How to Effectively Suppress Oscillation During the Startup of Synchronous Motors
Synchronous motors are widely used in mining, metallurgy, building materials, power generation, and large industrial drive systems due to their high efficiency, adjustable power factor, and stable operation—typical examples include 6 kV high-voltage synchronous motors (such as the TDMK series).
However, in practical operation, many users encounter a common issue: during startup, synchronous motors are prone to speed fluctuations, mechanical vibration, and even periodic oscillations of current and torque. If not handled properly, these problems can not only reduce startup success rates but also increase mechanical stress on bearings, couplings, and driven equipment.
Based on the manufacturing and field application experience of Shaanxi Lite Simo Motor Co., Ltd., the following provides a systematic explanation of synchronous motor startup oscillation—from root-cause analysis to practical engineering solutions.
I. Main Causes of Startup Oscillation in Synchronous Motors
Unlike asynchronous motors, synchronous motors must be “pulled into synchronism” after the rotor speed approaches synchronous speed. During startup, oscillation issues are usually caused by a combination of the following factors:
1. Insufficient or Unstable Electromagnetic Torque During Startup
Synchronous motors typically rely on damper windings (or squirrel-cage structures) for asynchronous starting. At low speeds, electromagnetic torque varies significantly with speed and cannot form a stable driving torque. This often leads to repeated acceleration–deceleration cycles, resulting in speed and torque oscillations.
2. Asynchronism Between Rotor Magnetic Field and Stator Rotating Field
During startup, there is always slip between the rotor magnetic field and the stator’s rotating magnetic field. When the motor speed is close to synchronous speed but has not yet fully synchronized, the magnetic pull fluctuates periodically, which can induce mechanical vibration and electromagnetic oscillation.
3. Load Characteristics Unfavorable to Startup
High-inertia loads (such as mills, fans, and large pumps) or applications requiring high starting torque can significantly amplify torque fluctuations during startup, making oscillations more pronounced.
4. Improper Starting Method or Control Strategy
Inappropriate selection of direct-on-line starting, reduced-voltage starting parameters, or improper timing of excitation application can all cause sudden torque changes, leading to unstable startup behavior.
II. Common Engineering Measures to Mitigate Startup Oscillation
In practical engineering, optimization can be implemented from electrical, mechanical, and control perspectives.
1. Increase Effective Torque During Startup
To ensure sufficient acceleration capability at low speeds, the starting method should be selected according to operating conditions:
Direct-on-line starting: Suitable when the power grid capacity is sufficient and the load is relatively light
Reduced-voltage starting (autotransformer, series resistor, etc.): Balances starting current and torque requirements to avoid excessive impact
Optimized damper winding design: During motor selection and design, improving rotor structure to enhance asynchronous starting performance fundamentally improves startup stability
At Shaanxi Lite Simo Motor Co., Ltd., starting torque is carefully verified during the synchronous motor design phase to avoid situations where rated power is adequate but starting performance is insufficient.
2. Proper Matching of Mechanical Parameters and Load Characteristics
The influence of the mechanical system on startup oscillation is often underestimated, yet it is critically important:
Control the total system inertia and avoid severe mismatch between motor and load inertia
Optimize coupling selection (e.g., elastic couplings) to absorb impact energy during startup
Ensure good shaft alignment to reduce mechanical vibration that can negatively affect electromagnetic stability
Introducing “mechanical flexibility” into the system can significantly reduce oscillation amplitude during startup.
3. Use Soft Starters or Intelligent Control Devices
Soft starters limit the rate of current rise during startup, allowing electromagnetic torque to build up smoothly and reducing startup shock.
For certain medium- and high-voltage synchronous motor applications, properly configured soft-start devices can effectively mitigate oscillation caused by sudden torque changes.
4. Variable Frequency Drive (VFD) Starting—One of the Best Solutions
Where conditions permit, VFD starting is one of the most effective solutions for synchronous motors. With a VFD:
Voltage and frequency are controlled synchronously to maintain stable magnetic flux
High torque at low frequency enables smooth acceleration without low-speed torque fluctuations
Excitation application timing can be precisely controlled to achieve smooth pull-in to synchronism
In high-inertia, high-power applications, VFD starting can almost fundamentally eliminate startup oscillation.
5. Optimize the Excitation System and Excitation Timing
Startup smoothness is closely related to the excitation system:
Excitation applied too early may cause sudden torque changes
Excitation applied too late may prevent successful synchronization
By optimizing excitation current magnitude, excitation application speed point, and control logic, the dynamic response during startup can be significantly improved.
III. Engineering Recommendations from Shaanxi Lite Simo Motor Co., Ltd.
In real projects, startup oscillation of synchronous motors is rarely caused by a single factor. Instead, it results from the combined effects of motor design, starting method, load characteristics, and control strategy. Therefore, we recommend:
Fully evaluating startup conditions during the selection stage, not just rated power
Choosing appropriate starting and control solutions based on load type
Implementing VFD starting or dedicated excitation control systems when necessary to solve the problem at a system level
Monitoring current, speed, and vibration comprehensively during commissioning and optimizing parameters in a timely manner
Shaanxi Lite Simo Motor Co., Ltd. has long specialized in the design, manufacturing, and technical service of high-voltage synchronous motors and special-purpose motors. We provide complete technical support—from motor selection and startup scheme design to on-site commissioning—to ensure smooth startup, reliable operation, and long-term stable service of synchronous motors under complex operating conditions.
