For instance, the TMS320C24x has 3 general-purpose timers, 3 full Compare Units, and 3 simple compare units, Programmable dead-band units and a dedicated space-vector PWM module that can be used for generating PWM outputs. It can generate up to 12 PWM outputs, of which half are complementary with programmable dead-band time.
The general-purpose (GP) timers can be configured to run in up and up/down count modes for generating asymmetrical and symmetrical PWM outputs. The period registers of the GP timers are shadowed to allow on-line change of PWM frequency, which in turn allows wabbling of PWM frequency, which can help spread out the spectrum of the PWM outputs.
The pulse widths of the PWM outputs are determined by values in the compare registers. The compare registers are shadowed, allowing the CPU to write to these registers at any time during the current PWM period. The new compare values can be programmed to become active immediately on underflow or on period match.
The polarities of the PWM outputs can be independently controlled by the action control register and the simple action control register. The polarity of a PWM output can be active high, active low, forced high, and forced low, allowing control of different types of power devices, such as IGBTs, power Mosfets, and bipolars.. The action control registers are also shadowed so users can write to these registers to change the polarities of PWM outputs at any time during a PWM period.
The space-vector PWM module automatically generates space-vector PWM patterns once a starting basic vector and a direction is given in the action control register.
The generation of such PWM outputs is entirely register-based. All the registers are data memory mapped so the CPU can access them as data memory locations. To generate a certain kind of PWM output, the CPU:
* Writes to the pin configuration registers to configure the pins as PWM outputs
* Writes to the GP timer control, compare control registers, the action control registers, and the dead-band control registers to configure PWM frequency, type of PWM waveform to be generated, polarities of the PWM outputs and the dead band
* Continuously updates the compare registers based on newly determined pulse widths.
Any of the mentioned PWM techniques can be used to determine the pulse widths.
One significant advantage of using symmetric space-vector PWM is that the technique applies about 14% more voltage on motor windings in comparison to sinusoidal PWM with the same dc bus voltage. This translates into a more efficient utilization of bus voltage, as well as a motor that can be rated at a higher voltage and lower current to achieve the same horsepower rating.
Using symmetric space-vector PWM results in 10% less phase current, with a reduced power dissipation and heat generation in the power converter and motor. Finally, symmetric space-vector PWM technique generates less harmonics in phase current for less power dissipation and less noise. It has been observed that symmetric space-vector PWM technique generates less audible noise, especially when the dc bus voltage goes above 100 V.
Depicted is the structure of a typical three-phase voltage-source power inverter--Va, Vb, and Vc are the output voltages applied to the windings of a motor (Fig. 2). Q1 through Q6 are the six power transistors that shape the output, which are controlled by a, a', b, b', c, and c'. When an upper transistor is switched on (a, b, or c is 1), the corresponding lower transistor is switched off (a', b', or c' is 0). The on and off states of the upper transistors Q1, Q3, and Q5, or equivalenty the state of a, b, and c, are sufficient to evaluate the output voltage.
The relationship between the switching variable vector [a, b, c]t and the line-to-line voltage vector [Vab Vbc Vca]t is given by Equation 1, from which one can easily arrive at Equation 2 that determines the phase voltage vector [Vab Vbc Vca]t.
As shown in Figure 3 , there are eight possible combinations of on and off patterns for the three power transistors that feed the three-phase power inverter. Notice that the on and off states of the lower power transistors are opposite to the upper ones, so they are completely determined once the states of the upper power transistors are known. The eight combinations and the derived output line-to-line and phase voltages in terms of dc supply voltage Vdc, according to Equation 1 and 2, are shown in Table 1.
Experimental Results
Experimental data results of using the TMS320 DSP, as well as the sinusoidal and space-vector PWM techniques are shown (Fig. 5 and Table 2). A three-phase ac induction motor, rated at 147V, 60 Hz and 1/2 hp is controlled, in this case, based on constant V/Hz principle with a PWM frequency of 25 kHz, sampling frequency of 12.5 kHz, and dc bus voltage of 180V. It can be seen that the space-vector PWM technique generates 14 to 25% more output voltage and an obvious reduction in harmonics in output currents.
