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[pnr/p]

I have just got a Graupner Speedgear 500 Race and the instructions state 'cell capacity 0.5 to 1.7 Ah - do not exceed'  I can understand not 'overcooking' the voltage but always thought Ah was just 'staying power' so why can't I use the bigger sticks - please help an old boy get his head round it.

[RipSlider]

The thing about Volts and Amps is a little misleading.

What the motor cares about is Watts. Too many watts and the motor will over heat and/or damage the magnets, depending on it's type.

Watts = Volts x Amps.

example:

Lets say you were going to use a 7.2Volt pack. We have the two packs discussed, a 0.5 Ah pack and a 1.7Ah pack.

At the lower end: 7.2x 0.5 = 3.6W/h  ( Watts per hour )
At the higher end: 7.2x 1.7 = 12.24 W/h

Remember that this is a sort of "steady state" equation. If is pefectly possible to use that entire Wattage in 5 minutes ( if you hammer the boat, or you point i hold it stationary and put the power on full ). In what case, you would be using, at the higher end:

7.2 x (1.7x 12 ) = 148.88 watts

On a 540 sized motor, that's about as high as you would want to go. However, on a 500 sided, which is, I think, a bit smaller, you are pushing the outer limits.

Remember also that this is the UNLOADED watts. I.e it's a motor running in a mount not doing any work. When you add a prop, and make it push a boat, you'll be doing even more work, so using more watts.

That's why it's useless to use either a volt meter or an ammeter to do anything useful when dealing with models. The only way to actually make any sort of useful calculations is to get a Watt meter, hook it up, then place your model in a bath and see what happens.


Hope this is of some help. If it's made you even more confused, then drop a PM or post here.

Steve

[Cargo]

Never heard of that...

if the akku has more capacity, you can longer drive. The motor gets the V from the akku and then he produces his Watt, depending on how much volts he gets. The A of the akku tells you only how much capacity the akku has.

[wombat]

Hi PNR,

I'm not sure that I agree with Steve that the critical factor for a motor is the power measurement. In terms of the defining parameter for a motor is the operating voltage range - once you have that you can determine the required current to achieve the required speed of rotation and torque. Essentially this is a secondary parameter. He is right that the power supplied to the motor is the product of the voltage and current - however it then gets more complicated.

Takes a deep breath

The motor operates because the current flowing in the coils of the armature creates mechanical force which causes the motor to rotate. However the rotation of the coils in the magnetic field generates a voltage that opposes the current (the back EMF). These are different - the back EMF is lower than the applied voltage - the difference between the two represents the power absorbed by the motor - this goes in four places: losses in the carbon brushes, losses in the copper wire of the coils, losses in the iron core of the armature and finally the power delivered by the motor. The peak efficiency of the motor is the point where the losses are at their lowest compared to the power delivered to the driveline. As an approximation, the copper losses are proportional to current - this means they increase with reducing speed and with increasing load, the iron losses are proportional to speed - thet reduce with increasing speed. The point of maximum efficiency is the point where the two lines cross.

Because of these complex relationships, the only fully predicatable parameter is the voltage because that is the applied parameter - a measurement of the power applied to the motor in watts is a quite misleading parameter.


But coming back to the original question  - I don't know why they put a limit of 1.7Ah on the motor, though there are two possibilities that I can think of....

First of all that if you run with a bigger battery pack the motor will get too hot while running - the various losses of the motor become heat (and noise). Running with a larger battery pack could allow the temperature of the motor to get too hot - especially the brushes. I assume if this is the case, the time to change the batteries is enough to allow the motor to cool suffieicently.

The other possibility is down to the battery structure - the capacity of the battery is dependant on the surface area of the plates. To get higher capacities the plates are "foamed" to get more surface area. The down side of this is that the internal resistance goes up - the higher the the resistance, the hotter the battery gets when it has current drawn from it - couple this with longer run time and the temperature of the battery can go up significantly. However, I suspect the 1.7Ah limit comes from NiCd batteries - it probably does not apply to NiMH batteries.

Wom

[sheerline]

I think a test meter is the most important tool in the box when dealing with this kind of problem. Running up a motor on or offload, you can first measure the voltage.. say 12 V across its terminals. Now with the meter set to read Amps and the leads inserted between one of the motor terminals and battery, you can measure amps.. say 5A for example. Once both of these readings have been obtained, multiply them both together (12x5) and you have the figure 60 which is watts. It should not be necessary to buy a wattmeter as this simple method is really all that is needed.

[wombat]

It is important to remember, though, that most meters especially cheap ones will read low on current because of the effect of the burden resistor used ot measure the current, soshould be relied on for indication only.

Wom

[Shipmate60]

Sorry RipSlider,
As Watts are Volts x Amps and the voltage is reasonably constant then the variable IS Amperage as a measure of the work being done.
Yes I accept that as the workload on a battery increases the voltage CAN decrease, but if you have a battery in good condition and the drain from the motor isn't excessive for the battery then it is imperative to know, therefore measure the current consumption in Amps.

Bob

[pnr/p]

Wombat got it right..here is the 'official' answer from Graupner importers...

It’s to stop it overheating as a larger AH battery is able to supply much larger currents (especially in a stall situation) . If you are pulling a very low current then you may want to risk it so as to get longer motor run.

 

[malcolmfrary]

You have to remember that a great many of these instructions are written with buggy users in mind, and they are disinclined to understand that electricity is involved.  Telling them what numbers should be on the labels of the various parts works for them, but is fairly meaningless for us boaties.

[wingertaz]

I agree with Malcolm  I always fit the size (Amp hour) of battery that will give me the running time i wish to achieve and not restrick the size of battery just in case the motor should stall. Even a small amp hour battery will supply a massive current if a motor is stalled. Fitting a relevant size fuse in your installation should prevent this.

Gary