# in DC machine model, when i take speed as input, i didn't need to enter back emf constant, inertia, and viscous friction coefficient etc but not in Torque input case.

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ABBAS HAIDER il 14 Mar 2024
in DC machine model, when i take Torque as input, i do need to enter back emf constant, inertia, and viscous friction coefficient etc but not in speed input case they do not ask for this.
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Dr.GADDALA JAYA RAJU il 15 Mar 2024
In a DC machine model, when torque is taken as an input, you typically need to provide additional parameters such as the back electromotive force (EMF) constant (often denoted as Kb), inertia, and viscous friction coefficient. This is because torque is directly related to these parameters through the machine's dynamic equations.
The basic dynamic equation of a DC motor relates torque (T), speed (ω), and back EMF (e):
=T=KtIBωTL
where:
• Kt is the torque constant (related to the back EMF constant Kb).
• I is the motor current.
• B is the viscous friction coefficient.
• TL is the load torque.
In this equation, the back EMF constant (Kb) is related to the torque constant (Kt) as =1Kb=Kt1. Therefore, specifying one parameter allows you to determine the other.
When speed is taken as an input, you may not need to explicitly provide these parameters because the dynamic equations governing the system are often linearized around an operating point where the back EMF, inertia, and friction effects are assumed to be constant or negligible. In many cases, speed control systems operate based on a simplified model where these parameters are implicitly included in the system's transfer function or state-space representation.
However, even when speed is taken as an input, these parameters may still be required depending on the level of detail and accuracy desired in the model. For example, if you're simulating a more detailed model of a DC motor system or if you're implementing a control algorithm that requires knowledge of these parameters, you would still need to provide them.
In summary, while torque input may explicitly require parameters like back EMF constant, inertia, and viscous friction coefficient due to their direct influence on torque, speed input may not explicitly require them depending on the level of abstraction and assumptions made in the model. However, in both cases, these parameters are fundamental to the dynamic behavior of the DC motor and may still be needed for more detailed or accurate simulations.
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