Help on connecting 24v Motors

Is one of these any use?

DC-MOTORCONTR_BTN8982 - Infineon Technologies

Hi Vandia,

How many Amps do you think it was taking at 12v then? Note, it did not just run slowly: The speed was actually "okay", it was the torque that I saw suffer.

A motor that is not under load (doing work) can run with a relatively low current draw. As we demand more energy from the motor by impeding the rotation of the shaft that power has to come from somewhere. The impeding of the shaft rotation actually causes the impedance of the motor to drop which causes the current to increase. If our motor was operating without a load at 12 volts one amp ( 12 Watts ) impeding the shaft slightly might cause the current to increase to 2 amps. Our motor would now be consuming 24 watts from our power supply. Ideally 12 watts would still be used for internal losses in the motor and 12 watts would be consumed by whatever is impeding the shaft rotation. As we continue to increase the amount of resistance that we apply to the shaft rotation the current continues to climb. At some point we approach the limits of our power supply. You said that your power supply was 12 volt 10 amps (120 watts). As the motor's current draw approaches the 10 amp level your power supplies voltage level will drop. The internal resistance of your power supply will begin to be a critical factor in the equation. At some point your current to the motor might be 12 amps but the voltage that is being applied to the motor may have dropped to only 8 volts ( only 96 watts ). The other 4 volts is being applied to internal resistance in your power supply. This is why manufacturer's of power supplies rate them with a voltage and an amperage. It allow the designer to know the practical limits of the power that can be obtained.  Also note that at 12 volts to the motor we have to increase the current by 2 amps to get an additional 24 watts while at 24 volts we only have to increase the current by 1 amp to get the same 24 watt increase. This is why your motor seems to have poor torque at 12 volts and may not give you the power you require.

John

It is.... but I didn't see a price to buy it. I'd also have to probably buy parts from eBay or something, because my budget ATM. I'm still paying for renovations on my house, plus it would be good to know how to search what I'm looking for. Thanks!

It sounds like your best approach would be to buy a 24V charger rather than try to do it yourself - I would have thought you could get a Chinese replacement for the scooter charger for a reasonable price. That will do the charging with the proper voltages, but you'll need to make sure that the maximum current that the charger supplies is suitable for the batteries you've got; it doesn't matter if it's lower - the charging just takes longer - but you don't want it higher or the batteries will heat up too much. (Batteries heat up when they're charging because of the chemical reaction that's taking place. If you want to impress people, just say that they're 'exothermic'.) Do the batteries have an Ah rating marked on them? [Ah stands for Ampere-hour and is a measure of the electrical capacity - a 1Ah battery can supply 1A for one hour.]

The commercial chargers will be designed to work with just the battery. I think it will mess up the change to trickle charging at the end if you steal current from the charging.

My first thought would be something like this (are you ok with circuit diagrams?). Others here may well have better ideas.

I don't have very much experience with schematics.... I presume to the right of the 12v converter are diodes?
I don't know what the triangle at the bottom means, and what's the 12v on the right?

Also, will these schematics allow the board to stay powered without interruption when being plugged in?

Thanks for the effort in drawing this, btw! <3

vandia wrote:

 

I presume to the right of the 12v converter are diodes?
I don't know what the triangle at the bottom means, and what's the 12v on the right?

 

Also, will these schematics allow the board to stay powered without interruption when being plugged in?Yes you are correct these are diodes.

The triangle at the bottom is a GND symbol, and whilst it's not strictly necessary on this schematic as the whole return path is shown as it's all wired together, if there was another part than needed connecting to this then using that symbol there too would imply the connection.

The 12V on the right appears to be an external 12V bench power supply. What Jon is doing with the two diodes is OR'ing the regulator output with a bench supply so you can plug in the bench supply to maintain a constant supply to your processor board before switching over to charge the batteries.

Best Regards,

Rachael

DCMOTORCONTRBTN8982TOBO1 INFINEON, DC Motor Control Shield | Farnell element14

<3 from your friendly neighborhood e14 Farnell Staff.

It isn't a finished circuit - it's just a quick sketch of a possible approach, an idea of how it might be done. I should have explained it better.

The diodes allow a current to flow only in the direction of the arrow that makes up the body of the shape on the circuit. So current can flow from the DC-DC converter output, through the diode, and power the processor. Similarly, current could flow from your bench supply, through its diode, and power the processor. So there are two ways to power the processor, the DC-DC converter OR the bench supply. The nice thing about the diode arrangement is when you consider the path from the DC-DC converter to the bench supply. Because the arrows face each other, there's no path for the current and you won't get one trying to power the other. That means you can have both supplies there at the same time and the processor will be powered from whichever happens to have the higher voltage. So you could apply the bench PSU whilst the processor was powered from the batteries, switch the batteries over to the charger, and have the processor continue like nothing had happened. [It would be a bit of a clunky arrangement for a commercial product, but for a personal project, where you knew what it does and why, you might decide to go with the simplicity.]

Just for future reference, diodes aren't perfect and have various difficiencies you need to take into account when designing. There is a small leakage current when a diode is in reverse, but it's very small and wouldn't worry us here. Another deficiency, that's slightly more of a nuisance, is that when the diode is conducting forward current there's a voltage drop across it. In this case, the voltage that the processor board sees will be less than 12V - I'm assuming that that doesn't matter and that the power circuit on the processor board will operate over a fairly wide range.

Putting in the 0V reference is just force of habit. I suppose some of it is history - partly because I'm old enough to remember when a good proportion of cars (automobiles) and motorbikes had positive grounds and also because of the early days of germanium transistors when circuit diagrams often had a positive ground and a negative power rail.