the vfd also knows current and stator frequency so it can calculate slip from equivalent circuit. From slip, we can find actual speed since we know stator frequency.
What is your opinion ? My thoughts are correct or close to correct ?
Thanks in advance.
 
We use a RPM meter and place it on the motor shaft.
we use 30, 60, and 90 hz. and look at the motor nameplate for the motor speed at 60 hz. we then adjust our speed calculation in Wonderware to match the motor speeds at those frequencies. it may not be an exact match, but it's very close.
james
 
Highly recommend reading reference #5 in the wikipedia article GaryS referenced.

TLDR; The drive stops powering the motor (for a VERY short period of time) and measures the voltage produced by the spinning motor. The voltage is proportional to the speed.

Who comes up with this stuff????
 
As the rotor bars pass through the magnetic fields in the stator, the CEMF disrupts the fields and that disruption results in minute changes in the stator current from what it should be, based on what the drive has learned in the motor modeling. That's why SVC cannot function correctly without a motor ID / Autotune procedure being done first. Then by "counting" the pulsations, the drive knows the rotor speed and relative rotor position from where it was previously, so it can do the vector algorithm calculations (more like intelligent estimates really in most low cost SVC drives).

The entire technology could not exist until there were microprocessors fast enough to crunch the numbers and sensors accurate enough to see these small disturbances. Hence SVC not existing until relatively recently.


Who comes up with this stuff????
I actually met the guy once. He was an electronics professor at Oregon State University, working on a contract from GE Locomotives who wanted to get away from using DC locomotive motors by using AC motors and vector drive technology (which already existed). But AC vector drive technology required shaft encoders to feed back the rotor position and that was not going to hold up on locomotive applications. This guy (forgot his name now) had dabbled in the the disturbance theory and got the contract from GE, but they only patented it for their locomotive drives. He was allowed to keep the patent himself for other non-locomotive applications, which he then licensed to basically all of the major VFD mfrs at the time (early 1990s) and retired very wealthy.


Remembered his name; Alan Wallace.
 
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