What is Essential Function of VFD While Starting a High Kw Motor

7/27/2019 What is Essential Function of VFD While Starting a High Kw Motor

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What is essential function of VFD while starting a high kw motor?

Does VFD reduces starting kick of the motor ? OR does it reduces current requirement of the

motor? I searched on websites, but seems to be confusing. Please share your expertise.

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EnriqueUnfollow Follow Enrique

Enrique Gallegos Try to explain in a few lines is a dream and im sure can be more confuse.

I would recommend you to check " Practical Variable speed drives and power electronics, by

Malcom Barnes" Elsevier editorial.

Is a very basic literature to start the study of VFDs, complete and clear.

I guess you can buy in amazon, etc.

Internet is just a reference to start the investigation but in my experience is not good idea to

believe it all.

good luck
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OmerUnfollow Follow Omer

Omer A.Osman In brief, yes , VFD capable to smoothly starting AC Drives (ramp from 0 up to

100% of the load) as well as the energy saving will be achieved, accordingly. Also may I

confirmed that VFD also acting as AC Motor starting means among (Star/Delat, Soft Starter,Auto Transformer, Electrolite, series resistance - wound rotor- etc,). The starting factor of VFD

is usually 1 up to 1.2 with respect to the rated load current while for Direct On line about 5-6.

Moreover and as you know the VFD can control the speed of the AC Drives in accordance to the

formula N=120f/P rpmwhere f = the supply frequencey and P = number of the Poles.

According to this formula , Motor Speed can be changed either by changing/control the

frequency or by changing the number of Poles of the Motor by which step changed in the RPM

will be given, while the former gives continous variable Speed as per application demand.

However, as per newly developed power Semi Conductor IGCT based on PWM VFD becamethe most smart , effective and efficient control device in Industries since is associated also with

protective and monitoring means.



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Christian ChineduUnfollow Follow Christian Chinedu
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3/20

Christian Chinedu Ohiaeri PMP R.ENGR,MIEAust,MNSE,MNIM,MBA Sunil,it better to find

more details through research as Enrique advised.From my experience, I know that variable

frequency drive plays around with the frequency which the motor operates. It starts at low speedand varies the frequency to attain maximum speed. This reduces the high starting torque usually

experienced when motors are started on DOL,Star/Delta etc. When you are driving delicate

materials through your conveyors or pumping liquid through pipes etc,VFD plays a useful role. Itreduces hammering in pipes usually experienced when using DOL. In large hotel application,VFD could be used with pressure switches to regulate water flow and reduce hammering when

guests are showing. The volume of water required will determine the speed at which the motor

runs through VFD control.However, very large KW motors at high voltage level are usuallystarted DOL due to the cost of VFD but that is when one has enough (power ) capacity otherwise

it will impact on other users in the network.You may wish to investigate more.


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Ing. OleUnfollow Follow Ing. Ole

Ing. Ole Knudsen A VFD can also serve to increase the starting torque of an induction motor,which can be an advantage in some applications, while still enabling a smooth acceleration, toavoid water hammer, fan and pump stall/cavitation, etc.



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4/20

Enrique Gallegos even when in theory VFD is able to reduce the motor starting current at very

low values is a fact that motor has limitations. I have seen many motors to stall because someone

limited the starting current to a very low value. That low value is an argument to sell theequipment. Limiting the starting current depends on the type of motor and load operation curve

reflected at motor shaft, for example a pumping systems has a different curve than a conveyor

and each application needs different starting torque and current.

that is why i recommend that book.

Regards



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DavidUnfollow Follow David

David Ruyle As Omer said the inrush/Locked rotor amps on a motor started across the line willtypically be 6 times the running load amps vs 1 to 1.2 with a vfd.This "surge" of current will

cause a voltage drop in the substation transformer, the primary voltage power lines, the stepdowntransformer etc.. All the customers on the same electrical circuit will see this. When a customerrequests that a large motor be started we simulate the situation in our modeling tool and calculate

the "flicker" and make sure it meets the IEEE specification for flicker to other customers. Even if

the flicker on the utility system is acceptable it may be too great for other loads inside the plantand the motors will drop out on load voltage. The regulators inside the utility substation typicall

take one to two minutes to step up to regulate the voltage.



