DC Motor Speed Goes Down When it is on Load

You need to show the forum the details of the motor rating plate and the position on the load's torque/speed curve at which you are trying to operate, if you want any serious help.

Hii

The details in the name plate are listed below

DC MOTOR DETAILS

MAKE :- AEG

Typ GF 20.02

Mot Nr 77 -785869

460v 135A

56KW - S1

1500 1/min

Err 340V 2.82 A

Isol KI F IP 23S

VDE 0530 / 11.72

What will be the field winding resistance supposed to be for this motor ?

Regards

JAGAN

Sorry! Field wounding 96 ohm/20 oC is good. At 85 oC will be 340/2.82=120.5 ohm.

Check the actual dc voltages-at armature and field terminals.

Since actual current is low, it must be the drive and/or it's settings. S.M.

Was it doing this before you sent it to the rewinder? If not then send it back!

I suppose there is a tachogenerator. Did you check it?

Check basics like drive settings, motor armature and field currents and voltages, tacho voltage, ajustment of the brush carrier (do collector and carbon brushes look fine, is there excessive sparking?)... As there's no runaway I suppose polarities are correct.

As I avoid Lenze products at any price I can't help you in detail regarding the used drive (not bashing a brand, it's just based on experience; usually I try to remind neutral but here I can't, sorry).

(BTW I'm not exactly a beginner, please feel free to check my other posts.)

Probably, a faulty winding. The drive tells you the truth.

Looks like faulty winding done as it is not taking load. Quality of winding wire may be an issue.

Either your motor winding works is no good or you have a faulty DC drives. Try to in change another DC drives and test it.

I first suspect the new winding job of the field. Did you give them all of the specifications you wanted for them to replace the field winding?

2nd issue, why did the field fail in the first place?

How is the rotor, armature winding? How is the condition of the commutator? What are the condition of the brushes?

According to Lenze dc motor drive catalogue:

http://www.lenze.org.ua/pdf/480_Speed_controllers_1002_EN.pdf

it seems to me it is a separate excitation dc motor of 400 Vdc.

The speed rule could be:

speed=(Vdc-Rarm.*Iarm.)/(k.Iexcit).

If the load [armature current] increases the excitation has to decrease in order to keep the speed constant.

If the measuring [shunt] shows an overload when the load is 12 A instead of- let's say -56000/400=140 A then it seems to me the supply voltage Vdc is low and Vdc-R*I is close to zero.

Check the dc voltage supplied by drive. If this is a SCR bridge rectifier it could be an overheating due to lack of ventilation [also].

However, the field resistance it seems to me double than it has to be.

I still know nothing about the load, Mildred. Please will you do something to change that state of affairs?

I'd also like to know what happens when you run the motor using the drive, with the load mechanically disconnected, please.

If the posted PDF links is the manual of the used Lenze DC drive it seems to be a very basic analogic model with fixed field voltage (didn't know that analogic models were still manufactured when the whole EC conformity crap was already required but with Lenze I'm not surprised). All larger DC drives I commisioned were able to handle field weakening (not that you always require that feature of course) but since about two decades new DC drives have become rare where I live (though there a still quite some in use).

Do you have a test bench protocol from those who've refurbished the motor? While full load benches are not always available you should at least get basic test data like armature and field voltages, armature and field current at nominal speed as well as tachogenerator voltage (e.g. at 1000 RPM or at nominal speed).

If the drive is running at maximum, one ought not to be surprised when the speed drops as the load is increased, for the drive has no facility to increase the speed to "catch up".

As indicated above, the load and the set speed of the drive is of immediate interest to the forum.

Armature (Ankerspannung): 460 V 135 A (at full load and nominal speed)

Power (Output at Shaft): 56 kW 100% ED (S1 = Continuous service)

Angular Speed: 1500 1/min

Field (Feld; Erregung): 340 V 2.82 A (check that you don't exceed the nominal current, in addition, is there a field weakening range mentioned?)

Insulation Temperature Class: F (relates to the design temperature, not necessarily the class of the used wire, it could be H class wire but design temperatures corresponding to F class or simply F class wire).

Degree of Protection (Schutzart): IP 23S

Standard: VDE 0530 / 11.72 (former German Standard, Edition Nov. 1972)

Put it back on whoever did the rewind job, since it sounds like your conditions are not near the motor's original limits, but may I ask, why did the motor need re-winding in the first place? Did something go haywire with the load, and over heat the windings?

I remember a DC motor driving a variable speed conveyor belt in one of my former employments, and there were always problems with the DC motors burning out, or the controllers burning out, and usually it always traced back to poor maintenance of the load system components (bad bearings on rollers, misalignment, foreign object, etc.

Go over the entire situation with a close watch on (1) how it looks, how it sounds, and (2) what you need your mover to accomplish whether you are asking it for too much.

Actually the problem was from the drive , the new cards i brought for the drive was faulty, i saw one resistor was absent from old card and saw one additional resistor was present .So i reworked on the card made as the old card and now the drive is working fine .Thanks for all the replies posted on this topic.

Don't you just hate it when OP puts forth some vague problem he is having, then gets all of us to "feel of the elepant". I think I know which part I was feeling of...LOL.

IT was big and stinky.

Then the OP magically dismisses all of our consternations in one fell swoop, with: "Oh well, I found a resistor missing on the control card..." DOH, why didn't you state that in the original question, rather than play hero?

Actually the problem was from the drive , the new cards i brought for the drive was faulty, i saw one resistor was absent from old card and saw one additional resistor was present .So i reworked on the card made as the old card and now the drive is working fine .Thanks for all the replies posted on this topic.