DC tach failure - output 75% of nameplate 1

[lyallst]

I have a 60v / 1000rpm tach in the control loop for a 30kW DC motor. Recently the printing press that the motor drives has been running faster than rpm gauge is indicating. I have taken a reading of 45v on the tack leads while the motor was running at 1060rpm (measured with an optical hand held tack on the motor output shaft.) the tack is connected to the DC drive and a rpm Gauge. I have disconnected the gauge and achieved the same results. however I can not easily disconnect the tack from the drive. My next step is to replace the tack. the tack brushes, commutator, cable and termination connections are in good condition.

Have you ever seen a tack fail in such a manner?

Past failures of a tack that I have come across are related to brushes and bearings or open windings. This situation seems strange to me!

thanks,

Steve

 

[Skogsgurra]

I have had this with Huebner tachos. Check with an oscilloscope if the DC is "continous" or if you have dropouts. It seems that the armature winding can have open turns and if there are four sections and one is open, you will have exactly this effect. You should also be able to locate the open part by connecting an ohmmeter and turning the rotor slowly.

 

[lyallst]

thank you,

your comments will allow me to purchase a new tach with more confidence that I am not wasting money.

Steve

 

[jbartos]

Suggestion: It may need a little as proper commutator maintenance. Check the commutator for proper insulation between lamellae and mica insulation between lamellae. There should be a small groove between each lamella. The mica should not be part of the same commutator perimeter as the lamellae or protruding outside of the commutator perimeter.

 

[simtricity]

Quick tip-If you are experiencing problems with a tacho,switch your drive to armature feedback this wiil give you some time until a spare arrives.

 

[jbartos]

Suggestion: Visit

http://www.goodmart.com/products/Commutator_Maintenance.htm

http://www.goodmart.com/products/electrical_supplies_commutator_maintenance_misc_commutator.htm

http://www.idealindustries.co.uk/index.cfm?pid=17

http://www.usbr.gov/power/data/fist/fist3_14/3_14_1.htm

etc. for more info on commutator maintenance, just in case the commutator is the culprit.

 

[Digitalwoodshop]

I agree with much of what was mentioned but most of all proper commutator maintenance. Another reason for the reduced output could be shorted turns in the copper wire windings due to a breakdown of the enamel coverings. Simply put, formerly 100 turns = 100% output. Then heat, shock, vibration cause some of the turns to short where they overlap. Now you have 75 turns, hence 75% output. The tach makes voltage from motion. I once had a 3 inch coil inside a Navy Radar Brake unit fail like that. The normal 45 volts DC didn't have the same magnetic effect causing the mechanical brakes to drag. By upping the voltage by 10 volts the problem went away. A replacement was ordered and the voltage went back to normal with the replacement. The temporary unsafe voltage was approved until the mission was over. Carefully unwinding of the defective coil turned up the shorted windings.
Replacement of the Tach was the best fix.

Al

 

[lyallst]

I have replaced the tachometer and all is well. It would appear that the tach was causing drive fluctuations that resulted in growling/vibration through the drive line of the printing press. after replacing the tach the press speed returned to normal as well as eliminating some noises that were causing concern from operators.

 

[Skogsgurra]

So, you didn't waste your money then? Good to know.

Did you measure the resistance? (all way round)

 

[lyallst]

I measured a resistance of 215 ohms with fluctuations from 192 ohms to 222 ohms through one complete revolution.

 

[jbartos]

Suggestion to the previous posting: The ohms may or may not be representative if the circuit with measured resistance does not have the rated current flowing.