Q2Advanced Electric Drives

Power Limitations in Armature Voltage Control of DC Drives

In armature voltage control of a DC motor drive, the field flux (phi) is held constant at its rated value while the armature voltage (and hence back-EMF Eb and speed) is varied to control speed below base speed. Since developed torque T = KphiIa depends only on armature current (with flux held constant), the maximum available torque remains constant (equal to rated torque) across the entire armature-voltage-controlled speed range, provided the armature current is not allowed to exceed its rated value - this is why armature voltage control is described as a 'constant torque' control region.

However, since the maximum available power is the product of torque and speed (P = Tomega), and torque is limited to its rated value throughout the constant-flux armature-voltage-control range, the maximum available output power increases linearly with speed as the armature voltage (and hence speed) is raised from zero toward the base speed - meaning at low speeds within the armature-voltage-control range, only a small fraction of the motor's rated power capability is actually available, even though full rated torque is available. This is the fundamental power limitation of pure armature voltage control: to obtain full rated power at any speed below base speed, the motor would need to be significantly over-sized in current (and hence physical size and cost) relative to what would be needed if it only had to deliver rated power at or near base speed, since the armature current required for a given power output at a low speed (low back-EMF) is much higher than the current required for the same power output at a higher speed (higher back-EMF), given that P = EbIa and Eb is proportional to speed.

This power limitation is precisely why practical DC drive systems combine armature voltage control below base speed with field-weakening (flux-reduction) control above base speed: armature voltage control alone can provide constant rated torque but only progressively increasing power output (from zero up to rated power at base speed) as speed increases from zero to base speed, while field weakening above base speed instead provides constant rated power (by reducing torque capability in inverse proportion to the further speed increase beyond base speed), together spanning the widest possible useful speed range while making full and efficient use of the motor's rated current, voltage, and power capability at every operating speed within that combined range.