Min. injector off time

[John Dammeyer]

hi,

Not quite!  A coil, whether on a motor or within an injector has a specific
DC resistance.  That DC resistance determines the maximum current that will
flow through the coil with a given applied voltage.  If your FAN motor draws
35 Amps on startup it's because the windings have a resistance of
12V/35A=0.34Ohms.  Similarily an injector that draws 4Amps has a resistance
of 3 Ohms.

Here the two types of devices diverge in behaviour.  The injector will
continue to draw 4Amps and dissapate 4A*12V=48watts and probably burn up.
The Motor Armature,  with that much current flowing through the windings,
turns and switches the current through the commutator to the next set of
windings. As the motor speeds up it starts to behave like a generator and
generates a voltage of the opposite polarity to the applied voltage (called
"Back EMF").  The faster you spin the motor the higher this reverse voltage.
On an unloaded motor when the back EMF equals the applied voltage the motor
has reached operating speed.  Raise the voltage, the motor turns faster;
lower the voltage,  the motor turns slower.  At some point the conduction
and convection of the iron core of the motor armature will no longer
dissapate the heat generated from the current flow through the motor
windings and the motor will overheat and melt either the insulation on the
windings or the commutator.  Load down the motor so that the back EMF will
not oppose the applied voltage and you draw extra current.  Stall the
armature and you burn out the motor.

Now on the the injector and a bit more theory. (Sorry).  When you first
apply a voltage across a coil of wire the inductance of the coil prevents
the current from flowing.  Over time current does flow until some point
later maximum current flows.  Until you have maximum current you don't get
maximum strenght out of the solonoid which is opening the injector valve.
If the time for the current to build up to maximum is longer than the
desired injector ON time then the injector will never fully open.  This
would be a problem in high RPM engines.  There is a trick though.

Just like the DC motor,  the higher the applied voltage,  the faster the
current builds up through the injector coil,  the sooner it reaches maximum
force and therefore the faster the injector opens.  However,  Now that it's
open there is too much current going through the windings for the injector
body to dissapate.  In reality, if you have an injector that must draw only
1 Amp at 12 volts but draws 4 Amps, then you really have a 3 volt injector.
The circuitry to run such an injector applies 12 volts to build up the
current as quickly as possible to the required 1 Amp.  At that point the
circuitry cuts back the voltage to 3 volts to maintain the 1 Amp current
flow for the duration of the injector on pulse period.  This, at the expense
of circuit complexity, gives the best of both worlds:  high speed injector
opening and low power dissapation.

Regards,

John

PS. I've left out the math but it's quite easy to determine, given coil
inductance, how quickly maximum current is reached so instead of using
current sensing circuitry you could pulse 12 volts for n microseconds and
then switch to 3 volts for the rest of the injector on duration.


At 01:06 AM 05/02/1998 -0800, you wrote:
>
>Bruce,
>  Well here's the deal,  When your engine temp get's to 215 or so and
>you cooling fan turns on, have you ever measured the current draw on
>fan startup? 35 amps or so, atleast mine does.  Then once it get's up
>to speed it only draw's 15 amp. right?  Same as an injector. With more
>avail. current their won't be as much voltage drop on inj. opening,
>which in turn will make it open faster.  This is kinda hard to explain
>so if you want more just ask.  Thanks
>  
>  Ward
>
>
>
>
>
>---bruce plecan <nacelp at bright.net> wrote:
>>
>> ref Ward Williamsom:
>> 
>> Another trick is to run peak and hold driver's with a saturated
>> switch injector,.........
>> 
>> How does that work?.  A peak and hold driver allows four amps
>> to flow, and then drops off to one, or so I've been told.  A
>> saturated injector has a resistance of 12-18 ohms, so it will
>> never "pass" four amps.  At 12V, a 12 ohm injector will pass
>> 1 amp,  Well some EEs will disagree, but cording to the ohms
>> law I know that sounds right.
>>   With some schematics I seen there is a resistor on the ground
>> side of the device that grounds the injector on the peak+holds
>> to monitor the current, at least that's my assumption.  In which
>> case that would be a current limiter on the saturated injector,
>> which would slow it down if anything.  Any EE who has this
>> figured out, I'd like to hear it....
>> Bruce      On the issue of Cone shaped hats and mice not
>>                getting cancer we need to know if it is cause or
>>                effect.  There may be an up side to wearing them
>>                after all....
>> 
>
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