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Reducing Volts and Amps

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Ron Laden11/08/2018 09:07:21
1035 forum posts
157 photos

Morning guys,

I am looking at a new project at the moment and I have a couple of questions.

If I have a 24 volt supply (2 x 12V - 75 amp batteries in series) and a non adjustable 100 amp motor controller is it possible to reduce the output voltage and amps from the controller.

Ideally I would want the voltage to be 18 volts and have the amps adjustable or if its simpler set to a maximum of 60 amps.

Also am I correct in thinking that if I have 4 motors wired in parallel from the controller they share the current, so at 60 amps each motor would see 15 amps..?

Many thanks

Ron

p.s. I forgot to say that the motors are rated at 12 to 24 volts and having spoken to the manufacturer a lot of people run them at 24 volts but you have to reduce the available current to under 20 amps so I am thinking 15 amps to be safe. I know if you double the voltage to a motor the speed doubles, the motors are rated at 2400 rpm so 4800 on 24 volt. If I go with 18 volts I am assuming the rpm would increase by 50% to 3600 or isnt it that simple.

Edited By Ron Laden on 11/08/2018 09:14:13

Edited By Ron Laden on 11/08/2018 09:32:29

Robin11/08/2018 09:22:28
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309 forum posts

There is a wonderful thing called Ohm's Law

Amps = Volts / Resistance

The three are inter-related. The current (Amps) flowing through your motor (Resistance) is determined by the power supply (Volts).

To get a meaningful answer, what are you trying to achieve?

Ron Laden11/08/2018 09:35:14
1035 forum posts
157 photos

Hi Robin, I have just added to my post

Ron

Les Jones 111/08/2018 09:42:43
2068 forum posts
141 photos

The motors will only share the current equally if they are providing the same torque. (Current is about proportional to torque. ) To answer your question properly we would need FULL details of the speed controller.

Les.

Russell Eberhardt11/08/2018 09:52:26
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2443 forum posts
83 photos
Posted by Robin on 11/08/2018 09:22:28:

There is a wonderful thing called Ohm's Law

Amps = Volts / Resistance

The three are inter-related. The current (Amps) flowing through your motor (Resistance) is determined by the power supply (Volts).

To get a meaningful answer, what are you trying to achieve?

Ohm's Law only refers to pure resistance (conductor at constant temperature in its original form). It certainly doesn't apply to a motor.

What you can do with them very much depends on the spec. of the controller and what sort of motors they are.

Russell

Russell Eberhardt11/08/2018 09:54:13
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2443 forum posts
83 photos

Deleted duplicate post

Edited By Russell Eberhardt on 11/08/2018 09:54:46

Robin11/08/2018 09:54:38
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309 forum posts

DC electric motors act as generators. The speed increases until the generated Voltage is the same as the applied Voltage.

Starting current is usually enormous.

Once up to speed, power consumption is down to load and making it hot.

Cheap motor controllers depend on turning it on/off many times a second and varying the on/off ratio.

That introduces an interesting 4th factor to the equasion, Inductance. This is the thing that blows up your electronics.

If you want 18 Volts, why not use three 6 Volt batteries?

Robin11/08/2018 09:57:11
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309 forum posts

Russell you have just volunteered to take over. Well done that man laugh

Ron Laden11/08/2018 09:58:26
1035 forum posts
157 photos

Thinking a bit more about it, it would probably be simpler to go with 24 volt and safeguard the motors by having a limit on the current.

Les, I cant provide full details of the controller, all I know is that it is rated at 12 to 36 volts and 100 amps maximum. I wasnt thinking of modifying the controller but wondering if I can control the amps at the output.

The 4 motors would each be driving a loco axle through the same gearing so the torque should be similar to each motor.

Ron

Robin11/08/2018 10:05:04
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309 forum posts

Limiting the current will limit the torque. Is that what you want?

Robin11/08/2018 10:05:59
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309 forum posts

Protection by limiting current is usually achieved with a fuse.

Ron Laden11/08/2018 10:19:25
1035 forum posts
157 photos

No I dont want to limit the torque but I would have thought at 24 volts and 15 amps the torque would be ok but I could be wrong.

With a 100 amp controller and say a 60 amp fuse, wouldnt the fuse blow each time the controller was switched to maximum, thats why I was hoping the max amps could be set or controlled.

Robin11/08/2018 10:50:37
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309 forum posts

A 100A controller can handle 100A before it starts to go wrong. It doesn't insist on 100A flowing through the motor.

Ron Laden11/08/2018 10:51:46
1035 forum posts
157 photos

Thanks Robin I do appreciate that but wonder what full power will deliver to the motors, though you are not constantly at full power. 

There is a simple answer to all of this.

I can buy a 120 amp controller with adjustable amps but it is over £200 which I was trying to avoid but if needs must I will have to go with that. I am ok with basic electrics but not with electronics and not knowing I was hoping there was a way of controlling or restricting the current after the controller but if not I will have to consider the adjustable controller.

Regards

Ron

Edited By Ron Laden on 11/08/2018 10:54:21

Robin11/08/2018 10:59:22
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309 forum posts

Volts times Amps equals Watts

746 Watts = 1 horse power

So 18V x 100A = 1800W = 2.4hp

Russell Eberhardt11/08/2018 11:08:53
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2443 forum posts
83 photos

If your controller can't control the current nor the voltage just what is it controlling? Is it perhaps for a brushless motor? Is it perhaps a PWM speed controller? I think more information is needed.

Russell

Keith Long11/08/2018 11:10:10
780 forum posts
10 photos

Ron you're thinking about this from the wrong end. Your controller will control the VOLTAGE to your motors which will then draw whatever CURRENT they need at that voltage setting. If the motors are running light - ie not driving anything and you set the voltage to 24 volts then they will draw little current. If however you were to lock the shafts and then apply the 24 VOLTS they will try and draw a lot more CURRENT. What blows up in the latter case could be the controller if the motors try to draw more than 100 amp or if could be the motors burning out if the controller can supply all that the motors demand.

Without knowing what current your motors will draw at full load it's pretty much impossible to say what size controller you need, you need to do more homework and supply more information. If you're worried about the motors demanding too much current from the controller then stick a fuse in the lead to each motor and a master fuse in the lead from the controller. With car batteries as the power supply then fuses are essential in any case - have you seen what happens when you short out a car battery?

Russell Eberhardt11/08/2018 11:11:07
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2443 forum posts
83 photos
Posted by Robin on 11/08/2018 09:57:11:

Russell you have just volunteered to take over. Well done that man laugh

Not trying to take over, just pointing out that Ohm's law is a minor part of the equation.

Russell

Robin11/08/2018 11:19:55
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309 forum posts

I have answered this many times, I find it best to start uncomplicated and then progress.

I'm betting Ron has a wicked idea for a killer, arena robot and has bought a bunch of Amp-Flow, rare earth motors to play with.

Journeyman11/08/2018 11:38:06
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590 forum posts
90 photos
Posted by Robin on 11/08/2018 11:19:55:

I'm betting Ron has a wicked idea for a killer, arena robot and has bought a bunch of Amp-Flow, rare earth motors to play with.

More likely for his 0-4-0 shunter in this ** THREAD ** smiley

John

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