- Electrical Drives Definition: Electrical drives are systems that control the operation of electric motors, including starting, speed control, and braking.
- Importance of Control: Controlling electrical drives is essential to prevent damage from sudden changes in voltage or current.
- Closed Loop Control: This system uses feedback to adjust inputs based on outputs, offering protection, faster response, and better accuracy.
- Current Limit Control: This method prevents excessive current flow during motor start-up by using a feedback loop to maintain safe current levels.
- Closed Loop Speed Control: This system uses feedback loops to regulate motor speed, ensuring smooth operation by balancing motoring and braking actions.
Electrical drives combine a motor, power converter, controller and any required feedback devices. They control electrical motors and their loads during three basic operations:
- Starting
- Speed control
- Braking
The control of electrical drives converts speed, torque or position commands into switching and motor-control actions. Starting and braking are transient operations, while regulated speed or torque can be steady-state duties. The controller must track the command, reject load disturbances and keep voltage, current, speed and temperature within the ratings of the converter, motor and load.
The selected control method depends on the motor, converter, load, required accuracy, response time and available feedback. The following sections describe common current, torque and speed loops.
Closed Loop Control of Drives
Drive control can be open loop or use a closed loop control system. An open loop control system does not use measured output to correct its command, although the motor and load still affect the result. A closed loop compares feedback with a reference and acts on the error. A well-designed closed loop control system in electrical drives can reject disturbances and reduce steady-state error. Limits and protective functions remain separate requirements, and poor tuning or faulty feedback can make performance worse.
- Correction of load disturbances
- Controlled dynamic response
- Lower steady-state error
Many electrical drives use nested control loops, but their implementation is not independent of supply or motor type. Sensorless AC vector control, encoder feedback, DC armature-current control and permanent-magnet motor control use different models and signals.
Current Limit Control
A motor can demand high current during acceleration, a sudden load change or a stall. A current controller measures or estimates motor current and adjusts the converter output to follow a limited reference. The drive can reduce the torque command when a configured current or thermal limit becomes active. The limit must match the converter rating, motor heating, duty cycle and required acceleration.
Closed Loop Torque Control
Torque control is used in traction, winding, tension and coordinated-drive systems, including battery-powered vehicles. An accelerator or supervisory controller sets the torque reference T. The drive controls torque-producing current so estimated or measured torque follows T* within current, voltage, speed and thermal limits. The reference-to-torque relationship also depends on motor flux and the selected control model.
Closed Loop Speed Control
A closed-loop speed drive commonly uses a slower outer speed loop and a faster inner torque or current loop.
The speed controller compares reference speed with feedback and sends a torque or current reference to the inner loop. The inner current loop follows that request within converter and motor limits. If reference speed Wm* rises, positive speed error ΔWm requests accelerating torque.
As the drive approaches its reference, the speed controller reduces the torque request. At steady speed, motor torque balances load torque and losses. A lower speed reference Wm can request braking torque, but the available deceleration depends on whether the drive can regenerate, dissipate energy or use mechanical braking. The controller does not need to switch continuously between motoring and braking when the load and reference are steady.
