Help me in my electric machines lab viva.?

Question

These are the qustions may be asked in my lab on coming tuesday. Please answer and help me.
THIS IS ELECTRICAL MACHINES LAB.
load test on compound dc genetator.
1. said that at the starting of experiment armature resistance should be kept at max. and field resistance at min. positions. why?

2.why speed must be maintained const. through out the experiment.?

3. why 4 point starter arm must be kept at the extreme left position before switching on the supply?

4. what is tachometer? why it should be kept in line with the shaft while measuring the speed of the motor?

HOPE SOME MAY EXPLAIN ME THE ABOVE REASONS, UR S BROTHER MADHU.

Answer 1

Without detailed knowlwdge of the exact setup in question it is diffcult to be certain in my answers, but my best guess at each would be as follows (examine my answers in light of your specific lab equipment and setup to make sure my answers make sense):

(1) By keeping the armature resistance at maximum and field resistance at minumum you should have the minimum current and therefore power being used/generated (since P=I^2R) and the maximum power generation capability (lower resistance = higher field current = more flux density = more potential power). You can get the generator speed stable and then apply greater laods without risk of either not being able to bring the generator up to speed due to the high load, or damage to equipment if the load is not noticed to be shorted.

2. Varying the speed varies the power -- presumably you are measuring something that is dependant on constant power either at the input or at the output.

3. Can't say without knowlwdge of your specific setup. What does keeping the starter arm at the leftmost position actually do in terms of the equipment set up? Why would you want the equipment set up that when first switching on the power? My guess is that it opens the circuit to the load (which is the safe and easy way to start a generator) but that is just a guess.

4. The tachometer tells you how rapidly the shaft is rotating (the RPM's). It must be aligned to the shaft because at an angle it can go through phases of rapid increase and decrease in RPM (look up "Universal Joint" and "Continuous Velocity Joint" for explanations of why this happens). Rapid changes in RPM might damage the tachometer, and they will certainly cause it to read a wrong value (usually higher than the true average RPM) which will affect your results.

Answer 2

1.By keeping the armature resistance at maximum and field resistance at minumum you should have the minimum current and therefore power being used/generated (since P=I^2R) and the maximum power generation capability (lower resistance = higher field current = more flux density = more potential power
2. Varying the speed varies the power,presumably you are measuring something that is dependant on constant power either at the input or at the output.
3.The four-point starter eliminates the drawback of the three-point starter. In addition to the same three points that were in use with the three-point starter, the other side of the line, L1, is the fourth point brought to the starter. When the arm is moved from the "Off" position, the coil of the holding magnet is connected across the line. The holding magnet and starting resistors function identical as in the three-point starter.

The possibility of accidentally opening the field circuit is quite remote. The four-point starter provides the no-voltage protection to the motor. If the power fails, the motor is disconnected from the line.
4.


Above: To monitor tach, one needs componets such as these: (left to right) hall-effect sensor, magnetic disk and tachometer.

Tachometers are used to determine the speed of a motor shaft for motor drives on conveyors, as well as to determine the speed of rotation of the screw shaft on a plastic injection molding machine. The speed of rotation of the screw on a plastic injection molding machine is important to control because the screw shaft is used to meter the amount of plastic that is drawn into the barrel of the machine for the next injection shot. If the speed isn't controlled, the screw will turn at different speeds and more or less plastic will be drawn into the barrel and the amount of plastic being used for each part will be inconsistent.