[SOLVED] what is Scalar (V/f) Control of 3-Phase Induction Motors

[abc_de]

Hello
i am trying to implement 3 phase ac motor control but i am not getting what what is Scalar (V/f) Control of 3-Phase Induction Motors.
what is advantage and how to implement.
:bang:

 

[mobinmk]

mainly two types of control

one is scalar control and other one is vector control

see the basic formula

N = 120f/p

N in rpm
F >> supply frequency
p >> no of poles.

u can vary the speed of induction motor by varying frequency.

but

in order to work with constant torque application.

T $ phi * I

Phi >> flux

Phi = E/F (derived from trnsfmr eqn E= 4.414*k*phi*f)

also motor should not operate in saturation of rotor flux..

if u vary the frequency only and voltage kept constant, Phi will increase>> fed to saturation region, magnetic losses..

 

[abc_de]

thanks for reply
it means it will heat the motor or may cause of loss of efficiency ?
then what about relation of scalar with torque of motor ?

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http://www.pumpsandsystems.com/topics/pumps/motor-horsepower-torque-versus-vfd-frequency

i have read this page i confuse me more

 

[mobinmk]

thanks for reply
it means it will heat the motor or may cause of loss of efficiency ?
then what about relation of scalar with torque of motor ?

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http://www.pumpsandsystems.com/topics/pumps/motor-horsepower-torque-versus-vfd-frequency

i have read this page i confuse me more

yes, you should operate the motor beyond saturation region.

if it work in saturation , magnetic losses, etc

torque is directly prportional to flux and current.

if u kept flux constant by E/F.

motor operate in constant torque..

 

[abc_de]

Thank u
Do you have any code example in which v/f principle has been used.
I just want to learn how do I can implement v/f ��

 

[chuckey]

I presume that you are generating your AC via PWM. So at the nominal frequency (so its nominal voltage output. You have a pulse of Z % on at the carrier frequency at some point in the AC output cycle. So if you increase the frequency by , say 10%, the PWM ON time decreases by 10% (as does the OFF time). To get the voltage to increase by 10% (to preserve the V/F), the ON time must be increased by 10%, this gets you back to the same ON time. So to change the frequency the OFF time must be changed.
Hope this makes sense.
Frank

 

[FlapJack]

So if you increase the frequency by , say 10%, the PWM ON time decreases by 10% (as does the OFF time). To get the voltage to increase by 10% (to preserve the V/F), the ON time must be increased by 10%, this gets you back to the same ON time. So to change the frequency the OFF time must be changed.
Hope this makes sense.
Frank

In actual practice wouldn't you just set the maximum current and pwm the voltage to keep a constant current.

 

[FvM]

In actual practice wouldn't you just set the maximum current and pwm the voltage to keep a constant current.

A V/f operated inverter is voltage controlled, the current varies according to the load. Current limiting may be implemented as an auxilary function.

It should be mentioned however that only few recent VFDs implement pure V/f control.

 

[abc_de]

Till I am not getting the concept. I have to study over it for couple of day's.
Thank to all for support

 

[schmitt trigger]

To prevent saturation, all magnetic steels must maintain a maximum volt-time product. Meaning that if you increase the voltage, the time must be decreased to maintain the constant relationship.

Since frequency is the reciprocal of time, then in the example above and decrease of time means an increase of frequency.
The opposite is also true

In plain english, if you increase/decrease the frequency (to vary the motor's speed) the voltage must increase/decrease accordingly.

It is a little more complicated than that, but that is the basic function.

 

[FlapJack]

abc_de, simple explanation.

220vac 50hz motor goes 1500 rpm.

To double the rpm to 3000 you double the frequency and double the voltage, 440vac 100hz.

To reduce the motor speed to half (750 rpm) you half the voltage and frequency, 110vac 25hz.

Torque and HP go lower as you decrease rpm. and increases with rpm.

 

[FvM]

Torque and HP go lower as you decrease rpm. and increases with rpm.

HP (power) yes, torque is roughly constant (proportional to motor current, which is also kept constant in linear V/f operation).

 

[abc_de]

Thank u ��
Lets assume single phase bridge rectifier gives roughly 340v dc from 230v ac
Now 340 volt DC Is dc bus voltage after apply full dc bus voltage it gives 238volts AC nearly.
After apply a full dc bus voltage if we increase frequency voltage remain same then torque may decrease right because magnetic flux of motor remain same. I am right????

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Sorry for my silly question
I am getting it the main thing is relation of v/f helps to keep constant.
Someone have an example code for scalar control implementation???
Thanks

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One more question what about load current if we keep v/f relation

220vac 50hz motor goes 1500 rpm.

To double the rpm to 3000 you double the frequency and double the voltage, 440vac 100hz.

To reduce the motor speed to half (750 rpm) you half the voltage and frequency, 110vac 25hz

 

[FvM]

The maximum phase-to-phase output voltage of a single phase 230 VAC supplied inverter is slightly below 230V (at least for sine output). You'll usually connect motors rated for 230V 50 Hz. This means, you can drive the motor with rated power at 50 Hz and with full torque at frequencies between 0 and 50 Hz. Operation above 50 Hz is possible in "field weakening" mode with reduced torque. The usable speed range depends on the load, 60 or 70 Hz is often feasible.

 

[abc_de]

Ok
Let us assume 230vac ,50hz motor is drives, as you told it gives good torque from 0to 50 hz, it means at 50hz inverter gives 230 vac to keep V/F constant
My requirement is motor should be work from 0-90 Hz with good torque
But if we change base frequency of motor by inverter up to 90hz and inverter gives 230vac at 90hz then torque of motor remain constant.
I am right if yes then my requirement will full fill ��

 

[FvM]

But if we change base frequency of motor by inverter up to 90hz and inverter gives 230vac at 90hz then torque of motor remain constant.

Nope. You either need a motor rated for 230VAC 90 Hz (surely not available from stock) or a motor rated for 127 VAC (phase-to-phase) 50 Hz. Or an inverter supplied by 230/400V.

In addition, consider that there are most likely increased motor losses at 1.8-fold power which may prohibit this operation if intended continuously.

 

[abc_de]

Ok so if we have motor rated 230vac,50hz RPM : 2800 and we want more RPM like 3500 then according to you torque will reduce. Then how do we maintain the torque constant of motor ???
How its possible ??

 

[FvM]

Then how do we maintain the torque constant of motor ???

If you want full torque above rated motor speed, you need to raise the inverter output voltage respectively above the rated motor voltage.

There will be also a power limit set by motor losses, and a meachanical speed limit, particularly for larger motors.

 

[abc_de]

Ok
I have seen many 3 phase vfd which can operate the motor from 0-400hz. Motor rating 230vac,50hz then definitely the torque of motor will reduce if we operate motor more then rated voltage and frequency.
I am right???

 

[FvM]

I have seen many 3 phase vfd which can operate the motor from 0-400hz.

The high speed range of recent VFD inverters is an option that can be only used with respectively designed motors. It doesn't say anything about the achievable speed with standard motors.