ELECTRICAL DRIVES How electrical drives are controlled

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Electrical drives have become one of the most essential pieces of equipment in the entire world. In applications that use them, matching drive to the motor is crucial for getting the best combination of key metrics like torque, speed, and efficiency.

An electric or electrical drive is an electromechanical system that utilizes an electric motor as its source of movement, rather than others such as diesel, steam, gas, or hydraulics, to control the motion and processes of different machines and mechanisms.

They can be found in everything from machinery in manufacturing facilities and Battery Electric Vehicles (BEVs) and have become one of the world’s most essential pieces of equipment, critical to the operations and overall functioning of virtually every industry on earth. They have also got several advantages over other types of drive systems, such as:

• Higher levels of efficiency

• Free from pollution

• Control can be adapted to different system and application needs

• Control methods are simple and easy to use

• Cleaner, less noisy, and less maintenance required

• A wide range of speed, power, and torque ratings

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A typical electrical drive system will include a controller, a transmission, an electric motor, and a load that needs to be driven by it. Together, the system accomplishes three main kinds of work: starting; stopping; and braking.

While electrical drives allow us to exercise full control over the motor, it’s also necessary to control the electrical drives because the functions carried out by the drives are impermanent operations, i.e. change in terminal voltage, that could cause damage if left unregulated and uncontrolled.

Open-loop systems

In an open-loop system, control parameters are either fixed or set ahead of time by an operator. When the drive is working, the system finds its own equilibrium state. In the case of an electrical motor, this operating equilibrium state is typically the motor speed or its angular position.

The controlling parameters - for example, the voltage supply or load on the motor - may or may not be under the operator’s control. If any parameters such as load or supply voltage are changed, the motor will find a new equilibrium state, i.e. settle at a different speed. The equilibrium state itself can be manipulated by forcing a change in one of the parameters over which the operator has control. In short, in an open-loop system, the output does not influence the input.

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Closed-loop systems

In contrast, a closed-loop system is much more sophisticated as it has an inherent degree of autonomy.

In a closed-loop system, once the initial operating parameters have been set, it is not responsive to any changes or disturbances in the system’s operating environment such as pressure and temperature, or to demands placed on the system by power delivery, load conditions, and speed.

Instead, the output is fed back to the input terminal which determines how much input the system needs. If, for example, the output is more than the predefined value, the input is reduced to compensate, and vice-versa. In electrical drives, these feedback (closed) loop controls help protect the system, improve response times, and increase accuracy.

Current limit control
This type of control configuration limits the amount of current fed to the motor when it starts up, thereby preventing damage. The feedback loop doesn’t affect the normal operation of the electrical drive but if the current exceeds the predefined “safe limit”, the feedback loop activates to reduce the current. Once current is reduced to a safe enough level, the feedback loop deactivates.

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Closed-loop torque control
This is usually found in BEVs such as cars and trains. The reference value for torque (T*) is set by depressing the accelerator, and this value follows by the loop controller and the motor. The electrical drive’s speed is controlled by the amount of pressure put on the accelerator by the operator.

Closed-loop speed control
In electrical drives, speed control loops are one of the most widely used control methods. Here, there are two control loops—inner and outer. The inner loop limits the converter and motor current or motor torque below the defined safe limit.

If the reference speed (W_m*) increases and there is a positive error (ΔW_m) this indicates that speed needs to increase. In this instance, the inner loop increases the current, allowing the driver to accelerate. When speed reaches either the desired level or the maximum allowable, motor torque is equal to load torque, and the reference speed decreases.

The system also recognizes the need for deceleration and braking is applied by the speed controller at the maximum allowable current.

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