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If DC generator excited by AC 3
- Thread starter harry641
- Start date
- Thread starter
- #4
I am testing DC traction motors which being supplied by two series Dc generators,when I am testing at High vlotage 1000 v 250 A most brushes start sparking If we suppose ARM,interpoles and brush boxes alignments are OK, you reckon it could be dc vlotage control board which exciting main DC generators is faulty?!!! it may generate AC which affect main DC generators output?!!!
Skogsgurra
Electrical
Why assume the most unlikely? And, why not check what the excitation voltage looks like? You got an oscilloscope, haven't you?
Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
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2
- Moderator
- #6
Interesting question.
Yes, you can get AC out of a generator by applying AC to the field.
HOWEVER, only at very low frequencies, measured in cycles per minute rather than cycles per second.
A good example would be the log carriage in a lumber mill.
These used to be driven by a DC motor direct connected to a DC generator. The motor field was connected to rated excitation voltage. The field of the DC generator was varied and reversed to vary the speed of the drive and the direction. Some were capable of 5 or 6 cycles per minute.
The wave form is closer to a square wave than a sin wave.
However in your instance, you suspect 60 Hz.
A generator field is very inductive. At 60 Hz. the field acts as a filter to smooth out the 120Hz. ripple resulting from full wave rectification of 60Hz.
As far as 60 Hz. AC resulting in the generator producing an AC output, the current and also the resulting field strength will be very low. The inductive reactance and the resulting impedance of the generator field at 60 Hz. will be many times the DC resistance. The current and the resulting field strength will be very low, resulting in a very low AC output.
Do I understand that you are testing large motors at 1000 volts? What is the rated armature voltage of the motors?
If you go much over rated voltage on a comutator you will get sparking. Go a little higher on the voltage and you can expect the "Ring of Fire" as the current takes a direct path across the surface of the comutator from brush holder to brush holder.
respectfully
Yes, you can get AC out of a generator by applying AC to the field.
HOWEVER, only at very low frequencies, measured in cycles per minute rather than cycles per second.
A good example would be the log carriage in a lumber mill.
These used to be driven by a DC motor direct connected to a DC generator. The motor field was connected to rated excitation voltage. The field of the DC generator was varied and reversed to vary the speed of the drive and the direction. Some were capable of 5 or 6 cycles per minute.
The wave form is closer to a square wave than a sin wave.
However in your instance, you suspect 60 Hz.
A generator field is very inductive. At 60 Hz. the field acts as a filter to smooth out the 120Hz. ripple resulting from full wave rectification of 60Hz.
As far as 60 Hz. AC resulting in the generator producing an AC output, the current and also the resulting field strength will be very low. The inductive reactance and the resulting impedance of the generator field at 60 Hz. will be many times the DC resistance. The current and the resulting field strength will be very low, resulting in a very low AC output.
Do I understand that you are testing large motors at 1000 volts? What is the rated armature voltage of the motors?
If you go much over rated voltage on a comutator you will get sparking. Go a little higher on the voltage and you can expect the "Ring of Fire" as the current takes a direct path across the surface of the comutator from brush holder to brush holder.
respectfully
harry641, as skogsgurra said "why assume the most unlikely?" If you got that pressumption, why not investigate it right away. You may use your resources, if you had it.
Waross, had a good example. that's nice.
When testing dc machines, i never seen a dc machine supplied with a armature voltage greater than its rating that never result to sparking. Maybe you're supplying voltage more than its limit.
Does 1000V really the maximum armature voltage rating? What is the field voltage rating?
Waross, had a good example. that's nice.
When testing dc machines, i never seen a dc machine supplied with a armature voltage greater than its rating that never result to sparking. Maybe you're supplying voltage more than its limit.
Does 1000V really the maximum armature voltage rating? What is the field voltage rating?
- Moderator
- #9
If the field of a generator is reversed the polarity of the output will be reversed.
Stated another way, the output of a generator will be proportional to the strength of the generator field and the polarity of the output will depend on the polarity of the generator field.
Google Ward-Leonard drives. The motor has full field excitation at all times.
In my previous post, it is probably more accurate and less controversial to call the field excitation and the output reversing rather than alternating.
