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24 volt bicycle motor switch

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McCoy

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Hello everyone, I am totally new to electronics so bear with me. I am trying to make a simple switch for a 24 volt electric bicycle project. I have a 24 volt .5 HP motor from ampflow.com.
I was thinking I should use a 24 volt relay switch for throttle, but I'm not sure. I'm concerned the initial torque of the motor will be too high and possibly damage the drive system. I was wondering what some possible solutions are? Can I wire 2 batteries to 1 switch, say one 3 volt and one 24 volt? so the switch would work off>3volt>24volt , that way the 3 volt battery could "take up the slack" a bit so the 24 volt isn't too much of a jolt. Is there another way to make a "soft start" type switch for this type of thing? Is a relay switch even the best way to go? I know there are PWM controllers available but they are pricey, I'd like to keep this project cheap and as efficient as possible. thanks all :)
 
I had an electric bike with a 12V motor and a plain switch, and always had soft starts under load.

Don't know where you plan to find a 3V battery capable of 20 Amperes, but you could rig a 3 position switch to four 6Volt batteries to make them all in parallel to start at 6V, then another position to make series-parallel for 12V, and a third position for all in series for 24V.
 
Thanks for reply Externet, are you saying you used a simple switch and not a relay switch?
Does the resistance of the wires come into play at all? The battery is on back rack and switch needs to be on handlebars, about 6ft of wire total.
 
Yes, the resistance of wiring takes a role, but 6 ft. of AWG 12 handles it fine. Yes, I used a simple switch.
Perhaps a plain 'silent' wall switch core (rated 15A or 20A) can make you happy. Try.
 
OK, this is probably a dumb question, but when you say "wall switch" do you mean 120v AC wall switch? or 24V DC wall switch? if you do mean 24V DC, where can I buy such a switch?
Thanks
 
OK cool, thanks again. I didnt know I could use AC switches with DC that bit of info is going to make this a whole lot easier :)
 
Did a bit of googling AC and DC switches and doesn't look like AC and DC switches are interchangebale, thanks anyway.
 
I can't seem to find a 24 volt momentary pushbutton switch anywhere plus it may be a pain to hold down the button of such a switch for extended periods while riding. I may try the relay again with a small pushbutton switch unless someone on here can enlighten me to a better idea.
 
Did a bit of googling AC and DC switches and doesn't look like AC and DC switches are interchangebale, thanks anyway.


Shhhh ! speak softly, do not tell it to the switch nor let it read your google findings !
If the switch is new, does not know what it is meant for, it will work ok if you never tell it is DC.
If it is an used one perhaps it has already noticed that has always worked with AC. Avoid those.
 
Although it may seem strange, but there can be a difference between switches designed for AC and those designed for DC.

Switches for AC only are generally designed to slowly open so that any inductive arc accross the contacts will be extinquished on the first cycle that the AC current goes through zero. If the switch opens rapidly the arc, on average, could be longer and provide more contact errosion before the current goes through zero. Thus standard AC wall switches usually have a slow break machanism. (Some old wall switches had fast break contacts which could be used for AC or DC).

Switches for DC are generally designed to switch rapidly to quickly extinquish any inductive arc across the contacts. A slow opening switch will tend to increase the time of the arc and thus the errosion of the contacts. Most toggle switchs are fast break for either AC or DC use.

For control I would suggest a small, momentary action lever type microswitch controlling a relay. That way you only need to run small wires to the switch. The microswitch lever is easy to hold down for long periods, but it will automatically shut off if you release your grip. If you Google "lever microswitch" you'll get many hits.

For switching between battery configurations (shifting gears) you could use a slide or toggle switch, also controlling relays.
 
Although it may seem strange, but there can be a difference between switches designed for AC and those designed for DC.

Switches for AC only are generally designed to slowly open so that any inductive arc accross the contacts will be extinquished on the first cycle that the AC current goes through zero. If the switch opens rapidly the arc, on average, could be longer and provide more contact errosion before the current goes through zero. Thus standard AC wall switches usually have a slow break machanism. (Some old wall switches had fast break contacts which could be used for AC or DC).

Switches for DC are generally designed to switch rapidly to quickly extinquish any inductive arc across the contacts. A slow opening switch will tend to increase the time of the arc and thus the errosion of the contacts. Most toggle switchs are fast break for either AC or DC use.

For control I would suggest a small, momentary action lever type microswitch controlling a relay. That way you only need to run small wires to the switch. The microswitch lever is easy to hold down for long periods, but it will automatically shut off if you release your grip. If you Google "lever microswitch" you'll get many hits.

I think you have that backwards. A DC switch that open rapidly will have a larger arc than one that opens more slowly- the faster you kill the current through an inductor, the larger the voltage spike. Regardless, I think we both agree that a DC switch is more rugged than an AC switch of the same rating because it must just "tough out" the spark and cannot take advantage of the zero-crossing point that AC signals have in order to kill the spark.
 
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I think you have that backwards. A DC switch that open rapidly will have a larger arc than one that opens more slowly- the faster you kill the current through an inductor, the larger the voltage spike.
Not backward. For contact erosion it's the arc duration, not length that's important. With DC, the arc will be maintained as long as the supply voltage exceeds the arc voltage (which is low for short distances), independent of any inductance. Thus you want to rapidly open the switch so that the arc voltage quickly exceeds the supply voltage. Opening the switch rapidly may give a longer arc, but it's the duration of the arc, not the arc length that erodes the contacts.
 
I wasn't referring to the physical arc length. When I mean larger arc I meant "higher intensity".
 
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I wasn't referring to the physical arc length. When I mean larger arc I meant "higher intensity".
Okay. It may be "higher intensity" (brightness?). But that is not a factor in contact errosion which, I believe, is the important parameter in determining the best way to open the contacts. And erosion is determined by arc current and arc duration. The arc current is fixed by whatever current the circuit happens to be carrying at the time the contacts are opened. The arc duration is determined by the inductive energy in the circuit and the length of the arc, where the inductive energy is dissipated. Thus a longer arc will dissipate the energy faster, giving a shorter arc duration.
 
Thanks crutschow, I'm going to use a relay and wire a switch up to the handlebars.I think the lever switch will work well after I figure out how to mount it :)
 
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