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Demag motors are devices that convert electrical energy into mechanical energy. They utilize energized coils (i.e., stator windings) to generate a rotating magnetic field, which acts on the rotor (such as a squirrel-cage closed aluminum frame) to form electromagnetic rotational torque. Motors are classified into DC motors and AC motors based on the power supply used. Most motors in power systems are AC motors, which can be synchronous motors or asynchronous motors (where the speed of the stator magnetic field does not remain synchronized with the rotor rotation speed). A motor mainly consists of a stator and a rotor. The direction of force-induced motion of current-carrying conductors in a magnetic field is related to the current direction and the direction of magnetic flux lines (magnetic field direction). The working principle of a motor is the force exerted by the magnetic field on the current, causing the motor to rotate.

Demag motor structure

The motor mainly consists of components such as the stator, rotor, and end covers. The stator is made of laminated stacks to reduce iron losses during motor operation and contains three-phase AC windings, referred to as the armature. The rotor can be made in a solid form or from laminated stacks, with permanent magnet materials mounted on it. Depending on the position of the permanent magnet material on the motor rotor, permanent magnet synchronous motors can be divided into two structural forms: surface-mounted and interior-mounted. Figure 1 provides the corresponding schematic diagrams. The surface-mounted rotor has a simple magnetic circuit structure and low manufacturing cost, but since starting windings cannot be installed on its surface, asynchronous starting is not possible. The magnetic circuit structures of interior-mounted rotors mainly include radial, tangential, and hybrid types, totaling 3 kinds. The main difference between them lies in the relationship between the magnetization direction of the permanent magnets and the rotor rotation direction. Figure 2 shows the magnetic circuit structures of 3 different forms of interior-mounted rotors. Since the permanent magnets are placed inside the rotor, the rotor surface can be shaped into pole shoes. Inserting copper bars or cast aluminum into the pole shoes serves starting and damping functions, resulting in good steady-state and dynamic performance. Additionally, due to the asymmetry of the interior-mounted rotor magnetic circuit, reluctance torque is generated during operation, which helps improve the motor's power density and overload capacity. Moreover, this structure facilitates field-weakening speed expansion.

Demag motor working principle

The motor is a rotating machine powered by a single-phase AC supply, with single-phase stator windings. When connected to single-phase AC power, it generates an alternating pulsating magnetic field in the air gap between the stator and rotor; therefore, single-phase asynchronous motors cannot self-start. In AC motors, when AC current flows through the stator windings, an armature magnetomotive force (MMF) is established, which significantly impacts energy conversion and operating performance. Thus, passing single-phase AC through a single-phase winding produces a pulsating MMF. This MMF can be decomposed into two rotating MMFs with equal amplitude and opposite rotational speeds, thereby establishing forward and reverse rotating magnetic fields in the air gap. These two rotating magnetic fields cut across the rotor conductors, inducing electromotive force (EMF) and induced current in the rotor conductors respectively. The interaction between this current and the magnetic field produces positive and negative electromagnetic torques. The forward electromagnetic torque attempts to rotate the rotor forward; the reverse electromagnetic torque attempts to rotate the rotor backward. The superposition of these two torques constitutes the resultant torque driving the motor. Regardless of whether it is the forward or reverse rotating magnetic field, their magnitude relationship with slip is the same as in three-phase asynchronous motors. If the motor speed is n, then for the forward rotating magnetic field, the slip is: s+=(n1-n)/n1=s. For the reverse rotating magnetic field, the slip is: s-=（-n1-n)/-n1=s. From the T-s curve of the single-phase asynchronous motor, the main characteristics are: (1) n=0, s=1, T=T++ T- =0, indicating that the single-phase asynchronous motor has no starting torque; without other measures, the motor cannot start. (2) When s≠1, T≠0, T has no fixed direction; it depends on the sign (positive/negative) of s. (3) Due to the existence of reverse torque, the resultant torque decreases, so the overload capacity of single-phase asynchronous motors is relatively low. Working principle of capacitor split-phase starting: During startup, switch K closes, causing the phase difference between the two winding currents I1 and I2 to be approximately 90°, thereby generating a rotating magnetic field and starting the motor. Once normal rotation is achieved, the centrifugal switch opens, disconnecting the starting winding. Working principle of shaded-pole single-phase motors: After current is applied to the stator, part of the magnetic flux passes through the short-circuit ring, inducing current within it. The current in the short-circuit ring opposes changes in magnetic flux, creating a phase difference between the flux in the shaded portion and the unshaded portion, thus forming a rotating magnetic field that causes the rotor to turn. In the T-s curve of the single-phase asynchronous motor, the motor rotation direction is clockwise. This is because the magnetic flux in the unshaded portion leads the flux in the shaded portion.

Demag motor control methods

Constant V/f control method for motors: The constant voltage-to-frequency ratio control method for permanent magnet synchronous motors is similar to that for AC induction motors. It controls the amplitude and frequency of the motor input voltage to change simultaneously, thereby keeping the motor flux constant. The constant V/f control method can meet the requirements of wide-range speed regulation systems. Without feeding back physical signals such as current, voltage, or position, it can still achieve a certain level of control accuracy, which is the greatest advantage of the constant V/f control method. The constant V/f control method features simple control algorithms and low hardware costs, and has been widely used in the field of general-purpose variable frequency drives. The disadvantages of the constant V/f control method are also obvious. Since there is no feedback of speed, position, or any other signals during the control process, it is almost impossible to obtain information on the motor's operating status, let alone precisely control the rotational speed or electromagnetic torque. The system performance is average, and the dynamic response is poor, especially when the target speed changes or the load undergoes sudden changes, where problems such as step-out and oscillation are prone to occur. Obviously, this control method cannot separately control torque and excitation current, and large excitation currents tend to exist during the control process, affecting motor efficiency. Therefore, this control method is commonly used in general-purpose variable frequency drives with lower performance requirements, such as air conditioners, conveyor belt drive control on assembly lines, and energy-saving operation of water pumps and fans.

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