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Motor Control Asistance — Parallax Forums

Motor Control Asistance

NWCCTVNWCCTV Posts: 3,629
edited 2010-03-31 04:26 in BASIC Stamp
I picked up a couple of Zooma 24vDC Motors with the controller that is provided in the below link. Am I correct in assuming that if there is no reverse connection, that the controller is uni directional and can not bechanged? http://tncscooters.com/product.php?sku=101180

Or is there some off chance the Basic Stamp can reverse direction? I am short on funds so can not get the HB-25 that I want. I picked up both motors and controllers for $10.00 and since I know at least·the motors are usable I did not want to pass it up. Thanks for the help.

Comments

  • W9GFOW9GFO Posts: 4,010
    edited 2010-03-30 00:39
    The Basic Stamp can't help you with that. You could however use a couple relays to change the current direction.

    Rich H

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    The Simple Servo Tester, a kit from Gadget Gangster.
  • NWCCTVNWCCTV Posts: 3,629
    edited 2010-03-30 01:46
    How would I go about doing that?
  • ercoerco Posts: 20,257
    edited 2010-03-30 03:41
    I'm normally a big proponent of relays, but I must inject a word of caution here. My sordid tale: I built an electric bike using TNC scooter parts. 36 volts worth of·SLAs and a strong 250W gearmotor. Wasn't positive that I wanted a speed control, so I first tried it with a 50-amp lighting relay to switch the motor on after I got moving. WOW,·she took off like a rocket and that motor inductive load WELDED the relay in the on position. For all practical purposes, it was...··· A TOYOTA! There WAS no turning it off, and the brakes were quite useless against the electric motor. Fortunately I was able to yank a wire off the battery and stop before I hit anything big...

    So, if you use relays to make a reversing switch, make absolutely sure the relays never try to switch unless the speed control is OFF and the vehicle is stopped. Mechanical and/or electrical safety interlock is required. You may not have a runaway like I did, but you could still weld your relays in place. Besides, switching from forward to reverse in motion would do silly things to your vehicle/robot (just what are you building?).

    My story's happy ending is that I did get·the controller & throttle from TNC and the bike works great!
    http://www.youtube.com/watch?v=eYh2JaHsdak

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    ·"If you build it, they will come."
  • W9GFOW9GFO Posts: 4,010
    edited 2010-03-30 04:17
    NWCCTV said...
    How would I go about doing that?

    Use two appropriately rated Dual Pole Single Throw relays.

    The center tap of each relay goes to a lead from the controller. The normally closed tap of one relay goes to the plus side of the motor, the normally open tap goes to the negative side of the motor. The normally closed tap of the other relay goes to the negative side of the motor and the normally open goes to the positive side.

    As erco said, the program will have to make sure that the controller is off before switching the relays or it will be really hard on things. Also, you want to make sure that both relays are switched at the same time. If only one relay is switched, you have a short and possibly a fire.

    A kill switch, in series with the main battery, is a real good idea.

    Rich H

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    The Simple Servo Tester, a kit from Gadget Gangster.
  • ercoerco Posts: 20,257
    edited 2010-03-30 05:04
    First choice would be a single DPDT relay IF you can find one that handles 50+ amps...

    Somebody (maybe Hitachi?) makes an H-bridge unit which functions like a relay. Saw it at Digi-Key. Have one somewhere... I think. Not sure what the current capacity is. Will advise if I find mine.

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    ·"If you build it, they will come."
  • allanlane5allanlane5 Posts: 3,815
    edited 2010-03-30 11:56
    50+ amps is a LOT of current.
  • ercoerco Posts: 20,257
    edited 2010-03-30 15:33
    Yes, it is! Automotive lighting relays (usually SPST or SPDT) come in 30, 40, and occasionally 50 amp versions. Two SPDT units wired together (coils in parallel to make sure they both switch simultaneously) could work, again, provided they are only switched with zero current flowing through the contacts.

    BUT any power glitch that momentarily blipped the relays while current was flowing through the contacts could lead to the arc-welded contact situation I experienced. Plus, motors could go from full forward to full reverse. As Daniel Powter sings, "You had a BAD DAY..."

    Safest thing: a giant mechanical DPDT switch to reverse the power. Flip it with a servo if need be.

