Control of Electrical Machines

Why it matters

Machine control adjusts speed, torque or position while respecting current, voltage, thermal and mechanical limits. Feedback compares the measured result with the command.

Main methods

For a separately excited DC motor, approximate steady equations are V = Ra Ia + kΦω and T = kΦIa. At fixed flux, armature-voltage control changes speed below the voltage limit. Field weakening can increase speed above base speed, but reduces torque capability for a given armature current. For induction machines, supply frequency sets synchronous speed. Scalar V/f control approximately maintains flux below base speed after accounting for low-frequency voltage drops. Above the available voltage limit, increasing frequency weakens flux. Vector control regulates components related to flux and torque using a machine model and measurements or estimates. A synchronous drive coordinates inverter frequency and rotor position. A grid-connected synchronous machine does not acquire an independent steady speed simply by changing field current.

Feedback structure

A common arrangement has a fast inner current loop and a slower outer speed loop. The speed controller requests torque/current; current limits and anti-windup prevent the controller from demanding unavailable actuation. Starting, braking and reversal must follow the drive and machine ratings.

Worked example

A DC motor has kΦ = 1 V·s/rad (also 1 N·m/A), Ra = 0.5 Ω, armature current 10 A and voltage 105 V. Back EMF E = 105 − 10 × 0.5 = 100 V. Speed ω = E/(kΦ) = 100 rad/s, or 955 rpm approximately. Developed torque T = 1 × 10 = 10 N·m. Converted power EIa = 1000 W equals Tω. Shaft output is smaller if mechanical losses are present.

Common mistakes

• Equating commanded and achieved speed despite saturation or overload.
• Treating V/f as exact flux control at every speed.
• Ignoring feedback sign, sensor scaling and current limits.

Quick check

1. What usually limits DC motor torque? Armature current and available flux, subject to thermal limits.
2. Why weaken the field above base speed? Voltage is limited and back EMF would otherwise exceed it.
3. Does field weakening preserve full torque? No; torque capability generally falls.