An electric motor consists of a rotor placed within a rotating magnetic field. Under the influence of this rotating magnetic field, the rotor acquires a rotational torque, causing it to turn. Asynchronous motors operate across a wide power range, from a few watts to tens of thousands of kilowatts, providing power for various mechanical equipment and household appliances.
An electric motor (commonly called a "motor") is an electromagnetic device that converts or transmits electrical energy based on the principle of electromagnetic induction. Its primary function is to generate driving torque, serving as a power source for electrical appliances or various mechanical equipment.
The primary function of a generator is to convert electrical energy into mechanical energy.
An electric motor primarily consists of an electromagnet winding or distributed stator windings for generating a magnetic field, a rotating armature or rotor, and other accessories. Under the influence of the rotating magnetic field produced by the stator windings, current flows through the armature's squirrel-cage aluminum frame. This current interacts with the magnetic field, causing the armature to rotate.
Stator (stationary part) Stator core: A component of the motor's magnetic circuit, upon which the stator windings are mounted; Stator windings: The electrical circuitry of the motor, through which three-phase alternating current flows to generate a rotating magnetic field; Frame: Secures the stator core and front/rear end covers to support the rotor, while providing protection, heat dissipation, and other functions;
Rotor (rotating part)
Rotor core: Serves as part of the motor's magnetic circuit and houses the rotor windings within its slots;
Rotor windings: Cut through the stator's rotating magnetic field to generate induced electromotive force and current, producing electromagnetic torque that drives the motor's rotation;
【Motor Principle Animation】
Permanent magnet motor ▼
DC Motor ▼
Quantum Magnetic Motor ▼
Single-phase induction motor ▼
Working Principle of Stepper Motors ▼
Balance Motor ▼
The Principle of Generating Electric Current ▼
Three-phase stator ▼
Motor Cross-Section ▼
Electric motor ▼
DC Motors▼
Differences Between Repairing Variable Frequency Motors and Conventional Motors
The repair methods for variable frequency motors are fundamentally the same as those for conventional motors. However, due to the unique characteristics of variable frequency power supplies, the insulation requirements for variable frequency motor windings are stricter than for conventional motors. The following measures should be taken to improve insulation conditions:
1. Select electromagnetic wire with excellent corona resistance to meet the motor's requirements for withstanding high-frequency pulses and partial discharge.
Typically, polyesterimide/polyamideimide composite enameled wire or corona-resistant electromagnetic wire is used.
2. Winding and slot insertion construction techniques.
Strict management is essential during winding, slot insertion, and binding processes for variable frequency motors. Particular attention must be paid to preventing conductor damage during winding and slot insertion. Slot insertion must ensure proper placement of slot insulation, phase insulation, and layer-to-layer insulation. Phase insulation should use materials easily penetrated by insulation varnish. Coil ends must be reinforced with binding and securing to ensure they form an integral unit.
Reinforcing insulation at the motor slot bottom, between phases, between layers, and at coil start/end turns enhances the motor's dielectric strength.
3. Main insulation must employ gap-free insulation.
Air gaps within the insulation structure of variable frequency motors are the primary cause of corona discharge. To ensure the absence of air gaps in the overall insulation structure, per the national standard GB/TZ1707-2008 "Insulation Specifications for Three-Phase Asynchronous Motors for Variable Frequency Speed Control," the impregnating varnish used must be no lower than F-class solvent-free varnish with volatile content below 10%, and the VPI process must be employed. This process also enhances the overall mechanical strength of the insulation structure.
4. Ensure proper matching between the inverter, cables, and motor, and limit the length of cables between the motor and power source.
Due to impedance mismatch in power lines, the overvoltage amplitude at the motor end increases with the length of cables between the inverter and motor, which can easily cause partial discharge. Therefore, based on the specific characteristics of the variable frequency power supply and actual requirements, the length of the connecting cable should be minimized as much as possible to reduce the overvoltage amplitude at the motor end and the amount of partial discharge, thereby extending the motor's service life. Power cables for variable frequency motors generally use specialized cables, also known as symmetrical conductor variable frequency cables, which are of the 3P+3N/E series. This means that the original 3+1 configuration splits the single neutral conductor into three separate conductors.
Stepper Motor
Figure 1.1 illustrates the operating principle of a two-phase stepper motor, which features two windings. When one winding is energized, its stator pole generates a magnetic field that attracts the rotor to align with this pole. If the windings are energized in sequence under control pulses-cycling through the states A`A→B`B→`AA→`BB-the motor rotates clockwise. When energized in the sequence A`A→`BB→`AA→B`B, the motor rotates counterclockwise. Each control pulse changes the energization direction, causing the motor to move one step (90 degrees). Four pulses complete one full rotation. Higher pulse frequency results in faster motor rotation.
