Variable frequency drive; Controlling AC motors

Motor’s speed & power rating been chosen according to the requirement. Speed is normally denoted with the general term ‘RPM(Rotation per minute)’ .

* A name plate of 15KW 3 phase induction motor is attached below which shows the rated speed of 2935 RPM at 50Hz power supply.

Name plate of 15KW; 3 phase Induction motor; Rated Speed 2800 rpm — Name plate of 15KW-3 phase Induction motor; Rated Speed 2800 rpm

Some functions or systems needs variable speed; different from the rated speed of the motor. This thing can be achieved in two ways; ‘Mechanically‘ or ‘Electrically‘. Mechanically by changing the gear system & Electrically By changing voltage or frequency or Both.

In Electrical, There are two types of power; AC & DC. AC as well as DC, Both the supplies have different mechanism of speed control. In DC power, By varying the voltage only; speed can be adjusted while In AC power, Speed is voltage & frequency dependent.

2.2KW 3phase 400VAC Automation VFD by Danfoss

To control the speed in AC motor, An electronic device been utilized which take rated supply as an input While produces a variable voltage-frequency supply as an output. That device is known as Variable frequency Drive. In this post, we will discuss VFD’s & its working mechanism.

VFD (Variable frequency Drive)

A VFD is a type of device; equipped with power electronics. VFD takes normal rated power supply as an input & produces an output with variable supply having variable voltage & frequency As per according to feed parameters in its microcontroller through a HMI .

By changing the voltage & frequency input to the motor; It changes the speed of the Electrical motor.

* An image attached below shows the different section of VFD .

A red rectangle doodle shows the input terminals.
A green rectangle doodle shows the output terminals.
A orange rectangle doodle shows the control terminals.
A blue rectangle doodle shows CAT-6 jack for HMI

HOW DOES THE VFD WORKS ?

Induction motor works on the principle of faraday law of electromagnetic induction. When a power supply of rated voltage & frequency is get applied across the stator’s winding. A rotating electromagnetic field is get created. Which further links across rotor.

Due to Induction, Current is get induced in rotor’s screw bars. In return, Rotor also produces a rotating magnetic field which interacts with the magnetic field of stator’s magnetic field. At last; In result; rotor starts rotating & starts following the magnetic field of stator.

The magnetic field of stator rotates at a speed; Known as “Synchronous speed”. While speed at which rotor rotates in known as “rotor speed”.

Synchronous Speed formula is, Ns=120F/P

Where f is frequency

P is the pole of motor winding

// When P is 4 & f is 50Hz

Than Ns becomes= 1500 RPM

// When P is 4 & f is 60Hz

Than Ns becomes= 1800 RPM

// When P is 2 & f is 50Hz

Than Ns becomes; 3000RPM

// When P is 2 & f is 60Hz

Than NS becomes; 3600 RPM

Clearly seen in the formula above, By changing the winding poles & frequency ; Speed can get adjusted. In fixed winding electrical motor, It is quite difficult to change the poles of the winding. Now Only option that left is; frequency .

VFD display shows the frequency 17.85Hz 4KW 3phase 400VAC VFD by invt

Before further understanding of VFD working mechanism, We should know about the frequency first !

FREQUNECY

The Electrical power supply from power generation utility; is a type of AC supply. AC ; The alternating current; continuously moving with respect to time & it also changes its direction in alternate manner at fixed intervals.

Shown in an GIF below:

A Gif shows the waveform of alternating current

A GIF shown above contains one positive & one negative half. Both the halves constitute One cycle. Frequency represent number of cycles with respect to time. In short number of cycles per second. frequency is denoted with the units Hertz. In USA, frequency standard is 60Hz; Which means power supply perform 60cycles in 1 Second.

WORKING MEHCANISM OF VFD

As discussed, by changing the frequency , speed of induction motor can get adjusted. A VFD is the mixture of power electronics & programmable electronics.

