Luna safety power switch with Kelly KDS

[151]

I am building an electric MX bike for my son. I am about to wire up the Kelly KDS and I am curious if with the Luna power switch do I still need to use the precharge resistor and the one way diode?

Thanks!


Chris

[Tommycat]

Addressing the diode installation... It's purpose is to protect the mechanical contacts of the relay, and electronic components downstream, from the excessive voltage potential produced by the collapsing magnetic field of the contactor coil winding by bleeding it off during shut-off. Since the Luna remote power switch is electronic and has no coil, I would say that the diode is not required.

[151]

Addressing the diode installation... It's purpose is to protect the mechanical contacts of the relay, and electronic components downstream, from the excessive voltage potential produced by the collapsing magnetic field of the contactor coil winding by bleeding it off during shut-off. Since the Luna remote power switch is electronic and has no coil, I would say that the diode is not required.

Thanks TC, I appreciate the info!

[Tommycat]

Your welcome! Looking into the pre-charge resistor...it's function is also to protect the mechanical contacts from inrush current arcing from charging the capacitors. IMO once again, with a solid state relay rated at 120 amps that has no mechanical contacts, the resistor could probably also be ignored... After reading the manufactures specification page...the pre-charge resistor IS required if your controllers total installed capacitance in micro farads is greater than 1500.

[AZguy]

I would suggest the diode may even be more important if it's the freewheeling diode that goes *across* the relay or contactor coil and regardless, a freewheeling diode will never hurt and always help (as long as everything is DC) since suppressing the transient as close to the source is always best - i.e. put the diode right across the coil it is providing the freewheeling path...

[Tommycat]

Yes AZguy, freewheeling diode is exactly what I'm referring too. But I need to add that it also protects the mechanical switch contacts that energize the relay coil.

[AZguy]

It also protects non-mechanical devices against excess reverse voltage generated by the field collapse - it is likely even more important for this unless there's protection at the switching device and even then, as mentioned, putting the suppression as close to the generator as possible greatly improves things...

[max_volt]

The thing is even without a relay coil you have an inductive load to contend with. Zener or MOV may be called for.

[Tommycat]

Max, saw this when doing research. Immediately thought of you...

[max_volt]

So another alternative. The Schottky as a clamp.

[AZguy]

The device internal to the SSR across the "load" pins is a TVS

[Tommycat]

I too will be wiring in a Luna Remote On/Off Solid State Switch shortly... Putting it between my bottle battery pack 52vdc output, and controller. Although my battery pack has an on/off switch (BMS E-switch).It has to be ON to charge the battery. I desire to charge the battery on the bike, which it's not recommended to charge with the controller connected...hence the solid state switch between the two. It will also give me a couple extra security shut-offs. I plan on securing it in 3m potting compound as it seems these devices are prone to moisture problems. Since it's recommended by Luna Cycle to serve such a purpose (battery shut-off). I do not plan on adding diodes of any type, or resistors. Sorry if I got too deep in the weeds...Have a great build Chris.

[Tommycat]

Double checked on the Magic Pie V5's controllers capacitance...total is 1207uf, under the recommended 1500uf max with-out resistor pre-charge circuit.

[151]

Thanks guys. I was able to wire everything up (less resistor and diode) and She seems to operate correctly. First thing, I am not running a relay of any sort. Is there a reason people use relays that I am missing? My last build was a custom 48v Oset dirt bike and it ran flawlessly for hours and hours. eventually something did give. Might have been the switch or the controller (kelly KDS). For this new bike we have 18s for a max of about 75 volts/180 amps. I have the battery xt90'd to the luna switch and then to the kelly 72/200 KDS controller.

If i can keep the wiring this simple I would like to. Even if it means replacing controllers often. Any suggestions are very welcome. Thank you.

Second thing, the luna switch can not handle the 75v. It worked fine for a few minutes then locked in the "on" position. Is there a similar switch that can handle 80v/180amps?

