Air Gaps and DC Motors

Working from first principles for a DC motor, lets assume shunt wound for now.

The voltage across the filed is constant and the current through the field is constant and generates a magnetic flux that flows through the poles and armourer/rotor in a loop. The "equation" for this is that, just like for current, the magnetic resistance of the circuit equals the magnetic driving force. The two things that mostly give magnetic resistance are magnetic saturation of the soft iron material that the poles/rotor body are made of AND the air gaps between the poles and rotor. The flux in the air gaps and so also through the rotor are very strongly determined by the air gap - the ratio is an inverse one -more gap gives less flux.

Looking now at the rotor. The current in the rotor starts out at a maximum at zero speed and as rotor speed (RPM) increases the back emf in the rotor windings increases, subtracts from the supply voltage across the rotor and so the current reduces as the remaining difference in voltage decreases. It is this difference between supply and back emf voltage that overcomes rotor electrical resistance and drives the current through the rotor windings.

The output torque of the motor is the product of rotor current and flux across the air-gap/s. The output (zero torque) speed of the motor will be higher as the air-gap increases because greater gap means less flux and therefore a higher rotor speed is needed to generate the equalising back emf.

Wiper motors in some automobiles are one form where different interconnects of the windings are used to effect the control of speed through flux variation.

For a DC series motor, or universal motor as they are sometime called, the fundamental are the same as above except that the current through both the rotor and field windings are the same in this case, and so now the factors involved interplay with each other.

In the series DC case, and lets start from an equilibrium case, increasing the gap will give lower flux, their will then be additional voltage available to increase rotor current BUT this will try and increase both the rotor current and field current, so the field flux will also tend to be increased back to where it was. The result of this is that there will be little or no resultant speed increase. Exactly what happens will depend on whether any of the magnetic circuit is saturated.

I can't recall if there is an interplay with any other parmaters/ratios but some simple equations based on the above will clarify that.

Thanx Trevor, that info helped alot