There is a lot of online information about this but there is also a lot of contradictory information. What is carrier frequency exactly and does it stay the same or does it vary? I understand the concept of PWM but if carrier frequency stays constant, then how does an AC motor see varying degrees of pulse width in order to operate on a simulated AC signal? I'm just trying to find the truth, in detail, about the relationship between carrier frequency, the speed reference signal, and PWM in order to control an AC motor.
Electronic – How does carrier frequency and pulse width modulation in a VFD work together to control AC motor speed
frequencyigbtinduction motorpwmvfd
Related Solutions
Shoot-through current refers to the condition where both switches/MOSFETs on one side of the H-Bridge are on simultaneously. Under normal conditions, the H-bridge is under one of the following conditions:
If, however, both switches on one side are on simultaneously, a huge current can flow (Only 34mΩ per leg, remember?), which is usually destructive. I'm not sure why both switches would be on - Possibly an issue with gate capacitance and switching time?
The datasheet says that the PWM pin is:
Voltage controlled input pin with hysteresis, CMOS compatible. Gates of low side FETs are modulated by the PWM signal during their ON phase allowing speed control of the motor.
You might get better luck with higher voltages if you modulated the PWM such that it was centered on the ON-pulses:
Motor on: ---- ---- -- -- S-Left-Top__/------\________/------\________ S-Rght-Bot___/----\___________/--\__________ S-Rght-Top__________/------\________/------\ S-Left-Bot___________/----\___________/--\__
But you'll definitely want to look very carefully at Figures 4, 5, and 6 of that datasheet.
The hobby servo needs a signal like this:
In a 20ms window, (50 times a second) a pulse with a width between approximately 1ms and 2ms (may vary, depending on servo) with a middle point of 1.5ms. So in the 20ms window, there is almost 90% of the time being "silence" with no signal level. The pulse is only high for a very short (relatively) period.
The Arduino pins are default output to either 500Hz and 3.9Khz. You cannot directly use the pin in PWM mode (with analogWrite), because it will be receiving pulses too fast, and will bug out/malfunction/won't do anything useful.
The Servo library available for inclusion to an Arduino sketch uses the built-in timers of the ATMEGA328P (or other chip, depends on which arduino you are using) and some fancy software and interrupts to get the proper 50Hz timing, and uses an ordinary digital output pin to send the required pulse.
You can do this yourself in a simple loop, if you only control 1 servo and have a simple program, by simply setting a digital pin HIGH and then delaying (using DelayMicroeconds) for between 1000 and 2000 microseconds depending on what position you want to move to, then setting the pin LOW and delaying for (20,000 - the time you delayed for the servo).
You can send the same signal constantly and the servo will stay at that point, or you can send it until it reaches there and stop sending a signal. The servo should stay at that point, but I do not think it will remember the point (you can move it with your hand and it will stay at the new point, until a new signal is sent again).
It's better not to try and use the PWM hardware outputs for the Arduino to control a servo, even if you do manage to pre-scale the timers down to the required 50Hz (I think this would be hard to get, but I believe you can do it!) as the PWM output registers will need to be set with 8 bit resolution in a tiny range of 10-20%, which only gives you 25 total positions available to the servo (that is pretty bad, 25 positions for a whole +-90 degree movement range!). So go with the Servo Library, basically :)
Best Answer
In a VFD, the carrier frequency is the frequency of switching the power devices. To start with a simple example, consider the H bridge circuit shown below. The four transistors can be switched sequentially to produce a crude approximation of a sine wave. There are two on/off switching cycles required to produce one cycle of the output waveform, so the switching frequency is twice the output frequency. If the output frequency changes the carrier frequency must change proportionally.
To control an induction motor, the voltage must be controlled to maintain a relatively constant ratio of voltage to frequency. To do that with pulse width modulation (PWM), one or more switching events must be added to the scheme as shown below. Adding PWM to control the voltage with this scheme makes the switching frequency six times the output frequency. The switching frequency is usually selected to be much more than six times the output frequency to reduce the harmonic content and provide a better quality effective output waveform. Various manufacturers describe their control schemes in different ways. In one way or another, the waveform required to produce the desired motor performance is calculated and the devices are switched accordingly within limits that the design has put on the switching frequency.
It is possible to use a higher number of modulated pulses at lower output frequencies in comparison to the number used at higher output frequencies. In that case, the switching frequency as a multiple of output frequency could rise and fall as the output frequency increases rather than just rising in proportion to output frequency. There are other factors in the overall control scheme and VFD design that add further complexity to the relationship between output frequency and switching frequency.
The following diagrams shows two possible PWM designs based on a triangle carrier wave intercepting a sinusoid reverence wave. These show how a variable frequency with proportional variable voltage PWM sine wave simulation can be implemented with either a carrier frequency that is a multiple pf the sine frequency or with a constant carrier frequency.
Note that the switching rate is only the count of on/off and off/on transitions per second. The "on" duration and "off" duration can be adjusted over a wide range without changing the switching frequency.
Various aspects of the “triangulation method” pulse width modulation schemes for VFDs are described in:
J. Zubek, A. Abbondanti and C. J. Norby, "Pulsewidth Modulated Inverter Motor Drives with Improved Modulation," in IEEE Transactions on Industry Applications, vol. IA-11, no. 6, pp. 695-703, Nov. 1975. doi: 10.1109/TIA.1975.349357
That paper cites:
K. Heintze et al., “Pulse width modulating static inverters for the speed control of induction motors,” Siemens-Z., vol. 45 (3), pp. 154-161, 1971.
and:
A. Schonung and H. Stemmler, "Static frequency changers with subharmonic control in conjunction with reversible variable speed ac drives," Brown-Boveri Review, pp. 555-577, Aug./Sept. 1964.
The triangulation method may have been used to some extent before microprocessors were used in VFDs. In many discussions, the triangle carrier wave is used to illustrate the basic principle rather than to describe the detailed implementation. With microprocessor control, it is possible to simulate sine waves with variable voltage and frequency in many ways with a fixed or variable switching frequency. Many schemes have been described and used. It is difficult to determine which schemes are popular today.
There may be schemes that, in effect, change the both the number and width of PWM pulses for every cycle of output waveform that is produced. In most modern VFD designs, the processor constantly recalculates the required output voltage and frequency.