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How can you run a 780 watt drill on 800 watt inverter generator? (startup 1,500 watts)

  1. Oct 2, 2014 #1
    Hi, I found a great generator, Earthquake 800 watt inverter style that is quiet and runs 14 hours on a gallon. BUT the Bosch SDS drill kills the engine, even if you barely pull the trigger.


    I have a 750 watt inverter I hook up to 12V battery and it starts the drill real slow but it after 10 secs goes full bore.

    So the Earthquake generator senses more that 800 surge watts on startup and just kills the engine electrically.

    Have 800 watt cheapo harbor freight old style gen, it runs the drill great, when the gen starts,

    What is the cheapest easiest way to get around this? I am fine with running the drill at 700 watts and waiting ten seconds for spool up. Understand startup might be 1500 watts on the drill. But the cheap gen and 12v invertor handle that by just giving what they have. This smart Earthquake gen shuts down when 800 surge watts is touched.

    Is there such a thing as a watt choke, that only allows 800 watts to be pulled, no matter what the demand?

    Any ideas, I am weak on electrical engineering. The gen only weighs 21 pound is cheap and whisper quiet, really what to use it.

    We might be able to bypass the trip thing, not sure.
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Oct 2, 2014 #2
    Ah, found out it is 700 watts continuous and 800w surge. So I have to reduce the watts pulled or let the gen just put out 700watts.
  4. Oct 2, 2014 #3

    jim hardy

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    Now you're finding out the fine print about inverters. Their overload behavior is a lot different from a generator.

    There's a device called "Ferroresonant Transformer" that'll limit AC current nicely.
    But a 500 watt one will cost a lot more than another Harbor Freight generator...

    http://www.emersonindustrial.com/en-US/documentcenter/EGSElectricalGroup/products_documents/control_power_solutions/power_protection_conditioning/power_conditioning/cvs_hardwired/CVS-Hardwired.pdf [Broken]

    myself i'd fix the old generator. Two stroke? Clean the carb and make sure plug is good...
    Last edited by a moderator: May 7, 2017
  5. Oct 2, 2014 #4
    Thanks Jim, Yea 2-stroke and very finicky. Thank you for advice. Was hoping for a cheaper way to "choke" it. The 4stroke inverter superior in every way, except not tripping. If I were to bypass whatever is telling the engine to kill, what do you think would happen? Damage?

    The clip-on inverter works just fine and is less watts.
  6. Oct 2, 2014 #5

    jim hardy

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    Impossible for me to even guess.
    Old timey inverters had to "commutate" else they'd blow an internal fuse.
    SIze of commutating capacitor determined how much load they could handle.
    I don't know how the newfangled ones work.

    A blown fuse is easily repairable, blown mosfets less so.
  7. Oct 3, 2014 #6


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    It may be worth while starting with a resistor in series, until the motor starts to roll and generates enough back EMF. Whatdya think Jim?
  8. Oct 3, 2014 #7


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    If you're handy with electrical wiring a electric fan controller speed controller might work to reduce the surge current. If it works I would only run it a full power once it starts.
  9. Oct 3, 2014 #8

    jim hardy

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    That's a variable speed drill - can't you just start it slowly ?

    A 500 watt light bulb in series should allow start. After start you can short out the lamp with an ordinary 79 cent wall switch . Those halogen work lights are pretty cheap.
  10. Oct 3, 2014 #9
    Interesting. I just came back after a day away. Another possible simple solution is Hitachi makes a 5.4amp 620 amp SDS drill for $60 recon. We are going to try to find a 5.5 amp or so power tool to test to see if it trips.

    Will check out your idea. and have thought of the resistor suggestion too, but resistors normally can not handle many watts, Thanks
  11. Oct 3, 2014 #10
    Aha, like that one. Found out the gen is 800 max 700 continuous so since drill is 760 can not get away wide open. But your concept is what was thinking should be possible for startup. If it does not solve the problem for the Bosch drill. It will solve it for lets say 699 watt or less tools. In fact the Hitachi 620 might require the same trick.

    Jim what do could do the trick for a 780 watt drill, to run continuously at lets say 100 watts less?

    Thanks folks, fun problem.
  12. Oct 3, 2014 #11
    Also, in terms of variable speed drill, we tried it as slow as possible and it tripped. No damage, just have to restart it, so some feedback thing that kills engine.

    Had even thought about dimmer too, but after watching video with a drill, shows empirically it likely would work.
  13. Oct 3, 2014 #12

    jim hardy

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    I've used plain incandescent lamp dimmers with plain single speed drills. They work great.
    I dont know what will happen when the variable speed unit in the drill, which is probably just another straightforward dimmer, competes with the external dimmer for control.
    Try it.

    I'm beginning to think your trouble is the cheap inverter misinterprets brush noise or chopping of wave by speed controller as an overload and trips.
    I had that trouble with an industrial inverter once. It couldn't tell a small steep wavefront from a gigantic sinewave.

    Have you a cheap two or three amp drill to test for that ?
    And an incandescent lamp with a built in dimmer ? Maybe a touch-lamp?
    Does inverter light a CFL lamp okay? They have surprising inrush....

    We have to find which demon to subdue - source or load.
  14. Oct 3, 2014 #13
    Hi Jim we will try CFL, have tried other things. As mentioned we will try a 5.5 or so drill/power tool. Its at the shop, may be tommorow before we know. Gen is cheap but highly rated, people love em.

