A rotating-field generator has a rotor inside a stationary stator with an air gap between them. This two-pole example lets you compare nominal gap, diametral bore allowance, resulting radial clearance and electrical frequency.
Electric Generator Construction Interactive Calculator
Compare a two-pole generator’s nominal rotor/bore geometry, entered diametral allowance and resulting radial clearance. Gap bounds define a design range, not eccentricity measurements.
Equation Used
- Two field poles and ideal sinusoidal flux.
- Radial design-gap mean, not measured eccentricity.
- Diametral bore undersize with unchanged rotor.
- Concentric geometry only.
Concentric circular parts. No runout, thermal growth, rotor tolerance or voltage/power rating is calculated.
A rotating field passes stationary coil pairs
The end view identifies the fixed stator, its colored phase-coil pairs, the rotating north/south field and the shaft. The two colored rotor halves indicate field polarity rather than a detailed excitation construction. End windings, bearings and the field supply are outside this sectional model.
The idealized three-phase display shows sinusoidal induced EMF 120 electrical degrees apart. Its values are normalized, not volts or amperes. With two poles, electrical angle advances through one cycle for each shaft revolution.
The main drawing keeps the entered rotor-to-bore diameter ratio. A separate enlarged gap detail makes small clearances visible without falsely enlarging the gap around the whole rotor.
Define what each gap input means
The two gap controls are bounds for a nominal design comparison; their mean is the selected radial gap. They are not measurements of opposite sides of an eccentric rotor. The bore control is a diameter in metres.
The allowance rate is an entered diametral bore undersize per metre of nominal bore diameter. It is not a universal generator tolerance. The calculation holds rotor diameter fixed while reducing the bore diameter by that allowance.
Rotation is slowed 300 times for inspection. Changing shaft speed changes the animation rate and the reported two-pole electrical frequency. Speed and geometry are independent inputs.
Diameter error changes radial gap by half
Let the entered gap bounds be g 1 and g 2. Their mean g=(g 1+g 2)/2 is radial clearance before the allowance. For bore diameter D in metres and rate c in mm/m, the diametral allowance is δD=cD millimetres.
The nominal rotor diameter is 1000 D−2 g. For concentric circular parts and an unchanged rotor, the reduced radial clearance is g−δD/2. A diameter change affects the clearance at each side by half that amount.
Electrical frequency is f=Pn/120. This example fixes pole count P=2, giving f=n/60. If the chosen bore and gap leave no positive rotor diameter, the drawing stays visible but the assembled-clearance result is withheld.
A one-metre bore and 2.5 mm nominal gap
The example uses gap bounds 2 mm and 3 mm. Their mean is 2.5 mm, so the nominal rotor diameter is 995 mm.
At an entered allowance rate of 0.05 mm/m, the bore-diameter allowance is 0.05 mm. That reduces the radial gap by 0.025 mm, leaving 2.475 mm in the concentric model. At 3000 rpm, the two-pole frequency is 50 Hz.
These example values are not clearance specifications for a real machine.
A clearance illustration is not a machine design
Real minimum running clearance depends on eccentricity, bearing motion, shaft bending, rotor and stator form error, thermal growth and additional tolerances. Those effects are not present in the two-radius calculation.
Frequency follows pole count and speed; voltage and power require additional electromagnetic, winding, thermal and load information. The phase bars assume an ideal sinusoidal field and do not establish output waveform quality.
Use the actual machine’s tolerances and operating limits when assessing running clearance. Input dimensions are design comparisons, not manufacturer limits.
Generator-construction questions
Why is half the allowance subtracted?
The entered allowance is a change in diameter. A centred radius changes by half that amount.
Why is the main air gap sometimes hard to see?
It is drawn at the actual diameter ratio. The separate detail enlarges the same gap.
Are the colored bars current?
No. They are normalized ideal induced EMF; no load current or voltage magnitude is calculated.
Why does the rotor disappear for some dimensions?
The rotor is replaced by an explicit unassembled preview when the required diameter is zero or negative. Those dimensions cannot form the concentric assembly illustrated here.
References
- STAMFORD Industrial Ratings Book — frequency, speed and pole-count relation; real ratings depend on specified machine conditions.
- ABB air-gap inspection for motors and generators — rotor, stator and winding inspection through the air gap. No inspection-tool dimension is used as a recommended generator gap.
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