Q20Electrical Machines and Drives

Armature Voltage Control vs Field Flux Control Characteristics

Armature voltage control (used from zero speed up to base/rated speed): field flux is held at its full rated value throughout, and armature voltage is varied to control speed. Since developed torque T=KphiIa and both K and phi are constant, the maximum permissible torque (limited by rated armature current Ia) remains constant across this entire speed range - this is why armature voltage control is called constant-torque control. Because power P=Tw and torque T is constant while speed w rises with increasing armature voltage, the maximum available power rises linearly with speed throughout this region, reaching its rated value exactly at base speed.

Field flux control (used from base speed upward to maximum permissible speed): armature voltage is held at its full rated value, and field current (flux) is progressively reduced below rated to raise speed above base speed. Since armature voltage and armature current both remain at their rated values in this region, the armature input power (and hence available mechanical power output, apart from small losses) remains essentially constant across this entire speed range - this is why field control is called constant-power control. Because torque T=KphiIa and Ia is held at its rated value while phi is progressively reduced to raise speed, developed torque correspondingly falls (approximately inversely with speed, since P=Tw=constant implies T is inversely proportional to w in this region).

The torque and power limitations in a combined armature-voltage-and-field-control drive arise from the physical rating limits of the machine: the armature current must never exceed its rated (thermal) value Ia,rated at any operating speed (in either control region), since exceeding it causes excessive armature heating and potential winding damage; similarly, applied armature voltage must never exceed its rated value Va,rated, since exceeding it risks insulation breakdown and commutation problems, and field current/flux must not be reduced below some minimum safe value, since excessive field weakening risks armature reaction effects becoming too severe relative to the weak main field (potentially causing commutation difficulties and, in extreme cases, a runaway condition similar in principle to the series-motor no-load runaway problem, though arising here from deliberate field weakening beyond a safe minimum rather than from load removal).

In the armature-voltage-control region (below base speed), the torque limitation is a hard, constant ceiling (Tmax = Kphi_ratedIa,rated, unchanging with speed) since flux is always at its full rated value and only Ia is limited by its rated thermal capacity; the power limitation in this region is not really a limitation at all in the usual sense but rather a natural consequence, rising linearly with speed up to the rated power value reached exactly at base speed (since P=Tw with T constant and w increasing).

In the field-control region (above base speed), the power limitation becomes the hard, constant ceiling (Pmax = Va,rated*Ia,rated, unchanging with speed, since both Va and Ia remain at their rated values throughout this region) while the torque limitation now falls with increasing speed (since T=P/w with P constant and w increasing, giving T inversely proportional to w) - this is why the field-control region can never be used to obtain high torque at high speed; if a load actually demanding high torque at high speed is encountered, the motor (limited by its rated armature current in this deliberately flux-weakened condition) simply cannot supply it, and attempting to force higher torque by increasing armature current beyond rated value would cause overheating, while attempting to restore full torque by increasing flux back toward its rated value would require reducing speed back toward base speed (since the two effects, flux and speed, are now coupled through the fixed rated armature voltage in this control region) - this fundamental armature-current/armature-voltage dual-rating constraint is precisely what defines the boundary between the constant-torque (armature-control) and constant-power (field-control) operating regions and dictates the practical torque-speed envelope within which any combined-control DC drive must operate.

A further important practical limitation to note is that the transition between the two control regions is not always perfectly sharp in a real drive: near base speed, some drive control schemes deliberately blend armature voltage and field control together over a narrow transition band (rather than switching abruptly at exactly one speed) to avoid control instability or excessive current transients that could otherwise occur if the controller attempted an instantaneous switch from pure armature-voltage regulation to pure field regulation, and this blended-transition strategy is a standard refinement found in industrial DC drive controllers used for rolling mills, paper machines, and other applications requiring wide, smooth speed range with well-behaved dynamic response across the entire operating envelope.

It is also worth noting the practical consequence of these two limitations for machine sizing and selection in an actual application: a drive application requiring both high torque at low speed (for example, during initial acceleration of a heavily loaded conveyor or hoist) and high speed at reduced torque (for example, during light-load, high-throughput operation) can be satisfied by a single, appropriately sized DC machine using combined armature-and-field control across the full torque-speed envelope, rather than requiring either a much larger, more expensive machine sized for the full torque at the full speed simultaneously (which the physical current and voltage rating limits described above show is generally unachievable without such oversizing) or two separate machines switched in and out for the two different operating regimes.

Finally, it should be emphasized that these torque and power limitations are not merely theoretical constructs but are the actual basis for the protective current-limiting and voltage-limiting circuits built into every practical DC drive controller: an armature current limiter (typically implemented as an inner current-control loop within the overall speed-control system) continuously restricts the commanded armature current reference to at most its rated value regardless of the outer speed loop's demand, while a field-weakening controller similarly restricts how far the field current can be reduced, together ensuring that the drive automatically and continuously respects the constant-torque and constant-power boundaries described above under all dynamic operating conditions, including transient overload or fault conditions, without relying on the machine's own thermal inertia alone to prevent damage.