Maybe I can help you to develop a little more confidence in your circuits. We are all here to learn a bit more! I have certainly learned a lot from HM and I try to give back a bit by my contributions.
A simple VFD description:
Excluding ground or neutral wires, a 3PH motor has three power wires rather than the two of simple single phase motor. Think of them as they are. Each has the same AC sinusoidal voltage but at three different phases. They are sinusoidal so oscillating between a positive peak, thru zero, to a negative peak, back thru zero, then returning to the positive peak. The easiest way to view 3 phase is as a Y connection. One wire, of the three, goes to each of the legs of the Y and the legs of the Y are the inductive windings in the motor. Letter them A, B, and C. When the voltage applied to A is at a peak, say 6 on a clock face, (or the bottom Y leg), the other two are voltages are at a less than peak "negative" values and are labeled B and C on the "Y". So current flows into the A leg thru the windings and out the B and C legs. At other times in the AC cycle current will flow in the other legs and out different ones. Only when the voltage on one leg is a maximum and current is flowing inward are the two currents in the other two legs balanced and flowing outward. Just as at this point in time, the rest of the time the three currents are summed to yield zero. We call it three phase because the peaks of each of the voltages (and currents) are out of phase by 120 degrees out of 360 degrees of the time cycle. So the currents are flowing in different amounts and directions in each of the motor windings but sum to zero (balanced). (PS. The three directions (arrows), each off set by 120 degrees, around a circle sort of forms a Y shape.)
To reverse the direction of a 3PH motor, just reverse the connection on any two of the legs. That is interchange B with C and the motor spins in the opposite direction. This is because the phases of the currents in the windings reverses with the connection change.
The VFD basically takes single phase 220Volt, 60 cycle per second (Hertz) power into two terminals, rectifies it to being mostly DC, and then has power silicon switching devices to reconstruct three phases out to the three terminals that go to the motor (A, B, and C). There is a computer built into the VFD to control this process. Because it is working from the temporally stored DC power, it can make the frequency of the new three phase power almost any value. Not just 60 cycle. It can use short burst of voltage/current to simulate the sinusoidal wave or to make the amplitude a different value from what came in from the power company.
The VFD computer looks to some control inputs to determine what it is doing and when. You control every thing by what voltages or currents you apply at these inputs. Typically there is a Forward and a Reverse input connection. When you apply a high signal to the Forward input terminal the output from the VFD creates the phase of the outputs such that the motor runs in the forward direction. When you turn this signal off the VFD out put turns off and the motor stops. When you apply a high signal to the Reverse input terminal the output of the VFD interchanges the phase to two of the motor wires to cause it to spin in the reverse direction. It is that simple, but the computer along with the programing to the VFD computer has predetermined how fast the voltage or frequency will ramp up or down. If you apply a high signal to both the Forward and the Reverse input terminals at the same time, the VFD programing decides what is to happen. Commonly it stops the output power. Just as the output power can be ramped up to reach full speed over a set period of time it can be ramped down to slow the motor. This control is referred to as the braking process. The Jog input simply turns the power on at a preprogrammed slow frequency. The direction of the jog is usually determined by a simultaneous application of a Forward or Reverse signal. Likewise, there is commonly protections built into the VFD to limit how rapidly the motor will go from being driven with a Forward to a Reverse signal. Afterall you will can program how fast it ramps down (brakes) to a zero speed.
For speed control, there are commonly multiple options for input. But the most often used is to simply apply a voltage between 0 and 10 volts DC to an input. This is commonly done by using a potentiometer (POT, variable three terminal resistor). So when a Forward signal is applied, the VFD computer looks to the voltage from the POT to determine what the final VFD output frequency should be. The maximum frequency us commonly programed to occur when the POT voltage is at a maximum. For example, a VFD might be capable of generating a 400 Hertz output frequency, which would be 400/60 = 6.667 times faster than the motor would run on 60 cycle power. However, you might decide you will never need to run the motor this fast and so you simply program the maximum frequency to be something smaller than 400 Hertz when the POT voltage is 10 volts. This would give you better resolution of desired low frequency values.
