Space Vector Modulation (SVM) is a common technique in field-oriented control for induction motors and BLDC motors. Space vector modulation is responsible for generating pulse width modulated signals to control the switches of an inverter, which then produces the required modulated voltage to drive the motor at the desired speed or torque. It is also known as Space Vector Pulse Width Modulation (SVPWM).
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If you were using 6-block commutation with your feedback controller, your current controller would look at the current running through your motor, would then compare it to the reference that has been provided, and then generate the magnitude of the voltage signal to be applied, which would then be used to find the duty cycle to be applied to the MOSFETs of the H-bridge.
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The resistor-inductor circuit that is a motor acts as a relatively fast low pass filter between voltage and current. So at low to medium rotation rates, you can produce a sinusoidal current vector which rotates with the motor’s angle by inputting voltage vectors which rotate at the same rate as the motor’s angle.
The output of your current control loop will specify a voltage magnitude to be applied. This would be divided by the supply voltage and saturate at a 100%. Now instead of applying the duty cycle fully to whichever MOSFET would be dictated by the block commutation, you would multiply this duty cycle by your A,B,C sinusoids which are functions of theta.
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This produces three duty cycles between -100% and +100%, which can then be output accordingly to the H-bridge. The downside to sinusoidal commutation is that at no point during sinusoidal commutation are you taking full advantage of your voltage range. This is because a voltage differential is being applied - the magnitude of the voltage does not matter, only the voltage difference between the three phases does. The phases do not span the full range of supply voltage possible. If we plot the voltage differential (highest voltage - lowest voltage), the maximum voltage being used is only 86.6% of what our voltage could do.
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SVM
Just like with sinusoidal modulation, space vector modulation maintains a voltage differential which rotates with the motor angle to stay in line with the Q axis.
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However, it takes full advantage of your supply voltage. The goal is to create three-phase voltages that are required to drive our PMSM motor, by way of using a three-phase inverter, which takes as an input a constant DC voltage. For properly converting DC to AC power, we need to control the on and off states of the inverter switches along with their switching sequence.
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Our voltage differential (in sinusoidal modulation) could theoretically be increased by 15.47% (shown below). This creates a new problem - while we would now be applying the max voltage differential our supply voltage could provide, we would also be required to command a duty cycle of 115% - which is impossible.
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We need to command the voltage to high for 115% of the time - but on the other side of the circuit we are only commanding the other two phases to ground at a 58% duty cycle.
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Accounting for the fact that the circuit dynamics will filter the PWM voltage into a pseudo-continuous voltage signal, the ability of the voltage differential to drive current would be the exact same for this location if instead of having the impossible 115% duty cycle and 58% duty cycle, we shifted all three phases down by 28.8% such that one was connected to high 86.6% of the time and the other two were connected to low 86.6% of the time.
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In the same way, we see that when the points on the low side would be required to connect to ground a 115% of the time, we could shift the common voltage of the three up by 28.8% such that the three duty cycles were again 86.6%. So in order to bring the -115% sections up, and the +115% sections down, we will add a triangle wave with peaks at these locations to all three of our sinusoids.
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Using this modulation scheme, we are taking full advantage of the voltage range made available to us, while keeping our required duty cycles at or below 100%.
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