Mailing List Archive Mailing List Archive
Alex: Cut out the middleman and run the compressor on whatever motor (gas, diesel, etc) you would use to run the generator. If your compressor uses a 3HP electric motor, you need about a 5 HP gasoline motor (you normally don't run gas engines at their full rated capacity) If for some reason you must run the compressor from an electric generator, you need to consider a couple of things to select your generator: 1. While running your compressor at maximum load (highest pressure), measure the current (borrow a clamp-on ammeter from an electrician buddy). This will tell you the maximum current that the compressor system will typically draw. It is not sufficient to check the data plate on the compressor's drive motor, because the compressor could be over or under powered. I can put a 10 horsepower motor in an application that requires a 5. The dataplate will say that the current draw is something like 50 amps, but it will only draw about 25 amps. Likewise, I can put 5 HP motor in an application that calls for a 7.5. The dataplate will say 25 amps, but the actual current will be around 35, and I risk burning up the motor. 2. Buy a generator that can deliver the amount of current that you measured continuously. Be careful of data sheets that quote peak or surge current. You need continuous. In the current ranges we're talking about, the generator should be able to handle that amount of current as an inductive load. But you probably want to verify that the current rating of the generator applies to an inductive and not a resistive load. Those little plastic hand-carry compressors that put out about 1000 watts (10 amps, 110V) are typically rated at 1000 watts for resistive loads (like light bulbs) -- the machine's rating for inductive loads (like motors), will be much lower. 3. If you find that a generator with enough capacity is out of your price range, or is not practical, you can cheat a little. Let's assume that your compressor uses a 3HP electric motor and is not over or under powered. Let's say you found a really good deal on a generator, but it will only put about about 10 amps at 220V instead of the 15 amps that the motor "wants" to pull. You can replace the pulley on the electric motor with a smaller one. This will cause the compressor to turn slower, which lightens the load on the motor, resulting in a lower current draw. In this example, the new pulley would need to be about 2/3 the diameter of the existing one. After changing the pulley, check the motor's current draw again, and make sure it is less than the 10 amp capacity of the generator. I still think it's easier and cheaper to buy a gas motor and set the compressor up so you can swap motors when you need to switch between "field" and "depot". A 5 to 8 HP motor is available for a few hundred dollars compared to over a thousand when you start talking about 220V, 15A (3-4 kW) generators. For all the real electricians that read this, cut me some slack on the current values and stuff, I'm just using some round and approximate numbers to provide a few examples to help answer Alex's question. I don't claim to be an electrician. In fact, I'm a licensed electrician's worst nightmare -- I'm a homeowner with and electrical engineering degree and a copy of the National Electric Code! Burn Baby, Burn! -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
Navigate by Author:
[Previous]
[Next]
[Author Search Index]
Navigate by Subject:
[Previous]
[Next]
[Subject Search Index]
[Send Reply] [Send Message with New Topic]
[Search Selection] [Mailing List Home] [Home]