With the inductance and the resistance of the generator field you may calculate a property of the generator field called the rise time. On a generator suitable for testing traction motors the rise time will be quite long compared to the period of a 60 Hz. wave. 60 Hz into a generator field will result in a very small 60 Hz. output. VERY small. Usually not enough to be useful, or a problem.
respectfully
Stated another way, the output of a generator will be proportional to the strength of the generator field and the polarity of the output will depend on the polarity of the generator field.
Google Ward-Leonard drives. The motor has full field excitation at all times.
In my previous post, it is probably more accurate and less controversial to call the field excitation and the output reversing rather than alternating.
With the inductance and the resistance of the generator field you may calculate a property of the generator field called the rise time. On a generator suitable for testing traction motors the rise time will be quite long compared to the period of a 60 Hz. wave. 60 Hz into a generator field will result in a very small 60 Hz. output. VERY small. Usually not enough to be useful, or a problem.
respectfully
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1
- #10
Harry641,
We need a little more information on the rating of your motor.
1000 Volts supply is this the rating of the motor, or is it higher or lower?
250 Amps. Where is this with respect to the motor rating ?
If you have the motor commutator sparking and this test point is within the normal operating limits of the motor, then the fact that the ".....ARM,interpoles and brush boxes alignments are OK....." is not enough to eliminate the motor as being the cause of the sparking.
Sparking at the commutator is caused by brushes not being at the neutral point of the motor. The neutral point moves as a function of load and the purpose of the interpols is to counteract this effect to keep it at the point where the brushes are located. Other windings in the motor such as pole face windings also strive to keep the neutral position correctly located.
A failure in the interpol insulation (Shorted turns) will reduce the effectiveness of the interpoles and lead to sparking at higher load currents. Over voltage can also do this as mentioned by Waross.
A failed or failing "Equalizer" or even a failed main pole can cause sparking at almost any load level.
I am a litle puzzled by your comment that the motor is ".......being supplied by two series Dc generators...." This seems an odd configuration, but, just because I havn't heard of it doesn't make it wrong.
181273, we apply 2,000 HP DC motors, 8 mechanically in parallel that can go from full speed in one direction to full speed the other in about 3 seconds, giving a full cycle time if 6 seconds, This would be similar to a 10 Cycles per minute" AC waveform.
About the only time we put AC in the order of 50 or 60 Hz on the gernerator Fields is for locating the generator neutral point. It is a rough way of doing it but it gives limited success. The inductance of the fields is such that at 50 Hz, the impedence is so high that only very small 50 hz field current actually flows.
Tom Grayson
We need a little more information on the rating of your motor.
1000 Volts supply is this the rating of the motor, or is it higher or lower?
250 Amps. Where is this with respect to the motor rating ?
If you have the motor commutator sparking and this test point is within the normal operating limits of the motor, then the fact that the ".....ARM,interpoles and brush boxes alignments are OK....." is not enough to eliminate the motor as being the cause of the sparking.
Sparking at the commutator is caused by brushes not being at the neutral point of the motor. The neutral point moves as a function of load and the purpose of the interpols is to counteract this effect to keep it at the point where the brushes are located. Other windings in the motor such as pole face windings also strive to keep the neutral position correctly located.
A failure in the interpol insulation (Shorted turns) will reduce the effectiveness of the interpoles and lead to sparking at higher load currents. Over voltage can also do this as mentioned by Waross.
A failed or failing "Equalizer" or even a failed main pole can cause sparking at almost any load level.
I am a litle puzzled by your comment that the motor is ".......being supplied by two series Dc generators...." This seems an odd configuration, but, just because I havn't heard of it doesn't make it wrong.
181273, we apply 2,000 HP DC motors, 8 mechanically in parallel that can go from full speed in one direction to full speed the other in about 3 seconds, giving a full cycle time if 6 seconds, This would be similar to a 10 Cycles per minute" AC waveform.
About the only time we put AC in the order of 50 or 60 Hz on the gernerator Fields is for locating the generator neutral point. It is a rough way of doing it but it gives limited success. The inductance of the fields is such that at 50 Hz, the impedence is so high that only very small 50 hz field current actually flows.
