Hi, i have a question that always stay in my mind.

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

Your help will be appreciated.

 

By the equation describing wear on a bearing as RPM x lateral force I must conclude that the friction increases with the load. By the equation concerning the sheer force in an oil, I must conclude that friction increases with RPM in a condition with no (or insignificant) lateral load, such as a fan blade.

I have no idea if these statements are significant in the study of motors because I suck at motors.

 

I never heard of a letter 'B" attached to a coefficient of friction. IIRC it comes in two flavors: static and moving. It relates the normal force between two surfaces to the force required move one surface against the other. As soon as the motion begins the coefficient changes to a lower value which means that less force is required to sustain the motion of one surface against the other.

 

IIRC it comes in two flavors: static and moving.

True. In this case, I assumed that mu static is irrelevant because a motor that isn't moving doesn't care about friction. (the other one is mu kinetic)

 

True. In this case, I assumed that mu static is irrelevant because a motor that isn't moving doesn't care about friction. (the other one is mu kinetic)

It does when the motor starts.

 

Is the OP talking about the 'windage' and bearing friction of the motor? There isn't enough information given in the question.

 

Is the OP talking about the 'windage' and bearing friction of the motor? There isn't enough information given in the question.

Yes.

 

To add a little to the current discussion, if we assume that the primary source of friction are the motor's bearings, I'd say that friction actually diminishes with time due to an increase in temperature, this is because the oil or grease inside the bearing will normally become less viscous with temperature. As for the brushes, I wouldn't be exactly sure, but I bet that temperature affects them in a similar way.

 

Hi,

Note the friction always increases with RPM, but the coefficient of friction does not necessarily change (much).

In one analogy we have:
T=f*w
i=G*v

where T is torque, w is angular speed, f is coefficient of friction in the rotational damper, i is current, G is conductance (of a resistor), v is voltage.

Solving the first for w:
w=T/f

so the smaller the friction coefficient, the faster the speed with the same applied torque.

If the coefficient of friction actually varies then we have to move to:
w=T/F(X)

where we now have the friction being a function of X variables.

Of course there is also the static friction, which means for some torques we get no movement at all so zero speed. You may or may not consider this to be a 'changing' friction.

 

Hi,

Note the friction always increases with RPM, but the coefficient of friction does not necessarily change (much).

In one analogy we have:
T=f*w
i=G*v

where T is torque, w is angular speed, f is coefficient of friction in the rotational damper, i is current, G is conductance (of a resistor), v is voltage.

Solving the first for w:
w=T/f

so the lower the friction, the faster the speed with the same applied torque.

If the coefficient of friction actually varies then we have to move to:
w=T/F(X)

where we now have the friction being a function of X variables.

Of course there is also the static friction, which means for some torques we get no movement at all so zero speed. You may or may not consider this to be a 'changing' friction.

You're right. I was confusing coefficient of friction with net friction force in my previous explanation.

 

To add a little to the current discussion, if we assume that the primary source of friction are the motor's bearings, I'd say that friction actually diminishes with time due to an increase in temperature, this is because the oil or grease inside the bearing will normally become less viscous with temperature. As for the brushes, I wouldn't be exactly sure, but I bet that temperature affects them in a similar way.

But there is a little understood thing called 'windage' in a motor. It does increase with the motors speed. But I don't know of any real formula to determine it. Think it's just measured for each design and size of motor.

 

But there is a little understood thing called 'windage' in a motor. It does increase with the motors speed. But I don't know of any real formula to determine it. Think it's just measured for each design and size of motor.

Are you referring to back EMF?

 

Are you referring to back EMF?

Hi,

I doubt it. That sounds like the air resistance friction from the armature turning.

 

Hi,

I doubt it. That sounds like the air resistance friction from the armature turning.

Right again... air is definitely another source of friction

 

Hi,

Note the friction always increases with RPM, but the coefficient of friction does not necessarily change (much).

In one analogy we have:
T=f*w
i=G*v

where T is torque, w is angular speed, f is coefficient of friction in the rotational damper, i is current, G is conductance (of a resistor), v is voltage.

Solving the first for w:
w=T/f

so the smaller the friction coefficient, the faster the speed with the same applied torque.

If the coefficient of friction actually varies then we have to move to:
w=T/F(X)

where we now have the friction being a function of X variables.

Of course there is also the static friction, which means for some torques we get no movement at all so zero speed. You may or may not consider this to be a 'changing' friction.

Thanks Ali, i appreciate your help. So what is the difference between Friction (f) and Friction Coefficient (B) ??

 

The coefficient is just a factor, typically between 0 and 1. The friction is a function of such a factor and various other things. A high value factor means more friction, a low value means less friction.
http://medias.schaeffler.de/medias/en!hp.tg.cat/tg_hr*ST4_102160011

 

Right again... air is definitely another source of friction

I have some books on motor design and rewinding that talk about windage in the design part, and MrAl answered before I could. It is a little talked about thing but is part of the design process. By finding ways to reduce it, it is like "free" power in a motor. They also do this in internal combustion engine design now.