VFD variable frequency units
To examine how to control the speed of an engine with a VFD driver (Variable Frequency Driver) we must first remember some basic terms. As we know, single-phase motors and three-phase motors operate according to the principle of operation of induction. They consist of a rotor and a stator without physical contact between the rotor and the stator. According to this principle as we see in the figure below when a frame is found in a changing magnetic field an inductive voltage will appear at the ends.
The motors have a constant speed determined by the supply frequency and the constant torque. The motor impeller rotates at speed n which depends essentially on the axle load. The ratio of the difference between the synchronous speed ns and the motor speed n to the synchronous speed is called the slip s and is given by the formula and the slip is not constant but changes with the load and increases with it.
n = Rotor speed, ns = Stator speed, s = slip.
The speed of the Rotor is given by the formula: Speed therefore for pole pairs it is standard and constant and for a network frequency of 50 Hz is given by the following table:
Obviously, a constant speed motor is not suitable for all applications, resulting in the need to adjust the speed as needed. Industrial machinery is often driven by electric motors with speed control devices. These motors are simply larger, more powerful than those driven by home appliances, such as food mixers or electric drills that normally run at a constant speed.
Although many different induction motor speed control techniques have been developed, the most popular control technique is to produce a variable supply voltage that has a constant voltage to frequency ratio. This technique is widely known as V / F control.
A very important observation here is why I change the voltage at the same time I change the frequency and I keep this reason constant what do I want to avoid something very important? Couldn’t I just change the frequency and adjust the speed?
As it is known, the frequency affects the impedance of a circuit since it enters the type of inductive resistance XL = ωL = 2πfL therefore if I reduce the frequency too much to values close to zero the reactive currents according to the law of Ωhm I = V / XL will take too much very high value with the result that I destroy the windings, the motor and the driving circuit, but if at the same time I reduce the voltage then I keep the idle currents constant and at low values.
Therefore V / F = constant.
Variable frequency drive systems.
Variable Speed Drives (VSD or Variable Frequency Drives – VFD) are used to control and adjust the rotational speed of a machine at will.
Typical A typical VFD system consists of three parts: the electric motor (electric motor)
the power converter (inverter), the control system.
Let’s look at the parts of a VFD Inverter one by one.
We will cover the parts and operation of (VFD) Variable Frequency
Drive. It is important to keep in mind that Drive is only one part of it
systemic. As you will see in the attached photos, we have the disconnect switch, the fuses, the bypass switch, the protection against thermal overload, the BAS, etc., which play an important role in the construction and proper operation of a VFD.
Inside the VFD there are 4 large sections: rectifier, DC link in the middle, inverter and control/adjustment. This fourth section controls
and setting, interfaces with the other 3 sections.
Generally, the operation is as follows. The three-phase or single-phase mains voltage begins to enter the rectifier, where it respectively rectifies it to DC voltage with some ripple. The intermediate circuits normalize and hold the DC voltage at constant levels and connect to the inverter. The latter, the inverter, uses DC voltage to propel the motor at the voltage and current levels depending on the control circuit. The configuration of the pulses going to the motor makes it look like a sine waveform.
Each of these sections is discussed in detail below:
Bypass switch and fuses for a 600Hp Inverter (450KW), in smaller Inverters these two parts are integrated inside the drive.
Converts alternating to continuous.
Thyristors (D1 to D6) allow current to flow only in one direction when their gate signal is activated. In this diagram, the AC power at L1 goes to thyristors D1 and D2. Due to the location of these thyristors, the current can only go up. Thyristor D1 leads when AC is positive and D2 leads when AC is negative. This leads the upper line (+) to a positive potential and the lower line (-) to negative. Thyristors D3 and D4 convert the power of L2 to DC and thyristors D5 and D6 convert L3. A voltmeter can be used to measure this DC voltage. In this type of circuit, the DC voltage is 1.35 times higher than the AC line voltage. Therefore if:
Vac = 240 V, then a voltage of 324 Vdc is produced.
Vac = 380 V, then a voltage of 513 Vdc is produced.
Vac = 400 V, then a voltage of 540 Vdc is produced.
Vac = 575 V, then a voltage of 776 Vdc is produced.
Because in one application the load power and mains voltage can change then the dc voltage will move between the values.
The rectifier section includes the power inlet, thyristors and heat sinks.
See the image below.
On large VDF drives> 22kW = 30 Hp a soft charge circuit is added which helps to charge the dc Capacitors before applying the supply voltage to the rectifier. As you will see in the image below this card is in the upper left corner just above the rectifier and its fuses are on the right. In drivers larger than 350 Hp, IGBT is used instead of resistors to limit the current flowing to the capacitors.
We see a Soft Charge for a 600Hp drive (450kW).
We see for a Drive 200 Hp the 2 big power protection fuses that go to the capacitors in the DC Link section
Using a series of capacitors and DC reactors the DC voltage becomes more stable.
