pic: How we cool our compressor

[Doug G]

Originally Posted by eshteyn

Also electricity is the flow of electrons, which are negatively charged. They flow from positive to negative, and since the black wire is the negative thats why it melts.

I can't help myself... There's a couple of things about this statement that bothers me and so I felt a teaching moment coming on ...

1) Electrons absolutely flow; but from negative to positive. Unfortunately the definition of "current" is the flow of positive charge (even though no positive charge carriers are flowing in these circuits, thank those dudes from the 1800's for defining current before they knew the existance of electrons), and thus we say current flows from positive to negative. This is a commonly misused/misunderstood concept. 2) The current in the black wire is no different than the current in the red wire and no different than the current in the motor windings inside the compressor. It is a series circuit (red wire --> Compressor --> black wire) and thus the current flow must be the same everywhere (conservation of charge). Perhaps the black wire melts first because it's insulation can't stand the heat and/or it is a higher AWG than the red wire resulting in more heating (I^2*R)?

[Al Skierkiewicz]

Ryan,
If this compressor is used as intended the only death will occur with old age. As others have pointed out in this thread, the motor occassionally is wired incorrectly and it runs backwards. The compressor is not as efficient and so takes longer to come up to shut off pressure. The result is added heat from the motor and added heat from the compressor running a longer time between cycles. The thermal breaker inside the motor is to protect the motor from burning up due to a locked compressor or from trying to start with a high "head" pressure. Both of which would exceed the design current for the motor windings. The compressor start current can be about 25 amps and run current will be about 10-12 amps under normal conditions. Remember that compressing air causes a significant rise in temperature and in a compressor there is frictional heat added as well. Under normal use, these temperatures are accounted for in the design. This compressor is not intended for continual use however.
A fan for cooling in this case would be acceptable under the rules for robot use. I would have to see your use of a thermistor to be able determine if it fits the robot rules. In any event the thermistor would have to be used as a sensor, insulated and returned to the RC or custom circuit as a sensor input. The Spike controlling the compressor can only be fed by the RC to be legal. Mounting the thermistor using one of the compressor head bolts might be an issue, but I can't think of anything off the top of my head that could consider that a modification of the compressor. If you drilled and tapped a hole, that is a modification. I could think of several ways to attach a thermistor that would not disturb any of the hardware and still work effectively.
If you are adding three compressors in parallel, I would think a check valve at the output of each one would be appropriate. This would prevent problems when one of the compressors starts later than the other two. It would also prevent starting with a high system pressure. Just a thought.
Doug, the black wire, I think, was referring to the PWM cable. This is a fairly common occurence when the PWM cable is connected inproperly or contacts the +12 volt line at the other end. Happens more often than I would like but is an effect of using RC components in this application.

[Adam Y.]

Originally Posted by Doug G

I can't help myself... There's a couple of things about this statement that bothers me and so I felt a teaching moment coming on ...

1) Electrons absolutely flow; but from negative to positive. Unfortunately the definition of "current" is the flow of positive charge (even though no positive charge carriers are flowing in these circuits, thank those dudes from the 1800's for defining current before they knew the existance of electrons), and thus we say current flows from positive to negative. This is a commonly misused/misunderstood concept.

This isn't entirly correct either. You have a positive charge flowing in a circuit. The correct term to describe this is holes. Holes move from positive to negative in a circuit. In reality it doesn't really affect anything in this example but once you get into semiconductors you get devices in which depending on how they made primairly move one type of charge (Holes or Electrons).

[RyanN]

Originally Posted by Al Skierkiewicz

Ryan,
If this compressor is used as intended the only death will occur with old age. As others have pointed out in this thread, the motor occassionally is wired incorrectly and it runs backwards. The compressor is not as efficient and so takes longer to come up to shut off pressure. The result is added heat from the motor and added heat from the compressor running a longer time between cycles. The thermal breaker inside the motor is to protect the motor from burning up due to a locked compressor or from trying to start with a high "head" pressure. Both of which would exceed the design current for the motor windings. The compressor start current can be about 25 amps and run current will be about 10-12 amps under normal conditions. Remember that compressing air causes a significant rise in temperature and in a compressor there is frictional heat added as well. Under normal use, these temperatures are accounted for in the design. This compressor is not intended for continual use however.
A fan for cooling in this case would be acceptable under the rules for robot use. I would have to see your use of a thermistor to be able determine if it fits the robot rules. In any event the thermistor would have to be used as a sensor, insulated and returned to the RC or custom circuit as a sensor input. The Spike controlling the compressor can only be fed by the RC to be legal. Mounting the thermistor using one of the compressor head bolts might be an issue, but I can't think of anything off the top of my head that could consider that a modification of the compressor. If you drilled and tapped a hole, that is a modification. I could think of several ways to attach a thermistor that would not disturb any of the hardware and still work effectively.
If you are adding three compressors in parallel, I would think a check valve at the output of each one would be appropriate. This would prevent problems when one of the compressors starts later than the other two. It would also prevent starting with a high system pressure. Just a thought.
Doug, the black wire, I think, was referring to the PWM cable. This is a fairly common occurence when the PWM cable is connected inproperly or contacts the +12 volt line at the other end. Happens more often than I would like but is an effect of using RC components in this application.

The thermistor by no means follows robot rules because it uses one of the bolts on the compressor to hold it in place, thereby modifying the compressor (but it will not have to compete in FIRST robotics). I think we are going to dump the thermistor idea though given the information above. I talked to our main engineer today and he explained everything to me so now I understand that the heat created cannot damage the compressor, and I learned that most of the heat is caused by the compression of air. I will post some pictures later of our t-shirt shooter bot. Here are some features that it will have:

• 2 Batteries
• 2 Air Compressors
• Large Steel Air Tank

We used our old T-Shirt shooter for football games and for robotic demonstrations. The new one will be much safer and more reliable than the old one.

[Al Skierkiewicz]

Ryan,
I can bring that question up to other inspectors but I still do not feel that using an existing bolt qualifies as a modification. I would again have to see it for sure.

Adam,
Hole theory as you pointed, out is a method to help in understanding the physics of semiconductor operation. Electrons do exist and they are the reason electricity works. They flow from the negative terminal to the positive terminal or from an electron source to a more positive terminal. (Remember opposite charges attract.) Think of a vacuum tube, the electrons generated by the filament and cathode material (thermionic emission) are attracted to and flow to the positively charged plate. As Doug pointed out, early pioneers did not know the actual physics and assumed that current flowing in a wire produced a magnetic field that would affect a compass, naturally flowed in one direction from positive terminal to negative terminal. Since there are no positive charge carriers there is no way for current to flow out of the positive terminal. These early electricians then defined a current flow as something they could measure and that is the "conventional current" flow we use everyday. When using an ammeter in a DC circuit connected so that the positive terminal is connected to the more positive source, an up scale meter movement will result. Hence the confusion between electron and conventional current flow.