The output torque of a stepper motor is proportional to the motor's effective volume, coil turns, magnetic flux, and current. Therefore, a larger effective volume, more coil turns, and a smaller air gap between stator and rotor result in greater torque, and vice versa.
Fig. 2 Stepper Motor Mechanism Structure Diagram
The structure of a stepper motor consists of a rotor (rotor core, permanent magnet, shaft, ball bearings), a stator (windings, stator core), and front and rear end caps. The most typical two-phase hybrid stepper motor features a stator with 8 large teeth and 40 small teeth, while the rotor has 50 small teeth. A three-phase motor's stator has 9 large teeth and 45 small teeth, with the rotor also possessing 50 small teeth.
The phase count of a stepper motor refers to the number of coil groups within the motor. Commonly used types include two-phase, three-phase, four-phase, and five-phase stepper motors. Different phase counts result in different step angles: typically, two-phase motors have step angles of 0.9°/1.8°, three-phase motors have 0.75°/1.5°, and five-phase motors have 0.36°/0.72°. Without a microstepping driver, users primarily select stepper motors with different phase counts to meet their required step angle specifications. When using a microstepping driver, the 'phase count' becomes irrelevant; users can simply adjust the microstep resolution on the driver to alter the step angle.
Whether it's a two-phase four-wire, four-phase five-wire, or four-phase six-wire stepper motor, the internal construction remains consistent. The distinction between four-wire, five-wire, or six-wire configurations depends on whether the A and ~A pairs, or B and B~ pairs, share a common terminal (COM) connection. If both the A and B groups have their own dedicated COM terminals, the motor is six-wire. If the common terminals for A and B are connected together, it is five-wire.
Therefore, to determine the wiring configuration of a stepper motor, simply separate the A and B groups and test them with a multimeter.
Four-wire: Since there is no common (COM) wire in a four-wire configuration, the A and B groups are completely isolated and non-conductive to each other. Thus, when tested with a multimeter, one group will show no continuity.
Five-wire: In a five-wire configuration, the common terminals of the A and B groups are connected together. When testing with a multimeter, if one wire shows a resistance value similar to the other wires, that wire is the common terminal. For driving a five-wire stepper motor, the common terminal can be left unconnected and the motor will still operate.
Six-wire: The common terminals of the A and B groups are not connected. Similarly, using a multimeter to measure resistance, if one wire shows identical resistance to the other two wires, that wire is the com terminal, and the other two wires form a group. For driving a four-phase six-wire stepper motor, the motor can also be driven without connecting the two common com terminals.
Stepper Motor Related Concepts:
Number of Phases: The number of excitation coil pairs generating different pairs of N and S magnetic poles. Commonly denoted by m.
Pulse count: The number of pulses or conductive states required to complete one magnetic field cycle, denoted by n. Alternatively, it refers to the number of pulses needed for the motor to rotate one pitch angle. For example, in a four-phase motor:
Step angle: The angular displacement of the motor rotor corresponding to one pulse signal, denoted by θ. θ = 360 degrees / (rotor teeth J × operating beat number). For conventional two- or four-phase motors with 50 rotor teeth: In four-phase operation, the step angle is θ = 360°/(50*4) = 1.8° (commonly called a full step). In eight-phase operation, the step angle is θ = 360°/(50*8) = 0.9° (commonly called a half step).
Holding Torque: The inherent locking torque of the motor rotor when de-energized (caused by magnetic field tooth profile harmonics and mechanical errors).
Static Torque: The locking torque on the motor shaft when the motor is under rated static electrical force but not rotating. This torque serves as a standard for evaluating motor size (geometric dimensions) and is independent of drive voltage or power supply.
Stepper Motor Drive: Driving a stepper motor essentially involves alternately applying continuous pulses to the motor's A and B groups, enabling the motor to operate.
Missed Step: The actual number of steps taken during motor operation does not match the theoretical step count.
Example: Differences Between Two-Phase and Five-Phase Stepper Motors
Stepper motors are primarily classified by the number of phases, with two-phase and five-phase stepper motors being the most widely adopted in the current market. Most two-phase stepper motors can be subdivided into a maximum of 400 equal steps per revolution, while five-phase motors can be subdivided into 1000 equal steps. Consequently, five-phase stepper motors exhibit superior performance characteristics, shorter acceleration/deceleration times, and lower dynamic inertia.
