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Gokulkumar

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Sirs,
Am facing problem in dc drive that is dc tacho fault . I have checked tacho and found values of armature resistance are same in all segments in commutator.
But motor speed not in control. It is automatically varying and suddenly stopped. Is there any bypass method available for tacho feedback ?
Pls help me.
 
TL;DR
Dc tacho generator fail
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darkwood

darkwood

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Hi Gokulkumer

We need a lot more info before we can advise.

What exactly is the set up here, is this a separate DC motor and tacho, is the tacho mounted and monitoring the speed or position something else for feedback to drive card or is it mounted on the motor itself to give closed loop control?

Under what conditions would the motor need to speed up and slow down, info needed to look at alternative solutions.
Have you monitored the output of the tacho, does it connect direct to the drive card or is there some form of signal conditioner or even a galvanic isolation set-up?

The more info you can offer the better the advise we can offer.

What are the details of the tacho, the drive and the motor?
 
G

Gokulkumar

  • Thread Starter Thread Starter
  • #3
Hi Gokulkumer

We need a lot more info before we can advise.

What exactly is the set up here, is this a separate DC motor and tacho, is the tacho mounted and monitoring the speed or position something else for feedback to drive card or is it mounted on the motor itself to give closed loop control?

Under what conditions would the motor need to speed up and slow down, info needed to look at alternative solutions.
Have you monitored the output of the tacho, does it connect direct to the drive card or is there some form of signal conditioner or even a galvanic isolation set-up?

The more info you can offer the better the advise we can offer.

What are the details of the tacho, the drive and the motor?

Dear sir ,

The dc motor 50hp was coupled with gear unit and gear unit connected with rotating table (3m Dia) . Vertical turning lathe machine. We wil machine the job. Operator was slowly increase the speed using pot upto required level. Then he started machining work. But it suddenly speed wil come and automatically increased
Some time alarm wil come TACH_F. Tacho was mounted in the dc motor shaft.
Today I removed the tacho and checked the resistance value of each segaments are same 290ohms. Two wires from tacho A1& A2 is directly connected to the drive PCB terminals. I have checked the carbon brushes and bearing are in good condition in tacho. But this proplem repeated

Tacho detail : radio energie model RE0122.
Motor : 400v dc motor
Drive : Alsando drive from Italy.

Expecting valuable advice
 
Lucien Nunes

Lucien Nunes

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The fault could be in the drive electronics rather than the tachogenerator itself. I would connect a DC voltmeter or even an oscilloscope across the wires from the tacho and check whether the voltage is steady at constant speed, and whether anything changes at the time when the drive goes out of closed loop control or the tacho error light comes on.
 
darkwood

darkwood

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We need to eliminate the cheaper often more common issues and we also need to try understand the causes of what you describe.

Let's assume the drive-card is working fine, I would suggest it is due to it flagging up and identifying tacho error and the fact you have been having speed up issues, It may be down to a lost signal which if it lasts long enough the drive card would go into error, if however it was short lived or a breakdown in voltage through a bad connection, damaged cable etc the drive card may assume the motor is going slower than it actually is and attempt to speed it up.

You need to monitor the voltage output of the tacho when working correctly and when it speeds up for no reason, if you see a voltage loss or drop then the drive is doing what it is designed to do, trying to speed up to get to the set value, if you are seeing a voltage drop then you need to establish why.

Several reasons I have found in the past -

Tacho slippage - the tacho's connection to the shaft has become loose, it sometimes slips and give the wrong feedback

Damaged cable or bad connection - self explanatory

Brushes are not running smooth, are worn down or are bouncing and catching in their holders so they intermittently break the circuit and possible vibrate loose again and make re-establish it, intermittent loss or voltage fluctuation caused by such problems as also with the above suggestions may also cause the drive to trip in error repeatedly or occasionally.

Personally I would be checking the tacho is secure on the motor shaft as it is a common issue to work loose over time and start slipping in my experience...
 
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pc1966

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Good advice already: systematically check for the cheap-to-repair faults first. The list from @darkwood is an excellent start.

