Question

3. A 25-kW, 230-V shunt motor has an armature resistance of 0.064 Ω and a field-circuit resistance of 95 Ω. The motor delivers rated output power at rated voltage when its armature current is 122 A. When the motor is operating at rated voltage, the speed is observed to be 1150 r/min when the machine is loaded such that the armature current is 69.5 A a. Calculate the rated-load speed of this motor In order to protect both the motor and the dc supply under starting conditions, an external resistance will be connected in series with the armature winding (with the field winding remaining directly across the 230-V supply). The resistance will then be automatically adjusted in steps so that the armature current does not exceed 200 p The step size will be determined such that, until all the external resistance is switched out the armature current will not be machine is to start with 200 percent of rated armature current and as soon as the current falls to rated value, sufficient series resistance is to be cut out to restore the current to 200 percent. This process will be repeated until all of the series resistance has been eliminated b. Find the maximum value of the series resistance ercent of rated current permi itted to drop below rated value. In other words, the c. How much resistance should be cut out at each step in the starting operation and at what speed should each step change occur? .M resistance of 4.6 Ω. With an applied armature voltage of 5 V, the motor is observed to achieve a no-load speed of 11,210 r/min while drawing an armature current of 12.5 mA. This motor will be used to drive a load that requires a power of 0.75 W at a speed of 8750 r/min Calculate the armature voltage that must be applied to achieve this operating condition easurements on a smal l permanent-magnet de motor indicate that it has an armature