I. What is starting current?
Starting current refers to the inrush current of electrical equipment (inductive load) when it is first started. It is the change in current during the short period from the moment the motor or inductive load is energized to when it is running smoothly. This current is generally 4-7 times the rated current. According to national regulations, in order to ensure the safe operation of the circuit and the normal operation of other electrical equipment, high-power motors must be equipped with starting devices to reduce the starting current.
Figure 1 Starting Current - Current-Speed
II. Characteristics of Starting Current
Different types of motors generally have different starting current characteristics. For example, the starting current of a squirrel-cage induction motor is typically 4 to 7 times its rated current, but this is not absolute. However, it is generally required that the starting current of a motor should not exceed 2 to 5 times its rated current. Motors with a power exceeding 30 kW are not suitable for frequent starting because the starting current of motors above 30 kW is generally 6 to 7 times the rated current. Frequent starting will increase the motor's temperature rise, potentially causing it to burn out. Furthermore, the starting method also has a significant impact on the starting current; for example, direct starting can result in a starting current that is 10 to 30 times the rated current.
III. How to accurately test the starting current
1. Use an oscilloscope to measure. Install a current transformer with a large ratio (selectable according to the motor's power or parameters provided by the manufacturer) in the motor starting circuit. Connect the secondary winding of the current transformer to an oscilloscope for measurement.
2. Use a fault recording device for measurement. Install a current transformer in the motor starting circuit, connect the secondary winding of the current transformer to the fault recording device, and start recording during motor startup to measure the fault.
3. Use a portable power quality analyzer for measurement. Install a current transformer in the motor starting circuit, connect the secondary winding of the current transformer to the portable power quality analyzer, and measure during motor startup.
4. Using a high-end motor testing system for testing, by setting parameters such as the dynamometer sensor ratio and synchronization source, the accuracy of parameter measurement can be accurately guaranteed.
During testing, due to the short starting current time, to accurately measure the starting current, it is necessary to ensure sufficient sampling points in a short period of time and a high update rate. The MPT1000 high-end motor testing system integrates a high-end dynamometer, and its 10ms data update rate can accurately guarantee measurement accuracy, accurately capture the maximum peak current, and perform half-wave RMS analysis.
IV. Methods to reduce starting current
Common methods to reduce starting current include direct starting, series resistor starting, autotransformer starting, star-delta reduced voltage starting, and frequency converter starting to reduce the impact on the power grid.
Direct Start
Direct starting involves directly connecting the stator windings of the motor to the power supply , starting it at the rated voltage . It features high starting torque and short starting time , and is the simplest, most economical, and most reliable starting method. Full-voltage starting involves high current but low starting torque, offering convenient operation and rapid start-up. However, this method requires a relatively large power grid capacity and load, and is mainly suitable for starting motors under 1W.
Series resistor starting
Motor starting with a series resistor is a method of reduced-voltage starting. During the starting process, a resistor is connected in series in the stator winding circuit. When the starting current flows, a voltage drop is generated across the resistor, reducing the voltage applied to the stator winding, thus reducing the starting current.
Autotransformer startup
Using a multi-tap autotransformer for reduced voltage can adapt to the starting needs of different loads and obtain a larger starting torque. It is a frequently used reduced voltage starting method for starting large-capacity motors. Its biggest advantage is the large starting torque. When the winding tap is at 80%, the starting torque can reach 64% of that of direct starting, and the starting torque can be adjusted by tapping.
Star-delta decompression start
For a squirrel-cage induction motor with a delta-connected stator winding during normal operation, connecting the stator winding in a star configuration during startup and then reconnecting it in a delta configuration after startup reduces the starting current and lessens its impact on the power grid. This starting method is called star-delta reduced-voltage starting, or simply star-delta starting (y-δ starting). When using star-delta starting, the starting current is only 1/3 of that when starting directly with a delta connection. The starting current is only 2-2.3 times that of direct starting with a delta connection. This means that the starting torque is also reduced to 1/3 of that when starting directly with a delta connection. It is suitable for no-load or light-load starting applications. Furthermore, compared to any other reduced-voltage starter, it has the simplest structure and is the cheapest. In addition, star-delta starting has another advantage: when the load is light, the motor can run in a star connection. In this case, the rated torque can match the load, which improves the motor's efficiency and saves power consumption.
Inverter start
Variable frequency drives (VFDs) are the most technologically advanced, feature-rich, and effective motor control devices in modern motor control. They regulate the speed and torque of a motor by changing the frequency of the power grid. Because they involve power electronics and microcomputer technology, they are expensive and require highly skilled maintenance technicians. Therefore, they are mainly used in fields that require speed regulation and have high speed control requirements.
V. Summary
Starting current is a crucial aspect of motor testing; its accurate measurement significantly influences the motor's starting structure, directly impacting its lifespan and its influence on the power grid. To better measure starting current, engineers must select instruments with higher refresh rates to ensure sufficient data is captured. Zhiyuan Electronics' MPT1000 integrated high-end power analyzer boasts a 10ms data update rate, 5MHz bandwidth, and a 2MS/s sampling rate, greatly guaranteeing data accuracy and waveform fidelity, helping engineers quickly and accurately capture starting current.
