Re: Tractor starters and magnetic fields

Posted by EX 450 Owner on January 13, 2012 at 11:32:00 from (50.96.220.226):

In Reply to: Tractor starters and magnetic fields posted by doug in illinois on January 11, 2012 at 08:29:08:

1. The magnetic strength of an electro magnet is determined by the Ampere Turns.



2. Increasing the Amperes or increasing the number of Turns will increase the flux density and therefore the magnetic force.

3. Every metal core has a maximum number of magnetic lines that it can support when it reaches this point the core is said to be saturated and increasing the Ampere Turns will not increase the flux density or increase the magnetic force.

4. Every motor has at least two interacting flux fields that cause rotation.

5. The torque developed by the motor is ɸ1 *ɸ2 *K (in a motor with 2 fields)

a. Where ɸ1 is the of one of the fields and ɸ2 is the magnetic flux of the second field and K is the design constants of the motor (the K of the motor are always the same for that given motor and do not change)

b. Increasing the flux density or either or both fields will increase the torque the motor can develop (ɸ1 *ɸ2 *K) if you double the flux density of one field the torque producing capabilities will double if you double the flux density of both fields the torque producing capabilities will increase by a factor of 4 as long as the field pieces are not saturated.

6. The current flowing in a motor coil is determined by the difference between the applied voltage and the Counter Electromotive Force (CEMF) produced by motor speed and the resistance to current flow.

a. In a DC motor this resistance is the DC resistance of the coils.

b. In an AC motor this resistance is the Impedance of the coils and the DC resistance.

c. Increasing the speed of the motor will increase the CEMF.

7. The CEMF is always less than the voltage applied to the Motor.

Let us look at a series DC motor like the tractors starter motor, if you run this motor under no load it will run so fast it will sometimes throw the commutating bars from the armature. As you apply a load to this motor it will slow down as it slows the CEMF of the motor decreases so the current in the motor increases.

Conversely to start cranking the starter must make enough rotations to run the Bendix out, once the flywheel is engaged the starter will pull high amperage and the starter motor will begin to gain speed so the current will be decreasing as CEMF increases, at some point the motor will be producing enough torque to turn the load (in this case it is cranking the tractors engine) and it will have some given amount of current flowing in the motors windings (since the windings are in series the same current flows in both windings) let us say for example the cranking amps are 200 amps on a 6 volts system.

Now if you double the voltage to the starter to get enough CEMF to control the current to 200 amps the motor will have to run twice as fast just to maintain the same Ampere Turns. With the same amount of Ampere Turns the starter will produce the same torque as the starter did on a 6 volts system.

Simply stated if you double the applied voltage to a series motor it will develop the same torque at twice the speed, to develop the same amount of torque it will have to maintain the same Amperage because the field strengths have to be the same.

If a starter motor is intended for a 12 volt system if will have twice as many turns of smaller wire when compared to a starter intended to be used on a 6 volt systems if both are designed to produce the same amount of torque.

If you test this theory do not use the figures of 6 and 12 volts but measure the voltage at the Starter terminal and compare by using the actual starter voltage not the nominal battery voltage.

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