LRC - Load Response Control for Alternators ...What is it???

Discussion in 'Tech & Repair' started by andrewsfm, Sep 5, 2008.


  1. andrewsfm

    andrewsfm New Member

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    From what I've read so far, my understanding is that it gradually increases the alternator output to maximum over a period of seconds to reduce the torque load the alternator puts on the engine.
    Is this how it works?

    Also, wouldn't an LRC device prevent an alternator from providing short high amperage bursts in the case of a high powered stereo installation with fast hard hitting bass?
  2. Newfie_dan

    Newfie_dan New Member

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    high power stereo installs

    A load control system for controlling the field current of an alternating current generator (alternator) and the idle speed control system for the engine that drives the alternator. A voltage regulator senses the output voltage of the alternator and controls field current by causing a semiconductor switch connected in series with the field winding to switch on and off in accordance with the magnitude of the sensed voltage. The voltage regulator is controlled to operate in a conventional manner when engine speed is higher than engine idle speed. When engine speed is in an idle speed range the duty cycle or on time of the semiconductor is controlled such that consecutive occurring on times are gradually increased to gradually increase field current when the output voltage of the generator is below a desired regulated value. The system operates to actuate an engine idle speed control system to increase the amount of fuel-air mixture supplied to the engine when the on times of the semiconductor switch are being gradually increased. In the event that engine speed decreases by a predetermined amount when the engine is operating in the idle speed range the on time or duty cycle of the semiconductor switch is reduced to a minimum to prevent engine stall.

    Now with that said for high power stereo installs you have 2 options to premature alternator failure and make your system stand up to abuse.
    1) install a second battery to cope with the ampage draw
    2) install a capacitor to lower the direct draw from your battery

    Capacitors are what I usually go with due to the compact size and ease of install. They help to cushion high draws on the system and also make for better overall amp performance. 1 Farad for ever 1000watts is a general rule ppl use but I like to go with 1 farad capacitors for every 500 watts of power you are going to be using. Make sure that you also opt for proper sized wiring for the power you want to put out. So in other words 8 gauge power wires wont supply a 2000 watt system properly. Bigger wire = less resistance and can provide more ampage. for a 1000 watt plus system I opt for 4 gauge as the smallest wire to be used for power and ground. Make sure that you match both (same size wire for both) also beef up your battery to chassis ground wire with the same gauge. All exterior connections should be soldered and shrunk tubed or molex connections. I solder and shrink tube all electrical work to ensure a good solid connection. Also use dielectric grease on all grounds and power connections that are exterior of the cabin. If you are not familiar with the rules for electricity then I recommend that you learn the basics. For a 12 volt system to put out 1000 watts that is (Watts=AmpageXVoltage) 83.33 Amps of draw, most alternators top out at 80 amps. So do the math. You will need a capacitor to help that out.
  3. andrewsfm

    andrewsfm New Member

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    I have a 2900W stereo (2500W subs/400W full range).

    The alternator is a 220A high output type with overdrive pulley.

    There is a 0 AWG wire run from the alternator positive terminal to the positive terminal on the battery, and another 0 AWG negative wire run from the alternator body to the battery negative terminal.

    The amp is fed from the battery terminals with dual runs of 0 AWG to the back of the car. One for positive and one for negative.

    Before passing through into the amp, they pass through an 18 farad capacitor.

    Is my setup an issue?

    I need to know if LRC will restrict the alternator from instantly pumping out 150A on the drop of a dime, and instead ramp up the current over time.
  4. Macgyver4

    Macgyver4 FEOA Member

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    150 amps from the alternator would probably require almost 3 horse power from your engine wouldn't it? Seems like it would be kinda weird if every time the base hit you instantly lost 3 HP. Might feel like driving through a bunch of small puddles or something. The battery can instantly provide the power can't it? I know that the battery voltage alone is not quite as high as the voltage you get from the alternator though.
  5. andrewsfm

    andrewsfm New Member

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    I've heard that for every 25 amps, you need 1 HP to drive the alternator.
    I don't know how true that statement is though.
    In theory, that means that the alternator would need 6 HP at 150 amps, or fully loaded at 200 amps needs about 9 HP to drive it.

    The battery can "instantly" provide power per se, but due to it's internal resistance, there is a slight delay which is why people use capacitors to fill in the gap between the microseconds between the demand and delivery from the battery.

    But keep in mind, that the voltage potential of a fully charged battery is about 12.9 volts, so unless the system voltage drops below that point, the alternator is supplying all of the power from 13 volts and up. Above 12.9 volts, the current is trying to flow INTO the battery, not out.

    If the function of LRC is to ramp up the alternators current output gradually over a period of seconds, I'm thinking it may allow the system voltage to dip below 13 volts when a bass note hits and start drawing from the battery.

    As a result of the voltage drop the output from the amp also drops. Amps are rated at 14.4 volts, so at 13 volts they only put out 90% of their power.

    Another note about capacitors, is that you need to know their voltage rating, and their farad capacity. Divide the farad rating by the voltage rating. That will tell you how many farads per volt you have to work with. My understanding is that a farad is 1 amp per volt.

    So if an 18 farad cap is rated at 18 volts, and is charged up to 14 volts, it can throw out 1 amp before dropping to 13 volts. Assuming my system draws 150 amps, the capacitor is only a factor for 1/150th of a second.
  6. zzyzzx

    zzyzzx FEOA Member

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    Re: LRC - Load Response Control for Alternators ...What is i

    That's what the battery is for.
  7. andrewsfm

    andrewsfm New Member

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    Re: LRC - Load Response Control for Alternators ...What is i

    Maybe so, but nobody has even made it clear what LRC does in laymens terms.

    I'm just assuming that's what it does.
  8. Newfie_dan

    Newfie_dan New Member

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    lrc

    in laymens terms it means that when there is a higher demand on the alternator it tries to match it and when its low demand it puts out low current/voltage. The response time to the demand depends on the alternator and the sensitivity of the lrc.
  9. andrewsfm

    andrewsfm New Member

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    Re: lrc

    So a 2.5 or 10 second LRC means it will delay full output by 2.5 or 10 seconds respectively?
  10. Newfie_dan

    Newfie_dan New Member

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    lrc

    yes, but the average lrc delay is around 1-2 seconds max from my experience.
  11. andrewsfm

    andrewsfm New Member

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    Found a webpage with a technical explanation on alternators and voltage regulators by accident the other night, weeks after I asked the original question.

    http://www.automotivedesignline.com/howto/171200074

    Be warned, that it is not all in laymans terms.
  12. needWheels

    needWheels FEOA Member

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    Sorry to bump this thread from the dead, but I am curious if it's safe and okay to replace a failing voltage regulator with a non-LRC one?

    See the non-LRC are half the price for some reason than the LRC regulators for Ford 3G alternators. Part is the same shape and fit. I suspect older cars had the non-LRC and LRC was added later.

    added: here's a lot of great info about LRC in an article about Ford's 4G alternator design:

    http://www.enginebuildermag.com/Article ... ew_4g.aspx
    and this probably answers my question about skipping LRC
    Now the question is, there is 2.5 second LRC and 6 second LRC units. Which is better?
  13. zzyzzx

    zzyzzx FEOA Member

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    Re: lrc

    That is my understanding as well, and it's the voltage regulator that determines that (I.E. - you can sometimes opt buy a voltage regulator with a shorter delay if you want to)

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