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| Scheme: A VFD panel view |
What is variable frequency driver (VFD)?
A variable frequency driver (it is also knows as Drives, VFD and inverters) is a type of motor starter that drives an electric motor by changing the frequency and voltage of its power supply. A VFD works according to principle of V / f policy. The VFD has the ability to control the acceleration and de-acceleration of the motor during start or stop, respectively.
These drives are the main reason why the DC motor, which was used extensively in factories in the past, has completely shifted the AC motor from our focus to the essential. VFD has brought the use of AC motors back to important among us. The AC-induction motor can change its speed by changing the frequency and voltage of the power supply used to operate it. This means that if the voltage applied to the motor is 0 Hz, then motor is running at zero speed. When a frequency of 50 Hz is applied to the motor, the motor will run at its rated speed, and if the frequency of the supply voltage is less than 50 Hz, the motor will run less than its rated speed. According to the V / f principle in which the variable frequency drive operates, it is a complete electronic controller (BJT, IGBT and Thyresistor) designed to change the voltage and frequency supplied to the motor.
One of the biggest advantages of VFD is that it ensures that the motor does not draws too much current during starting, so the overall energy consumption of the factory can be controlled and keep the electricity bill as low as possible.
The block diagram of a VFD:
The block diagram of a VFD can be divided into four major sections:
a. Converter or rectifier section
b. Filter section
c. Inverter section
d. Controller/switching section
The rectifier section contains six diodes. In this diodes D1, D2, and D3 are connected with the positive bus and the diode D4, D5 and D6 are connected with the negative bus. Those 6 diodes are connected as a bridge which converts the three-phase A.C supply into a DC supply. The three-phase R, Y, and B are connected across the diode bridge. Depending on the sinusoidal wave the diodes triggered in forward bias or reverse bias thus provides a positive or a negative pulse in both positive and negative bus.
b. Filter section
We know that standard rectifier diodes convert AC distribution to DC supply so that the DC supply obtained is not sufficient because the frequency-dependent AC ripples are also present with it. Some kind of ripple rejection filters are used to remove the AC ripples and create a pure DC output. The standard component of the filter consists of different types of capacitors and inductors. In the filter section, mainly the capacitor filters the AC ripple and provides pure DC output.
c. Inverter section
The switching or inverter section inverts the DC to AC. In this section, different types of electronic switches are used, they are high power transistors, SCRs, IGBT or MOSFETs. The switches are rapidly switches between on or off thus the load receives a alternating voltage that is very similar to AC. Of this output frequency is proportional to the switching rate. If the switching rate is high it provides high-frequency output whereas low switching rate provides a low-frequency output.
d. Controller/switching section
All operations of a drive are controlled by an embedded microprocessor. A programmed microprocessor controls the whole control circuit. It gives control signals to the rectifier and inverter based on reference input signal for their operation.
Layout of Drive:
The layout of VFD is shown below. Here we take the ABB-ACS880 layout to better understand the layout of VFD
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| Scheme: VFD layout view |
1. Hood in frames.
2. Auxiliary cooling fan.3. Clamps for securing the FSO wiring mechanically.
4. Power cable connection terminals behind the shroud.
5. 360-degree grounding clamps for power cable shields.
6. 360-degree grounding clamps for control cable shields.
7. Control unit with I/O cable connection terminals.
8. Clamps for securing the control cables mechanically.
9. Input power cable entry behind the 360-degree grounding clamps.
10. Control cable entry.
11. DC cable entry.
12. Motor cable entry behind the 360-degree grounding clamps.
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| Scheme: Layout view control connections |
- XPOW External power input
- XAI Analog inputs
- XAO Analog outputs
- XD2D Drive-to-drive link
- XRO1 Relay output 1
- XRO2 Relay output 2
- XRO3 Relay output 3
- XD24 Start interlock connection (DIIL) and +24 V output
- XDIO Digital input/outputs
- XDI Digital inputs
- XSTO Safe torque off connection
- X12 Connector for safety functions modules (optional)
- X13 Control panel connection
- X202 Option slot 1
- X203 Option slot 2
- X204 Option slot 3
- X205 Memory unit connection
- X208 Auxiliary cooling fan connection
- J1, J2 Voltage/Current selection jumpers (J1, J2) for analog inputs
- J3, J6 Drive-to-drive link termination jumper (J3), common digital input ground selection jumper (J6)
How a variable frequency drive works?
A variable frequency Drive (VFD) works by taking AC supply either single or three phase (Here we consider three phase supply) and in the first step rectifying it into DC, then DC is usually smoothed with Capacitors and often a DC choke before it is connected to an inverter to turn it into three phases variable voltage for the motor.
The control method is known as Pulse Width Modulation (PWM). In this method the DC voltage is switching between on and off very fastly, in other words it is called as chopping. This is done by the Transistor switches. A three phase simulated form of motor current is built by a series of DC pulses. Here the first pulse has a very short on period, followed by a longer on period, then longer until the widest pulse appears in the center of the positive sine wave, then smaller until the DC is inverted. Now the same process is repeated in the negative part of the sine wave.