For instance, the TMS320C24x has 3 general-purpose timers, 3 full Compare Units, and 3 simple compare units, Programmable dead-band units and a dedicated space-vector PWM module that can be used for generating PWM outputs. It can generate up to 12 PWM outputs, of which half are complementary with programmable dead-band time.
The general-purpose (GP) timers can be configured to run in up and up/down count modes for generating asymmetrical and symmetrical PWM outputs. The period registers of the GP timers are shadowed to allow on-line change of PWM frequency, which in turn allows wabbling of PWM frequency, which can help spread out the spectrum of the PWM outputs.
The pulse widths of the PWM outputs are determined by values in the compare registers. The compare registers are shadowed, allowing the CPU to write to these registers at any time during the current PWM period. The new compare values can be programmed to become active immediately on underflow or on period match.
The polarities of the PWM outputs can be independently controlled by the action control register and the simple action control register. The polarity of a PWM output can be active high, active low, forced high, and forced low, allowing control of different types of power devices, such as IGBTs, power Mosfets, and bipolars.. The action control registers are also shadowed so users can write to these registers to change the polarities of PWM outputs at any time during a PWM period.
The space-vector PWM module automatically generates space-vector PWM patterns once a starting basic vector and a direction is given in the action control register.
The generation of such PWM outputs is entirely register-based. All the registers are data memory mapped so the CPU can access them as data memory locations. To generate a certain kind of PWM output, the CPU:
* Writes to the pin configuration registers to configure the pins as PWM outputs
* Writes to the GP timer control, compare control registers, the action control registers, and the dead-band control registers to configure PWM frequency, type of PWM waveform to be generated, polarities of the PWM outputs and the dead band
* Continuously updates the compare registers based on newly determined pulse widths.
Any of the mentioned PWM techniques can be used to determine the pulse widths.
One significant advantage of using symmetric space-vector PWM is that the technique applies about 14% more voltage on motor windings in comparison to sinusoidal PWM with the same dc bus voltage. This translates into a more efficient utilization of bus voltage, as well as a motor that can be rated at a higher voltage and lower current to achieve the same horsepower rating.
Using symmetric space-vector PWM results in 10% less phase current, with a reduced power dissipation and heat generation in the power converter and motor. Finally, symmetric space-vector PWM technique generates less harmonics in phase current for less power dissipation and less noise. It has been observed that symmetric space-vector PWM technique generates less audible noise, especially when the dc bus voltage goes above 100 V.
Depicted is the structure of a typical three-phase voltage-source power inverter--Va, Vb, and Vc are the output voltages applied to the windings of a motor (Fig. 2). Q1 through Q6 are the six power transistors that shape the output, which are controlled by a, a', b, b', c, and c'. When an upper transistor is switched on (a, b, or c is 1), the corresponding lower transistor is switched off (a', b', or c' is 0). The on and off states of the upper transistors Q1, Q3, and Q5, or equivalenty the state of a, b, and c, are sufficient to evaluate the output voltage.
The relationship between the switching variable vector [a, b, c]t and the line-to-line voltage vector [Vab Vbc Vca]t is given by Equation 1, from which one can easily arrive at Equation 2 that determines the phase voltage vector [Vab Vbc Vca]t.
As shown in Figure 3 , there are eight possible combinations of on and off patterns for the three power transistors that feed the three-phase power inverter. Notice that the on and off states of the lower power transistors are opposite to the upper ones, so they are completely determined once the states of the upper power transistors are known. The eight combinations and the derived output line-to-line and phase voltages in terms of dc supply voltage Vdc, according to Equation 1 and 2, are shown in Table 1.
Experimental Results
Experimental data results of using the TMS320 DSP, as well as the sinusoidal and space-vector PWM techniques are shown (Fig. 5 and Table 2). A three-phase ac induction motor, rated at 147V, 60 Hz and 1/2 hp is controlled, in this case, based on constant V/Hz principle with a PWM frequency of 25 kHz, sampling frequency of 12.5 kHz, and dc bus voltage of 180V. It can be seen that the space-vector PWM technique generates 14 to 25% more output voltage and an obvious reduction in harmonics in output currents.