Potential Divider Criteria and Pot Selection

....which isn't a pot; it's a big rheostat.

I didn't want to stir the rheostat.

Thanks for your help, thats something to work with. I might use individual high power wire wound resistors to create a network.

I would seriously recommend you look at something like a DC/DC converter that can give you the output voltage and current you need, or an actual motor controller. It will be much much much more efficient, and really not all that more expensive, considering the cost of high power resistors.

Quite.

I'm going o look up a dc/dc converter thanks,

I was also considering two 24V supplys with the two -ve terminals together as my common motor connection that should get rid of the heat distribution problem.

I just realised D'oh! that putting the he two negatives together wiil give me two outputs the same for my motor which isnt what I wanted. looked at dc/dc converters and i dont think thats the way. Thought about putting two mains dc power supplies in series but think there would be issues when joining the +ve of one to the -ve of the other just in case the -ve is earthed to the ac supply earth.

I'm to far down the path now so I'm going for sealed lead acid cells in series on permanent trickle charge then i can just tap off supplies where needed and no need for potentiometer.

If you charge the sealed lead acid batteries in series and discharge from different points you will end up overcharging those you use least, which I suspect will not be not good for their durability.

Running motors from a resistive network is not a good idea either. Motors often need to draw greater currents at start-up (to avoid stalling) and resistors could prevent this.

I'm not certain that I understand what you are doing. I think we need more details. Does your motor have a permanent connection between the field coil and the rotor? That would be most unusual, and suggests that the commutator may not be designed so that the motor can be reversed. If you can completely separate the field coil and the rotor connections, why not use a two-pole change-over relay? (If you cannot wire your existing relay in that way, the cheapest option will still be to buy a new and more suitable one)

Regarding DC power supplies, it is very unusual for the DC source to be permanently connected to ground. If one terminal is grounded, the connected terminal will be specified, and you will be able to purchase a pair that are different. If the supply is isolated, the isolation resistance and breakdown potential should be specified. (If these are not there, you haven't got the proper specification)

Warning: if you switch or disconnect the motor while it is running, there will be energy stored in the windings - certainly enough to damage the coils in the motor unless you have a suitable quenching method.

It sounds to me as if you need someone with relevant experience to check whatever system you come up with - and not only on paper, but physically, before you turn on.

Regarding the batterys I was proposing 8 x 6v lead acid 12ahc batterys and the motors would be only on intermittently (about 30 seconds in a 2hour period), during maintenance batterys could be rotated, if one or two do fail I dont have to replace all of them.

I agree the resitive network is not a good idea.

I was thinking along the lines the dc motor was permanent magnet field with rotating field coil. so no connection there.

I was thinking of protection by signal diode across relay.

this is my version of getting experienced views. I think this is a really good site.

Only one coil! You do not need two supplies. One supply and a two-pole change-over relay (to swap both connections between the two terminals) will do everything you need. Even if you have to buy a new relay (I'd be surprised, because that's a fairly basic configuration) it should be cheaper than a second battery or a second power supply.

In any case, I would not recommend trickle charging two batteries in series unless you can guarantee that the motor will be used in both directions - and with similar loads and for similar amounts; this balance would need to be in a time that is short compared with the charging time of the batteries - which means that the battery would have to be over-specified for the purpose.
[Imagine discharging one battery only; you then have to recharge it with the current passing through the other still-fully-charged battery in series. The fully-charged battery can only pass current by dissociating the electrolyte, which will give off hydrogen at one electrode and (usually) oxygen at the other. If these remain within the battery, you will have an ideal explosive mixture on your hands. If the battery is suitably vented, it may withstand this, but will need topping up regularly with distilled water; however, this will always have an adverse effect on battery life - though in some cases that effect can be quite small]

I was going to use the sealed gel batteries so topping up and ventilation won't be a problem.

in all I will be powering 6 pumps one at a time (max 6.5W) and 5 motors one at a time

going forward and reverse equally.

I like the two pole change over relay idea, but i,ve also got 6v and 12v transistor switches running at the same time.

Surely the transistors will be on the same side of the relay as is the power supply? So I don't see the problem.

Equality in time is most unlikely to be the same as equality in total charge - you would need equality of load as well.

Although safe as regards explosion, sealed gel cells are the worst possible choice if you are going to have any asymmetry between the charge and discharge. The problem is that they use the minimum liquid in the electrolyte; as each overcharge depletes the water in the electrolyte, you can see that the electrolyte concentration will increase more rapidly than for other batteries - and you can't top them up. To be explicit: forget about using batteries unless you can control the charging so that each battery sees its optimum cycle.

Your question is vague. Is your power supply DC with +24, -24 and common? Or is it 48 volts AC with center tapped transformer connected to common?

A potentiometer is nothing more than a resistor with a slider that makes a variable two-resistor divider. All considerations for pots would be the same for two fixed resistors, such as power dissipation.

If the power supply is DC +24, -24 and common, then the motor can be connected directly to +24 and common, or (using the same lead formerly connected to the +24) -24 and common, which will cause it to reverse direction.

The proposed supply will be 8 x 6V lead acid cells in series. one terminal of motor is permanently connected to +24V (ie between battery 4 and 5) the relay will switch between 48 V (+24V wrt 24)and 0V (-24V wrt 0) to five rotatin in either direction.

Why don't you call the connection between battery 4 and 5 zero? That way, battery 1 is -24 and battery 8 is +24. All a matter of perspective I guess . . .