The blue or black capacitors we can find the store a lot of energy. In the photo, we see three rows of 12 capacitors so a total of 36 capacitors.
There are 2 series of coils as shown above. DC Link coils are always with 2 terminals, shown here on the left. Coils are DC, also known as DC Chokes.
When drivers are asked to have a dynamic brake, the drive comes with an IGBTs brake. When the voltage reaches a very high value in the DC bus, the IGBT brake is activated and sends power to
brake resistance. This is not an option for HVACs.
This brake option, also known as dynamic braking, is used in devices that need to be stopped or changed quickly, such as conveyors, lifts and centrifuges. In brake units, the IGBT transistor is used to remove the extra power that returns to the unit when the engine, which is at high standstill, stops or changes direction. The only HVAC application that can use dynamic braking is for boiler burner fans.
Inverters receive the voltage from the DC bus and use the pulse amplitude (PWM) modulation to send the signal displayed on the motor as an AC signal.
Current sensors monitor the current going to the 3 phases of the motor. These sensors detect and alert when I have a short circuit or ground. Some manufacturers only test for a short circuit or ground at the first command to run the circuit. The process is supervised by some software that gives us an Alarm and there is a switch that disconnects the engine from the drive. Some inverters do not have this control resulting in damage to the motor in the event of a short circuit. Of course, a driver does not remove the other protections of the engine from overheating or overheating and they must coexist.
The Inverters, the IGBT and the snubber card are mounted on a heatsink under each of the 3 rows of capacitors.
All Drivers manufacturers use a PWM configuration with a different pulse width that goes to the engine.
Note : Why we use IGBT:
The semiconductor we use is IGBT because it has a very fast response and can reach high frequencies.
The diagram above shows only 7 pulses on each side, but in reality, 1750 pulses or more should be displayed. This PWM frequency can range from 3.5 kHz to 15 kHz, which means it sounds. It is also known as carrier frequency, which is variable by most VFD manufacturers. A low carrier frequency can make annoying noise, but a higher carrier frequency produces more heat in the driver and engine. If the carrier frequency noise is very loud especially with the supply fans, LC filters are placed between the VFD and the motor and the noise stops in this filter.
Without a driver, the engine can run at full speed or be stopped. With a driver, the engine can go to a number of different speeds.
In the photo, we see a PWM configuration that goes to the motor without control and to the right with VVC + control by Danfoss.
The control and regulation unit monitors all 3 sections, making many calculations and corrections to the output signal. The test is performed with a PID controller and a 4-20 mA or 0-10 V sensor. The photos for the analysis of the Inverter circuits are from the large Inverters> 400Hp of the Danfoss company based in Denmark. https://www.danfoss.com/en/products/
Troubleshooting on a VFD Driver
If we know what the components of a VFD Drive are made of, we can very easily find some damage by measuring dynamically with a multimeter.
Step 1: Check the input rectifier.
Before starting, do not forget to always turn off the power and wait until there is no voltage on the DC bus. Then measure the voltage drop across the diode-thyristor and it should have values according to the manufacturer’s leaflet eg (0.3 V to 0.6 V) at each input terminal. One possible fault is that the entrance bridge is most likely short-circuited. However, if you read the open circuit with both + – or upside down terminals, then the charging resistor is probably open.
Step 2: Check the output of the VFD driver
Holding the red multimeter terminal on the “-“ negative bus, insert the black cable into each of the three motor output terminals. You will need to read again, a small voltage drop across the diode. Now, reverse the black cable on the positive “+” bus and the red cable on each motor output terminal. It will bring out a diode voltage drop upwards. If this does not happen in both cases then the output device is shorted. If you read an open circuit, then either the output device is damaged or the Bus fuse is open.
Step 3: Check the bus capacitors
First, physically inspect the capacitors for signs of damage, such as a cracked or deformed housing, or a pressure plug coming out of the top. Then set the multimeter to Ohm measurement and start measuring the resistance in the capacitor as shown in the Figure below. For a short-circuited capacitor, the multimeter will display a reading close to 0 Ohm. In this case, you should look for a short circuit. The only way to be sure of this is to put an oscilloscope on the Bus and watch the ripple.
Basic diagram for small Inverter motors of some Hp.
As we see in the photo above, the wiring of such an Inverter is very easy, a three-phase power supply and three cables power the motor. Many manufacturers allow us to connect three-phase motors with single-phase supply since the capacitor circuit that converts the single-phase network to three-phase is integrated into the board.
We also see that at inputs DI1, DI2, DI3 we can put the start of the motor Stop-Start, make a reversal in the motor but also get some error. We can also achieve variable speed regulation with analogue voltage 0-10 Volt or analogue current 0-20mA.
In the seminar that will be held in Athens in September, we will connect the Inverter with a PLC eg Logo8 and we will fluctuate the engine speed from the PLC