what is Low frequency vibration for vfd?
What is low-frequency vibration in a VFD-driven motor?
Low-frequency vibration is a relatively slow oscillation in motor speed, torque or the connected mechanical system. On a machine controlled by a variable frequency drive, this behaviour can come from the mechanical system, the motor-control algorithm or an interaction between the two.
The most important distinction is that VFD output frequency and vibration frequency are not the same quantity. A drive may be operating a motor at 15 Hz while the shaft speed slowly hunts at 1 Hz. A machine may also vibrate strongly only when it passes through one particular operating-speed band because that speed excites a mechanical resonance.
For that reason, low-frequency vibration is closely connected with V/F control, Sensorless Vector Control, Vector Control, motor torque and the mechanical dynamics of the driven machine.
Explore the OtomasyonAVM knowledge base: V/F Control · Torque Boost · Slip Compensation · Auto-Tuning · Jump Frequency · Switching Frequency · Encoders
Why are motor frequency and vibration frequency different?
If a VFD is producing 20 Hz fundamental output, that electrical frequency primarily determines the rotating magnetic-field speed according to the number of motor poles. It is therefore closely related to average motor speed.
The mechanical system can still oscillate around that average speed at an entirely different and much lower frequency. Slow torque pulsation, control-loop instability, shaft elasticity or a mechanical natural frequency can create an oscillation of its own. Drive manufacturers may describe this behaviour as hunting or V/F oscillation.
VFD output frequency vs low-frequency oscillation
The compact demonstration below keeps these two values separate. VFD output frequency changes the average motor speed. Oscillation frequency controls how quickly the shaft or mechanical load moves back and forth around that average operating point.
The example uses a simplified four-pole induction motor. The vibration frequency is deliberately shown independently from VFD output frequency.
Editor's note: If someone says, “The motor vibrates at 18 Hz,” my first question is: what exactly is 18 Hz? Is that the drive output, or has the mechanical vibration itself actually been measured at 18 Hz? A drive can be running at 18 Hz while the machine slowly hunts at 1 Hz. That distinction can completely change the diagnosis.
Why can hunting occur in V/F control?
V/F control is simple and robust, but standard open-loop V/F control does not directly measure actual rotor speed. Under certain combinations of light load, low mechanical inertia and operating speed, the motor and drive can develop a slow oscillatory behaviour.
Many drives therefore include functions called Hunting Prevention or V/F Oscillation Suppression. These functions do not simply change average motor speed; they try to damp periodic current and torque oscillation.
Accurate motor data is important for these algorithms. Before aggressively changing suppression gain, check the motor nameplate information and, where appropriate, perform a proper Auto-Tuning procedure.
Can Torque Boost make the vibration worse?
At low speed, Torque Boost increases applied voltage to compensate for stator resistance drop and improve low-speed torque. If the boost is set too aggressively, however, motor current and flux can become excessive.
When an application genuinely needs strong and stable low-speed torque, simply adding more boost is not always the answer. Sensorless Vector Control or more advanced Vector Control may provide a better motor model and more stable torque regulation.
Slip compensation and oscillation
Slip Compensation adds a small frequency correction when induction-motor speed falls under load. Properly adjusted, this improves speed regulation.
But slip compensation and oscillation suppression are not the same function. Incorrect motor data or an overly aggressive compensation setting can make a control system respond too strongly to changing load and introduce additional speed or current variation.
How is mechanical resonance different?
The drive is not always the cause. Motors, couplings, gearboxes, shafts, frames and driven loads all have their own natural mechanical frequencies. If operating speed excites one of those modes, a relatively small force can produce a large vibration.
A typical clue is vibration that appears only in a narrow speed range. If a machine runs smoothly at 10 Hz, shakes badly around 18 Hz and becomes smooth again above 23 Hz, investigate a possible resonance band.
A VFD's Jump Frequency or Skip Frequency function can prevent continuous operation in a known resonance region. It is a useful operating tool, but it does not repair loose foundations, poor alignment or an unbalanced fan.
Does switching frequency matter?
Switching frequency, also commonly called carrier frequency in this context, is different from the fundamental output frequency that determines motor speed.
Changing carrier frequency can affect motor acoustic noise, current waveform, switching losses and, in some systems, vibration behaviour. It is therefore worth checking when a machine hunts or resonates. However, increasing it blindly is not a universal cure because higher switching frequency normally increases semiconductor losses and drive heating.
Will Vector Control cure low-frequency vibration?
Not necessarily. But SVC and Vector Control manage motor flux and torque in more detail than basic open-loop V/F operation, so they can provide better low-speed performance and load response.
Where very accurate speed information is required, an encoder can provide actual shaft feedback. That does not mean encoder feedback magically removes mechanical resonance; it simply gives the controller much better information about what the shaft is doing.
What can cause low-frequency vibration?
Control-related causes include an unsuitable V/F curve, excessive torque boost, incorrect motor data, poorly adjusted slip compensation, unsuitable control-loop gains or genuine drive hunting.
Mechanical causes include imbalance, poor alignment, loose foundations, coupling problems, gearbox backlash, belt tension, flexible shafts and structural resonance. Bearing damage can certainly produce vibration as well, but not every slow oscillation should automatically be diagnosed as a bearing fault.
Process-related causes can include pulsating pump flow, reciprocating compressor torque, irregular conveyor loading or material accumulating on fan blades.
How should the problem be diagnosed?
First establish the operating condition. Does vibration appear only within one speed band? Does it begin when load increases? Is the motor smooth when uncoupled? Does changing from V/F to SVC materially change the behaviour? Does a change in switching frequency change the noise or vibration?
Actual shaft-speed variation can be monitored using an encoder or a suitable tachometer, while mechanical vibration requires an appropriate vibration or acceleration measurement method.
If drive data is available over communications, a PLC or HMI can trend output frequency, current, estimated torque, DC-bus voltage, faults and actual speed together. Seeing those values on the same time axis often makes it much easier to tell whether the disturbance is coming from the drive, the load or a particular machine speed.
A note from the field: When a motor begins hunting at low speed, I would not start by turning the oscillation-suppression gain all the way up. First verify the motor data, control mode, auto-tune, torque boost, motor current and mechanical coupling. Software can hide a symptom while excessive current or heating appears somewhere else.
In short: low-frequency vibration is not simply “the VFD is running at low frequency, therefore the motor vibrates slowly.” The control algorithm, motor model, torque production, mechanical resonance and process load must be considered together. Continue through the OtomasyonAVM knowledge base with V/F Control, SVC, Torque Boost, Slip Compensation, Jump Frequency, Switching Frequency and Encoders.