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Sensorless Vector Control In Vfd


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Hi all,

Could you please throw some light on Sensorless vector control in VFD ? Any pointers / websites would help. Also, what is the difference between scalar control of vfd and this sensorless vector control ?




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Hello SNS


Welcome to the forum.


A standard scalar or V/Hz drive produces an output voltage that reduces with frequency below the rated motor frequency. This is in an attempt to keep the flux in the iron within a reasonable range of the design flux of the motor.

If the stator exhibited purely inductive characteristics, then this would hold very well as the current would be determined by the reactance of the inductance.

In reality, the stator circuit comprises both inductance and resistance and so the constant V/Hz model causes a reduction in flux at low frequencies where the resistance becomes significant relative to the reactance of the stator circuit.

The reduced flux at low frequencies, results in low torque at low frequencies.

Vector drives were developed as a means of increasing the flux at low frequencies and in response to transient loads, thereby improving the torque characteristics of the motor when driven by a VFD.

The first vector drives used a shaft encoder to measure the speed of the motor and the system operated to control the rotor slip.

More recent developments, mathematically model the motor based on certain motor parameters which can be measured and use this model to control the voltage at any given frequency. This system is known as a sensorless vector system. The results are very variable, and in some cases the torque below 5 Hz is still very poor. - not all sensorless systems are equal!! Most sensorless systems work well above 10 Hz.


There are two major alternative techniques employed to create a sensorless system, one system uses "historical data" measured during the previous cycle to determine the variation in voltage (most common) and the other uses fast measurements and a predictive model to determine the flux level. The second method is commonly referred to as direct torque control (DTC) as it is determining and controlling the torque at any instant in time. This system requires very fast measurements and calculations, but provides a better transient response than the other approach. The DTC system is far more sensitive to the motor characteristics and you can not change the motor without recalibrating, whereas the standard system will work, albeit less effectively.


Best regards,

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  • 3 years later...
  • 2 weeks later...

Thanks for the informative post Marke,


A couple of questions:


It seems that the main benefit of SVC is performance at low end. What other advantages does SVC provide?

If SLV control relays heavily on knowing the specific electrical characteristics of the motor, can SVC be used with either an output choke or output sinewave filter?



How does the VFD measure both magnetisation current and torque producing current, and how does it monitor the variance of the two during operation?

I have heard the term "Space-wave topology" when considering an SVC waveform - is this a real term, something just randomly in my head, or possibly advertising jargon?





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