Thanks again!

[AZguy]

180A is a *lot* of current for a solid-state device. You'll need sub mΩ resistance in order to not turn into a glowing slug. Not impossible but a tall order...

[Tommycat]

Wow what a difference specifications make! :-0 Just reading your O.P. I got the idea of just a small lower powered mini motorcycle. Yikes...this is a serious motocross electric beast! If my calculating's right, over 18 horse power at peak, and running continuous at 12 horse power. Using my 1965 Honda Trail 55 as reference, at continuous h.p. that would be about a 130 cc motor!

That said...:-)

Yep, that poor Luna Remote switch is only rated for 60 vdc max and 120 amps. And pressing it 25% over voltage...well. And with your peak draw , ouch. No surprise there.

For your question...Is there a reason people use relays that I am missing? Probably because it's difficult and expensive to find a switch that can handle that much amperage...as well as safety.

For new parts...the company that makes the Luna SSR (Solid State Relay) also makes a heavy duty one, but it tops out at 70 vdc max. But the amp rating is good. This is a must read--> HERE!
You'd have to drop a couple cells to 16s...Also note that at these controller capacitor values, a pre-charge resistor would be needed and perhaps a support capacitor also. A support e-mail address is supplied.

Sticking with what you've got...? I think something like this HERE. would work well. Have your diode and resistor ready! :-)

Speaking of sticking with what you've got... Do you think a XT90 rated at 90 amps will handle a 180 amp peak load? :-(

If I may ask a few questions? Is your battery pack store bought or home made? How many cells in parallel...Do you plan on setting the controller values? Dialing it down a bit? What motor are you powering?

Best regards,
T.C.

[151]

We are running three six cell 6000 mah lipo packs. With his first little bike we used two 5 cell 7400 packs.

I will for certain limit voltage and current with the Kelly controller for a while. The 40v oset was quick so even with the weight increase this ktm should be very quick.

the motor is a 48v mottor from Oset. It's stock in their 20" bike.

[max_volt]

I heard of coffee table books before but never a coffee table cycle.

[Tommycat]

Lol...Max you got to get out more.

Chris, is that a 1400 watt 48 volt motor?

Good SSR Info https://www.phidgets.com/docs/Solid_State_Relay_Primer

[151]

TC, I think it may be 1200W. I don't know much about motor tech, but the oset motors seem to be pretty tough. We ran the 36v motor on the small oset. It had plenty of power and speed.

[Tommycat]

OK, so with the main parts in place lets see what we have...

Controller-72volt nominal-200 amps max
Battery- 3 in series equals 65volts with a 180 amp max discharge. (30c?)
Motor- 48volt 1200 watt (continuous or peak?)

So I guess it boils down too how much abuse your going to give the motor? As it looks like the other parts are capable of dishing it out. This will determine your max amp draw, and your selection of shut-off relay/switch ,wiring ,connectors, and fuses, adding a bit of headroom for durability and safety.

By the book that motor should run carefree at 48volts and 25 amps. Can it handle the 35% voltage increase? And how hard to push it? At 75 volts amperage would be 16 amps. And with a 50% increase it would be 24 amps.

[151]

Awesome!

How did you figure the current capability like that? I don't expect it do be pushed to hard, at least not for a while

I read recently that KTM is ending production of 2 stroke 50cc bikes soon. All 50cc size bikes will be electric. So pretty soon we can just get obe from the factory...

[Tommycat]

Figured the current capability using electrical formulas. As an example... Watts=Volts X Amps So if your motor can handle 1200 watts continuous, and your putting 48 volts to it. 1200=48 X ?amps, or 1200/48=?amps, so 1200 watts running at 48 volts will draw 25 Amps. As voltage goes up, amp draw goes down.

I'm new too this field, so I'm trying to learn how much increase is too much? Seems it would be a good investment to have the capability to monitor motor temperature, and then control current to keep below motor specifications if you plan on pushing the envelope.