Re: [CAD_CAM_EDM_DRO] Re: [DIY-CNC] need transmission ideas / advice re: gantry

Sorry I didn't read that well enough.

The 130VDC motor won't run well at all on 24V. In fact, it can easily
overheat because at the slow speed the motor's fan will be ineffective!

You probably want a 12VDC to 110VAC inverter, and rectify the inverter
output, and use a high gear ratio.
The starting may be a problem. An inverter typically doesn't carry a
huge surge capacity and the DC motor will try to get up to speed
immediately. This is a problem that is difficult to avoid.

A treadmill speed controller might help, except I think those are
SCR-based and may not work at all on the Modified Sine Wave output of an
inverter.

The 11,760 watt-second load per lift remains, regardless of gearing. A
single 12v 200AH batt can do this 700 times if the speed is reasonable.
A 1KW inverter should probably not exceed 500W the way they rate the
cheap ones today. If you gear it for a 24 sec lift that'll meet the
500W criteria, you can only use BIG cables between the batt and inverter
and hope the inverter doesn't overcurrent and shutdown.

Be aware that DC motor, once the power is removed, PROVIDES LITTLE TO NO
BRAKING FORCE. Your load could get 3/4 the way up, the inverter shuts
down, and the load will come sliding back and accelerating. A winch is
desirable because I'm fairly sure it'll have a manually unlocked ratchet
to prevent the cable from playing back out if there's a thermal shutdown.

Be aware that electric winches have "funny" ratings as far as pulling
force. A 2000lb winch usually does NOT mean it can pull 2000lbs of
cable tension. It means it can drag a 2000 lb trailer rolling on
wheels. Which is far easier than lifting the trailer! IIRC the
standard for that was something like 10% or 15% of its actual weight- so
it's claiming it can pull like 200lb or 300lb of cable tension. Very
misleading, IMHO.

Danny

Danny Miller wrote:

> A 12VDC starter motor is a terrible idea. It's powerful for short
> bursts. It will quickly overheat in any other duty, so it's actually
> quite a weak motor for continuous duty. Even if you run it at low
> power, it has no means of cooling itself so the heat will build up.
> It's also inefficient.
> You may be underestimating you load requirements.
> Your lift requires a total energy of 11,760 watt-seconds, or 15.76
> hp-seconds. One hp motor out takes 15.76sec to lift. 2.5hp needs
> 6.3sec. That does not include the kinetic energy required, which can
> be very significant as higher speeds are used. The kinetic energy
> goes up with the square of the velocity used.
>
> The treadmill motor is probably an ideal, economic yet effective
> load. How are you getting the 12v to power the 130VDC motor? An
> inverter?
>
> The 200ah batts- or the cables connecting them, or the inverter- may
> be "bogging down" and dropping voltage under this sort of load, which
> kills motor torque. But it's not the motor's fault. If the batts
> were both under a 1C discharge- which is a lot of load for lead acid,
> in good condition, and they can only handle this sort of thing
> intermittently- then put through a 120v inverter and motor- then it'd
> be able to lift in about 4sec, but that's nearly "flooring" the
> batteries.
>
> How fast were you expecting to move this load?
>
> Danny
>
> Bob Muse wrote:
>> Get a 12VDC winch or possibly a 12VDC starter motor with gear reduction.
>> This type of series wound DC motor will produce a lot of torque at
>> low rpm.
>> Will also require a lot of current-should work fine with your battery.
>> ----- Original Message ----- From: "gcode fi (hanermo)"
>> <yahoog@...>
>> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
>> Sent: Monday, May 11, 2009 11:55 AM
>> Subject: [CAD_CAM_EDM_DRO] Re: [DIY-CNC] need transmission ideas /
>> advice
>> re: gantry
>>
>>
>>
>>> I am making a large cnc-driven gantry, which is done.(4.4 m wide, 4 m
>>> lift, 8 m rails).
>>> At the last moment, I ran into some difficulties at the transmission;
>>> 2-stage, HTD 5 mm 15 mm wide steel pulleys and timing belts, 1:3 to 1:3
>>>
>> for 1:9 total.
>>
>>> I have no way to get electricity to the site, we run from 2 large
>>> batteries of 12 V DC (200 amp hours each).
>>>
>>> I am using a DC treadmill motor, from surplus center, rated upto 2.5 HP
>>> and 130 V DC.
>>> Unfortunately, the motor seems to be too weak on torque !
>>>
>>> Any ideas on how to move the gantry ?
>>>
>>> The gantry and payload are 300 kg in mass, its a rolling non-dragging
>>> application, up a steep gradient of 45 degrees, 8 m length, total
>>> lift 4
>>>
>> m.
>
>