1
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5/20

SIVAKUMARUnfollow Follow SIVAKUMAR

SIVAKUMAR K Yes! All VFDs have soft starting as an inherent feature & thus they willdefinitely reduce the starting kick of motors.

And, VFDs will reduce current requirement of motors provided the load is less. But, that couldhappen without VFDs even. What VFDs do is energy conservation, if the process flow

requirements are variable.



0

DineshUnfollow Follow Dinesh

Dinesh Segaran Hi Sunil,

Let me try and give a brief overview of the entire problem before explaining the operation of theVFD.

At startup, a motor's backEMF is (near) zero, so a Direct-On-Line start causes a large current toflow (up to 6x rated). As current translates into torque, this also gives high starting torque.

However, in cases where such a high starting current is undesirable (such as high powered

motors), the current can be reduced in two basic ways. The first is to increase the impedance,achieved using resistors in series with the motor during startup, switching them out during

normal operation to maintain efficiency. The second is to reduce the applied voltage to the

motor, i.e. star/delta startup.

An alternate approach is to actively regulate the current into the motor, which is what a motor

drive does. It uses controlled switches (thyristors, IGBTs, etc.) to manage the flow of current into

the motor, which gives it the ability to limit startup current. However, as torque is still directlyrelated to current, the starting torque must also be affected.

I hope this clears up the problem a little for you, all the best!

Thanks,

DINESH
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6/20



6

GaryUnfollow Follow Gary

Gary Fox While you can use a VFD to soft start a motor, it's expensive compared to a solid

state starter if you intend to run the load a full speed all the time and there is no need for flow or

speed control. A solid state starter achieves the same current limit and reduced water hammereffects by controlling just the voltage to the motor, typically using thyristors. Many soft starters

have a contactor that bypasses the thyristor bridge once the motor gets to full speed.



3


Ing. Ole Knudsen Gary, one essential difference between a soft starter and a VFD in this regard

is, that the VFD delivers "nearly" sinusoidal voltages (and currents) to the motor, which makes it

possible to develop high starting torques during the acceleration, even higher than nominal full

load torque, depending on the application, while a soft starter only supplies fractions of the basic

waveform, which serves to reduce the current to the motor significantly, but still at the nominalfrequency. This will reduce the available starting torque dramatically until the motor is up to

around two-thirds of nominal speed, or maybe even higher.

There is very advanced version of this which is called a cycloconverter, but that is even moreexpensive than a VFD.

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7/20


0

AHSANUnfollow Follow AHSAN

AHSAN KHAN Definitely by changing frequency as per formula N = 120f/P the load of themotor can be adjusted.So as per demand kW is being consumed saving energy.KW= VI cosQ.

VFD always changes input current of the motor.Though the VFD is expensive but it saves a lot.

Eventually the owner runs the motor in cost saving way.



0

adnanUnfollow Follow adnan

adnan ali what ever the kilowatt is,, motor will run smooth like b/c we control torq through ourhand.



0
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8/20

HalimUnfollow Follow Halim

Halim Bensmaia In order to understand better the starting requirements using a VFD and theselection of an ASD for your motors, a number of motor starting simulation studies (ETAP or

other software) can be conducted for the load types and ASD control mechanism of the motor, in

order to determine the ASD size, control method and settings. These studies can verify that thedrive and motor system meet the operational requirements. There are usually a set of speed-

torques curves for each application. During the acceleration, frequency and voltage are generally

controlled such that full voltage is reached at the design rated RPM of the motor following aramp. Motor voltage and current are regulated to provide speed and torque control to the load.

The voltage to frequency ratio is held roughly proportional. In practice torque developed at very

low frequencies falls off and extra voltage above the value of the volts / HZ ratio (or other ctl ) is

automatically applied to regain the lost torque.The VFD modify the speed but also the inputvoltage in direct proportion . The issues that need to be considered include:

Type of load the motor is driving: Does it require a high starting torque?

