Q1. a) What is the expression for calculating the acceleration of a motor given the moment of inertia and the accelerating torque?
b) What is the accelerating torque required to accelerate a motor with the moment of inertia of the system being 100 kg m2 to be accelerated at 100 RPM per second?
Q2. In a Variable Speed Drive (VSD) the region between 0 speed and the rated speed of the motor is called the constant ......... mode and above the rated speed the motor is on a constant ......... mode.
Q3. Normally, the constraint for lower speed operation of standard self-cooled induction motors driven by a VSD is .........
This problem can be overcome by using .........
Q4. Critical Question. This Critical question is mandatory. It should be answered and students are expected to get this question completely right to be assessed as competent with a score for this module.
a) What is the main reason for suggesting a VSD in a pump application where part load operation is possible for long periods?
b) Why is the same reason not valid for a pump which runs at full load constantly?
Q5. Why is it not possible to have a VSD running at higher than its base speed to drive a pump motor whose rated speed is equal to the base speed of the drive?
Q6. a) If a VSD/motor combination has a torque of 100 N.m at the rated speed and full power, what is the maximum torque it can supply at 120% rated speed? (Answer in %)
b) At what speed does the motor torque get limited to a maximum value of 80 N.m?
Q7. Write a brief note on maintenance program of a motor.
Q8. Analyse the probable cause if there is Frequent coil failure in motors and suggest remedial measures.
Q9. A 3-phase 400V 50Hz 6-pole cage induction motor of rating 20 kW, with slip at 4% of the rated load, has a starting torque equal to 120% of the rated torque. The motor is coupled to a pump through a reduction gearbox of efficiency 85%. The breakaway torque is 24N.m at the motor shaft. Assume that the torque available for acceleration remains constant for the entire duration and the motor accelerates to the rated speed in 6 seconds.
Based on this data, please answer the questions below.
a) What is the rated speed of the motor?
b) What are the values of the rated torque and starting torque of the motor?
c) What quantity of torque is available for acceleration?
d) What is the total Moment of Inertia of the rotating parts of the system (seen at the motor shaft)?
Q10. Analyse the probable cause of the following problems in motors and suggest remedial measures.
a) A motor driving a pump occasionally fails to start and trips with an indication of overload. The electrical parameters are normal. The observation is that every time this happens the delivery valve is fully open.
b) The motor driving a rotary cement kiln fails repeatedly and every time the insulation is found to have burnt out. It is observed that this happens only during summer when the temperature is close to 45 Deg C.
c) A compressor motor being switched on for the first time rotates in the reverse direction.
d) A bulk ore handling conveyor motor has repeated failures. After repairs the motor runs normally for a few months until it again suffers a burn out. It is a totally enclosed fan-cooled motor and every time it fails it is observed that the outer fins have a thick and hard deposit of ore dust.
Q11. a) List and briefly explain the selection of VSD settings based on their application.
b) Which one of these settings can be used to control the starting torque?