# How to find Friction or Viscous force b (nmsec) in DC motor

PLease guide me How to find Friction or Viscous force b (nmsec) in DC motor for a particlar speed. The motor is connected with a gear and the ration is 26:1 I want to find for 200 rpm and the motor no load speed is 4900rpm please guide me

## 1 Answer

When no other load is on the motor, any torque that the motor is producing is to overcome the friction. If you know how to calculate your friction from the torque needed to overcome it (I don't), then the rest is easy.

You need a way to control the speed, a way to measure the speed, and a way to measure the current. You also need the motor specs, specifically the torque constant, Kt. A variable power supply should allow you to dial in 200 rpm. When you're at the correct speed, measure the current. Then multiply your measured current by Kt, and you now know the torque. Then simply convert the torque to friction using the equation $T = b\omega$ (rearrange for the friction coefficient to get $b = \frac{T}{\omega}$) where $T$ is the torque in N m and $\omega$ is the angular velocity in rad/s to get $b$ in N m s.

• @BarbalatsDilemma, Thanks for the equation. That really completes the answer. – Mark Jun 28 '16 at 0:56