eeinpa
Electrical
- Nov 12, 2006
- 65
Greetings!
We located a rebuilt 30hp shunt field DC motor for a customer to put on an air compressor which "kept burning up motors". Hmmmm. Upon installation, they reported it "runs way too fast" compared to its next door neighbor. They wanted to disassemble the motor and move the pole pieces closer to the armature. Yikes! Hold your horses, folks.
Nameplate specs:
Westinghouse, 30hp, 1750/2100 rpm, 104 FLA, 240Vdc, field 2.25A, field 88 ohms
My first thought was that they had too little field current, but upon questioning them, they said they had the field connected directly to the line. Actually, they said the starter (two step resistance type) has a field loss relay, but they had jumpered that out.
The motor shop reports that after rebuilding, the motor was tested at 1825rpm (no load) at 240Vdc with the field connected to line. I calculate 240/88 = 2.73A field current.
The customer reported they were seeing 2200rpm when running the motor on a 255Vdc bus (6% hot, hmmm) and measuring a field current (inline Fluke meter) of 2.0 amps. They reported that the motor was drawing 150 amps (~50% overload!) trying to run at that speed! The field current appears too low, though I can't understand how 255/88 = 2.0 amps field current. They measured the field resistance as 84 ohms outside in the cold, which seems plausible, but still 255/84 != 2.0.
Ultimately, the rebuild shop questioned EASA, who opined that they were initially providing excessive field current which (in combination with the overload on the motor) was causing the field to heat quite rapidly, increasing its resistance and decreasing its current and field, which increased speed, which increased overload and heating, etc. etc.
The last suggestion I made was that they get a rheostat and set the field current to the nameplate value (2.25A) and see what happens. Or put on a smaller pulley
Questions:
1) Does anyone agree with EASA's assessment?
2) Why the 1700/2100rpm spec on the motor? I would think that means rated field current will get you 1700, and reduced field current can get you up to 2100.
3) Is 2.0 amps on a 2.25 rated field enough to get you 20% overspeed? With 6% overvoltage on armature?
Since our customer isn't calling hourly, maybe they decided to take my suggestion and use a smaller drive pulley
Still, I'd like to understand what was/is going on. Thank you for thoughtful comments.
We located a rebuilt 30hp shunt field DC motor for a customer to put on an air compressor which "kept burning up motors". Hmmmm. Upon installation, they reported it "runs way too fast" compared to its next door neighbor. They wanted to disassemble the motor and move the pole pieces closer to the armature. Yikes! Hold your horses, folks.
Nameplate specs:
Westinghouse, 30hp, 1750/2100 rpm, 104 FLA, 240Vdc, field 2.25A, field 88 ohms
My first thought was that they had too little field current, but upon questioning them, they said they had the field connected directly to the line. Actually, they said the starter (two step resistance type) has a field loss relay, but they had jumpered that out.
The motor shop reports that after rebuilding, the motor was tested at 1825rpm (no load) at 240Vdc with the field connected to line. I calculate 240/88 = 2.73A field current.
The customer reported they were seeing 2200rpm when running the motor on a 255Vdc bus (6% hot, hmmm) and measuring a field current (inline Fluke meter) of 2.0 amps. They reported that the motor was drawing 150 amps (~50% overload!) trying to run at that speed! The field current appears too low, though I can't understand how 255/88 = 2.0 amps field current. They measured the field resistance as 84 ohms outside in the cold, which seems plausible, but still 255/84 != 2.0.
Ultimately, the rebuild shop questioned EASA, who opined that they were initially providing excessive field current which (in combination with the overload on the motor) was causing the field to heat quite rapidly, increasing its resistance and decreasing its current and field, which increased speed, which increased overload and heating, etc. etc.
The last suggestion I made was that they get a rheostat and set the field current to the nameplate value (2.25A) and see what happens. Or put on a smaller pulley
Questions:
1) Does anyone agree with EASA's assessment?
2) Why the 1700/2100rpm spec on the motor? I would think that means rated field current will get you 1700, and reduced field current can get you up to 2100.
3) Is 2.0 amps on a 2.25 rated field enough to get you 20% overspeed? With 6% overvoltage on armature?
Since our customer isn't calling hourly, maybe they decided to take my suggestion and use a smaller drive pulley