Of course all of this is a bit simplified. For example, you have a motor that is suppose to run on 3 phase 60Hz power. If the power sinewaves were exactly in sync with the motor rotations you would expect the motor to be spinning at a multiple of the 60Hz. For example, 1800 rotations per minute might make sense. 1800 RPM is the same at 1800/60(sec/min) = 30 revolutions per second (RPS). So, 30RPS/60Hertz = one revolutions of the shaft for each half cycle of the input power. But you look at your motor and it rated rotation rate is some other number, maybe 1750RPM or 1140 or etc. It is not a multiple or submultiple of 60 so it is not synchronous with the line frequency. It is the characteristic of an induction motor that the rotation slips relative to the drive frequency. Furthermore as the load becomes greater then slip is also greater and so the motor actually turns slower. It is not meant to run at exactly 1750 rpm.
In my lathe VFD conversion document, I had a small section describing the inputs to the VFD. (see the part 2 subsection "C. Short VFD (Inverter) Description and Chosen Design Functions:") Maybe now they will make more sense.
https://www.hobby-machinist.com/att...-vfddescript-links-dnl-l910_1440b-pdf.378083/
To start to understand the VFD you do not really have to understand all of the circuits to achieve all of the VFD features. For a mill, all you really have to have to get started is the inputs for Forward, OFF, Reverse(if you want it at all), and the POT (0-10volt) signal. Most VFDs even supply the 10volts DC power. You just need to hook up the three terminals of the POT. The ends of the pot go to the 0 and to the 10 volt terminals of the VFD and the POT wiper goes to the speed input of the VFD. It is common that the input signals to the VFD (Forward, Reverse, Jog, etc want to see 24 volts to be turned on. It is also common that the VFD will supply the 24V DC power that is connected via a switch to the VFD input. Examples of how to hook this up are common in the VFD manuals. However, they may seem more complex than this because they are sometimes driven by more complicated controls.
As an example, down load the Hitachi manual. While it is long it is pretty good and you can use it to learn from.
https://www.hitachi-iec.cn/ch/product/trans/wj200/images/WJ200 Series Instruction manual.pdf
Look at page 4-5 and 4-6 (pdf pages 209-210) for a over view VFD connection diagram. This shows a box labled WJ200, which is suppose to be the VFD insides with various connections around the outside. You will note at the top left of page 4-5 the input power connectors. Likewise at the top right the motor is connected via U, V, W terminals. At the lower left corner you will see a drawing of the POT. Note it is connected to 10Vdc at terminal H and at 0Vdc at terminal L. The POT wiper is connected at terminal O. That is all that there is to hook up the speed control. The other items around the POT are just alternative ways of controlling the speed. Now, at the middle lefthand side are the input signals. Terminal 1 is noted as the Forward control. These are programmable and can be interchanged so you need to see what is assigned in the code, but 1 and 2 are commonly Forward and Reverse. Now just above this is a circle with +24- written in. This is the internal power supply sysmbol for these inputs. Outside of the VFD it shows the P24 connected to the PLC terminals. On the inside of the drawing shows the PLC is connected to the box called input circuits which are also connected to the other 7 input terminals. This P24 supplies the power to the internal input box of electronics. So with this the 7 input terminals simply want to be connected to the low voltage point called, L. Closing any of the switches shown along the left then activates that input by connecting the input electronics for that particular terminal. All that is reqired are switches. They are activated by current flowing out of the terminal to the low voltage (0 volts) connector L. Close the switch connected to terminal 1 and the Forward condition is made active. The VFD then outputs power to the motor to casue it to rotate. These input electronics are make rather universal, and can be set up (programed) to work with current flow in either direction to cause an input to be activated. In my write up I turned it around so that when current is flowing into one of the input terminals the input is activated. This is done by connecting the PLC connector to the L connector instead of it being connected to the P24. (You will see that in my Part 2, Figure 4. ) Hence, while I used an external 24 volt supply rather than the P24, the activating current flows into the numbered input terminal rather than out of it. By the way on this Part 2, Figure 4 you will see which inputs I used for Forward, Reverse, Jog etc. The terminal 6 labled Brk for Brake does not cause the motor to break. I simply tells the VFD to use the normal braking or faster braking. The VFD does braking based upon what the other inputs are telling it to do.
Anyway, maybe some of this you will find useful.
Dave L.
