TABLE OF CONTENTS The second electric machine that is used in series and combined hybrids, primarily acting as a generator, must be modeled according to a causality that is the reverse of that of the main traction motor. This becomes clear observing the flow of power factors in Figs. 4.9 4.11.
In quasistatic simulations, the input variable is the electric power P 2( t), while the output variables are the rotational speed ? 1( t) and torque T 1( t). One relationship between these variables is given by the definition of efficiency or by the Willans approach. The missing degree of freedom is typically a control signal. In fact, in series hybrids the rotational speed of the APU (engine plus generator) is determined by the control unit that tries to optimize the operation of the engine. In combined hybrids, the rotational speed of the generator is the variable that regulates the transmission ratio of the planetary gear set. Therefore, in any case a speed command 
is supplied to the generator model, so that 
and from a power balance the torque T 1( t) also can be calculated [217].
In dynamic models, the input variables are the voltage U 2( t) and the torque T 1( t), while the output variables are the speed ? 1( t) and the current I 2( t). In this case the motor equations introduced above still...