    Again, are we talking about a robot (how heavy?) or a ride-on?

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    ·"If you build it, they will come."
  • ercoerco Posts: 20,257
    edited 2010-03-30 15:35
    http://www.tncscooters.com/YK48-2.php·· has reverse for $30

    attachment.php?attachmentid=74011

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  • W9GFOW9GFO Posts: 4,010
    edited 2010-03-30 17:10
    erco said...
    BUT any power glitch that momentarily blipped the relays while current was flowing through the contacts could lead to the arc-welded contact situation I experienced.

    Just asking... If the relay is appropriately rated for the amount of current that may pass through it, is welding of the contacts really an issue? Isn't such a relay supposed to be able to be switched while carrying current?

    Rich H

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    The Simple Servo Tester, a kit from Gadget Gangster.
  • stamptrolstamptrol Posts: 1,731
    edited 2010-03-30 19:11
    The issue with DC motors is that rated current (say 20 Amps in this case) is only a fraction of what can flow during startup or reversal. ( perhaps 100 + in a high quality motor).

    Its the startup current that welds the contacts. Thats why pre-solid state motor starters for DC motors used several steps with resistors in series to get the motor from standstill to operating speed.

    Erco's example of a controller should work well in both directions.

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    Tom Sisk

    http://www.siskconsult.com
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  • W9GFOW9GFO Posts: 4,010
    edited 2010-03-30 19:31
    stamptrol said...
    The issue with DC motors is that rated current (say 20 Amps in this case) is only a fraction of what can flow during startup or reversal. ( perhaps 100 + in a high quality motor).

    Its the startup current that welds the contacts.
    Right, but the relay should be sized for what the controller can supply. Understood that a motor connected directly to a battery can draw huge currents at startup but I would think that since there is a controller in between that the max current would be limited.

    Rich H

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    The Simple Servo Tester, a kit from Gadget Gangster.
  • ercoerco Posts: 20,257
    edited 2010-03-30 20:35
    Startup draws huge current, and switching the relay off is equally destructive. A motor is an inductive load, as opposed to a lamp, which is a resistive load. The difference is that a motor (inductive load) will arc the relay contacts when they try to break the circuit.

    As a result, relays are rated in switching current (usually for a resistive load), which is considerably lower than their maximum contact current (non-switched).

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    ·"If you build it, they will come."
  • W9GFOW9GFO Posts: 4,010
    edited 2010-03-30 20:57
    Also understood about the arcing.

    What I don't get is how can the startup current weld the contacts if the relay is sized to handle the current that the controller can supply?

    Is it not possible to use a relay that can handle the max current of the controller and then not be concerned with welded contacts?

    For example, the HB-25 has a max (surge) current of 35 amps and it has an over current protection circuit that I assume will shut it down if more current tries to flow through it. So, would it be possible to weld the contacts of a 50 amp relay using the HB-25, regardless of the size of the motor?

    note: using relays to reverse a motor with the HB-25 would be pointless as the HB-25 is reversible.

    Rich H

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    The Simple Servo Tester, a kit from Gadget Gangster.

    Post Edited (W9GFO) : 3/30/2010 9:07:23 PM GMT
  • ercoerco Posts: 20,257
    edited 2010-03-30 22:44
    AFAIK, if the relay doesn't switch midstream and the relay contacts are properly rated for an inductive load, everything should be OK. But if an electrical glitch makes the relay try to open with 50A flowing, arcing (& possible welding) may happen.

    Edit: But at least in this case of the relays used only as a reversing switch, arcing or welding·should only result in loss of function or being locked in FWD or REV and not a full throttle runaway, since the speed controller is·on the case.

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    ·"If you build it, they will come."

    Post Edited (erco) : 3/31/2010 12:15:59 AM GMT
  • NWCCTVNWCCTV Posts: 3,629
    edited 2010-03-31 04:26
    WOW!!! Thanks for everyone's help. erco, I am building a robot. Not sure but I may end up using my granddaughter's old barbie jeep. It is hopefully going to have remote control AND be autonomous. I was looking at that same controller but would need 2 and if I could afford it I would rather get an HB-25. I was just hoping for an easy/inexpensive solution, but that never seems to happen for me!!! Thanks everone for all the great info.
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