AC motors 9

[jacktheripper]

what are the predictive technologies and preventive maintenance use to monitor their operation status.

 

[electricpete]

Here's a list, with the most useful in each category (on-line/off-line) near the beginning of list (my opinion).

On-line:
Vibration analysis
Lube oil monitoring
current signature analysis to check for rotor problems
monitoring of winding rtd's and bearing thermocouples if so equipped
current magnitude and speed as indicator of loading (particularly if temperatures are not monitored)
infra-red and contact temperature measurements
power analysis for imbalance, harmonics, etc (pdma)
partial discharge monitoring (usually only 13.2kv and above)
ozone sniffer (Draeger tube)

Off-line
regrease bearings and change oil using time-based or condition-based strategy
insulation resistance
hi-pot (controversial)
winding resistance test
visual inspection
surge comparison test

 

[elecman]

electricpete has covered just about everything. Something I don't specify myself, because very few end users tend to bother with them, are Rogowski Coils. Only practical for large machines, these are potentially a good method of preventive maintenance in getting signatures etc.

 

[jbartos]

Suggestion to electricpete (Electrical) Jul 8, 2002 marked ///\\Here's a list, with the most useful in each category (on-line/off-line) near the beginning of list (my opinion).

On-line:
Vibration analysis
Lube oil monitoring
current signature analysis to check for rotor problems ///The current signature would be more suitable to apply during transition from off-line to on-line.\\monitoring of winding rtd's and bearing thermocouples if so equipped
current magnitude and speed as indicator of loading
(particularly if temperatures are not monitored)
///Also, the motor terminal voltage for undervoltages and overvoltages, sags, swells, spikes, etc.\\infra-red and contact temperature measurements
power analysis for imbalance, harmonics, etc (pdma)
partial discharge monitoring (usually only 13.2kv and above)
ozone sniffer (Draeger tube)
///Acoustic monitoring.\\
Off-line
regrease bearings and change oil using time-based or condition-based strategy
insulation resistance
hi-pot (controversial)
winding resistance test
visual inspection
surge comparison test
///Painting, maintenance, etc.\\

 

what is the advantage of a ac motors over dc motors

 

[jbartos]

Suggestion: Since some advantages may be viewed relatively, the following are potential advantages of the three-phase motors:
1. Many three-phase motors start with full-voltage across their terminals. The dc motors often have to be started over some resistor adjustments.
2. DC power supply often has to be obtained from ac power supply.
3. AC motors tend to be more reliable, with higher availability and lower downtime.
4. AC motors have somewhat simpler windings.
5. AC motors tend to run more quietly.
6. AC motors are more available in a wider range of sizes, e.g. from very small ones in HP to over 20000HP.
7. AC motors can have easily reversed their direction of rotation which is good for plugging.
8. AC motors tend to comply with industry standards in wider variety than dc motors.
9. AC motors have better service than dc motors.
10. AC motors of the same HP as dc motors tend to be less expensive.
11. Etc.

 

[electricpete]

ac induction motors are easier to maintain... no brushes.

 

[micjk]

electricpete & jbartos,
Would you consider thermography as a helpful proactive/ predictive/preventive tool for online testing or more of a troubleshooter.

 

[electricpete]

EPRI does recommend infrared scans of motors on a routine basis. However, we don't do routine thermography on any motors on our plants. Here is my reasoning:

For a large motors, we already have winding rtd's and bearing thermocouples, which provide a good indication of most thermal-related problems.

Smaller motors generally aren't as critical, so may not warrant the attention of doing an infrared scan and studying that data. If I was really interested in watching a critical small motor without installed temperature monitoring, I might consider contact temperature readings at one point on stator and one point each bearing and/or clamp-on current readings as a quicker easier alternative to infra-red. Also when I look at my vib spectra I sometimes check my running speed for indication of possible overload.

One potentially useful area for infra-red checks is on motor lead terminations for critical motors. However there is very little sensitivity to detect motor termination problems unless the motor lead box cover is removed.

As you point out thermography can be useful for troubleshooting.

 

[jbartos]

Suggestion: It could potentially be used for nonimpact transfer of heat info via heat impressions, e.g. bearing heat. Compare the thermographic printer principle, IEEE Std 100-2000 Dictionary.