DC motor speed 3

[oftenlost]

300 HP Reliance shunt DC 850/1400 RPM 500/240VDC. Rebuilders nameplate only. 16.8 amp max field amps. Suffering overcurrent trip at full speed.(fan pump application)Armature checked good, fields drop test good, motor runs 200 RPM faster than base speed at max field amps. On load test,motor reached nameplate armature amps at 280 HP. Rewound fields. Added shims to .030 existing shims in steps until .060 added. Motor now achieves 300 HP at nameplate amps, barely. At 310 HP, armature amps increased from 475 to 530 amps.Base speed changed only 10 RPM per .020 shim change. Varying field current from 7 to 24 amps changes speed only 100 RPM. Normal??

 

[waross]

Have you checked the condition and connections of the compensating poles? It may be over compounded. Is there any sparking at the brushes, and does the sparking pattern change with loading? If the sparking moves from one side of the brushes to the other, a compensating winding problem is a prime suspect. If the compensating windings were designed for parallel connection and inadvertently reconnected in series, you could expect problems.
yours

 

[oftenlost]

My thoughts exactly. Ran the motor on the armature alone with no change in speed. The motor is compensated shunt but these were disconnected and later removed during rewind. I thought of an improperly rewound armature but the speed not high enough

 

[waross]

That's a pretty big motor to run without compensating poles.
Without the field contribution of the inter poles, you may be running in a weakened field condition which will give you over speed and high armature current.
There's an old trick for setting brush positions. I've only used it on exciters, but with an appropriate voltage it will probably work on any size motor. Connect AC to the field. Rotate the brushholders while measuring the voltage at the brushes. Zero volts is the best brush setting that you can determine without running the motor. You can also use the voltmeter leads directly on the commutator.
We had an exciter that failed when it was hot. The internal heating patterns were different depending on whether it was in service or in an oven and it would not fail in the oven in the shop. The shop was reluctant to rewind it when they could not detect a fault. I agreed.
When the exciter was hot I could not find a brush position that would eliminate sparking. I was using the AC test to reset the brushes, and every time the set was stopped, the optimum brush position appeared to have changed.
I then cranked the set with AC on the field and the voltmeter connected.
The voltage at the brushes would fluctuate as the machine rotated.
I relized that I had arranged an "In Place Growler".
Back at the shop, I described the test and the tech ageed that the armature had been proved faulty, and proceeded with the rewind.
It's a quick and easy test of an armature in place.
1> Disconnect the field and the brushes.
2> Connect a voltmeter to the brushes, and an AC voltage to the field.
3> Rotate the machine and watch the voltage. It should be zero. Speed of rotation is unimportant. The test is position dependent and not speed dependent. To check to see if you have enough induced voltage for a reliable test, move the brushes, or test the voltage directly on the commutator between the brush holders.
In your case, I would expect the brushes to be set for full load condition and the no-load brush position will be significantly different.
I would expect the AC field test to show your brushes out of position, but the voltage will be stable as the machine rotates. I don't recommend resetting the brushes until the compensating poles are reinstalled.
Without compensating poles, this test will tell you Two things;
1> Condition of the armature.
(I expect that you will see a small but steady voltage. Steady means good armature, A voltage other than zero indicates that the brushes are off the neutral position.)
2> If the actual brush position is significantly different than the brush position indicated by the test, it is another indication that the motor wants compensating poles. The indicated shift in brush position is an indication of the shift in the uncompensated field flux under load.
yours

 

[oftenlost]

I did set the brush position with AC and verified by reversing rotation while monitoring armature voltage, field current and rotational speed. I have never applied AC and rotated armature but I will try it tomorrow morning. Thanks for the input

 

[waross]

Do you have to adjust the brushes at full load to reduce sparking/arcing?

 

[oftenlost]

The unit has never suffered from arcing except for the short period when I had my technicians run and lightly load it with the interpoles disconnected. There is no arcing at all. The motor just will not slow down to the nameplate speed and required halving the airgap to achieve nameplate horsepower without overcurrent. I know that I cannot leave the airgap where it is. .060 air gap on this type machine is probably less than half design.

 

[waross]

Sparking under light load may suggest that the brushes were set under load. Normally as the load increases, the flux field distorts. This moves the optimum position of the brushes. One of the purposes of the compensating poles is to "Push" the flux back into place to improve commutation. With a variable load on a motor with no compensating poles, the brush position under load is different from the brush position under no load.
With the relatively steady load of a fan pump, you can adjust the brushes for good commutation at that load, but I would expect them to spark under light load.
Over speed, over current, sparking at light load, this all points to an inter-pole problem.
At full load, there is a considerable increase in field flux supplied by the inter-poles.
I am puzzled by the small change in speed with the change in field current. I would have expected a much larger speed change. Was the change in field current from 7 amps to 24 amps under load or running unloaded?
If the motor was loaded, the characteristics of the pump may have been increasing the load so that the increased load held the speed change to a minimum. Did the armature current increase as the field current was reduced?
I may be missing something. Maybe some of the experts can make some suggestions.
yours

 

[DickDV]

If this motor is nameplated "comp shunt", it is a compound wound motor with a series field and a shunt field. If the series field has been disconnected from the armature circuit (this is often done to convert a motor with non-symetrical torque to symetrical), the output torque and therefore hp is reduced somewhat for a given armature current.

In order to acheive the nameplate hp on a comp shunt motor, the series field must be operable and the nameplate hp will only be available in the forward direction.

