1. What is a variable frequency drive (VFD)?
A variable frequency drive (VFD) is often referred to as an inverter, is a motor controller used to control the speed of an AC motor by varying the power-supplied frequency to the induction motor. It functions in the AC-DC-AC pattern. VFD is also known as AC drive, AFD (adjustable frequency drive), ASD (adjustable speed drive), VSD (variable speed drive), variable frequency inverter (VFI), micro drive.
- The first section of VFD (variable voltage control) not only converts the AC supply voltage into DC but also controls the required pulsating DC output voltage to maintain V/f constant ratio by using SCR/IGBT rectifier.
- The second section of VFD (DC-Link/DC Bus) consists of inductance and capacitors to convert pulsating DC into pure direct current. Here, inductance smooths the current, and the capacitor smooths the voltage.
- The third section of VFD (referred to as variable frequency control) not only converts DC voltage back into AC voltage but also controls the variable frequency to maintain the V/f ratio by using electronic devices such as SCR/IGBT transistors.
2. How to select the correct VFD for induction motor application?
Never easy to find a suitable VFD for your applications if you do not know how VFD functions and what is the requirement. Following are the basic steps to choose the correct VFD from Peaco support for your need:
1) Check the power line supply voltage and phase.
2) Check the adoptable induction motor voltage, phase, current, and frequency.
3) If both the input and output phases are three phases, then ensure the adoptable motor-rated voltage should be in the range of input power voltage +- 20%.
4) Ensure the rated current on the motor label is equal to or less than the VFD adoptable current.
5) Choose the suitable control method for your applications.
6) The final step is to proceed with the order.
7) Please contact the sales department with the motor label and power supply details to make you choose the suitable VFD for your applications.
3. Does VFD provide any savings in kWh consumption?
In industry, 80~90% of motors are induction motors. Therefore, we can save lots of power consumption by reducing the motor speed with the help of a VFD. Let us understand it with the help of the following technical formula and details:
Power = Torque*angular speed
Hence, in the VFD, torque supplied to the motor is constant within the range of operating frequency 0Hz~60Hz to the induction motor, and angular speed will decrease with decreasing the frequency and vice versa.
Now, torque is directly proportional to the total current that will remain constant in the case of VFD up to 60Hz frequency. For example, you have a 20 kW induction motor with a rated current of 30A and a rated frequency of 60Hz. Now, it supposes to be operated by the V/f control method by setting a frequency of 30Hz. Hence, the induction motor will consume nearly 50% of the rated power and rotate at half the rated speed.
4. What are the applications of variable frequency drive?
Peaco Support supplies a wide selection of VFDs, which mostly meet all requirements of customers. They are suitable to operate the three-phase motors by delivering 220V, 380V, 415V, 440V, 460V, 480V, or 690V voltage.
Peaco Support VFDs are suitable to use with induction motors and are widely used for pumps, compressors, elevators, hoists, lifts, off-grid solar pumps, etc.
5. What are the environmental conditions for VFD?
To enhance the life and functionality of the AC drive, it is a good suggestion that users consider the following environmental conditions.
- Cooling method-forced air.
- Keep away the ac converter from corrosive gasses, liquid, moisture, and dust.
- Installation location altitude below 1000m (In the case above 1000m, VFD is to be de-rated. Please contact for technical support).
- Storage-temperate -20℃ to +60℃.
- Ambient temperature -10℃ to +40℃ (non-condensing and not frozen).
- Ambient humidity below 90% RH.
- Vibration under 20Hz 9.8m/s(1G), over 20Hz 5.88m/s (0.6G).
6. Different types of variable frequency drive control methods.
The control method determines how VFDs control motors. Usually, there are four types of motor control methods for induction motors powered by using VFD:
- V/f (volts per hertz) or V/hz control.
- V/f with encoder or closed loop V/f (PG card).
- Open loop vector control.
- Closed loop vector control (PG card).
7. Difference between V/f control and vector control of VFD.
Generally speaking, a standard VFD has a V/f control mechanism. Such an AC drive is used widely due to its simplicity. It maintains a V/Hz ratio to the motor all the time. For example, a three-phase 380V AC induction motor with a frequency of 60Hz and a V/f value 6.33 (380/60) is to make run by setting a frequency of 40 Hz then the induction motor will draw only 253 V from the VFD output to maintain the same 6.33 (253/40) V/f ratio.
A vector control AC drive is applicable for empowering the more dynamic motor control. It independently controls motor speed and torque, similarly DC motor control mechanism. A Vector control VFD can generate more starting torque than a V/f control VFD. This type of VFD control method delivers high performance and is expensive too.
With encoder feedback, a PG card is introduced separately into V/f control and Vector control methods VFD to operate precisely.
8. What are the causes of motor heat and noise?
The delivered output voltage of the VFD is PWM, not pure sine wave contains a certain amount of harmonics so that the noise, temperature, and motor vibration will be more than those operated with direct grid power frequency 50/60 Hz.
9. What is PG Card?
PG abbreviates from the pulse generator. Our PG card is an optical encoder to be installed at the AC drive side for the closed-loop vector control drive method. PG card is fitted in VFD to get the feedback from an encoder for high-accuracy speed control or torque control applications such as high-speed paper-making-machine, cranes, elevators, etc. One AC drive can operate only one motor.
An encoder generates the motor feedback signals for the VFD and the matching PG card receives those signals to communicate with VFD.
10. Can I run the multiple motors with a single VFD?
A V/f control method VFD can run several motors at a time. In such circumstances, Motors with VFD will start and stop simultaneously following the same speed reference. The following factors are supposed to consider for selecting the suitable VFD for your motors.
VFD capacity is to be sized more or equal to the sum of the capacity of all connected motors.
The sum of the distance between the VFD and all connected motors should be less than 500m. Introduce an output load reactor to avoid motor failure due to voltage spikes, current surges, or harmonics.
Install circuit breaker MCB or with extra protection MPCB between each motor and VFD.
The voltage of each motor should be the same and ensure the VFD adoptable current is higher than the sum of all rated currents.
Connect all motors with VFD output in parallel.
11. Does VFD output pure sine wave or PWM wave?
The output of VFD is a PWM wave, not a pure sine wave. Therefore, to enhance the service life of the induction motor. You may introduce a suitable harmonic filter externally at the output of VFD as a load reactor, filter, and sine wave filter. In case, the line supply to the VFD is not a pure sine wave and consists of harmonics that are not good for sensitive components of VFD. We recommend using a suitable line reactor to enhance the life of VFD.
12. What is the minimum frequency a VFD can operate?
Different countries have different off-grid frequency supplies. Countries like India, China, Dubai, and England use 50Hz frequency. Many other countries like the USA, Canada, Brazil, and Mexico use 60Hz frequency. Our Variable frequency drive can be easily operated with the line input frequency 47~63 Hz and marked with 50/60Hz rated frequency at VFD label.
13. Does capacitor improve power factor?
The capacitor in the DC link/DC bus of VFD not only smoothes the voltage but also supplies the reactive current to the motor whenever requires resulting in the improvement of the motor power factor. Without variable frequency drive, the motor draws the reactive current directly from the power source.