Comparison of Differences Between Two-Phase and Five-Phase Stepper Motors:
| Two-phase stepper motor | Five-phase stepper motor | |
| Resolution | 1.8°/0.9° (200, 400 microsteps) | 0.72 degrees/0.36 degrees (500, 1000 microsteps), 2.5 times higher than two-phase stepper motors |
| Vibration characteristics | Low-speed resonance range between 100-200 PPS, significant vibration | No significant resonance points, low vibration |
| Speed & torque characteristics | Lower speed | High speed, high torque |
1. Differences in Control Accuracy
Two-phase hybrid stepper motors typically have step angles of 3.6 degrees or 1.8 degrees, while five-phase hybrid stepper motors generally feature step angles of 0.72 degrees or 0.36 degrees. Some high-performance stepper motors offer even smaller step angles. For instance, a stepper motor produced by Sitong Company for slow-wire cutting machines has a step angle of 0.09 degrees. The three-phase hybrid stepper motors produced by Germany's Berger Lahr can have their step angles set via DIP switches to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, or 0.036°, offering compatibility with both two-phase and five-phase hybrid stepper motor step angles.
The control accuracy of AC servo motors is ensured by rotary encoders. Taking Panasonic's fully digital AC servo motors as an example, for motors equipped with standard 2500-line encoders, the pulse equivalent is 360 degrees/10000 = 0.036 degrees due to the quadrature frequency conversion technology implemented internally in the driver. For motors equipped with a 17-bit encoder, the drive receives 2¹⁷ = 131,072 pulses per revolution, resulting in a pulse resolution of 360 degrees / 131,072 pulses = 0.002746 degrees per pulse.
2. Different Low-Frequency Characteristics
Stepper motors are prone to low-frequency vibration at slow speeds. The vibration frequency depends on load conditions and driver performance, generally considered to be half the motor's no-load starting frequency. This low-frequency vibration, inherent to the operating principle of stepper motors, is highly detrimental to normal machine operation. When stepper motors operate at low speeds, damping techniques should be employed to mitigate low-frequency vibration, such as adding dampers to the motor or using microstepping technology in the driver.
AC servo motors operate exceptionally smoothly, exhibiting no vibration even at low speeds. AC servo systems incorporate resonance suppression capabilities to compensate for mechanical stiffness deficiencies. Additionally, the system's internal frequency analysis function (FFT) detects mechanical resonance points, facilitating system tuning.
3. Different Torque-Frequency Characteristics
Stepper motors exhibit decreasing output torque with increasing speed, experiencing a sharp drop at higher speeds. Consequently, their maximum operating speed is typically limited to 300–600 RPM. AC servo motors deliver constant torque output, maintaining rated torque within their rated speed range (generally 2000 or 3000 RPM). Above the rated speed, they transition to constant power output.
4. Different overload capabilities
Stepper motors generally lack overload capability. AC servo motors possess strong overload capability. Taking the Panasonic AC servo system as an example, it features both speed overload and torque overload capability. Its maximum torque reaches three times the rated torque, enabling it to overcome the inertia torque of inertial loads during startup. Stepper motors lack this overload capability. To overcome inertial torque during startup, larger torque motors are often selected during specification. However, such high torque is unnecessary during normal operation, resulting in wasted torque.
5. Different Operational Performance
Stepper motors employ open-loop control. Excessively high starting frequencies or excessive loads can cause step loss or stalling. Excessively high speeds during stopping can lead to overshoot. Therefore, to ensure control accuracy, acceleration and deceleration must be properly managed. AC servo drive systems employ closed-loop control. The driver directly samples feedback signals from the motor encoder, forming internal position and velocity loops. This design generally avoids the step loss or overshoot issues common in stepper motors, delivering more reliable control performance.
6. Different Speed Response Performance
A stepper motor requires 200–400 milliseconds to accelerate from rest to operating speed (typically several hundred RPM). AC servo systems exhibit superior acceleration performance. For example, the Panasonic MSMA 400W AC servo motor accelerates from rest to its rated speed of 3000 RPM in just a few milliseconds, making it suitable for applications requiring rapid start-stop control.
In summary, AC servo systems outperform stepper motors in numerous performance aspects. However, stepper motors are still commonly used as actuators in less demanding applications. Therefore, during control system design, factors such as control requirements and cost must be comprehensively evaluated to select the appropriate motor.