| ??? 07/29/05 11:03 Read: times |
#98436 - More questions... Responding to: ???'s previous message |
Sun said:
The way this was done previously is;
The test technician applies a current of anything of between 200 and 300A to the commutator with the brushes at about 90 deg from each other. As I mentioned previously, the supply is about 10V/300A and the resistence of the armature is about 0.0148 ohms. The technician then uses a millivilt meter to measure the volt drop between each pair of bars. The readings should not vary much if there are no shorts or open cct windings. I have several questions: 1. Does the test current make the motor turn around, does it set the motor in motion? 2. How is the current applied, and more interesting how is it removed? Normally, there are heavy voltage spikes, if the current through a large inductance is suddenly switched-off!! How do you handle these voltage spikes? How high can they be? How much time is needed for the current through motor winding to reach its final value, when being switched-on? 3. So, you are only interested in the ratio of voltage drops, not the actual voltage drop? This would allow that the test current isn't actually known, but is stable while jumping from winding to winding? Now I need to impliment this test digitaly and save the readings. What does this mean exactly? What is needed to be implemented digitally? Is still a technician going to the motor and applying the test current through the several windings? What all is to be done 'digitally'? Sun said:
Kai, I plan to use the INA117, but my worry is the current.
I dont need to measure the currrent but I am worried that large currents from the test supply (300A) might flow into my low current control cct. Whether current is flowing depends on your application. How is the test current being generated? Is there any connection to safety earth? Does your circuit have any connection to safety earth? Normally, if everything has a connection to safety earth somehow, means if everything is referenced to safety earth, then, the only two wires running from your DC motor to your test circuit would be the probe wires, which are going to the 380kOhm resistors of INA117. So, there cannot flow any high current! You told, that the current generator does only apply a voltage of 10V. Then it's this voltage what drives a current into your circuit. And the 380kOhm resistors prevent any high currents from flowing into your circuit. By the way, can you elaborate a bit more, how the test current generator works? Is it a voltage generator using a current limiting resistor? Kai |
