Voltage Control
The most basic technique for torque control of three-phase motors is the voltage control method. Its working principle consists of changing the voltage applied to the stator windings of the motor, aimed at changing the value of magnetic flux inside the motor and, consequently, the output torque of the motor. In the case of a motor whose torque is proportional to the square of the applied voltage, if the voltage is reduced to 50% of the rated voltage, then the output torque of the motor will be reduced to 25% of the original value.
This method of control is utilized for basic small power motor startups and speed regulation for those motors whose loads are relatively constant, such as in fans and pumps. In these applications, torque regulation by voltage control can be done at minimal costs by merely adjusting the input voltage. However, since the method of voltage control inherently causes fluctuations in the magnetic flux of the motor, the magnetic flux inside the motor will fall with the reduction in voltage, which would decrease the operating efficiency of the motor and can result in overheating of the motor or saturation of the magnetic circuit.
In practice, the voltage control method shows limitations in torque adjustment capability and is unsuitable for cases that involve a wide range of torque adjustments or fast dynamic response. Voltage control may also be applied to high-power motors; however, this would result in very high starting currents that may impact the power grid. Consequently, the method of voltage control normally finds its application coupled with other torque control strategies to result in more effective torque regulation.
Frequency Control
Frequency control, sometimes called the “V/f control” method, is one of the most popular approaches to torque and speed regulation. It relies on the simple expedient of changing the input frequency of the motor. Very often, frequency control is used in combination with voltage changes—i.e., maintaining a constant voltage-to-frequency ratio (V/f = constant). This approach aims to keep the stator magnetic flux constant, such that over its full speed range, the motor retains the same torque output capability.
If the frequency of a 50Hz industrial motor is reduced to one-half (e.g., 25Hz), the voltage should also be supplied at half the rated voltage to keep the stator magnetic flux the same. This is to avoid magnetic saturation and loss of torque due to insufficient magnetic flux at low frequencies. In practice, the V/f control method can ensure relatively smooth adjustments in torque and is suitable for applications with constant load characteristics, such as fans and pumps.
Frequency control has some shortcomings: at low speeds, the voltage drop due to internal resistance in the motor is relatively large and directly influences the stability of magnetic flux. Because the magnetic flux is unstable, it results in unstable torque at low speeds. In addition, due to the inability to separate magnetic flux and torque components using frequency control, dynamic response and highly precise torque control are unsatisfactory. Therefore, frequency control is mainly used in applications with modest requirements for dynamic performance and torque control accuracy.
Vector Control
Vector control, also known as field-oriented control (FOC), is an advanced control technology based on the direction of the motor’s magnetic field. Vector control decomposes the stator current into two components: the “torque current component” related to torque and the “magnetic flux current component” related to magnetic flux. By independently adjusting the motor’s torque and magnetic flux, three-phase AC motor control becomes similar in many aspects to DC motor control, achieving high accuracy of motor torque.
Another important advantage of vector control is that it can achieve linear and fast torque adjustment over the entire speed range and ensure high dynamic response and high precision of torque control even at low or zero speed. For this reason, vector control is applied to many applications that have very high torque accuracy and dynamic response requirements, including but not limited to CNC machine tools, industrial robots, and servo systems.
In practical applications, vector control usually requires precise rotor magnetic field position detection and complex mathematical models for current decoupling. Therefore, high-performance controllers and precise current and position sensors are required. Although vector control can provide excellent torque control capability, its implementation cost is high and the algorithm complexity is significant, so it is mainly used in high-performance scenarios.
Direct Torque Control
Direct torque control (DTC) is a method identified by its direct control of the stator magnetic flux and torque. Unlike vector control, DTC does not require the decoupling of current components—real-time stator magnetic flux and torque are monitored, and then the appropriate voltage vector is chosen to adjust both the torque and magnetic flux. The main concept is to estimate the change rate of magnetic flux and torque in real time and dynamically select a corresponding switching state according to the estimation to adjust torque output.
DTC has some advantages, including a simple structure, intuitive control strategy, and fast response speed. Compared to vector control, DTC has much lower algorithmic complexity but can achieve similar torque control effects with a very fast dynamic response speed. DTC is widely used in high-speed motor drive systems and high-precision servo systems, and it is especially suitable for industrial applications requiring fast response and high dynamic performance.
However, the major disadvantage of the DTC method is that it has a tendency to create torque ripple at low speeds and high loads, resulting in uneven torque fluctuations that can cause motor vibration, noise, and unstable speed. Therefore, in practical applications, space vector modulation (SVM) is often combined to smooth the torque output.
Using a Variable Frequency Drive (VFD)
A VFD is one of the most common devices for implementing control functions related to the torque and speed of three-phase motors. In operation, the VFD controls the operating state of the motor through changes in the motor’s supply voltage and frequency. Modern VFDs usually integrate multiple torque control strategies, such as V/f control, vector control, and direct torque control, and can select an appropriate control strategy based on different application needs.
VFDs can offer good performance in speed regulation and thus enable accurate control of motor torque and speed. In energy-saving control applications like fans and pumps, the reduction of frequency by VFDs can reduce the motor speed and ensure energy-saving operation. A fan system, for example, might achieve up to 50% energy savings when its speed is reduced by 20%. In addition, VFDs can eliminate the high current impact that occurs during direct motor startup by providing soft-start functions.
In applications requiring high precision and complex control, VFDs can be combined with vector control or DTC to realize highly accurate torque regulation and rapid dynamic response. Thus, VFDs are widely used in industrial automation, building HVAC, petrochemical, and other fields and are essential equipment for three-phase motor control.
Slip Control
Slip control is a special torque control technique mainly applied in wound-rotor induction motors (slip-ring motors). The principle is to change the motor’s slip rate and thus adjust the motor’s torque by changing the additional resistance in the rotor circuit. The greater the slip rate, the greater the output torque of the motor will be. This method provides good performance for heavy-load startup and constant torque load scenarios, such as mine hoists and cranes, which require high starting torque and smooth operation at low speeds.
While mine hoists can realize high-torque, smooth start and precisely control the motor at low speed by adjusting the rotor resistance with slip control, this system can avoid torque instability caused by load fluctuations. However, slip control needs additional rotor resistance in the circuit, resulting in significant energy losses and possible rotor overheating. Therefore, slip control mainly applies to wound-rotor induction motors and is not suitable for squirrel-cage induction motor torque control.
Closed-Loop Control
One approach for precise motor torque control is real-time monitoring of motor output parameters and comparing them with target values. This is usually referred to as closed-loop control. The closed-loop control usually uses sensors to estimate the torque, speed, or position of the motor and corrects the input voltage or frequency in real time via a controller, such as a PID controller, to precisely regulate the output torque of the motor.
Closed-loop control can be used in high-precision machining tools to maintain constant cutting force by adjusting the input current through real-time measurement and comparison of motor torque with the target value. Consequently, closed-loop control can compensate for load fluctuations, external disturbances, and other factors that may disturb the stable operation of motors under complex working conditions. Therefore, it is widely used in applications where high precision is desired, such as precision machining and industrial robot control.
However, closed-loop control is expensive because it requires high-precision sensors and complex control algorithms. Simplified closed-loop control strategies or open-loop control schemes can usually meet the requirements for general industrial applications. In high-precision control fields, such as CNC machine tools and high-end servo drive systems, closed-loop control is an indispensable technology for achieving high precision and high dynamic response.