According to an A.C motor characteristics the applied voltage to be direct proportionally adjusted by the inverters whenever the frequency is changed. For e.g. if a motor is designed to operate at 415 Volts at 50 Hz, the applied voltage must be reduced to 220 Volts when the frequency is reduced to 25 Hz. Thus the ratio of volts per hertz (i.e v/f) must be regulated to a constant value.
Since the Transistors can be switch to ON and OFF to any time base the frequency of the pulses being turned on is known as the Switching Frequency. The designed switching frequency for VFD is usually around 3 kHz to 4 kHz, so the pulses it makes for 50Hz will be 3000/50=60 pulses per full sine wave or half phase. When the fixed Voltage pulses are applied to the induction motor, the result is control of both Voltage and Frequency (means motor speed N is varied).
By programming parameter settings in the VFD you can see the Inverter drive will control Voltage and Frequency over virtually any range. This means when setting up all the motor parameters (as per manufacturer name plate detail) an
Inverter drive the motor accordingly. For ex. a Delta connected 230V, 50Hz, 1500rpm motor is supplied from a 230V single phase supply with a base frequency set at 50Hz means motor run at 1500 rpm. If we change the set frequency to 30Hz in drive parameter motor run at 900 rpm. By changing arrangement of Voltage and frequency we choose that will correctly flux the motor.
The motor will be correctly fluxed when its Voltage curve rises from around 0Hz to its rated frequency. The rated frequency and rated Voltage being what is shown on the motor nameplate.
And an important application in VFD is electrical braking. For this in the Inverter Drive installed this provision and a braking resistor (DBR) is present. The input of the Inverter Drive is flows in a single way direction, while the output permit power to flow in both the directions. It is known as regeneration process. When a motor attempt is made to slow its speed at a greater rate than it would achieve for deceleration or coast down it follows stored energy back to the Inverter Drive.
The bus voltage is likely to rise during this regeneration, when there is no suitable control arrangement. The capacitors are charged during the bus voltage, which provides a small amount of braking to the motor. Usually it is about 5% to 10%, but it depends on the capacitor size.
A brake switch or chopper is used to convert the braking power into a braking resistor. This braking resistor is usually connected externally and is selected so that the brake switch can send enough current that it is not too high and that it is not overheated.
The drive can operate in several operating modes with different types of reference. The mode is selectable for each control location (Local, EXT1 and EXT2) by programmable parameter.
Speed control mode:
The motor operate based on a speed reference given to the drive. This mode can be used either with estimated speed used as feedback (without encoder), or with an encoder or resolver for better speed control accuracy.
Speed control mode is available in both local and external control (EXT1 & EXT2). It is also available both in DTC (Direct Torque Control) and scalar motor control modes.
Torque control mode:
Motor torque follows a torque reference given to the drive. This mode can be used either with or without an encoder or resolver. When used with an encoder or resolver, this mode provides for more accurate and dynamic motor control.
Torque control mode is available in both local and external control (EXT1 & EXT2).
Note: in this control mode motor is follows torque reference not speed reference.
Frequency control mode:
In this control mode the motor follows a frequency reference given to the drive. Frequency control is only available for scalar motor control.
Special control modes:
In addition to the above-mentioned control modes, the following special control modes are available:
Process PID control: In process PID control, a set point is connected to the drive instead of a speed reference. An actual value is also brought back to the drive. The process PID control adjusts the drive speed in order to keep the measured process actual value at the desired set point.
Emergency stop modes: Drive stops along the defined deceleration ramp and drive modulation stops.
Jogging mode: Drive starts and accelerates to the defined speed when the jogging signal is activated.
How the driver is connected:
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| Scheme: connection diagram |
The output of mcb or mccb is connected to the input of VFD (U1, V1, and W1). The output of the VFD (U2, V2 and W2) is connected to the terminal of the motor. When the cable length between the VFD and the motor is too long, we use a choke in between.
Use this as an output choke for long cable runs between 90 to 150 meter cables run is the accumulated actual cable length, connected to VFD output terminals feeding single or multiple motors. This choke can be used to reduce motor noise and will help to reduce voltage resonance at the motor, when it occur.
Output choke help to reduce this voltage peak, and increase the acceleration time, to reduce the stress applied to the motor insulation and prevent damages.
Output choke can be used to protect variable frequency drives and motors. Ac output choke install on the output side of the VFD, the motor can be reduced noise and vibration when the inverter and motor cable length is longer.
Insulation is one of the most important for VFD and earthing (PE) is considered to be the most important when the insulation level of the cable is low.
- The inverter has very large range of torque, speed and power control.
- Their working is independent of the environmental condition.
- The drives are free from pollution.
- The electric drives operate on all the quadrants of speed torque mode.
- The drive can easily be start and stop and it does not require any controls.
- The efficiency of the drives is high because losses occur on it is very less.
- https://library.e.abb.com/public/b5bd3587579a47949b326959681491d2/EN_ACS880-11_hw_E_A5.pdf
- https://library.e.abb.com/public/bcdbf7774d700195c1257a0f003c16cf/ACS850-04_AtoD_quickguide_H.pdf
- https://cache.industry.siemens.com/dl/files/000/99673000/att_51983/v1/GH6_0414_eng_en-US.pdf
- https://en.wikipedia.org/wiki/Variable-frequency_drive
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