Does the Friction Coefficient "B" of AC Motor changes with the change of a load connected to an AC motor? OR it is always constant?

  • \$\begingroup\$ Friction Coefficient "B" is probably nomenclature specific to a particular book or text. I'm not even going to try to guess which book. \$\endgroup\$ – user_1818839 Jul 14 '17 at 9:49
  • \$\begingroup\$ None of the tags attached to this question are appropriate. This is mostly a mechanical engineering question. It would be off-topic here except that friction is an important aspect of electric motor design. I will revise. \$\endgroup\$ – Charles Cowie Jul 14 '17 at 10:59
  • \$\begingroup\$ We have no way of knowing what the "friction coefficient B" of the motor is. \$\endgroup\$ – Olin Lathrop Jul 14 '17 at 11:08
  • \$\begingroup\$ Lack of explanation of "B" certainly diminishes the quality of the question considerably, but how could it refer to anything but bearing and brush friction? \$\endgroup\$ – Charles Cowie Jul 14 '17 at 11:47

The coefficient of friction is the ratio of the force of friction between two bodies and the force pressing them together. The only friction in most ad motors is the friction in the shaft bearings. That is probably what the "B" refers to in the question. The coefficient of friction is normally considered to be a constant, but it can change over time due to inadequate or failing lubrication and normal wear. There could also be a small change due to a change in operating temperature.

The force due to friction is independent of speed. The force on the bearings is often only the weight of the motor's rotor, so the force due to friction may not change due to load. If the load is belt driven, the load will cause a side force on the bearings called "overhung load." That could cause a change in the force due to friction to change due to load changes. If the shaft supports a pump impeller or a fan, the load could cause a thrust force on the bearings that would change with speed and load.

In addition to the kinetic friction described above, the bearings have some static friction that only causes a force that acts at the transition between transition and motion. That would be pretty much constant except for changes in the effectiveness of lubrication.

In addition to bearing friction, motors have aerodynamic drag or "windage" that is a type of friction between the moving parts of the motor and air. Most motors have some windage that is deliberately added by incorporating a fan or rotor fins for cooling purposes.

AC motors with slip rings have brush friction in addition to bearing friction.

  • \$\begingroup\$ Thanks, @charles, I appreciate your help, I have one more question if you know that what is the approximate value of friction coefficient for an AC motor. \$\endgroup\$ – Abdul Wali Jul 14 '17 at 11:45
  • \$\begingroup\$ The value of the bearing friction coefficient varies with the size, type and quality of the bearings. You should be able to find values in bearing sales literature or some kind of bearing application guide. You really should explain the use of "B" in your question. \$\endgroup\$ – Charles Cowie Jul 14 '17 at 11:53
  • \$\begingroup\$ i have the model of ac servomotor where the dynamic equation of mechanical system is Tc= J(theta)(t) + B(theta)(t) + TL and from further analysis i get Motor Time Constat = Tm = J/K+B (where K is motor Constant) ... So in this scenario i wanted to know that whether the J(moment of inertia) and B(Friction Coefficient) changes with the change of load or No?? \$\endgroup\$ – Abdul Wali Jul 14 '17 at 12:12
  • \$\begingroup\$ My answer covers friction as related to the motor itself. The "mechanical system," would generally refer to the motor plus the load. The load would have its own bearings, which would have constant coefficients, but possibly variable forces on the bearings. In addition, the load process may be frictional in nature but may or may not have constant coefficients. If the model is just for the motor and not the load, I believe by answer covers the nature of friction as it applies. \$\endgroup\$ – Charles Cowie Jul 14 '17 at 12:45

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