I'm running a 12v bilge pump on a 24v system

[Bill Seal]

I=E/R
The same motor has the same R regardless of the supply voltage (sort of), so doubling the voltage doubles the current and quadruples the wattage.

[coopec43]

Originally Posted by fivecapes

Please explain how the internal resistance of the pump went from 0.6Ohms to 2.4Ohms?

Maybe you can answer this for me?

A DC electric motor spinning at 4500 RPM draws 3 amps of current with 110 volts measured at its terminals. The resistance of the armature windings, measured with an ohmmeter when the motor is at rest, unpowered, is 2.45 ohms. How much counter-EMF is the motor generating at 4500 RPM?

How much “inrush” current will there be when the motor is initially powered up (armature speed = 0 RPM), once again assuming 110 volts at the terminals?

It's all very well to test the resistance of the electrical motor at rest but resistance increases with load. A motor not under load would draw very few amps.

ANSWER
Current and back-emf
Some basic DC motor concepts: (Try and grasp each one before going on to the next.)

• DC motors spin when a voltage is applied to the terminals.
• When unloaded the motor speed will be proportional to the voltage applied.
• If you disconnect the motor (from the supply) and spin it then voltage appears at the terminals. It is acting as a dynamo.
• The voltage that is generated in dynamo mode is proportional to speed. We call this "back electro-motive force" or "back-emf" for short.

[fivecapes]

Originally Posted by coopec43

Maybe you can answer this for me?

A DC electric motor spinning at 4500 RPM draws 3 amps of current with 110 volts measured at its terminals. The resistance of the armature windings, measured with an ohmmeter when the motor is at rest, unpowered, is 2.45 ohms. How much counter-EMF is the motor generating at 4500 RPM?

How much “inrush” current will there be when the motor is initially powered up (armature speed = 0 RPM), once again assuming 110 volts at the terminals?

It's all very well to test the resistance of the electrical motor at rest but resistance increases with load. A motor not under load would draw very few amps.

ANSWER
Current and back-emf
Some basic DC motor concepts: (Try and grasp each one before going on to the next.)

• DC motors spin when a voltage is applied to the terminals.
• When unloaded the motor speed will be proportional to the voltage applied.
• If you disconnect the motor (from the supply) and spin it then voltage appears at the terminals. It is acting as a dynamo.
• The voltage that is generated in dynamo mode is proportional to speed. We call this "back electro-motive force" or "back-emf" for short.

Why should I care about no load scenarios? There's always a load on bilge pumps the moment they're on. Why are you even looking at startup vs. steady state?

Are you confusing mechanical load with motor internal electrical resistance? The winding resistance is fixed.

[coopec43]

Originally Posted by fivecapes

Why should I care about no load scenarios? There's always a load on bilge pumps the moment they're on. Why are you even looking at startup vs. steady state?

Are you confusing mechanical load with motor internal electrical resistance? The winding resistance is fixed.

NO I am NOT confusing internal electrical resistance to mechanical load but OP seem to be confused.

Why do you ask?
"Please explain how the internal resistance of the pump went from 0.6Ohms to 2.4Ohms?"

Why even mention Internal Resistance? If the 12V bilge pump is 120W then the current will be 10A. If that same 120W pump is run on 24V the current will be 5A.

Why do electric motors draw X3 times the amperage on start up?

"Why should I care about no load scenarios?:

Good question why? They seem to think an electric motor has a fixed resistance.

[sstuller]

If you double the RPM of your diesel according to some replies you will reduce your fuel consumption by half. What a wonderful world we live in. Current is the fuel of an electric motor. Draw your own conclusions.

[cal40john]

Originally Posted by coopec43

Watts = Amps X Volts

To do the work of pumping let's say it takes 240 watts.

So with a 12V pump it will draw 20 Amp.

With 24V (and the same pump doing the same work) it will draw 10 Amps.

But in reality the pump will run faster on 24V and pump more water but to pump the same amount of water (as the 12V pump) it will be on for a shorter time.

You're mixing up work and power.
Work is a type of energy. To get 100 gallons of water out of your boat it will take a set amount of energy.

Power is the rate of doing work. If you get the water out in less time, you have done the same amount of work, but the rate of work done, which is power was greater. In electricity we measure power in watts.

If you apply a higher voltage you will increase the power. Some of that power is used up doing work on moving the water and some is dissipated as heat.

The pump is designed for heat to dissipate below a certain rate. Increasing the voltage increases the power which increases the rate of heat produced (and water flow), leading to higher temperatures. Too high a temperature destroys the pump.

Yes the pump runs a shorter time for a certain quantity of water, but is the quantity of water small enough to run the pump a short enough period of time to not overheat the pump? Depends on how much water is in the boat.

As a kid I had enough slot car track that the far side of the track, away from where the wires connected, had enough resistive losses that the cars would either slow greatly or come to a stop. My fix was to hook up two transformers in series doubling the voltage. The cars screamed at the near side of the track and made it all the way around.

After melting down a few motors we learned to only run a lap or two before swapping out cars to let them cool down.

[coopec43]

The thread is "I'm running a 12v bilge pump on a 24v system"

"Motor voltage
If you run a 12v motor from 24v its current drain and speed will still depend on the mechanical loading. However under no load it will now run at twice the speed at which it originally ran with 12v. Heating in the motor is still related to the current – so you can still run it at its full rated mechanical load/current. However if the motor is badly balanced you may expect noise and vibration as the general construction may be inadequate for the faster speed. There may also be a problem with brush wear since the brushes are being asked to switch the current twice as fast. These effects are, however not very likely and usually the speed increase is quite OK

If you overload the motor, its current rises in the same way whether the motor is running from 12v or 24v. However on stall the current from 24v could be twice that from 12v, so the motor could get four times as hot (heating is proportional to the square of the current).

