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Power Master
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A speed control is rated at 50 Amps. This means that I can run 50 Amps continuously through it. Gold plated circuit boards have much less resistance than copper boards. There is no advantage in purchasing a higher rated speed control than one that will do the job. A large heatsink or a fan on a speed controller makes it much better. You can draw 80 amps off of a 6 cell 3300 mAh battery pack. Speed controls run fine without Schottky diodes or motor capacitors. Speed controllers with reverse go just as fast as ones with forward/brake only.
A speed control is rated at 50 Amps. This means that I can run 50 Amps continuously through it. Most speed controls are not rated at their continuous current level. In fact, the best speed controls (most expensive) appear to rated at about the current rating that could be achieved for about 10 seconds, some a little longer. My Mini P controller is 40 Amps and Power Master is 150 Amp on this basis. Worse still, some speed controls claim the mosfet spec on the manufacturer's spec sheet as the current rating, which disregards the package or mounting heat dissipation limits. This would make my Mini P2 161Amp, and the PM6 644 Amp. I have changed mosfets a number of times trying to the best available. I recently found a spec on a mosfet which was much better than the one I was using , but when I bought some, they just were not as good. Sometimes the ratings claimed for speed controllers are just laughable. So here's a very rough guide. If you can work out how many drive mosfets in a forward/brake control (excluding brake mosfets) or drive mosfets divided by 2 in a forward reverse control (excluding reverse mosfets), multiply it by 15 for very little heatsink, and 20 for large heatsinks. This should be close to the maximum continuous or average current. And for something on heatsinks. The little push on ones that fall off during racing are not large or properly connected to the mosfets, and are next to useless. More to make an impression.
Gold plated circuit boards have much less resistance than copper boards While it's true that gold is a better conductor of electricity than copper, the large track sizes and short distances on the boards make the difference in resistance to be in the 1 ten thousandths of an Ohm area. Hardly worth paying for gold, and with a good board layout, copper can be much better. After all, I've seen gold boards burn off those little tabs where they solder the wires. Now there's got to be some resistance there.
There is no advantage in purchasing a higher rated speed control than one that will do the job. This is definitely false. A speed control that will handle 20 amps will have an internal resistance of ~20 milliohms, and an unlimited will have an internal resistance of around 3 milliohms. The voltage drop from each at 20 Amps is 0.4V and 0.06V respectively, and power loss is 8W and 1.2W respectively. So the more expensive control gave an extra 0.34 V to the motor, and wasted less battery power as well. You should see the extra performance you get from an extra 0.34V.
A large heatsink or a fan on a speed controller makes it much better. This is almost the same answer as the previous question. Yes, it allows more current to pass through the speed control before it overheats, but it doesn't address the real problem. Reducing the internal resistance by adding extra mosfets is much better, as it reduces the wasted heat, increases the voltage to the motor, and is much lighter.
You can draw 80 amps off of a 6 cell 3300 mAh battery pack. Well you can because I've seen it done, but here's the problem. Sanyo RC-3300HVs and GP330SCH batteries have a roughly level voltage of 1.45 V and an internal resistance of 6 milliohms. The voltage drop at 6 milliohms and 80 Amps is 0.48V, which gives a cell voltage of 0.97V or a pack voltage of 5.82V. Add in some small resistance for leads, and you're below 5V. If you are on variable throttle then your receiver is getting some high frequency switching of the power supply, and they just don't like it. Besides the batteries don't last either.
Speed controls run fine without Schottky diodes or motor capacitors. Motor capacitors are used for a purpose. As the commutator rotates the brushes make and break contact with the segments of commutator. This is very sudden switching at high current. any sudden switching causes voltage spikes of very high voltage, which in turn produce radio waves. These radio waves can and usually do drown out or interfere with the signal coming from the transmitter, which is where most radio interference comes from. Small capacitors remove the high voltage spikes and reduce the commutator arcing, and therefore reduce radio interference. This high current switching is also done by the speed controller. Speed controls should have capacitors built in, but some interference can still come from them. This is why the motor leads should be kept short. The shorter the lead the less of an aerial you have. This problem can also be reduced by twisting the leads together. Schottky Diodes. If you have ever been running a hose and turned it off quickly. you'll be familiar with the term water hammer. The water once running does not like to be stopped quickly because of inertia. The pressure that builds up on the tap is huge. Electricity is the same as water in a pipe. Once the electrons are flowing, if you suddenly turn them off they build up at the gap, and the voltage can be huge. sometimes in the thousands of volts. In a speed controller, this is the drive mosfet, which will avalanche (overflow the maximum voltage) and produce heat, or it could show up across the battery (depending on the configuration). These high voltages can be extremely damaging to electronic components and motors. Normally a Schottky Diode is placed across the motor terminals. and acts like a pressure relief valve that allows the current flowing in the motor to flow out the negative terminal and into the positive terminal of the motor during the short period after each turn off of the drive mosfets in the speed control. Some speed controls have them inbuilt, and forward reverse speed controls use the diodes inherent in the reverse mosfets to do the same task. But if a speed control requires a Schottky, then you'd better have one or face the destruction of your speed control.
Speed controllers with reverse go just as fast as ones with forward/brake only. This is impossible. Speed controllers with reverse require a configuration called a 'H' bridge. This is where both sides of the motor have connections to + and - so that the motor can be reversed. Whereas, in a forward only control the + is connected directly to the motor and the negative only has mosfets. As the mosfets are one of the greatest sources of resistance, it can be seen that if a forward only controller has 3 drive mosfets, then a F/R controller is going to have 6 drive mosfets, 3 positive and 3 negative (each side of the motor) for the same current. The resistance will then be double with twice the voltage drop. This could be overcome by using 6 + and 6 - mosfets plus more for reverse. This would be a huge controller, in size and weight. |
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This website last updated Monday, 06 November 2006 |