# Viscous damping in BLDC

I am simulating a BLDC model and one of the parameters I need to enter in my simulation is Viscose Damping (in units of [N.m.s]). I am not familiar with this term, and after some research, I came across a formula that defines it as the ratio of a) the product of the constant torque and back emf constant, and b) the motor resistance. However, that was defined as viscose damping coefficient in units of [N.m/rad/s].

I did not find this value in the motor spec I'm trying to simulate.

My question is this: 1) what is viscose damping - in intuitive terms? What is it dependent on, and how does it affect the current consumed by the motor? 2) what is the difference between viscose damping and viscose damping coefficient and how are they different from each other (beyond the formula above). I assume that the difference in the units defined above is the presence of radians, but still need a clearer understanding. 3) If I know the viscose damping coefficient, how do I go to the viscose damping? Would a simple 2pi take care of the conversion, or is dependent on the rotational speed (omega) of the motor? 4) I also came across viscose damping torque when reading previous answers in this forum. How does the viscose damping torque differentiates itself from all the other 'viscose damping' terms mentioned above. What does it mean in terms of current drawn from the motor?

Thanks much!

• what does google or wiki tell you? – Andy aka Sep 9 '16 at 12:40
• Note that radians are (not accidentally) dimensionless. – Brian Drummond Sep 9 '16 at 12:53
• I first saw rotary stepper HDD's in early 80's using rotary viscous damper about 30x3mm brass oil filled in plastic. Makes a big difference on settling time even with microsteps and ramped velocity. This is inertial mass slew rate control or torque speed change constant.For BLDC, surge current limit reduction from 8x rated current slows acceleration like viscous, mass torque rate of change in RPM – Sunnyskyguy EE75 Sep 9 '16 at 12:58

## 2 Answers

1) what is viscose damping - in intuitive terms? What is it dependent on, and how does it affect the current consumed by the motor?

Viscose damping, aka rotation damping is the mechanical rotational equivalent of resistance. It restricts rotational freedom.

The bearings, additional magnetic drag, windage, fluid etc... All contribute to additional loading and loading which is related to velocity. Sometimes it is directly proportional, sometimes it isnt (airflow as a drag affect is a square law iirc)

How does it affect the current? Well with increased rotor velocity the viscose drag increases and so to maintain a given operating point more current is needed to offset this loss of torque

2) what is the difference between viscose damping and viscose damping coefficient and how are they different from each other (beyond the formula above). I assume that the difference in the units defined above is the presence of radians, but still need a clearer understanding.

One is the concept, the other is the concept of proportionality. When dealing with electrical machines all the equations are only valid in radians and rad/s.

3) If I know the viscose damping coefficient, how do I go to the viscose damping? Would a simple 2pi take care of the conversion, or is dependent on the rotational speed (omega) of the motor?

Double-check the units (it should be in Nm/rad/s). Determine rotor velocity in w (rpm -> w is a multiplication factor of 2π/60 , roughly 1/10th). Multiply the coef by the velocity in w and this will result in the lost torque at that velocity

4) I also came across viscose damping torque when reading previous answers in this forum. How does the viscose damping torque differentiates itself from all the other 'viscose damping' terms mentioned above. What does it mean in terms of current drawn from the motor?

viscose damping is the term used to describe the effect

viscose damping coefficient is the constant of proportionality associated to the effect

viscose damping torque is torque due to viscose damping

Ideally, for viscous damping the force (torque in your case) is a constant times the (angular in your case) velocity.

You can calculate the coefficient from other motor parameters.

To find the viscous damping torque at a given RPM you must convert from RPM to radians/second so a factor of $\frac{2\pi}{60}$ (about 0.105) is involved. Then multiply the angular velocity is s^-1 by the coefficient to get torque (Nm).