(e) The power rating P of an electrical machine with rotor diameter D and length
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Question
(e) The power rating P of an electrical machine with rotor diameter D and length l can be represented by the equation P = omega sigma pi D^2 l/2, where omega is angular speed and sigma is shear stress. Briefly discuss the practical limitations of increasing the power rating by: (i) increasing angular speed; and (ii) increasing rotor volume. (f) A self-commutated fully controllable semiconductor power device with rated voltage and current of 50V and 20A is required for a dc-dc converter circuit. If the device will be switched at 50kHz, what device would you choose and why? (g) An IGBT produces 100W power loss during operation. If the total thermal impedance from the IGBT junction to ambient is 0.75 degree C/W and the ambient temperature is 25 degree C, what is the junction temperature of the IGBT? (h) Draw the circuit diagram for a four switch H-Bridge chopper supplied by a dc voltage source V_dc and show how, with appropriate switching, the circuit can apply voltages of +V_dc, 0 and -V_dc to the load terminals. (i) Show how carrier-based, sinusoidal pulse width modulation (SPWM) of a voltage source inverter may be used to synthesize an ac output voltage. (j) Draw the block diagram of a typical speed feedback system applied to separately excited dc motor and explain the main function of each block.Explanation / Answer
Q.(e) The power rating P of an electrical machine with rotor diameter D and length l can be represented by the equation P = ((D2l)/2),where is the angular speed and is shear stress. Briefly discuss the practical limitations of increasing the power rating by:
(i) increasing angular speed;and
(ii) increasing rotor volume
Answer (e)
Practical limitations of increasing the power rating –
The physical size of electrical machines is primarily determined by their torque capability. It will be shown that the output torque is proportional to the product of the rotorvolume and the shear stress. The shear stress in turn is proportional to the product of the electric and magnetic loading. Once the approximate rotor volume has been
determined, other design decisions include the rotor aspect ratio, the stator slot diameter ratio and the number of poles.
Though the torque rating is the primary fact or affecting motor sizing, the motor speed rating is important in determining factors such as the maximum allowable rotor
diameter (due to mechanical stress), the maximum number of poles (due to excessive iron losses) and the minimum rotor mechanical critical speed.
Now since the output torque is proportional to the product of the rotor volume and the shear stress and also output power is equal to the product of angular speed and torque .
Thus
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