Help on connecting 24v Motors

Hi Vandia,

The replies may be slow as we are scratching our heads a little to digest and analyse your questions and situation. Let me restate my understanding of your resources. You have two 100 Watt 12 volt motors from scooters. You have (2) 12 volt batteries in each scooter and they charge from a 24 volt supply. I am not sure whether the batteries are connected in parallel and the 24 volt charger voltage is being dropped to charge level for the parallel batteries or if the Batteries are in series and the output of 24 volts is being controlled to run the motor.  Regardless here are some observations that you can think about and then get back with more questions. #1. The motors will draw about 8 Amps when they are hooked to a 12 volt supply. Amps times Volts = Watts. If your supply is weak or your connections are poor then you may not get the full 12 volts across the motor and the Amps will drop accordingly. #2 To get good control of your vehicle using differential motor control you will not be happy with relays. The relays will put full power to the motors whenever they are switched. You will go from full power in one direction to full power in the other. This will be hard on the system and it will likely throw the rider off. You will have to learn how to use MOSFETs and PWM to control the motors. This will allow you to gradually increase and decrease the power and directions of the motors. There are some good H Bridge circuits that are designed to do this. #3 The computer control will be able to be run off the same power system as the motors but you may have to regulate and buffer the supply so that interference and spikes do not disrupt the logic. Draw out some schematics of what you have so we can better understand your resources and ask some more questions.

John

Thanks for the response! The bumps was one before I noticed it was in the wrong area, then one because it was an old post just moved to this area, so I thought it may be down a page in the list due to the time.

The motors are 24v 100W. I have two of these. The batteries were either series or parellel and stepped by a control box that was in the scooter, I'm not sure which, but it used two 12v acid/lead core batteries. It uses proprietary connectors, so I had to cut them off to test the motors. The scooter originally charged form a 24v supply, but I don't have that chargers, so I need to be able to charger batteries with my 12v supply, and turn the 12v from the batteries/supply into 12v going to the motors.

#1: I was told before that you could not take amps to substitute voltage in motors. I figured it would be about 8A, but noticed it was still acting sluggish with 12v/10A. I thought this may be partly because the chain, but this could not push an ironing board, much less a persons standing on it. I will still try to double check my connectors anyhow.

#2: I'd be the only one riding it, and was planning on turning off power to the motors before switching directions. I've nothing against using a motor controller.... except IDK where to start, and finances are an issue. Recommendations? I currently plan to use one bi-directional motor per side.

#3: Even if I could regulate the power to the board down to 5-12v, I can only charge the batteries at 12v. For your method, I'd need on stepper to step the 12v/10A to ~24v/5A, ignoring loss in conversion. I felt it was better to run the main circuit off the 12v supply/batteries, then connect a converter stepper for the motor controls.

My problem is, I just don't know the parts out there, or really anything past powering 5v operations. So, what would be ideal is a motor controller that takes in 12v, but steps it to 24v, and controls it bi-directionally.

If the motors are 24 volts and the charger input was 24 volts then the (2) 12 volt batteries were hooked in series. You can charge them with your 12 volts but they would have to be done individually or switched out of the series circuit and put in parallel. I think you may be confused on Amps and Volts. If I have a 24 volt 100 Watt motor I know that when I put 24 volts on that device it will draw about 4 Amps though this wattage rating is probably for when the motor is under load and if you hook it up without a load it will probably draw a lot less amperage. The voltage is like the pressure that is being applied and for an individual device that is all we can control. More voltage = more amperage and less voltage = less amperage. We can not make the device take more amperage with less voltage. When you hooked up the motor with your 12 volts you saw the result of putting half the rated voltage and it ran slowly.

You asked about testing the batteries to see if they are any good. Put the batteries on your 12 volt charger for several hours and then hook them up to a load and see how long the run before they run out of power. You can use an old automotive tail light or head light for the load. Do this for each battery and see how they compare at taking and holding a charge. A good 12 volt gel cell should be able to run an automotive brake light for 1 to 2 hours.

This is wonderful that you are interested in and attacking this project. Whether it succeeds or not you will learn a lot from trying. One of the bad things about being an old guy like me is that we have had too much experience with things and we think we know what will not work so we don't try. Keep on trying and don't be discouraged. Also use your questions as an inspiration to learn more about the basics of electronics.

John

Couple of thoughts to add to John's. You may know this anyway, but just in case not:

1. Safety

A lead-acid battery can deliver a lot of current very, very quickly if you ask it to. By a lot of current, I mean hundreds of amps if you were to short the terminals. If you shorted the terminals with a thin piece of wire, the wire would vapourise like a fuse does. If the short were more substantial, like a screwdriver or a spanner, the current would flow, the metal might weld itself to the terminals, and the battery would get very hot, very quickly. The acid would start to boil, the battery would vent if it's a sealed type, and the lead plates would deform. Needless to say, it doesn't do the battery much good. To avoid that, you can do two simple things. Put covers over the terminals so that things can't come in contact with them. Have a fuse in-line with the cable to one of the battery terminals [the fuse deals with the situation that you have a wiring problem in your circuit] - automotive fuses [the 'blade' type] are good for this.

2. Charging

Do you have a proper battery charger or are you talking about an ordinary power supply? You can charge a lead-acid battery with a bench power supply [it MUST be of the kind that does proper constant-voltage/constant-current operation], but the voltage needs to be higher than 12V [set it to 14.5V], and the current would probably need to be limited to less than 10A (or you'll overheat and damage the battery when it's charging). If you only charge to 12V, you won't get a full charge into the battery [I can't quite remember now how it goes capacity-wise, but I think it would be less than a half charge when the charging gets up to 12V]. You also need to remember to take it off charge when it's done [a proper charger would sense the end, drop the voltage to 13.6V, and then continue trickle charging the battery at that lower voltage]. If you show us what's written on the battery, we could advise on a safe maximum current for the charging, or you could look it up in the datasheet of an equivalent battery (it doesn't matter if the current is limited to less than that maximum, it then just takes longer to do the charge).

Please note this post is for John from earlier today. I started on this earlier, and never finished. I may tackle the other comments before bed, but it's getting late/early.

For the batteries being in series, that was my conclusion, BUT I'm also not declaring it 100% for certain, because AFAIK, that control box could have charged them at 12v in parallel. Unlikely, I know, but I prefer to say "IDK" than be wrong, lol.

I am aware of the difference between amps and watts.... I just kind of forgot as I tend to get on here when I first wake up or right before bed (like now).

For how many amps it's taking.... I'm pretty sure it's going to have a load, lol. So, I guess, IF I CAN make work at 12v.... I guess I should find some WD-40 to fix that chain and see if I can make it work. Again, I only have serious experience on ~5v applications, so I'd like to ask: 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.

I am still hoping to run the main system on 12v somehow. You said about charging the batteries. I don't have a 12v charger yet, just a 12v power supply. Is there an easy way to charge a battery with that, or do I need something special? I typically have used rechargeable USB battery packs, so never hard to worry about the circuit design. Thanks!

Thanks for the advice I only have a 12v10A power supply not adjustable so I guess I should look for a charger I asked for advice on that on the comment before this but with the detail you explained may I also ask Depending on the circuit when charging does it switch off the power TO the rest of the circuit when the battery is charging I have 5v(USB battery packs which disable power output when charging

I'm hoping to be able to keep the unit on when charging the batteries, without rebooting, but it's only like 80% important, not like 100%, if that feature is in a much higher price range to have.

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

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