Discussion
Please can a boffin help with a puzzlement I have.
The alternator charges the battery, when the car engine is running, and increases its output as demand on the battery increases. I think that statement is right, but welcome any correction if it isn't.
My question: Does the increase in output charge from the alternator to the battery (should the battery fall under optimum charge level) require additional power from the engine, or is the increase in alternator output within its normal charging ability?
Regrettably no prizes other than the knowledge imparted will provide an unbroken night's sleep.
Thanks.
The alternator charges the battery, when the car engine is running, and increases its output as demand on the battery increases. I think that statement is right, but welcome any correction if it isn't.
My question: Does the increase in output charge from the alternator to the battery (should the battery fall under optimum charge level) require additional power from the engine, or is the increase in alternator output within its normal charging ability?
Regrettably no prizes other than the knowledge imparted will provide an unbroken night's sleep.
Thanks.
HumbleJim said:
An alternator with no load will need very little power to drive it, as the load increases the power to drive it increases proportionally.
Thanks for that. My uneducated mind thought that what drives the alternator is the belt from the crank wheel, and only that. And that a) all demands for the charge it generates come from the battery, and b)the alternator only supplies the battery (not any other components)?Thorodin said:
HumbleJim said:
An alternator with no load will need very little power to drive it, as the load increases the power to drive it increases proportionally.
Thanks for that. My uneducated mind thought that what drives the alternator is the belt from the crank wheel, and only that. And that a) all demands for the charge it generates come from the battery, and b)the alternator only supplies the battery (not any other components)?The alternator operates as a "voltage controller".
As more load (current) is demanded by the electrical consumers in the system, battery voltage would, if no alternator was present, fall (because the battery has both an internal resistance, and a characteristic voltage vs state-of-charge curve). However, the alternator senses the fall in voltage (see note below), and increases it's current output in response. During all normal engine running operations, it is the alternator that provides the electrical power for the systems on the car, not the battery. And in fact, typically it will be also recharging the battery (current flowing into the battery) as the battery SOC will be low due to the power required to start the engine, and the small power drain when the car has been parked (immobilizer etc).
The alternator is a device that converts mechanical energy into electrical energy. If has a magnetised rotor that is spun by the engine, and static coils to generate an electrical output. If the alternator must provide more electrical output, then more power must be taken from the crank to do that. Typical alternators are only around 60% efficient, and so the drive power required is significantly higher than the electrical output power.
As more load (current) is demanded by the electrical consumers in the system, battery voltage would, if no alternator was present, fall (because the battery has both an internal resistance, and a characteristic voltage vs state-of-charge curve). However, the alternator senses the fall in voltage (see note below), and increases it's current output in response. During all normal engine running operations, it is the alternator that provides the electrical power for the systems on the car, not the battery. And in fact, typically it will be also recharging the battery (current flowing into the battery) as the battery SOC will be low due to the power required to start the engine, and the small power drain when the car has been parked (immobilizer etc).
The alternator is a device that converts mechanical energy into electrical energy. If has a magnetised rotor that is spun by the engine, and static coils to generate an electrical output. If the alternator must provide more electrical output, then more power must be taken from the crank to do that. Typical alternators are only around 60% efficient, and so the drive power required is significantly higher than the electrical output power.
- * Modern cars often have a so called "smart" alternator, which is actually controlled by logic in the engine ecu. The system senses the battery voltage, and when it falls below the necessary "float" voltage (between 13.7 and 14.4v typically) it commands the alternator to increase it's current output. This is done in a "closed loop" fashion to maintain a fairly steady system voltage across a wide range of electrical loads and engine speeds. The latest systems go one step further and only allow the alternator to supply power when the vehicle is braking, and as such help to increase fuel economy (they use a bigger battery as a temporary energy store, and use a coulomb counting method to achieve an average battery SOC during operation.
Max_Torque said:
The alternator operates as a "voltage controller".
As more load (current) is demanded by the electrical consumers in the system, battery voltage would, if no alternator was present, fall (because the battery has both an internal resistance, and a characteristic voltage vs state-of-charge curve). However, the alternator senses the fall in voltage (see note below), and increases it's current output in response. During all normal engine running operations, it is the alternator that provides the electrical power for the systems on the car, not the battery. And in fact, typically it will be also recharging the battery (current flowing into the battery) as the battery SOC will be low due to the power required to start the engine, and the small power drain when the car has been parked (immobilizer etc).