Finally, if you see no problem with the tacho operation back to the drive system, then also check the speed control pot as it might be a fault there is commanding the drive system in error.

Replacing the tacho or drive electronics will be very expensive compared to (hopefully) finding a repairable fault in the mechanical links, replaceable brushes, cabling, or command potentiometer.

Good luck! Please inform us of what you find.
 
Lucien Nunes

Lucien Nunes

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Key point from Darkwood there - if the voltage drops as the machine speeds up it's probably the tacho at fault and the drive is responding to an apparent loss of speed. If the voltage rises it's the drive malfunctioning and the tacho is simply reporting the increase in speed.
 
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pc1966

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Reading your post again:
Then he started machining work. But it suddenly speed will come and automatically increased
Some time alarm will come TACH_F.
That certainly sounds like a loss of tacho feedback.

I don't know the equipment to know if a TACH_F alarm is from a loss of voltage or an open circuit. Loss of feedback voltage (or probably a big enough difference between the tacho and the armature voltage) could be from any sort of fault:
  • Mechanical coupling lost/badly slipping
  • Brushes worn/sticking so contact poor or missing
  • Tacho windings going open circuit
  • Cable / connector / joint going open circuit
  • Cable shorting out
If it is detecting an open circuit then it would be a limited number of those possibilities (basically brushes, windings or cable/connector/joint open).
 
G

Gokulkumar

  • Thread Starter Thread Starter
  • #9
Dear sir,

Today we have removed the tacho and put some CTC ,and clean the Bridgers and armanture surfaces. Now tacho fail Alram gone. New alarm came. Phase failure. But incoming phase is ok. Motor run for 2nts and getting off. That time I checked the motor current it's varying 0 to 10 amps.
Now I am confusing it is motor fault or drive fault or tacho fault. Pls some one clear the doubts.
Post automatically merged:

Can we by pass tha tacho feed back in the drive. Is it possible. ? If yes. How to do ?
 
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pc1966

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Do you know if the phase failure warning also applies to the drive-motor connections?

Check the motor cables/connections in case work on the tacho has disturbed something.

Also double-check that various things are properly grounded (motor chassis, drive system ground, tacho cable shield if fitted, etc) as electronics can produce strange errors if there are interference signals on cables.
Post automatically merged:

Also check the supply at the motor drive in case there is any unexpected voltage drop on the feed cables (i.e. your incoming factory supply may be fine, but it might not be correct at the drive itself).
 
darkwood

darkwood

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When you say new alarm, where is this alarm coming from, is it the same card the tacho error was given or is it on something else, if it is on the same card then as speculated earlier Lucien it may be the drive card at fault, having said that if you have a breakdown in the supply to the card which is intermittent it in itself could be causing transients that is giving false alarm conditions. (Has the card been replaced, please can you confirm yes or no).

You really need to take this stage by stage ensuring all connection and contactors upstream of the card are sound, it is hard to advise too much through a forum as diagnostics are best done on the job and we may lack info you have missed or not considered as important which could be vital to the advice we give.

An intermittent volts drop on the drive card input could manifest into control and monitoring issues, depending on the cards age the cap's may be old and not as efficient to smooth out short lived transients, other issues is if the card is converting AC to DC, if any of the rectifiers have failed we could be seeing AC waveforms on DC control and power, you really need to go through all the power and control system with a fine toothbrush starting upstream with power to the card and whatever switches it to the output to the motor and control circuits.. If possible put the card in a trial run with direct power connection on the input and you manipulating the card through its control terminals to see if it runs fine uncoupled from the rest of the system.

A background of knowledge will be needed and the correct test equipment otherwise I would suggest bringing in an engineer who is familiar with this set-up.
 
G

Gokulkumar

  • Thread Starter Thread Starter
  • #12
Dear sir ,

Is the any method available to by pass the techo generator ?

I want to run the machine immedietly. pls let me know.

If tacho bypassed , what will happened?

Pls
 
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Reply to Dc tacho fail dc motor in the Electrical Forum area at ElectriciansForums.net

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