AC drive or VFD is mainly consists of 3 sections; that is ‘Rectifier circuit’, ‘intermediate circuit’ & ‘inverter circuit’. Standard Input power is connected to a Rectifier first which changes the AC power into DC supply. Then Rectified DC supply passes through intermediate circuit Which is a type of filter circuit or a tank circuit which feeds the power to inverter circuit.

Inverter circuit contains IGBT’s which convers DC supply into AC supply.

Microcontroller in VFD triggers the IGBT’s according to the feed parameters to achieve desire frequency . Which results variable AC supply get converted into a regulated & controlled AC supply with variable voltage & frequency .

SCHEMACTIC DIAGRAM SHOWS THE WORKING OF VFD

COMMUNICATION INTERFERENCE WITH VFD

HMI-Human Machine Interference is a type of Digital display. HMI is equipped with multiple button which are used to feed the parameters in VFD. This HMI also displays the alarms & error signals.

VFD PARAMETERS

VFDs are designed according to the rating of induction motor. Each & every motor is unique with unique characteristics like voltage, Current, Frequency, Power rating & torque. An Electrical motor; which is going to be connected; is unknown to VFD . That’s why it is important to feed the motor data in VFD.

VFD is also unknown to the values like what will be the acceleration & deacceleration time? What are the types of start & stop signal ? On which method; Speed is going to be varied ? What is the speed range ?

All these questions of VFDs been updated through PARAMETERS. VFD in default contains the factory parameters.

List of parameters for VLT micro drive FC-51 By Danfoss

SPECIAL FUNCTIONS OF VFD

VFD’s not only changes the speed of the motor but it also protect the motor is every possible manner. It continuously monitors the winding resistance, the input voltage, Line current across the motor terminals. When any of the parameter goes out of the range; It produces the fault or error signal.

Advance automation VFD drives also monitors motor’s winding temperature through thermistor .

Fault or error code; Every VFD manufacturers has its own fault code or number code which can easily be traced out through the manual of the VFD.

HMI shows alarm of under voltage | 4KW-400VAC 3phae VFD by invt |

Fault/Alarm Code list for ACS drive by ABB

VFD INPUT METHODS

VFD takes some of the input signals for some of the functions like start, stop, reverse & speed regulation. Different VFD have different input methods, It all depend upon the requirement of the system.

Start, stop & reverse – input methods

By switching VFD’s own control signal; Through defined input port.
By producing a input signal to VFD from another external power source.
Through communication port.

*An image attached below shows the control terminals in Factory setting for VLT micro drive FC-51 by Danfoss. In this, VFD’s own +24VDC used to control the Start, Stop & Reverse Functions. While its +10VDC is used in speed regulation through 1Kohm potentiometer.

Control Terminals with Factory Setting for VLT micro drive FC-51 By Danfoss — Control Terminals in Factory Setting for VLT micro drive FC-51 By Danfoss

Speed regulation methods

Through its own Potentiometer
From external potentiometer
Through communication port
Through digital input
Through analog input

Danfoss Automation VFD's working parallelly through communication cable — Danfoss Automation VFD’s working parallelly through communication cable

SUMMARY

This post discusses the selection of motor speed and power ratings, emphasizing the rated speed typically denoted in RPM (Rotation per minute). It describes the need for variable speed functionality that can be achieved mechanically or electrically.

Specifically this post introduces the Variable Frequency Drive (VFD), an electronic device that adjusts motor speed by varying voltage and frequency. The VFD operates based on Faraday’s law and comprises rectifier, intermediate, and inverter circuits for power conversion. Additionally, it outlines the importance of parameter settings within VFDs for optimal motor performance and safety monitoring against faults or errors.

Hope, You like the post. If you have any doubt, Suggestion or Query please do comments !

Tagged Devices, VFD

Variable frequency drive; Controlling AC motors

VFD (Variable frequency Drive)