You're mixing up work and power.
Work is a type of energy. To get 100 gallons of water out of your boat it will take a set amount of energy.

I don't believe I am. Work is energy.

Power Formula. Power is a rate at which work is done, or energy is used. It is equal to the amount of work done divided by the time it takes to do the work. The unit of power is the Watt (W), which is equal to a Joule per second (J/s).


To get the 100 gallons out of the boat you may need a 12V 120W pump drawing 10A OR
a 24V 120W pump drawing 5A

[coopec43]

Maybe Fuss needs one of these!!

[Fuss]

Sofar I don't see a problem and for me it works fine.

better than a 24v pump for sure


I dont feel like testing it at full speed out of the water...maybe it doesn't like that much.

On my short testing I would say that if you always run it with water in it and only briefy run it without water (less than 10 sec maybe) then it is a big improvement.

Does it really last less time than one run at the correct voltage..

I know that someone on here must have done this before, this forum as a few very innovative people, maybe one will see this thread and give some feedback!!

[coopec43]

Fuss I can't answer that. On anything else other than a bilge pump I'd take a risk.

I've switched my yacht over from 12V to 24V and intend running my lights, oscillating fans, macerator and maybe a small diaphragm pump on 24V and I don't expect problems. (I'm not the first to do it)

My Adler Barber freezer unit will run on 12V but they say it works better on 24V ????

[becrux]

Originally Posted by coopec43

Watts = Amps X Volts

To do the work of pumping let's say it takes 240 watts.

So with a 12V pump it will draw 20 Amp.

With 24V (and the same pump doing the same work) it will draw 10 Amps.

But in reality the pump will run faster on 24V and pump more water but to pump the same amount of water (as the 12V pump) it will be on for a shorter time.

The error here.is in the assumption that the same pump will draw the same power whether operated at 12v or 24v. It will not. Power ratings for pumps assume a single fixed voltage. Doubling the voltage quadruples the power for a given pump design.
Where does that extra power go? Some goes to moving the water faster (remember there is flow resistance in the discharge hose, which will be higher when the flow is faster). So some power is used for useful work in a shorter time. The rest goes to heat. Exactly how much goes to each depends on several factors and is open to discussion. [Some of those factors have nothing to do with the pump, e.g. discharge hose diameter and run, and lift height.]
As others have pointed out, heat dissipation may/may not be a problem. Low duty cycle is probably fine. High duty cycle, not so much, but that is exactly when the pump is needed most.

Best option? Use appropriately rated pumps for your system, and ensure that there is not too much voltage drop in the leads when the pump is under load.

[Side note: the guy who suggested relays to mitigate voltage drop seems to have assumed that some pumps are activated only from the bridge and do not have an automatic switch. For automatic operation the relays would always have to be on. They could be wired to be on by default, but then why have the relay?]

[a64pilot]

How about buy a 24V pump and do not put additional components in the system?
Additional components are adding complexity to a safety system where the simplest system is desirable to reduce failure possibilities

[cal40john]

Originally Posted by coopec43

The thread is "I'm running a 12v bilge pump on a 24v system"

"Motor voltage
If you run a 12v motor from 24v its current drain and speed will still depend on the mechanical loading. However under no load it will now run at twice the speed at which it originally ran with 12v. Heating in the motor is still related to the current – so you can still run it at its full rated mechanical load/current. However if the motor is badly balanced you may expect noise and vibration as the general construction may be inadequate for the faster speed. There may also be a problem with brush wear since the brushes are being asked to switch the current twice as fast. These effects are, however not very likely and usually the speed increase is quite OK

If you overload the motor, its current rises in the same way whether the motor is running from 12v or 24v. However on stall the current from 24v could be twice that from 12v, so the motor could get four times as hot (heating is proportional to the square of the current).

You're mixing up work and power.
Work is a type of energy. To get 100 gallons of water out of your boat it will take a set amount of energy.

I don't believe I am. Work is energy.

Power Formula. Power is a rate at which work is done, or energy is used. It is equal to the amount of work done divided by the time it takes to do the work. The unit of power is the Watt (W), which is equal to a Joule per second (J/s).


To get the 100 gallons out of the boat you may need a 12V 120W pump drawing 10A OR
a 24V 120W pump drawing 5A

You can have any size pump you want to get the water out of the boat. Pumps with different power ratings will change the rate of water leaving the boat.
If you buy a 24 v 120 w or a 12 v 120w pump it will take the same time.
If you put 24 volts on a 12 volt pump it is no longer a 120 w pump.

(Repeating Becrux)

From this link near bottom of page 117 -
https://books.google.com/books?id=nz...0motor&f=false

Compared to a 12 volt motor a 24 volt motor will have twice the windings with half the cross sectional area, the resistance will be 4 times as great.

Your 12 volt pump will not have the resistance of a 24 volt pump, so will no longer be consuming power at the same rate, so will no longer be a 120 watt pump, it will be a higher wattage pump.

[billknny]

Originally Posted by Fuss

Your absolutely right SV, I read your post and I immediately changed it for a proper 24v pump so that I don't kill anyone.

But now I'm confused , lets say its 2 oclock in the morning and the correct 24v bilge pump has failed, I don't know why it failed , it was only 2 years old and it had hardly had to pump anything over this time...must have been a faulty one...anyway, its 2 oclock in the morning and a storm is raging and some water is coming in from somewhere, I'm holding a 12v bilge pump in my hand.... what shall I do now?

shall I connect it to the 24v system?

If you need a bulge pump to keep your boat afloat you have bigger problems than voltage issues.

[Crocboy]

I'm assuming the 24v is supplied by 2 x 12v or 4 x 6v batteries.
What I would do is run cabling to one 12v battery and just connect the 12v pump directly to the one battery.