Question

Consider a separately excited dc motor, Machine 1: rated at 10hp,850rpm. Using the same datasheet of Assignment 1 and neglecting friction. Use the "hot" resistance for all calculations =1.2× Ra 925⁰ C. The motor is connected to a 270 V dc source through a H-bridge converter. The converters operate at 10kHz switching frequency without a filter. Assume a connected load inertia equal to the motor's inertia (Jₗ=Jₘ) and assume rated flux operation, 1. Create a model for the machine in PLECS. Simulate the model for 300 ms :

- a step change in the armature voltage from 0 to the rated value at 100 ms.
- a step change in the lond torque from 0 to the rated value at 200 ms.
a) Sketch the speed in rpm and compare the sketch to the step response of the transfer functions we obtained in class.

Answer 1

The task is to model and simulate a step change in armature voltage and load torque in a separately excited DC motor using PLECS, observing and comparing the motor's speed response to theoretical step function responses.

Step-by-step explanation:

The exercise involves creating a model for a separately excited DC motor on PLECS and simulating various step changes. Specifically, one should simulate a step change in the armature voltage from 0 to the rated value at 100 ms, and a step change in the load torque from 0 to the rated value at 200 ms, while observing the motor's speed in RPM. In such simulations, the speed response will initially show a rapid increase due to the sudden application of rated armature voltage. Afterward, when the load is applied at 200 ms, the speed will decrease momentarily before reaching a steady state as the motor adjusts to the new load conditions.

Comparing the simulated response to the step response of transfer functions discussed in class should reveal similarities in the dynamics of the motor's speed adjustment, particularly in the ramp-up phase and the settling period after the application of load. The primary differences would be accounted for by the physical parameters and settings in the PLECS model, such as motor resistance at a hot temperature and the use of a H-bridge converter without a filter operating at a 10kHz switching frequency.