Vibration when using V/F Control

ushidayo

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Here is a question for the variable speed drive experts:geek:

Why will a drive using proportional Volts/frequency control cause or amplify vibrations in the rotated equipment when the same drive using sensorless vector control does not?

I have witnessed a situation where the rotated equipment was vibrating noisily at certain motor frequencies but when the drive was changed to sensorless vector control, the vibration became unnoticeable.
 
Because the vector control (with sensor or sensorless) is a more advanced type of control. In this way we are controlling the current in a current loop, and according to the motor model calculated using the measured output current. That's why the control is smoother and vibration is less. In fact, this is the torque profile which is more homogeneous, without big ripples.
On the other hand, the V/f control is just following a linear equation between Voltage and Frequency. This results in overshooting and ripples in the torque, causing vibrations.
 
In addition to the comments by Poet, most drives include a frequency range avoidance feature that will allow the user to avoid frequncies that cause harmonic vibrations to occur in a particular machine. The feature may be called "skip frequency" or "Jump Frequency", or somethign to that effect.

Paul
 
Because the vector control (with sensor or sensorless) is a more advanced type of control. In this way we are controlling the current in a current loop, and according to the motor model calculated using the measured output current. That's why the control is smoother and vibration is less. In fact, this is the torque profile which is more homogeneous, without big ripples.
On the other hand, the V/f control is just following a linear equation between Voltage and Frequency. This results in overshooting and ripples in the torque, causing vibrations.

Please can you explain what is causing these torque ripples? If the rotated equipment has a "natural vibration" how does the V/F control amplify the vibrations when the vector control doesn't?
 
In fact, this theory can be treated in books and books...
But in summary, the vector control is able of controlling the instantaneous torque, in transitory mode, by controlling the flux of the motor. However, V/f control which is a scalar control cannot operate in transitory mode.
 
I understand that the required voltage to maintain the motor flux is pretty much constant independent of the frequency. Vector control methods incorporate this base voltage plus a voltage calculated from the load torque to give a final voltage. But I still don't understand how vector control would give negligible vibration compared with V/F control. I'm missing some key information that explains it all. Please be patient with my ignorance, I would really like to understand the "why".
 
Here is a question for the variable speed drive experts:geek:

Why will a drive using proportional Volts/frequency control cause or amplify vibrations in the rotated equipment when the same drive using sensorless vector control does not?
REPLY It is not the drive that is creating the problem it is the machine. Something is out of balance or maybe two and when they get into synch their impact from imbalance adds up or harmonics in the machine which can add up.

I have witnessed a situation where the rotated equipment was vibrating noisily at certain motor frequencies but when the drive was changed to sensorless vector control, the vibration became unnoticeable.

REPLY when the machine gets into the vibration RPM area it may increase torque
this increase in torque will increase slip in the motor.
A normal V/F VFD will not correct for this. This is why they allow for a no operate in this speed range option.

A vector will see the increased slip and compensate to get motor RPM back up to where it should be. I can see two possibilities the vector VFD will have the motor operate right at the vibration RPM OR it will be either slightly less or more than the vibrate RPM.

I would say that if the VFD you have is operating well do not change it. ASSUMING you can operate machine slightly above or less than the "vibrate RPM" IF so put in a command to VFD to skip that RPM range or set top speed to less than "vibrate RPM"

DICK DV you can jump right in here at any time.

Dan Bentler
 
Well, shucks, I can't say I can explain this behavior either. I've never witnessed it in my experience.

One thought tho, if the drive happened to be an ABB ACS600 or 800, the explanation could very well be due to the fact that DTC does not use a carrier frequency. The output pulses tend to be randomly spaced, the spacing determined by the calculated needs of the motor to develop torque in the shaft.

As far as I know, all other drives use a fixed carrier frequency, whether in V/Hz or sensorless vector mode.
 
If I may:

I think the key to explaining the phenomenon scientifically would have to do with the relationship of Voltage to Frequency with the two different signal control methods.

The V/F control has a STABLE voltage at a given frequency, so if torque varies (due to uneven resistance in the way the machine is rotating for example), the motor speed will vary accordingly. This VARIANCE can cause the machine to rotate less smoothly, causing more rotational VARIANCE and so on, actually AMPLIFYING the slight vibration.

With VECTOR control, voltage is adjusted as needed in VERY short order, less than a revolution, less than a cycle. This microsecond by microsecond adjustment of voltage, manifests itself as microsecond by microsecond adjustment of torque and therefore speed compensation.

In short, the vibration is corrected for by very fast variation of the voltage, therefore current, therefore torque, therefore speed. The speed is evened out, and vibration is reduced.

Stationmaster
 
If I may:

I think the key to explaining the phenomenon scientifically would have to do with the relationship of Voltage to Frequency with the two different signal control methods.

The V/F control has a STABLE voltage at a given frequency, so if torque varies (due to uneven resistance in the way the machine is rotating for example), the motor speed will vary accordingly. This VARIANCE can cause the machine to rotate less smoothly, causing more rotational VARIANCE and so on, actually AMPLIFYING the slight vibration.

With VECTOR control, voltage is adjusted as needed in VERY short order, less than a revolution, less than a cycle. This microsecond by microsecond adjustment of voltage, manifests itself as microsecond by microsecond adjustment of torque and therefore speed compensation.

In short, the vibration is corrected for by very fast variation of the voltage, therefore current, therefore torque, therefore speed. The speed is evened out, and vibration is reduced.

Stationmaster

So for V/F Control: Rotational vibration causes change in load torque causes speed change causes more vibration.

and for vector Control: Rotational vibration causes change in load torque which is compensated for and speed change is reduced hence no (or less) vibration.

I think I understand now!
 
It is the current that drives a motor not the voltage. So if the current is controlled(vector) it will work, when voltage is controlled the torque will vary with each switching.
Please also note the bearings will give pitting due to current flowing thru it.
 
You can try to make a recognision run with the motor without load in the VSD. Most of the new converters have this function and it allowes the VSD to try the motor and thereby getting all the data about the motor and optimise the control.

It is the current that drives a motor not the voltage.

Current is the resitance the motor gives when it is working. The heavier you load the motor the higher current you get so no its not the current that drives the motor its the voltage combinded with the frequency that rotates the motor.
 
First off, trying to separate current from voltage is nonsensical. Current produces the torque, and voltage determines how much current is pushed through a given impedence. (V=IR, V=IZ). Just like flow rate determines the speed of a hydraulic motor (flow makes it go), and pressure determines the torque of the motor and how much flow you can push through a given restriction.

My experience agrees with Dick's. The only time I have ever seen this is the cogging you sometimes get at very low frequency, usually below 10 Hz.
 
Last edited:
Well said Tom, you can't have current without voltage no matter HOW low the resistance is.

It is the management of the RELATIONSHIP of voltage to frequency that distinguishes the operating characteristics of the motor.

I was wondering if the "flow makes it go" controversy would come up again. I never weighed in on that before but I will now.

In hydraulics, the fluid has little (basically no) compression or expansion, so Peter's position that "pressure" makes it go is only true until the piston moves ANY. The tiniest movement of the piston will cause the pressure to drop. That movement has caused an increase in the volume of the hydraulic fluid chamber, and that increase must be refilled with pressurized fluid to maintain pressure. That replacement of fluid is called "flow".

The earlier raging debate over whether "Flow" or "Pressure" makes a piston move, is just as "nonsensical" as whether "voltage" or "current" makes an electric motor go.

It takes BOTH.

IF your hydraulic pump won't produce adequate FLOW and PRESSURE or FLOW at PRESSURE, the result will be unsatisfactory.

$.02

Stationmaster
 

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