Open-circuit characteristic of a DC shunt generator
Open-circuit characteristic of a DC shunt generator: no-load magnetisation curve, critical field resistance, critical speed, conditions for voltage build-up, and viva questions with answers.
Aim
To obtain the open-circuit characteristic (no-load magnetisation curve) of a DC shunt generator at constant speed, and from it determine the critical field resistance and the critical speed.
Apparatus required
| Apparatus | Specification | Qty |
|---|---|---|
| DC shunt generator with prime mover | 220 V, 5 kW, 1500 rpm | 1 set |
| Field rheostat | For separate excitation of the generator field | 1 |
| Voltmeter (MC) | 0–300 V | 1 |
| Ammeter (MC) | 0–2 A, field current | 1 |
| Tachometer | Digital or contact type | 1 |
| DPST switch | For the field circuit | 1 |
Theory
The generated emf of a DC machine is E = φZNP/60A — for a given machine, simply E = kφN. Hold the speed constant and the emf becomes a direct picture of the flux, and therefore of the magnetisation curve of the iron.
The open-circuit characteristic is that picture: generated emf against field current, at constant speed with the armature open so no load current distorts anything. It starts at a small residual voltage even with zero field current, because the poles retain some magnetism. It then rises almost linearly, bends over at the knee as the iron begins to saturate, and finally flattens.
For self-excitation the field must be fed by the machine's own armature. That works only if the residual flux is present, the field winding is connected so that its current strengthens the residual flux rather than opposing it, and the field circuit resistance is below a critical value.
The critical field resistance is the slope of the straight line drawn from the origin tangent to the OCC. The operating voltage settles where the field resistance line crosses the curve. If the resistance line is steeper than the tangent, it never crosses the curve except at the origin, and the machine simply never builds up.
The same argument works on speed. The OCC scales directly with speed, so lowering the speed pulls the curve down until the fixed resistance line no longer cuts it. The speed at which that happens is the critical speed, N_c = N × R_f/R_c.
Circuit connections
Check every point below against your board before switching on. There is no diagram here on purpose — a wrong diagram is worse than none, and this is the list a demonstrator actually walks through with you.
- The generator is driven by a prime mover at constant rated speed.
- The generator field is separately excited from a DC source through the field rheostat and the field ammeter — separate excitation is used so that the field current can be set independently and read directly.
- Voltmeter across the armature terminals, which are otherwise open-circuited.
- Field rheostat at maximum resistance and the field switch open before starting.
Procedure
- 1Start the prime mover and bring the generator to its rated speed. Hold that speed for every reading.
- 2With the field switch still open, record the residual voltage on the armature voltmeter at zero field current. This is the starting point of the curve and the record asks for it.
- 3Close the field switch and increase the field current in equal steps, recording the generated emf at each step, up to about 125 % of rated voltage.
- 4Increase the field current in one direction only — never reduce it and go up again.
- 5Bring the field current to zero, open the field switch and stop the prime mover.
- 6Plot E against I_f, draw the tangent from the origin, and read off the critical field resistance. Compute the critical speed for the field resistance actually in use.
Work out your readings
Type in the numbers off the meters. This fills the tabular column, works the calculation through step by step, plots the characteristic — and tells you when a reading cannot physically be right, which is the part a manual can't do. Everything stays on this device, and it works with the network off.
Nameplate and machine data
Field winding + regulator
Open-circuit readings
Raise the field current in one direction only — going back down traces the other side of the hysteresis loop.
| # | Field current I_f(A) | Generated emf E₀(V) |
|---|---|---|
| 1 | ||
| 2 | ||
| 3 | ||
| 4 | ||
| 5 | ||
| 6 | ||
| 7 |
Fill in the machine data and at least one complete row of readings to see the results, the worked calculation and the curve.
Precautions
- Hold the speed constant throughout. Emf is directly proportional to speed, so a drifting prime mover puts a drifting error into every point.
- Increase the field current monotonically. Going back down traces the descending branch of the hysteresis loop, and the two branches do not coincide.
- The armature must stay open-circuited — any load current produces armature reaction and voltage drop, and the curve is no longer an open-circuit one.
- Keep the field rheostat at maximum resistance before closing the field switch.
Sources of error
Every record asks for these, and every record gets the same three lines copied from the one before. These are the errors this particular experiment actually has.
- Hysteresis: the curve obtained going up differs from the one going down, so the OCC is really a band rather than a line.
- Speed variation on the prime mover between low and high field currents.
- Reading the residual voltage on a range where it is a tiny fraction of full scale.
- The tangent from the origin is drawn by eye, so the critical resistance read from a hand-drawn plot carries the error of the draughtsman as much as of the machine.
Viva questions with answers
What are the conditions for a shunt generator to build up voltage?
Three. There must be residual magnetism in the poles; the field winding must be connected so its current aids the residual flux, not opposes it; and the field circuit resistance must be less than the critical value at the running speed. The speed must also be above the critical speed.
What is critical field resistance?
The slope of the line drawn from the origin tangent to the open-circuit characteristic. Above this resistance the field resistance line lies entirely above the curve, so there is no stable crossing point and the generator fails to build up.
A shunt generator refuses to build up. What are the possible causes and remedies?
No residual magnetism — flash the field from a battery for a few seconds. Field connections reversed relative to rotation — swap the field leads, or reverse the direction of rotation. Field resistance above critical — cut out some of the field rheostat. Speed below critical — drive it faster.
Why does the OCC start above the origin?
Residual magnetism. Even with zero field current the poles retain flux from the last time the machine was excited, and that flux generates a small emf — typically a few volts.
Why does the curve bend over at the knee?
Saturation of the iron in the magnetic circuit. Below the knee the reluctance is dominated by the air gap and flux is proportional to field current; beyond it the iron needs disproportionately more MMF for the same flux increment.
How does the OCC change if the speed is increased?
Every ordinate scales directly with speed, because E = kφN. The curve keeps its shape and simply stretches upwards, which is why a higher speed can rescue a generator that would not build up at a lower one.
Why is the machine separately excited for this test?
So the field current is an independent variable that can be set to any value and read directly. With self-excitation the field current is whatever the generated voltage and field resistance happen to produce, and points below the knee cannot be reached at all.