--- In electricboats@
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> Aren't relays basically a remote switch?
ANS: Yes, a relay is simply a switch that is moved by an elecromagnet, and returned usually with a spring. Although there are relays that are moved in one direction with the electromagnet, and returned by energizing another electromagnet.
>On the schematic for the Javelin the batteries are close to the >motor and the wiring is all heavy gauge/ I think what he is using is >just regular switches. A relay is when you have a low amperage >switch (such as on the dash of a car) that controls a remote relay >(under the hood, closer to the end use) that is a higher amp switch >that actually closes the high amp circuit. A starter solenoid is the >ultimate example. Modern cars have a whole series of 12 volt relays >for controlling high output devices, remote control, and delayed >control. Switches sound cheap. relays may not be needed.
ANS: On the Javelin, I used relays, because they were much cheaper than the manual switches that I had originally intended to use. At the time of building the project, the relays were about $22 each, and double pole double throw manual switches, rated for 30 amps, cost far more.
The relays are mounted between the two batteries, there is no photo of this in the presentation file. The gray control box next to the motor on slide 14 shows two (red) rocker switches which actuiate the relays which are near the batteries. The main on/off switch on the gray panel (to the left of the red switches)is a single pole single throw, rated at 50 amps, and serves as the main on-off switch. This simple switch was fairly low cost, but not a typical Radio-Shack item.
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> Carrying this intellectual exercise a bit farther... Javelin has two settings - 12 v full on and 24 v full on. We always talk about motors being rated as continuous use and utlimate use. How does this factor in? Seems like when the switch is thrown you get full power at whatever voltage is selected.
ANS: As motor duty ratings, I will try to explain: The Javelin motor is rated for continuous duty, 3/4 horsepower, 24 VDC, 29 amps. Thet means that the motor is capable of operating continuously at that shaft power level, assuming the load matches the 3/4 horsepower. If the propeller is exactly sized right, it will absorb the 3/4 horsepower and load the motor to it's continuous rating.
If the prop is smaller, or has lower pitch, the shaft will not absorb as much power, and the motor will operate at a lower power level. The applied voltage is still 24 volts, but the amperage will go down somewhat.
If the prop is bigger, or has higher pitch, the shaft will absorb more than rated power, and the motor will be overloaded. The applied voltage is still 24 volts, but the amperage will be higher than the rated continuous amperage. In general, this overloaded condition will result in high motor internal temperatures, and a shortened lifetime.
Many motors can survive this overload condition for a limited period of time, that is why some motors are rated for "intermittent duty".
The point here is to realize that a 3/4 horsepower motor does not always put out 3/4 horsepower, the actual load on the motor is entirely dependent on the nature of the applied load.
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> -----Original Message-----
> From: hardy71uk <p0054107@..
> To: electricboats@
> Sent: Thu, Jul 9, 2009 4:10 am
> Subject: [Electric Boats] Re: Relays to replace a controller
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> in the 19th century they sometimes had complicated switching to give speed control for example switching 24 times 2 volt cells in various series and parallel combinations .Also series and parallel motors. Nowadays it is normally cheaper and simpler to use an electronic controller unless youve been given a pile of free relays to play with or only want a couple of gentle speeds.
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> Chris S
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> --- In electricboats@
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> > resistors = cheap, relays = expensive, controllers = more expensive...
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> > Remember that the increase in torque is nearly instantaneous - a large prop on a small boat and something might tear off - though there is probably some way to introduce a capacitor to slow the current rush down.
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> > I've considered it in the context of systems redundency - I have a brushless but could carry a brushed motor and rig such a setup in a pinch.
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> > Controllers eat about an amp, relays probably much less and so I would think the efficiency would actually be higher - except the loss of fine-grained control means you will burn amps when you don't need to (you are more likely to over-throttle than under) - that could amount to a 20% overall loss in range for example. So now you are carrying more batteries than needed - and where is the savings in that?
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> > -Keith
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> > --- In electricboats@
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> > > What does the group think about replacing a controller with relays that reduce the voltage from 24 to 12 volts? What is the efficiency loss and what other problems would be expected? And could the same thing be done with a 48 volt system, reducing it to 36, 24 and 12? There must be some reason why older golf carts used resistors to reduce speed rather than reducing the voltage.
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> > > Thank you
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