What are static and dynamic friction in a VFD-driven machine?

Friction is resistance to relative motion between contacting surfaces. Static friction acts while the surfaces are not sliding against each other; dynamic friction, also called kinetic friction, acts when they slide. An AC drive does not remove mechanical friction. It controls the motor so that the torque available at the shaft can overcome the resistance to starting and meet the demand during operation.

A conveyor can need more torque to start from rest than to keep running at a steady speed. If available motor torque is below the initial resistance, increasing the speed command alone will not make the shaft turn. A held brake, a jammed mechanism, incorrect motor data and a drive current limit can give similar symptoms, so friction is only one part of the diagnosis.

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Static friction  ·  Dynamic friction  ·  Starting torque example  ·  What the drive can do  ·  Stick-slip  ·  Field checks  ·  FAQ  ·  Related articles

Static friction and breakaway torque

Static friction is not a single fixed force whenever the machine is stationary. It responds to an applied force up to a maximum. In the simple dry-sliding model, F_s ≤ μ_s × N: μ_s is the static friction coefficient and N is the normal force pressing the surfaces together. Sliding begins when the applied force exceeds the limiting value. This is a model for surfaces that would slide, not a complete torque calculation for every bearing or gearbox. OpenStax explains the limit and the distinction between static and kinetic friction.

The torque needed to get a stopped mechanism moving is commonly called breakaway torque. The total shaft torque needed to start may also include a suspended or process load, drag from a brake or seal, losses in the transmission and torque for acceleration. A slide that has rested for hours, a tight pump seal or a heavily loaded conveyor can therefore behave differently at startup.

Static friction is defined by the relative motion at the contact. The whole machine does not have to be at rest: a belt gripping a pulley without slipping is another example of static contact friction. That grip can be essential for transmitting torque.

Dynamic friction once surfaces slide

When two contacting surfaces slide against one another, their resistance is kinetic or dynamic friction. In a basic dry-sliding example, F_k ≈ μ_k × N. Kinetic friction is often below the maximum static friction for the same surfaces, but that observation is not a universal rule for a complete machine. Lubrication, temperature, seals and speed-dependent drag all affect actual running torque.

Separate friction torque from the rest of the load. A pump also works against fluid pressure; a fan works against airflow; a conveyor may be carrying material uphill. Rolling contact is not identical to dry sliding, either. Specify the load torque characteristic before choosing a motor and drive from a friction estimate.

Why can a machine run but fail to restart?

Illustrative example: Suppose friction referred to the motor shaft requires 18 Nm to break free and 10 Nm after motion begins, while the motor can provide only 15 Nm under the current conditions. It cannot break away from rest, although 15 Nm could overcome the 10 Nm running friction once it is moving. These numbers isolate friction to explain the principle; real sizing must include process load, acceleration, gearing and motor/drive limits. The Rockwell PowerFlex 700S reference manual uses separate friction compensation settings for breakaway and running conditions.

At a stationary shaft, mechanical shaft power torque × angular speed is zero. The motor can still draw current and generate heat, so a stalled machine should not be held against a torque command indefinitely. A speed or frequency command does not prove that the motor has delivered enough starting torque. Compare commanded speed, actual speed where feedback is available, current, limits and the mechanical state.

What can the VFD do about friction?

Depending on the drive and motor, torque boost, IR compensation, sensorless vector control or a dedicated friction compensation function can support low-speed operation. They are different functions. In the Rockwell example, “Stick” represents breakaway torque, “Slip” represents friction at very low running speed and “Rated” describes friction at base speed. Use the settings and motor data specified for the actual equipment.

Do not increase boost blindly to overcome a stiff mechanism. Check the motor nameplate data, control mode, current limit and mechanical condition first. Excessive compensation may bring high current or a sudden lurch when the load breaks free. An S-curve ramp can soften acceleration but does not create missing breakaway torque. At prolonged low motor speeds, also check the motor's cooling capability.

Why does a machine stick and slip at low speed?

A moving part may stick while force builds, slip suddenly once the threshold is crossed, then stick again. The result can be jerky travel, speed ripple or oscillation during fine positioning. A Rockwell motion control manual describes this behavior when breakaway friction is much higher than running friction. Low-speed vibration can have other causes too, including backlash, resonance, feedback faults and unsuitable control settings.

Dynamic friction is not dynamic braking. The former describes a mechanical contact during relative motion. Dynamic braking refers to removing energy from a decelerating load. Similar wording does not make the brake setting a friction compensation setting.

Practical checks before changing parameters

1. Identify when the problem occurs: at the first movement, only at crawl speed or throughout the run. 2. Inspect the load path, transmission, couplings, bearings, slides, seals and whether the mechanical brake releases as intended. 3. Verify motor wiring and nameplate data, drive mode, trip history and current limiting. 4. If the hardware supports it, tune the appropriate starting torque or friction compensation feature within the manufacturer's instructions while monitoring the machine.

Lubrication is application-specific. Lubricate a bearing or slide according to its manufacturer's instructions. Keep brake friction surfaces and similar torque-transmitting surfaces free of oil and grease; Ogura's brake installation instructions explicitly prohibit oil or grease on the friction faces. Polishing an arbitrary surface is no substitute for correcting misalignment or a worn component.

Frequently asked questions

Is static friction always at its maximum while the shaft is stopped?
No. It adjusts to oppose an applied force up to its limit; the maximum is reached just before slipping begins.

Is running friction always lower than starting friction?
Often in a simple dry-sliding model, but a machine's total running torque may rise with speed or load. Measure or calculate the actual application requirement.

Will a higher VFD frequency free a jammed mechanism?
No reliable conclusion follows from a higher frequency command. Verify the mechanical cause and the torque/current available from the selected motor and drive.

Current flows, but the motor will not turn. Is friction the only explanation?
No. A held brake, seizure, incorrect motor connection, low-speed torque limit or a drive protection setting may also be responsible. Check the fault history and the mechanical assembly.

More from OtomasyonAVM

Motor torque and loads: VFD guide  ·  Torque control  ·  Speed control vs torque control  ·  Constant-torque loads

Starting and low-speed control: Torque boost  ·  IR compensation  ·  Sensorless vector control  ·  Current limiting

Movement and troubleshooting: Jog frequency  ·  S-curve  ·  Low-frequency vibration  ·  Heavy-duty drive

Türkçe içerik: Statik ve dinamik friksiyon nedir?