 

[oftenlost]

Thank you for your inputs. I probably have not explained the problem and steps taken adequately. There is a hole in my logic but I do not know what.
Motor weak and runs too fast.
-Simplex/duplex armature winding. Possibly but speed should be 425 or 1700 RPM
-fields connected wrong. Possibly but the current is too close to nameplate (16.8 at 260 volts cold as opposed to an expected 19 or 20 at 240 cold)
-Interpoles parallel vs series. Perhaps but motor exhibits problem with the poles completely removed
-Stabilizing winding out of circuit. Maybe but many of these motors are operating locally with such a configuration due to the confusion of what to do with the S leads on reversal. Besides, the air gap was halved, effectively quadrupling the field flux.
-equalizer problem. Don't think so. No arcing.
-I have seen Reliance motors in this frame size up to 500 HP so I do not believe that it is too small. Maybe two problems caused by the rerate of a higher horsepower, higher speed motor to lower speed lower horsepower.

 

[edison123]

My guess would be the problem is in the shunt field winding

1. Too small a wire cross-section (because of 22% higher field winding resistance).

2. Too small no. of turns in the coils (because of higher speed at the rated field current).

Both of the above would result in significant copper weight and may be the rewinder cut corners here and didn't inform you.

I wouldn't fool around with machine air-gaps.

Armature winding has to be simplex because of the HP.



"Most people stop working when they find a job"

 

[waross]

A couple of points about interpoles.
1> The ones that I have worked on have been connected directly to one side of the brushes and were not shown on the diagrams.
The Armature leads included the interpoles.
Don't confuse interpoles with series poles. Thre is a big difference.
2> I am wondering about the source of the concerns about reversing. No problem. Almost all the DC drive systems that I have worked on (up to 1300 HP) were reversing and used interpoles. When the current through the armature is reversed the current through the interpoles is reversed.

3> Weak field. The interpoles are in series with the armature current and add considerable field strength at full load. Without interpoles your field will be weak.

4> No arcing. Without interpoles you can set the brushes for no arcing at a constant load such as a fan pump. However the brushes will then spark at light load, and I understand they do spark at light loads.

5> There was reference to "S" leads. These are series leads on a compound machine. There are no "S" leads on a shunt machine but there are interpoles. On the diagrams the interpoles are often not shown, but they are included in the armature circuit and are permanently connected.
5>

 

[aolalde]

For satisfactory performance a DC motor must be connected exactly as per the original motor design. If you remove windings, modify the air gaps, increase or decrease the strength of the field, etc. How to expect a performance per the name plate?
By the way, the function of the Interpoles and the pole phase windings is to cancel the armature reaction and allow proper commutation for a wide range of armature current.
I do not mean modifications are not possible but those must be calculated by a DC motor designer expert, trial and error will lead to a catastrophic failure.

 

[oftenlost]

Thank each of you for your posts. I appreciate your time and expertise. We do many DC motors and this is the first which did not offer much field control. I found the manufacturer's ID stamped on the machine and determined that the rebuilder modified the fields on a machine designed for 1150 to 1500 RPM to attempt to make it run at 850-1700 RPM. Strangely, he decreased turns, decreased wire size, added shims and increased field current. I had already rewound the fields with larger wire as the density was high and guessed pretty closely the required turns so it appears that I just need to reestablish the proper air gap to see if he also modified the armature. I appreciate your input. This machine was not responding according to my expectations. I will post the results of my full load test later this week. Again, thanks

 

[waross]

Hello oftenlost
The smaller wire will give a weaker field. Within reasonable limits the machine will run faster. There is a technique called "Field weakening" that is used to run a machine over base speed. A resistor is added in series to reduce the field current and weaken the field. That was a long time ago. Today you can probably just dial the field voltage down to get the same effect. Rather than rewinding the shop could have just added a resistor in series with the coil.
The number of turns may not be as important to the speed as you think.
For simplicity this example will consider that the average resistance per turn remains the same when turns are added.
Consider a coil with 100 turns with a resistance of 100 ohms at 100 volts.
we have 1 amp times 100 turns equals 100 amp turns.
Now add another 100 turns.
The resistance will double to 200 ohms and the current will drop to 1/2 amp.
200 turns times 1/2 amp equals 100 amp turns.
1000 turns, 1000 Ohms, .1 Amps, = 100 amp turns.
From this we see that wire size is the main factor in the field strength that a coil can produce. The more turns, the higher resistance and the lower the current. The more turns the less power required, and the less heating.
Because of inductance this relationship doesn't hold true on AC.
Wire gauge or average resistance per turn determines the field strength.
More turns results in less current and less power required.
yours

 

[aolalde]

Hello Waross; My opinion is that the field winding modification is not that easy. In your example if you add another 100 turns the mean length of the extra 100 turns is longer than that in the original 100 turns. Now the resultant resistance is more than double (R = p*L/A). Consider now the heat generated and transfer conductivity due to that modification and the temperature rise will need to be higher, increasing again the temperature factor for the resistance of this new construction {R2 = R1*(234.5+T2) / (234.5+T1) assuming copper windings}. I suggest that any attempted change should be engineered and evaluated with detail.

 

[waross]

Hi aolalde
You are completely correct. The example was simplified to demonstrate that wire size was more important than the number of turns in determining field strength. The space for the field coil in a motor will dictate the number of turns of a given size of wire that can be wound. The bigger the wire, the stronger the field.
Please consider a field with two coils wound side by side on the same pole so that the average turn length is the same in each coil. The amp turns will be the same with one coil singly or two coils in series. My amp turns comments will be verified. And I agree with you that with only one coil in the circuit it will run hotter, increasing the resistance and lowering the field strength marginally and your comments are verified.
I can agree with you, I hope you can agree with me.
yours

 

[aolalde]

Waross; Yes I concur with your theoretical example, it produces the very same total ampere-turns.