Articles
Beware of the These Myths About 3-Phase Variable Frequency Drive
by Chen Yajun Executive3 phase variable frequency drive is an electronic device which is precisely used to control the speed of AC induction motors (single as well as three phase) without causing any impact to the electric consumption, torque, impedance, magnetic flux, etc.
Variable frequency drives may be industry-standard technology, but that does not mean they are immune to myths and misconceptions. One common misconception is to believe that they severely affect the development of VFD solutions for OEMs looking to implement them. It’s high time to dispel these myths once and for all related to 3 phase variable frequency drive, starting with the most common ones engineers will encounter.
1. All VFDs are created equally
Most VFDs consist of the same components:
- Bridge rectifier
- oft-charging circuit
- Dc bus capacitor bank
- Output inverter section
While the necessary elements are the same, still VFDs are distinct from each other in various factors such as:
- How the inverter section switches
- The reliability of the components
- The effectiveness of the thermal dissipation scheme
Some VFDs also have a three-level-output section that gives authority to the output pulses to switch between half-bus, voltage-level pulses and full-bus level pulses. Three-level output enables for mitigation in voltage amplification in the motor.
At last, some VFDs have an inverter of matrix-style that doesn’t consist of a DC-bus or bridge rectifier. These types of VFDs use bidirectional switches that connect any of the incoming phase voltages to any of the three output phases. This alignment permits power to flow freely from line-to-motor or motor-to-line, enabling for fully regenerative four-quadrant operation.
2. VFDs eradicate power factor issues
VFDs utilise their internal capacitor buses to provide the reactive current that is compulsory for a motor. This reduces the displacement PF and safeguards the AC line from producing the reactive flow. While input displacement PF does boost when a VFD is installed, displacement PF does not thoroughly explain PF calculation.
To calculate PF accurately, it’s obligatory to incorporate the reactive power required by the harmonics created when AC voltage is reformed to DC.
It’s also essential to remember that the current regulated by the diode bridge from the AC line to the DC bus is sporadic. Since diodes only conduct when the voltage on the anode side is higher than that on the cathode side, diodes are only on at the peak of each phase of the positive and negative portions of the sine wave. It causes the input current to become disrupted due to a ripple-like voltage waveform.
3. VFD enables motors to run at any speed
VFDs can alter their output voltage and output frequency, they allow a motor to run faster or slower than their rated operating speeds. It doesn’t mean that VFDs have the potential to run motors at any speed; there are certain limitations.
- Cooling: When totally enclosed fan cooled (TEFC) motors run at very low speeds, less cooling air reaches to them. It is not advisable to run these motors at a full load below 15 Hz.
- Speed: Generally, motors come with a maximum safe operating speed. To exceed this speed even with a VFD is not recommendable.
- Power: Motors that are closer to their maximum operating speeds may run out of torque. We can also say that the VFD will run out of voltage.
Overlook VFD myths and misunderstandings that can wreck your application design whether it is about AC motor speed control or other specification. Work with professionals who can assist you to choose the suitable drive and motor products to make your application a success.
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About Chen Yajun 
Executive
2 connections, 0 recommendations, 19 honor points.Joined APSense since, December 13th, 2017, From China, China.
Created on Jun 22nd 2018 07:55. Viewed 486 times.
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