  15. Oct 4, 2014 #14
    Sounds like a good application for an inrush limiter circuit. A typical one consists of a resistor and relay.

    The resistor is wired in series with the load and serves to initially limit the current.
    After the load current reduces due to speed / charging, the relay contacts close, shorting across the resistor and allowing full voltage to be applied to the load.

    The relay's coil is wired across the load so that the relay closes when the voltage across the load becomes sufficient. (i.e. a 120VAC relay closes at say 85VAC.)

    I really like the Idea of a halogen lamp as a resistor for the reasons mentioned above and how well the lamp would handle high wattage should the drill stall.

    All that said, I wouldn't recommend bothering to build one unless you have an excess of time, tools, etc... I'd lean towards buying a good cordless with a couple of batteries and allow one to charge while working.

    - Mike
  16. Oct 4, 2014 #15

    jim hardy

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    another approach is an inrush limiting thermistor.


    tutorial here


    If it works you could put it in an outlet box on end of an extension cord, in series with one half of a receptacle.
    to separate the two halves break off the little metal tab on hot side(hot has shorter slot & brass colored screws),
    and replace with one of these:


    Be aware it'll run hot so give it plenty of clearance and leave the leads long. Use a double box.
    Paint that half of receptacle same color as drill so you'll remember which one to use.

    I'm afraid this will be a trial and error project. I did find a wiring diagram of the inverter at Earthquake's site but nothing about how the overload sensor works.
    http://www.getearthquake.com/foundations/store/shopdetail.asp?params=11613 [Broken]

    Mike has a point - that little inverter should run a charger for cordless version of the drill.
    Last edited by a moderator: May 7, 2017
  17. Oct 5, 2014 #16
    Thank you all. I am 100 miles from shop, my partner who is close to it has been busy building product, but should get to it soon. He has a pulse device he thinks might work too. It is used in machinery to reduce speed but retain about 80% of motor torque, think it pulses the power on and off very fast, so half the current used. Between all these ideas am confident it will be solved.

    BTW I do have a battery drill, just got a Milwaukie 12v SDS drill. But it is not at headquarters for demos. It works fine. Also its sorta a shame to run a generator for 6 hours to recharge a cordless for 25 mins of drilling. Our need is being in wilderness areas drilling rocks for micro blasting. If you are interested in doing small blasting without a blasting license, we have the first ATF exempted blasting system. There is a video of us using it here. www.SierraBlaster.com. The 2 stroke is so finicky we do not trust it for demos anymore. So as mentioned, there are so many good ideas here, think we will solve it. One thing is in drilling you start and stop a lot. So something automatic may be best, whereas dimmer would require action each time.

    Am guessing about 3 of these suggestions will work, we will test for one that is easiest and report back.
  18. Oct 6, 2014 #17
    Update: Good news. It is not just the inrush issue. We tested with various power tools, it handles 6 amps or 660 watts as is. So the Hitachi 620 watt will be fine. Still will try the other things, as it would be nice to have a setup to handle anything up to about 1000 watts on the 700 watt generator with a watt limiter.

    Had some bad results with the Milwaukie cordless 12 v SDS, With 4X battery only get two 16" 10mm holes in rock. Does not look good although we are thinking of building our own litium battery pack to solve.
  19. Oct 7, 2014 #18

    jim hardy

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    I connected a set of old cheap jumper cables to a 12 volt drill and use a lawnmower battery. Not so portable but a lot cheaper than those lithium battery packs.

    Congratulations on your success !

    old jim
  20. Oct 16, 2014 #19
    We built the Dimmer switch box yesterday and unfortunately after about 1/8 rotation it tripped the generator. Same as barely pulling the trigger on the drill. So maybe it is an inrush issue.

    We did get the lower amp Hitachi drill and it does work with the continuous 700 watt invertor generator. It did trip once when motor stalled in hole, but acceptable. The Hitachi however drills about 40-50% slower that the bigger drill which pulls about 20% more watts. We would still like a solution for running slightly larger watt devices that rated capacity of gen. A way to restrict the watts delivered to device.

    What about a capacitor in line? Bigger motors use capacitors for startup. Could an external capacitor be put inline to get past the startup issue? Thought is that the dimmer setup would work as watt limiter after startup. But we can not know till we get past startup.

    Thanks in advance.
    Last edited: Oct 16, 2014
  21. Oct 16, 2014 #20
    Not surprising that the light dimmer behaved similar to the drill. Many variable speed products use the same phase control method as the lamps. It works, and it's cheap. Go here, and they show the wave forms. The 100 degree one is particularly interesting, because it's what you'd see when starting the motor slowly'
    http://www.dv-fansler.com/Computer Lights/dimmers.htm

    I used to design motor drives and power supplies at home, so I kept a variable transformer for test ("variac"). Having a cheap, single-speed drill, I often used the variac to cut down the line voltage so that I didn't have the drill going berserk on me while trying to drive screws. Of course this isn't as convenient as a trigger.

    Jim's method of bringing along a serious battery for the cordless really impressed me. If you have a battery that is designed for rapid discharge, like a car / truck battery or high discharge LiFePO4, you can get some serious short term power, while charging the battery more slowly to account for the intermittent nature of tool use.
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