Hello,

Im using DC motor to run the conveyor but suddenly the armature shows high current 130 A for 30 V power and the speed cant increase anymore. The motor power stated in the name plate is 93.8 KW, Armature voltage is 453 V and Armature ampere is 225 A. The motor can run free load after disconnected from the load but fail to run on load. We have checked the field coil, armature coil and the brushes . The are no insulation damaged since it can be run free load.

What are the possible causes to the motor that fail to run on load? Is it because not strong magnetism or another mechanical failure?

Possibly a failure of the drive or possibly changed settings.

Bill
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"Why not the best?"
Jimmy Carter

If you check DC motor from 30V power supply instead 453V nominal one, then problem will be in low bus voltage (U=backEMF + I*R)

Obviously the DC motor may "run" at 30 Volt input when mechanically disconnected rom the load, bur how can you expect it zo operate the conveyor at such a low armature voltage? If this is an electronic drive system, did it ever work properly or is this something new just being tested? See above last two posts.

rasevskii

Some more background info is needed:

Is this an existing installation that worked before and now does not?= Then something may be jammed up in the conveyor, and the motor cannot break it away. The current limit in the drive may be correctly limiting the armature amps according to its original settings (at commissioning) but now this current limit is too low to break away the load.

Do not increase the settings beyond the original ones to resolve the problem, if it is mechanical. You may damage the motor beyond easy repair (flashover or burned commutator).

I am assuming (never assume...)that this is an existing installation, not a new one. Pls elaborate.

rasevskii

Thanks for the reply...

Actually this is an existing motor. The field coil is just rewound. The problem occur when we want to install back the DC motor to the machine. After all the connection are connected to the load, we did test run to the motor. We found out the armature did not rotate and the voltage only can run until 30V input voltage only instead or 453 V which is the actual armature voltage. The current of the armature increase to 130 Ampere while the voltage just only 30 V. We have swapped new motor to the same machine and the motor worked fine. Means there is no problem with the drive and the control panel setting. The motor can run with no load. What is the possible cause that can produce high amp to the armature?

Call an electrician. Send the motor to a good rewind shop.

Bill
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"Why not the best?"
Jimmy Carter

I think rewound of field coil was made wrong or connections of sections (if there are 2 or more); if sections was wrong connected, field is canceled, going to low field value that impose very low back-emf and low torque also; try to disconect field coil and maybe nothing change; this motor don't have field (very weak more corectly).

I think it is obvious your motor repair shop reconfigured the field windings wrong (if not rewound them wrong). There are often multiple windings as well as compensating windings all in series to make up the field; if wired in reverse you will get results you have. If you have 2 clear sets of field windings maybe they just marked one set backwards - might be worth reversing one set and seeing if that fixes it to save trip back to rewind shop. If not, I would suggest taking it back to the place who rewound it for free warranty fix - ask them to run it before sending back to you this time.

Uh No field often means motor runaway and possible explosive dis-assembly. The compensating windings (in the armature circuit, NOT the field circuit) will often provide enough field that the motor will become a series motor of sorts. Unloaded series motors will reach higher than normal speeds.

Bill
--------------------
"Why not the best?"
Jimmy Carter

We have made surge test and winding test to the field coil. Phase to phase coil showed balance. We got the rated voltage and ampere. Still facing the same problem.. I have doubt here.. Will it possibly cause from tachometer? We have change the new tachometer from 20 A 1000 RPM to 20A 7200 RPM(max)..

P/S : Need advice because im new in this field.

funny how the little missing data comes up later :)

I assume you mean 20volts/1000rpm (typical good tach) changed to 20v/7200rpm tach (typical crappy cheap tach).

If so, and you did not rescale the drive tach input, then of course this would account for your issue and explain why you only get 30v across armature.... I did not go back to look at details but if this was 230v armature with base speed of 1800rpm, the old tach scaling was near 30v at base speed of 1800rpm. Now you feed it only 10 volts at same 1800rpm so the output armature voltage will be 3x lower or down at 70-ish volts. this motor will not be happy and do stuff you said earlier like pull way too much amps trying to make up for the missing Kt!

rescale the tach input per manual.

I would have thought that the controller would try to drive the motor 7.2 times faster for a given set point if the tach ratio alone was the issue. That is not to rule out other possible issues with the tach installation.
Another thought; If this was a tach issue the motor would not run at full speed unloaded.

Bill
--------------------
"Why not the best?"
Jimmy Carter

SOrry for the mistake.. The old tach is 20V/ 1000 RPM and the new tach is 20V/7200 RPM (max)... The rated voltage is 453 V 3000 RPM.. We have made some test to the tach, we have made free load test for old tach.... It is ran 57 V for 3000 RPM and for new tach 61 V for 3000 RPM....

so you are NOT saying the tach puts out 7.2x less than the old one; you are just saying the old tach was rated 20v/krpm and the new one is rated 7200rpm mechanical max speed limit with out listing the actual output rating. All tachs are rated in v/rpm AND a max speed limit. In case of your old tach, you list the v/rpm rating but not the max speed limit. in the case of your new tach you list the max speed but not its v/rpm rating then - just "20v" which is not a tach rating.

But even not knowing your new tach v/rpm rating you lucked out using it as you imperically proved it is also about 20v/krpm after all with your measurements. so ignore my suggestion of adjusting the tach input since you have now said it outputs virtually the same as the old one.

The problem solved! The armature has been rewound and it successfully run. Thank you for all supported.