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Types of drives


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Hello everyone,

I have a few questions on the different types of drives. I was told that the best drive for my application was a sensorless vector since I need a drive that can keep the same amount of torque at very low frequencies.


1) Can someone explain to me what is the difference between a sensorless vector, a V/Hz, and a flux vector control drive? I read the explanation on this site between the V/Hz and the vector drive. They are probably taking about the same things but with different terms.


2) The 115 V, single phase VFD that I looked at don't have any internal or optional filters. Is this because there is a transformer in the drive and there is less RFI/EMI or because the manufacturer just don't bother about the interference?


Thanks for your help,

Luc Landry

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V/Hz (a.k.a. Scalar) drives just provide a variable voltage and frequency. The motor does what it can, but the drive has no idea if that is working.


Vector control implies that the drive is watching the motor's performance and reacting to it so that speed or torque can be maintained with excellent accuracy. Within the Vector Control group are Senesorless Vector and Flux Vector.


Sensorless Vector, also know as Open Loop Vector, uses internal current sensors in the drive and a very sophisticated microprocessor to follow the motor's performance and adjust the PWM output to maximize performance. It is good for speeds down to 1 Hz. some less than that, but because it relies on maintaining a mathematic motor model in the microprocessor memory, it can lose track of the motor if it comes to a stop, so it is not suitable for winch and hoist applications that require full torque at zero speed.


That final performance step, long the realm of DC dives, can be accomplished with a Flux Vector drive, also called Closed Loop Vector and Field Oriented Control (FOC). They use a feedback signal from an encoder on the motor to provide the VFD's microprocessor with a hard signal indicating the rotor position at any time, even if the motor is at a dead stop. This way, the VFD output can provide full torque at zero speed, as well as be even more precise. That added precision can sometimes even equal the performance of Servo drives for positioning and load profiling, although in small HP applications a Servo drive is a better choice.

"He's not dead, he's just pinin' for the fjords!"
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As to the other issue on 115V drives, there usually is no transformer inside, they use what is called a "voltage doubler" circuit that converts the 115VAC to 150VDC, then doubles the DC to 300VDC and feeds that directly into the VFD DC bus for filtering. if the VFD does not include filtering that's because it is being cheap. many of the 115V input VFDs designed for the world market do now include the filtering as standard, you just need to look.
"He's not dead, he's just pinin' for the fjords!"
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