Sensorless Vector Vs Space Vector Drive
Posted 05 December 2007 - 08:46 AM
Can anyone please tell me what are the major differences between 'Sensorless Vector Controlled' & 'Space Vector Controlled' AC drive?
What are the typical applications that need these 2 different technologies?
Posted 06 December 2007 - 05:34 AM
"Space Vector Control" has to do with how the basic output signal algorithm is coded for firing the transistors to create the PWM signal used by the inverter output itself. The "opposing" technology would be Sinusoidal (or Sine Coded) PWM, abbreviated SPWM, an older technology. SPWM was a less efficient method of generating the PWM output, so the power that actually got to the motor was only about 78% of the available power (compared to a square wave). Space Vector PWM coding came along and boosted that efficiency to 90%, which meant less switching loss, thus more available motor power with the same amount of input power. Manufacturers still like to throw this out as though it's something special, but I'd venture to say that most inverters on the market now have been redesigned in the last 5 years and would all be using Space Vector coding, so it's no longer as special as it once was.
"Sensorless Vector Control" is a reference to how the VFD controls the motor speed and or torque. The opposing technologies are Scalar Control, a.k.a. V/Hz control, and Closed Loop Vector Control, a.k.a. Flux Vector Control. The differences are, in order of hierarchy of capabilities;
Scalar Control (V/Hz) means that there is no feedback from the motor to the VFD, so the output is just "scaled" to the commanded signal. The problem is that if the motor's response to the speed command is not what you wanted, there is no way to know it (unless you were watching and tweaking the dial yourself). This works just fine for things like pumps, fans and non-critical speed applications, which make up probably 85-90% of all VFD use.
"Vector Control" is when you have some sort of feedback of the motor's performance into the VFD so that it can interpret why it is not performing to specification, then adjust the vector algorithms of voltage, current and frequency that is applied to the motor so as to bring it into line (so you don't have to be there tweaking the dials). But within the idea of Vector Control, there are two divergences: Sensorless Vector Control (SVC) and Closed Loop Vector (CLV) control. Technically, both are closed loop systems because they involve a feedback loop, it's just an argument as to how that information makes it into the microprocessor.
"Sensorless" Vector uses the motor current itself as the feedback method. The microprocessor creates and maintains an accurate mathematic model of the motor's best performance, measures the performance with current sensors duriung use, then makes adjustments to the PWM output on the fly in order to affect a change to improve that performance. SVC is good for the majority of the remaining applications where more precision is necessary, but has a final limitation: it relies on that motor current feedback to adjust to the model, so if the motor is not moving, it can't determine what to do. Not a problem, until you want to release the mechanical brake on a hoist and not let the load start dropping! So no hoists with SVC.
Closed Loop Vector is the original, and still the most accurate, form of Vector control. It closes the loop with actual motor speed indication, typically a shaft encoder with thousands of pulses per revolution mounted directly to the motor. So when the VFD tells the motor to move, it checks to see if it did, then adjusts on the fly to make sure it moved the right amount. CLV then comes closest to matching the performance of DC drives and even some servo drives.
Within the Scalar, SVC or CLV drives however, most (if not all now) use Space Vector coding of the transistor firing pattern to create the PWM signals.
Posted 14 December 2007 - 08:26 PM
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