• The drive is said to be in equilibrium if the torque developed by the motor is exactly equal to the load torque.
  
  
• If the drive comes out of the state of equilibrium due to some disturbance, it comes back to steady state for stable equilibrium but for unstable equilibrium the speed of the drive increases uncontrollably or decreases to zero. When the drive coming out of the state of equilibrium preserves it steady state at different speed (lying in small range), it is said to be in neutral range.
  
  
• The stability of the motor load combination is defined as the capacity of the system which enables it to develop forces of such a nature as to restore equilibrium after any small departure therefore.
  
  
• Equilibrium state of the drive mainly disturbs because of the following two types of disturbances,
  
  1.Changes from the state of equilibrium takes place slowly and the effect of either the inertia or the inductance is insignificant – Steady state stability.
  2.Sudden and fast changes from the equilibrium state so effect of both inertia and inductance can not be neglected- Dynamic or transient stability