The roller door motor joke is actually a bit of a trap. The 4 being larger than 2 is the funny punch line, but the real difference is the spin speed of the motor and the ratio of the motor speed to drum roll speed. The 1/2HP motor relies on peak torque and has a slow spin speed an very little gearing down between motor speed and drum speed, but it requires a lot of current to get it spinning in the first place when the door is all the way down.
On the other hand, the 1/4HP motor spins like a demon and has a lot of turns of the motor compared to one turn of the drum. The lift is actually slower than the 1/2HP motor, yet the 1/4HP motor has less load and therefore less heat generated and requires less current to drive it.
So, although the repair man didn't know why the 1/4HP was better, he was actually probably correct because the switches and cabling used would be generic and the cheapest option suited to the 1/4HP motor and not really up to the requirements of the 1/2Hp motor. The system works for a while, but as the overheated components start to increase their resistance as the age faster with the overload, the 1/2HP motor gets starved of both the voltage and amps it needs, over heats and fails .......
Don't ya just hate it when a good joke gets wrecked by a logical explanation
The take away is making sure you use cabling that can carry the current (amps) expected with as little voltage drop as possible. The amp rating sometimes attached to the cable description e.g 6mm auto cable (4.35mmsq) is 50 amps, this actually means you can drag 50 amps through 1 mtr of that size cable, strung out in free flowing air for cooling, before the insulation melts and either catches fire or starts to fall off leaving the conductor exposed the short against and cable close to it. To get the min expectable voltage drop required for solar to controller connection over a 5 mtr length, under 2%, this size cable can carry the max output of a 180w panel allowing for the losses due to heat on the panel etc. This is the size cable caravan manufacturers use through the roof to the controller and from there onward to the battery. If they fit 2 x 12w panels they don't double the cable runs or use bigger cable, but they seem to think they can connect 240w of solar and get 2 times the battery charging ability as they got from the single 120w panel. I've seen 4 x 150w panels connected up the same way more than once here at the workshop ......
T1 Terry
