ScootDawg Forums - View Single Post - wires to stator and rectifier warm to hot

bnc · 06-09-2014, 04:28 PM

What is true is that if you start the car, then all that has been said can happen.

In simple terms, the electrical systems in cars and bikes are different. Turning on the car starts a war between the two regulation systems and the car is going to win.

Here is why. The alternator in a car uses an electromagnet instead of a permanent magnet. Because of this, the regulator in the car can control the strength of the magnetic field in the alternator by adjusting the current to the field coil. Lowering the current to the alternator field coil reduces the magnetic field so the alternator output drops.

Because the bike has a permanent magnet in the alternator, the car technique cannot be used. Instead the regulator uses a device called a Triac ( same device used in light dimmers ) to short the output of the alternator to ground until the average voltage is what the regulator wants. Obviously, this causes the stator and alternator wires to get hot and the regulator gets hot too. This is why bike regulators have big heatsinks. Under normal circumstances, the shorting is within design parameters so nothing blows up.

The problem is created when the car is connected up and it is turned on so its regulator starts to work. If there is a slight difference in regulated voltage between the two, for example, the bike regulator wants to regulate to +13 volts and the car regulator wants to regulate to +13.5 volts; the voltage regulator on the bike will think the bike's alternator is putting out too much voltage so it will do everything it can to lower the +13.5 volts the car regulator is supplying. It does this by shorting out the bikes alternator completely which can burn up the stator wires and itself because the car alternator can supply more power to maintain +13.5 volts than the bike regulator can absorb trying to lower it to +13 volts.

Cars can be safely connected together with both running even if they have slightly different regulator set points. The car with the +13 volt regulator set point will see the +13.5 volts from the other car and think its own alternator is producing too much voltage so it will reduce the current to the electromagnet of its own alternator leaving the other car's alternator in control. This harms neither car.

I would also like to dispel one other confusion. This regards battery voltage and battery capacity ( CCA ). There is a direct analogy between volts and water pressure and current and water flow in gallons/minute.

Let's look at this example. Suppose you have an outside faucet capable of 12 psi ( analogous to 12 volts ) and 14 gallons/minute ( analogous to 14 CCA ) and a fire hydrant with the same 12 psi pressure and capable of 200 gallons/minute. If you connect an ordinary garden hose ( analogous to a bike ) to the outside faucet and after one minute measure six gallons flowed into a bucket then the hose is only capable of taking 6 gallons/minute from the 14 gallon/minute faucet. If you hook this same hose to the fire hydrant, it will still only deliver 6 gallons/minute.

06-09-2014, 04:28 PM	#14
bnc Join Date: May 2014 Posts: 49	What is true is that if you start the car, then all that has been said can happen. In simple terms, the electrical systems in cars and bikes are different. Turning on the car starts a war between the two regulation systems and the car is going to win. Here is why. The alternator in a car uses an electromagnet instead of a permanent magnet. Because of this, the regulator in the car can control the strength of the magnetic field in the alternator by adjusting the current to the field coil. Lowering the current to the alternator field coil reduces the magnetic field so the alternator output drops. Because the bike has a permanent magnet in the alternator, the car technique cannot be used. Instead the regulator uses a device called a Triac ( same device used in light dimmers ) to short the output of the alternator to ground until the average voltage is what the regulator wants. Obviously, this causes the stator and alternator wires to get hot and the regulator gets hot too. This is why bike regulators have big heatsinks. Under normal circumstances, the shorting is within design parameters so nothing blows up. The problem is created when the car is connected up and it is turned on so its regulator starts to work. If there is a slight difference in regulated voltage between the two, for example, the bike regulator wants to regulate to +13 volts and the car regulator wants to regulate to +13.5 volts; the voltage regulator on the bike will think the bike's alternator is putting out too much voltage so it will do everything it can to lower the +13.5 volts the car regulator is supplying. It does this by shorting out the bikes alternator completely which can burn up the stator wires and itself because the car alternator can supply more power to maintain +13.5 volts than the bike regulator can absorb trying to lower it to +13 volts. Cars can be safely connected together with both running even if they have slightly different regulator set points. The car with the +13 volt regulator set point will see the +13.5 volts from the other car and think its own alternator is producing too much voltage so it will reduce the current to the electromagnet of its own alternator leaving the other car's alternator in control. This harms neither car. I would also like to dispel one other confusion. This regards battery voltage and battery capacity ( CCA ). There is a direct analogy between volts and water pressure and current and water flow in gallons/minute. Let's look at this example. Suppose you have an outside faucet capable of 12 psi ( analogous to 12 volts ) and 14 gallons/minute ( analogous to 14 CCA ) and a fire hydrant with the same 12 psi pressure and capable of 200 gallons/minute. If you connect an ordinary garden hose ( analogous to a bike ) to the outside faucet and after one minute measure six gallons flowed into a bucket then the hose is only capable of taking 6 gallons/minute from the 14 gallon/minute faucet. If you hook this same hose to the fire hydrant, it will still only deliver 6 gallons/minute.
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