Speed control of a DC shunt motor by armature voltage control
Armature voltage and field flux control of a DC shunt motor: connections, readings, back-emf calculation, why armature control gives constant torque and field control constant power, plus viva questions with answers.
Aim
To control the speed of a DC shunt motor by varying the armature voltage at constant field current, and to verify that speed is proportional to back emf.
Apparatus required
| Apparatus | Specification | Qty |
|---|---|---|
| DC shunt motor | 220 V, 5 hp, 1500 rpm | 1 |
| Three-point starter | Matched to the machine | 1 |
| Armature rheostat | Heavy-duty, rated for armature current | 1 |
| Field rheostat | As specified for the machine | 1 |
| Voltmeter (MC) | 0–300 V | 1 |
| Ammeter (MC) | 0–10 A, armature | 1 |
| Ammeter (MC) | 0–2 A, field | 1 |
| Tachometer | Digital or contact type | 1 |
Theory
For a DC motor, E_b = V − I_a R_a and E_b = kφN. Putting the two together gives N = (V − I_a R_a)/kφ. Every method of controlling the speed of a DC motor comes out of that one equation: change the voltage applied to the armature, change the resistance in series with it, or change the flux.
Armature voltage control holds the flux constant and varies V. Since the armature drop I_a R_a is small, speed follows the applied voltage almost linearly. The available torque, which depends on φ and I_a, is unchanged — so this is a constant-torque method, and it can only take the speed below the base value.
Field control does the opposite. Weakening the flux raises the speed above base, but the same weakening reduces the torque the machine can produce for a given armature current, so the power stays roughly constant. It gives a wide upward range cheaply, since the field current is small and the rheostat need not be heavy.
Armature-series-resistance control is the crude version of the first method: it drops voltage in a resistor and wastes it as heat, so the efficiency falls in proportion to the speed reduction. Modern drives replace it with a controlled rectifier or a chopper, which vary the armature voltage without the loss.
The measurement worth taking here is E_b/N. Since E_b = kφN and the flux is being held constant, that ratio should be the same at every operating point. If it drifts, the flux moved — which almost always means the field current was disturbed.
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.
- Supply through the starter, then through the armature rheostat, to the armature, with the armature ammeter in series.
- Voltmeter across the armature terminals, so it reads the voltage actually applied to the armature and not the supply voltage.
- Shunt field across the supply through the field rheostat, with the field ammeter in series.
- Armature rheostat at maximum resistance and field rheostat at minimum resistance before starting.
Procedure
- 1Check the connections. Field rheostat at minimum resistance — that is maximum field current — and armature rheostat at maximum.
- 2Start the motor with the starter and let the speed settle.
- 3Note the field current and keep it fixed at that value for the whole experiment.
- 4Cut out the armature rheostat in steps. At each step record the armature voltage, the armature current and the speed.
- 5Continue until the full supply voltage appears across the armature and the motor is at its base speed.
- 6Compute the back emf at each step and plot speed against armature voltage, and against back emf.
- 7For field control, restore the armature rheostat and instead increase the field resistance in steps, recording speed against field current.
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
Flux must not move while armature voltage is varied
Armature-control readings
Keep the field current fixed at the value above for every row.
| # | Armature voltage V_a(V) | Armature current I_a(A) | Speed N(rpm) |
|---|---|---|---|
| 1 | |||
| 2 | |||
| 3 | |||
| 4 | |||
| 5 | |||
| 6 |
Fill in the machine data and at least one complete row of readings to see the results, the worked calculation and the curve.
Precautions
- Never open the field circuit while the motor is running. The flux collapses, the back emf falls, the armature current rises and the speed runs away.
- Start with the armature rheostat at maximum and the field rheostat at minimum.
- The armature rheostat carries the full armature current — use a heavy-duty one, not a field rheostat.
- Keep the field current constant while varying the armature voltage. Changing both at once makes the readings uninterpretable.
- Do not exceed the rated speed of the machine during field weakening; the commutator and bearings have a limit.
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.
- R_a is taken as a fixed value, but it rises as the winding heats, so the back emf calculated from it drifts through the experiment.
- Reading speed and current at different instants while the machine is still settling.
- Brush contact drop — typically about 2 V for carbon brushes — is not included in V − I_a R_a, and matters most at low armature voltages.
- Supply voltage variation on the bench, which changes the field current along with everything else.
Viva questions with answers
State the speed equation of a DC shunt motor.
N = (V − I_a R_a)/kφ. Speed is proportional to back emf and inversely proportional to flux — those are the two handles every control method uses.
Why is armature control called a constant-torque method?
Torque is proportional to φ·I_a. Armature control keeps the flux fixed, and the machine can still carry rated armature current at any speed, so the torque available is the same throughout the range. Output power, being torque times speed, falls as the speed is reduced.
Why is field control called a constant-power method?
Weakening the flux raises speed but reduces torque for the same armature current, and the two changes roughly cancel in the product. So the machine can deliver about the same power at any speed above base.
Can armature control raise the speed above the base value?
No. It works by applying less than full voltage to the armature, so it can only reduce speed below the value obtained at full voltage. Going above base speed needs field weakening.
What happens if the field circuit opens while the motor is running?
The flux collapses to the residual value, so the back emf falls sharply. The armature current rises to restore the torque, and the speed climbs dangerously — a lightly loaded shunt motor can reach a destructive speed. This is why the three-point starter has a no-volt coil in the field circuit.
Why is a starter needed at all?
At standstill the back emf is zero, so the armature current would be V/R_a — R_a is a fraction of an ohm, so that is many times rated current. The starter puts resistance in series with the armature and cuts it out as the motor picks up speed and generates its own back emf.
What is the function of the no-volt coil in a three-point starter?
It holds the starter handle at the run position magnetically, carrying the field current. If the supply fails or the field circuit breaks, the coil releases and a spring returns the handle to off — which both protects against restarting without resistance and against the field-open runaway.
How does the Ward–Leonard system fit in?
It is armature voltage control done properly: a motor-generator set supplies a continuously variable armature voltage, giving smooth control in both directions and regenerative braking, with none of the loss of a series rheostat. Solid-state drives have replaced it, but the principle is identical.