vvvf control

[electrics]

hello, i have a question. Now we use vvvf control at the lifts. This means it has a speed 1 m/sn and it is constant(ı am not sure of this) isnt it? according to the load = torque the controller will vary the voltage and frequency so that for that specific load the mechanical speed will be constant is it true?
or is it just it has a constant electrical frequency for that speed (from the curve of this motor) and as the load changes the speed will change slightly??
which one i cant figure out? it has an always constant speed or slightly constant speed?

 

[winnie]

You are probably going to need to rewrite your question, because it is tremendously unclear.

'at the lifts"...what sort of lifts? Elevators? Sewage pumps?
'it has a speed'....which it? the lift, the vvvf, the motor?
etc. It is simply hard to understand what you are asking.

Here is my stab at an answer:

VVVF simply stands for 'variable voltage variable frequency', and describes a device commonly used to control an AC motor by adjusting both the voltage and frequency of the AC power supplied to that motor. The term VVVF can describe a whole range of different types of device, with vastly different capabilities.

The simplest sort of VVVF controller is an 'open loop' controller. This means that it makes no measurement of the speed of the motor, and has no feedback. The VVVF controller is simply set to a designed frequency and voltage, and hopefully the motor performs in the desired fashion. The speed versus load characteristics of a motor supplied by an open loop VVVF controller will depend upon the specific characteristics of the motor; if you supply an induction motor then the speed will vary slightly with load until the load is so great that the motor stalls.

More complex VVVF controllers have various different types of feedback loop; you might hear the terms 'sensorless vector' control, 'encoder feedback', or even 'servo control'. With suitable feedback, the controller can adjust the supply voltage and frequency so as to maintain a desired constant speed, a desired constant torque, or even a specific speed versus load curve.

The better the VVVF controller's feedback component, the more the speed versus load characteristic is set by the controller.

Hope that helps.
Jon

 

[electrics]

yeah it seems quite reasonable, what i mean is just human lift, and vvvf is said to be used with these lifts but what i wonder is this:
we control the speed holding v/f ratio is it true? since air-gap flux = v/f
it means we cant change this true? if we change the load of a constant speed lift so will air-gap flux change? i think no it wont.
So this is my second and more important question.
how can a driver know the characteristics of a motor which it drives?
İ mean all the motors have characteristically the same values or for a specific motor some infos are loaded to or defined bt the driver? or is it just trying and failing method using closedloop technique..??
best regards..

 

[electrics]

here feedback means a lot, is it necessary that somehow driver has the parameters of the motor or is it not necessary so that the driver will change the v and f using current vector space analysis?

 

[Besoeker]

yeah it seems quite reasonable, what i mean is just human lift, and vvvf is said to be used with these lifts but what i wonder is this:
we control the speed holding v/f ratio is it true? since air-gap flux = v/f
it means we cant change this true? if we change the load of a constant speed lift so will air-gap flux change? i think no it wont.
So this is my second and more important question.
how can a driver know the characteristics of a motor which it drives?
İ mean all the motors have characteristically the same values or for a specific motor some infos are loaded to or defined bt the driver? or is it just trying and failing method using closedloop technique..??
best regards..

The change in speed from off-load to full is normally not much - typically less than 2%. On a lift motor it may be more by design - better torque curve for the application.
Given that you need precise position control for a lift your motors are most likely fitted with a shaft mounted encoder and the drives have specific application programming enabled.
I have no experience with lifts. We make motors and drives for machine tool applications. Accurate position control is mandatory. Typically, this is 10,000 rpm to zero in half a second and to within +/- 0.5 deg positional accuracy. And comparable acceleration. We use resolvers for feedback.
But lift users might find this a little harsh.

 

[electrics]

can we say that with vector control the parameters of the motor are needed, what i want to know is this,is air gap flux changing with vector control? in v/f it is always constant true? these are urgent questions for me best regards...

 

[electrics]

i think it is very difficult to know these answers anyway i hope i can learn one day in the future...

 

[winnie]

Different motors will have different characteristics, and for really tight control the VVVF system and its feedback loops will need to incorporate a very good model of the motor (lots of characteristics modeled).

But if you don't need the tightest possible control, then you don't really need to spend all that effort on the most accurate model of the motor.

All motors of the same horsepower, voltage, and speed will have somewhat similar characteristics. For simple V/F control, all you really need to know is the motors base voltage and frequency rating. In the ideal case of a motor with no resistance, V/F control will maintain the same flux density at all frequencies. In the real world we have resistance and V/F control does not maintain constant torque.

Slightly more complex V/F with low voltage boost adds one more parameter, the limit voltage at 0 frequency that will push current through the finite resistance of the windings and maintain flux as the speed goes down. This is again only an approximation.

Vector control is a way of thinking about how to develop a motor model, and instead of calculating the correct voltage to apply to get the desired torque, the current flowing in the machine is measured and evaluated in terms of torque producing current (D current) and flux producing current (Q current). You then run feedback loops (all calculated) to maintain the optimal current flow (and thus flux level) for a given operation.

-Jon

 

[electrics]

i think at least so know that the flux is independent of the resistances. Since the net flux is proportional to the v/f as extracted from the basic formula i guess that if v/f is constant flux will be all the same. u say it is not constant excuse me but i cant see why...

 

[winnie]

The magnetic flux in the motor is set by the magnetizing current flowing through the motor.

The magnetizing current is _mostly_ set by the drive frequency, the drive voltage, and the inductance of the motor. For an ideal inductance, it is quite simple to show that the current is proportional to voltage and inversely proportional to frequency. This means that for an ideal inductance (one with zero resistance) the current flow is constant if V/F is constant. If the current flow is constant than the peak flux created by that current will be constant.

But a real motor is made of real copper which has resistance. In a real motor at normal operating frequency, the magnetizing current is mostly limited by the the inductance, and very slightly limited by resistance. But at low frequency (and thus low voltage) the resistance starts to dominate. What this means is that if you simply hold V/F constant, as the frequency gets lower the magnetizing current will get lower, and the flux will decrease.

-Jon

 

[electrics]

oh u are all right, thank you.

 

[Besoeker]

i think at least so know that the flux is independent of the resistances. Since the net flux is proportional to the v/f as extracted from the basic formula i guess that if v/f is constant flux will be all the same. u say it is not constant excuse me but i cant see why...

Winnie is correct in that the flux does not stay the same.
Here is the equivalent circuit often used for motor performance calculations.

The voltage across Xm or, more accurately the voltage time integral, determines the flux. The elements R1 and X1 are the stator resistance and stator leakage reactance respectively. At higher currents (greater loading) the voltage drop across these elements is obviously greater resulting in a lower voltage across Xm, the magnetising reactance and lower flux. For constant V/f, the drop isn't a great deal but enough that it has to be taken into account for more accurate calculations.

FWIW, for the couple of motor models I checked, the X1 component of the voltage drop is significantly larger than the R1 component.