Q2Electric Drives

Armature Voltage Control

In armature voltage control, the field flux φ is held constant at its rated value (by maintaining constant, rated field current/voltage), and motor speed is controlled purely by varying the armature terminal voltage Va, typically via a chopper (for a DC supply) or a phase-controlled thyristor rectifier (for an AC supply). The steady-state DC motor speed equation is:

Since φ is held constant, speed N is directly proportional to (Va-IaRa), so reducing Va proportionally reduces speed (for a given armature current/load torque), while the maximum available torque capability, T=Kφ·Ia(max), remains unchanged across the entire armature-voltage-controlled speed range (since φ and the motor's maximum rated current Ia(max) are both unchanged) — this is why armature voltage control is called 'constant torque' control, since the motor can deliver its full rated torque at any speed within this control range, from very low speed up to the motor's base (rated) speed, which is reached when Va reaches its maximum rated value.

Practical application: armature voltage control is the standard method for controlling DC motor speed from zero up to base speed, used in applications requiring full torque capability across a wide low-to-medium speed range, such as electric vehicle traction motors during acceleration from standstill, rolling mill drives, and general-purpose variable-speed industrial DC drives operating at or below the motor's rated (base) speed.

Flux Weakening for High-Speed Operation

Once the armature voltage has reached its maximum rated value (base speed reached), further speed increase can only be achieved by instead reducing the field flux φ (since Va cannot be increased further without exceeding the motor's/converter's voltage rating) — this technique, called field weakening or flux weakening, reduces the field current (and hence flux) below its rated value, which, from the speed equation above, directly increases speed for the same armature voltage and current (since N ∝ 1/φ for fixed Va, Ia).

Trade-off — reduced torque capability: however, since torque T=Kφ·Ia, reducing flux φ (while keeping armature current at its rated maximum, to avoid exceeding the motor's thermal current rating) directly reduces the maximum torque the motor can deliver at these higher, flux-weakened speeds — this is why flux weakening is called a 'constant power' control region: since power P=T×N=(Kφ·Ia)×((Va-IaRa)/(Kφ)) ≈ Va·Ia (approximately independent of φ, for fixed Va and Ia at their rated maximum values), the maximum power the motor can deliver remains approximately constant across the flux-weakened speed range, even as maximum available torque decreases proportionally as speed increases beyond base speed.

Practical application: flux weakening is used whenever a drive application requires operating speeds beyond the motor's rated base speed but can tolerate correspondingly reduced torque at those higher speeds — common examples include electric vehicle traction motors at highway cruising speeds (where high torque is no longer needed once the vehicle has already accelerated to speed, but continued speed increase is still desired), and certain machine-tool spindle drives requiring a wide speed range while accepting reduced torque capability (but roughly maintained power capability) at the highest spindle speeds. Together, armature voltage control (constant torque, below base speed) and flux weakening (constant power, above base speed) provide the classic, widely-used combined speed-control strategy giving a DC motor drive its full practical operating speed range while making the most efficient possible use of the motor's voltage and current ratings at every operating speed.