Tom Grayson
in traction motors,keeping all brushes black is an essential,if we suppose the motor that Harry641 mentioned it's motor volatge rating is 1050 V 250 A at Hi speed stage and 700v 450A at HI current stage and the motor in Hi current stage, every things is Ok"no sparking" but in Hi speed low current 1000v 250A, the motor starts sparking at 800v up to 1000v, (which is applicable volatge)sparking level 1/5 intermittedly.after checking interpoles mains and brushes alignments,every things are fitted correctly.what else could be reasons for sparking in a number of traction motors at same time.
TomG33,why 2 generators are series,because every generator is generating 500v 250A so we had to series them for our purpose.
TomG33,why 2 generators are series,because every generator is generating 500v 250A so we had to series them for our purpose.
- Moderator
- #12
What type of sparking do you have?
A line of intense sparks onlong one edge of the brushes indicates that the brushes are not at the proper neutral point. Try shifting the brushes slightly in the direction to cover the sparks.
Lines of sparks extending away from the brushes may indicate over voltage, high mica, dirty comutator, the wrong grade of brushes, or some other problem.
remember that the higher the speed and voltage, the higher the voltage between the comutator segments. A very slight misposition of the brushes may only show up when the voltage is increased. I would first "cover the sparks" to avoid sparking at high voltage and then observe at lower voltages. I will be surprised if, after an adjustment, there is sparking at low voltages but not at high voltages.
respectfully
A line of intense sparks onlong one edge of the brushes indicates that the brushes are not at the proper neutral point. Try shifting the brushes slightly in the direction to cover the sparks.
Lines of sparks extending away from the brushes may indicate over voltage, high mica, dirty comutator, the wrong grade of brushes, or some other problem.
remember that the higher the speed and voltage, the higher the voltage between the comutator segments. A very slight misposition of the brushes may only show up when the voltage is increased. I would first "cover the sparks" to avoid sparking at high voltage and then observe at lower voltages. I will be surprised if, after an adjustment, there is sparking at low voltages but not at high voltages.
respectfully
thank you waross,
in low voltage we have no sparking but at 800v up to 1000v sparking start at leaving adge of brushes,swing onlong of brushes intermittedly,as glowing point travelling onlong brushes then disappearing and coming back in couple seconds.we checked all brush boxes alignments and interpols,they seem OK.I have to say in D29 traction motors brush boxes are fixed and can not be shifted around.
in low voltage we have no sparking but at 800v up to 1000v sparking start at leaving adge of brushes,swing onlong of brushes intermittedly,as glowing point travelling onlong brushes then disappearing and coming back in couple seconds.we checked all brush boxes alignments and interpols,they seem OK.I have to say in D29 traction motors brush boxes are fixed and can not be shifted around.
- Moderator
- #14
My brush experience is many years ago. I can't recall such a problem with any brushes. My most common problem was fine dust between the brushes. If the brushes are tight or firm in the brush boxes you may experience sparking. The brushes must be completely free to float in the boxes.
Brushes out of position would give a steady bright line of arcing on one side of the brushes.
I would consider skogsgurra's suggestion of a "scope.
I would look at both the voltage supplied to the motor and the current. Both at once if you have a dual trace 'scope.
Check the inside of the brush boxes for smoothness. Try a different grade of brushes. Check that the motor is not being unintentionally overloaded.
Is this one or two motors or almost all motors tested? The answer to this will direct us to look for a fault in one motor or a problem with the test setup.
respectfully
Brushes out of position would give a steady bright line of arcing on one side of the brushes.
I would consider skogsgurra's suggestion of a "scope.
I would look at both the voltage supplied to the motor and the current. Both at once if you have a dual trace 'scope.
Check the inside of the brush boxes for smoothness. Try a different grade of brushes. Check that the motor is not being unintentionally overloaded.
Is this one or two motors or almost all motors tested? The answer to this will direct us to look for a fault in one motor or a problem with the test setup.
respectfully
- Moderator
- #16
waross,
I mean most recent we overhauled and rewound motors, as usual, the motors are GE D29 EMD traction motors.link below I found a sample of motor image, top image in the page:
I mean most recent we overhauled and rewound motors, as usual, the motors are GE D29 EMD traction motors.link below I found a sample of motor image, top image in the page:
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