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DC motor voltage recomendations for a bike and power tools

  1. May 16, 2017 #1
    I have been working on some projects, I build a glass grinder, faceter with a 120v induction AC motor.
    Its not possible to effectively change the speeds with electronic controls being a single phase motor.
    It is using belts and pulleys for now.

    So I am wanting to build several tools, rock saw,belt sander,wood lathe with a variable speed. Also rig up my trike for electric power that has the potential to be charged with solar panels. looking for 0.5HP-2.5HP max.

    What voltage motor and controllers would be the sweet spot so I can use the same motor and control on all my tools and contraptions so I can keep some continuity of component?. making the parts all interchangeable so I dont have a mix . A voltage I could swap power from a solar or grid for direct power or run on battery power.

    I find the right motors and controls I will buy 6 or so to make my projects and have spare parts.

    This is my bike I would like to turn into a solar charged electric/pedal bike.

    Attached Files:

  2. jcsd
  3. May 17, 2017 #2


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    Welcome to PF.
    I see no reason why you cannot use a VFD to control the speed of a small single phase induction motor.

    You can usually supply a VFD with single phase if you do not push it too hard. You can remove the start&run capacitor from your motor which makes three terminals, then you can run it on 3PH, but with lower voltage. There are also VFDs available that are designed to produce two phases in quadrature to drive single phase motors without needing any capacitor.

    Controlling the speed of DC motors is more difficult since speed is highly dependent on load. But a tacho on the motor and a controlled H-bridge to adjust the power would close the loop and hold the speed to what you set.

    You can run a VFD directly from DC solar panels. The rectifier does not need AC, it will pass DC. Just keep the open circuit voltage, Voc, of the PV string below the VFD maximum input voltage. The switching of the HV DC must be done with a switch rated to break the PV short circuit current, Isc.
  4. May 17, 2017 #3
    simple PWM using mosfet makes nice speed controller for a DC motor. the right freq has to match motor to get best performance.
  5. May 17, 2017 #4


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    Any old DC motor can be matched to a PWM through the use of an external LC low-pass filter. That will protect a motor from the higher transient voltages expected and reduce the eddy current losses in older thicker iron components.

    A PWM sets the average voltage across a DC motor. That sets the unloaded RPM. The DC motor speed will still be highly load sensitive and so will vary from the maximum, 100%, to 0% when stalled. There needs to be feedback from RPM to PWM duty-cycle so as to compensate for changing load.

    Induction motor speed varies only over a narrow band due to changing load. Motor slip below synchronous speed will only vary between about 2% and 12%. Motor speed variation between 88% and 98% is quite acceptable for most power tools.
  6. May 18, 2017 #5
    A big part of the decision here will be economic - for example good options for motors are: Treadmills, (90-150VDC), Car starters (12V), golf carts (probably too big - but 36 or 48V) - I have electric lawnmower with 36V motor, fuse is 25A - so the motor is probably ~ 15-20A rating. (I was toying with the idea to build a speed controlled for this, but a 15A inductor was looking expensive.)

    Then batteries size, weight, running time - etc. Lead Acid are available and cheap, but bulky and heavy - LiPO $$$.... and harder to charge if you do not buy a preconfigured set. ( like RC Car batteries -) - A trike can carry weight better than a bike - so I would lean to LA. Also -- LA may be better to build and tst on and then if it works well - upgrade the batteries to LiPo for the ones that matter.

    For the shop items a small AC drive probably is fine ( Automation direct) - but AC induction motors do not make good tool motors becasue their torque is reduced with speed.

    THEN once you figure out what to use - figuring out how is easier.

    Also - google DIY Electric bike, lots-o-hits
  7. May 20, 2017 #6

    jim hardy

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    Check the tools aisle at a Home Depot or Lowe's or Sears. There's a variety of DC circular saws, drills and batteries.
    Check yard sales for secondhand fishing boat trolling motors. They're variable speed. Sometines they're discarded just for a broken propeller .
    Don't overlook the humble lawn tractor battery. They're only about twenty-five bucks.
    I use one for my trolling motor on the small lake where i live.

    An old fashioned car battery charger(not the newfangled electronic inverter type) plugged into a Variac adjustable transformer works wonderfully for a high amp low volts variable DC supply.
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