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DC drives, common problems and troubleshooting

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Joined: 25 Mar 2006
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PostPosted: Wed Mar 29, 2006 9:15 pm Post subject: DC drives, common problems and troubleshooting Reply with quote

The link below provides good, problem-solving approaches to DC drive troubleshooting. The engineer leaves out unnecessary technical detail (i.e. theory) and offers bottom-line advice. Here's a snip from this site:

Quote:
Motor runs at uncontrolled full speed

Motor can go to full speed due to faulty tachogenerator, faulty or loose coupling, wrong polarity of tachogenerator, wrong polarity of field or armature connections, defective reference signal, defective RAMP circuit, or even defective "firing" circuit. In case of faulty tachogenerator, a glance at speed indicator, which in many cases connected to the same tachogenerator, can give a clue. The indication will be zero.

If firing circuit has no control over armature voltage, then usually the fuses will blow first before speed increases. If fuses escape from blowing, then the speed will be attained very fast with a jerk.

Faulty reference will usually take the motor to full speed rather smoothly if everything else is okay.

If speed indication is seen, one can check the voltage divider used for scaling down the tachogenerator voltage to low value. Normally a chain of resistors is used for this purpose.



Resistor divider in a good design will make use of many small value resistors rather than a few high value resistors. Also the variable preset potentiometer provided for adjustment of speed takes the following correct form in a good design. Note the shorting of wiper of the potentiometer with the one end. Just a small track on the PCB, but shows a real deep thinking on the designer's part. When the wiper is not shorted, as seen in the image on the right hand side, the drive will go to uncontrolled full speed when the wiper makes poor contact after years of use. With the arrangement as shown on the left hand side, bad contact on wiper will not take the motor to full speed. It will, on the other hand, decrease slightly and therefore not damage the mechanical parts connected to the motor.


More info at: http://freeideas.bizhat.com/tips/dcdrives.htm
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