The alternator is a device that converts mechanical energy into electrical energy. If has a magnetised rotor that is spun by the engine, and static coils to generate an electrical output. If the alternator must provide more electrical output, then more power must be taken from the crank to do that. Typical alternators are only around 60% efficient, and so the drive power required is significantly higher than the electrical output power.
Most comprehensive, thanks for that - fully understood. My problem was I couldn't understand how an alternator drew additional power from the engine when the alternator has no independent means of turning. In other words the alternator cannot spin any faster (to generate more electrical power) than the limits of the engine rpm transferred to it by the belt. I assumed that the energy from the engine was always constant (subject to engine rpm) at the alternator, irrespective of the amount of current needed by the battery to satisfy the demands on it. I thought that if the battery could not serve the requirements of the system it would just eventually go flat. I also assumed that as the alternator it is not a condenser and so can not store electrical power - any excess power to immediate needs being wasted to 'ground'. Although I seem thick, I can't half bake a good cake!As more load (current) is demanded by the electrical consumers in the system, battery voltage would, if no alternator was present, fall (because the battery has both an internal resistance, and a characteristic voltage vs state-of-charge curve). However, the alternator senses the fall in voltage (see note below), and increases it's current output in response. During all normal engine running operations, it is the alternator that provides the electrical power for the systems on the car, not the battery. And in fact, typically it will be also recharging the battery (current flowing into the battery) as the battery SOC will be low due to the power required to start the engine, and the small power drain when the car has been parked (immobilizer etc).
The alternator is a device that converts mechanical energy into electrical energy. If has a magnetised rotor that is spun by the engine, and static coils to generate an electrical output. If the alternator must provide more electrical output, then more power must be taken from the crank to do that. Typical alternators are only around 60% efficient, and so the drive power required is significantly higher than the electrical output power.
- * Modern cars often have a so called "smart" alternator, which is actually controlled by logic in the engine ecu. The system senses the battery voltage, and when it falls below the necessary "float" voltage (between 13.7 and 14.4v typically) it commands the alternator to increase it's current output. This is done in a "closed loop" fashion to maintain a fairly steady system voltage across a wide range of electrical loads and engine speeds. The latest systems go one step further and only allow the alternator to supply power when the vehicle is braking, and as such help to increase fuel economy (they use a bigger battery as a temporary energy store, and use a coulomb counting method to achieve an average battery SOC during operation.
For any electrical machine, the mechanical power it is making (for a motor) or absorbing(for a generator) is proportional to the speed of the machine multiplied by the drive torque.
For an automotive alternator, the machine itself has no control over how fast it is being spun (because the engine speed is a function of road speed), but it does have a say over the torque required to drive it.
In the case of a conventional "claw pole" alternator, the rotor is energised with a low current (<5amps) supply (via some slip rings) that creates a spining magnetic field, that creates a much larger current in the stator windings, and hence an appopriate output current. Increasing the rotor excitation current increases the electrical output, and also creates a larger drag torque on the mechanical input. The voltage requlator in the alternator is responsible for controlling the rotor energising current to create the appropriate output current. A completely non energised rotor, has little of no magnetic field, and as such no drag torque (other than a tiny amount in its support the bearings).
Modern EMS system automatically increase engine torque in conjunction with alternator demand to maintain a steady idle speed
For an automotive alternator, the machine itself has no control over how fast it is being spun (because the engine speed is a function of road speed), but it does have a say over the torque required to drive it.
In the case of a conventional "claw pole" alternator, the rotor is energised with a low current (<5amps) supply (via some slip rings) that creates a spining magnetic field, that creates a much larger current in the stator windings, and hence an appopriate output current. Increasing the rotor excitation current increases the electrical output, and also creates a larger drag torque on the mechanical input. The voltage requlator in the alternator is responsible for controlling the rotor energising current to create the appropriate output current. A completely non energised rotor, has little of no magnetic field, and as such no drag torque (other than a tiny amount in its support the bearings).
Modern EMS system automatically increase engine torque in conjunction with alternator demand to maintain a steady idle speed
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