The time required for the motor to reach speed

The best control method of an ASD for the load: Is voltage boost required in order to obtain therequired starting torque? What is the minimum operating frequency of the ASD? Will it cause

high starting current?Also, we must not get confused and think very simply that soft-start and VFD have the same role. VFD are more expensive and soft start are not able to regulate frequency !. There are many

other considerations before deciding if a VFD or soft -start is needed for your load.


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HalimUnfollow Follow Halim

Halim Bensmaia Another formula is the rated torque of the motor which is the design value on

the motor nameplate that is developed with rated voltage and rated frequency at the rated slipRPM below the synchronous RPM. In English units, this rated torque can be calculated as:

Rated Torque in lb-ft = Rated HP x 5252 / Rated RPM

With VFD control, the motor can deliver constant torque from zero speed to rated speed.

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9/20


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adnanUnfollow Follow adnan

adnan ali VFD Saves INPUT Power from I/P breaker of VFD and Supplies the routinedVoltage and Current to Motor, Actually It(VFD) is the Device which SAVES

SYSTEM Power.



0

SunilUnfollow Follow Sunil

Sunil Wadekar Dear All,

Thanks for intelligent inputs and views. The practical problem is to start 1500kW 11KV motor.

What is the solution? Please share views. Regards.



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10/20

OMERUnfollow Follow OMER

OMER HAFEEZ ITS simple man, vfd basically reduces the inrush phenomena by delaying in

providing full freq .. you can see on vfd that it will take 5secs to reach max freq. in this way

current is reduced



1

OMERUnfollow Follow OMER

OMER HAFEEZ initial kick is reduced by delaying freq reach its max value, 'you can checkthe parameter of acceleration and deceleration and can check by using am meter to load the

current can more be reduced



0


Enrique Gallegos Sunil are u electrical engineer?
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11/20

At that voltage level and for that motor size you do not need a VFD to reduce the starting current

is very easy to start it directly on line unless your system have no capacity.

Is the VFD necessary for you process?

A 25000 hp motor needs a VFD to start but a 1500 kw motor is a joke.

May be u need check this with your electrical plant engineer.

regards



0

OmerUnfollow Follow Omer

Omer A.Osman Mr.Sunil, in practice the suitable means that can be used for Starting MV

Electrical Drives (3.3 kV -11kV) either Direct on line or Auto transformers or adequate softstarter. Regardless of the adopted means, vacuum / oil / SF6 contractors are recommended to be

used along with starters for switching purposes - taking into consideration that the main issuehere is not the current only but the dielectric strength and torque.



0

true 39 39 groupItem?seeM

Tony CutnerUnfollow Follow Tony Cutner
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12/20

Tony Cutner BSc CEng MIET There is plenty of electrical discussion here I'm guessing at a

concern of yours. You mention "kick", now you can use a S-shaped ramp curve, instead of linear

to reduce the rate of acceleration on large inertia loads to reduce mechanical "kick", whichreflects as a nice current ramp up on an electrical system.

A lot of my mechanical colleagues don't expect electrical engineers to tell them that, so go and

shock them with your cross discipline knowledge. It certainly reduces the stress on bearings etc.and reduces the maintenance issues.In fact I don't understand why linear ramps are used at all, maybe some one can tell me.



0

PhilUnfollow Follow Phil

Phil Mullins Simple question is your problem the voltage drop from a locked rotor start of your

motor?



0


Ing. Ole Knudsen Sunil,There can be a lot of reasons for using a VFD for such a motor, but in that voltage / power range,

there are LOTS of other starting methods that are nearly as good, and a lot cheaper.

Just tell us the characteristics of your supply system, and what your concerns are during start,and I am sure quite a number of electrical engineers will be able to advise which starting mode

will be cheaper, easier to run, and easier to maintain, than a VFD. I have done a few myself.
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Documents

What is Essential Function of VFD While Starting a High Kw Motor