Why does a single-phase motor require more current at starting than running?

Single-phase motors require a larger current during startup compared to when they are running primarily due to the following reasons:

1. Inertia During Startup

During startup, the motor must overcome its own static inertia. Since the motor is stationary before startup, a larger torque is needed to overcome static friction and accelerate to operating speed. This process requires a higher current to provide the necessary starting torque compared to normal operation.

2. Change in Flux Density

At startup, the flux density within the motor needs to be established from zero. This means that the motor requires a larger current to quickly build up the magnetic field necessary to generate sufficient starting torque. As the motor begins to rotate, the flux density stabilizes, and the required current decreases.

3. Phase Difference

Single-phase motors, at startup, only have one phase of power, which does not naturally produce a rotating magnetic field. To simulate a rotating magnetic field, capacitors, resistors, or PTC (Positive Temperature Coefficient) thermistors are commonly used as starting aids. These components provide an additional phase difference at startup, making the current distribution more uniform to generate a rotating magnetic field. This process requires a larger current to activate.

4. Mechanical Resistance

In addition to overcoming the motor's inertia, the motor may also need to overcome the resistance of the load it is driving. If the motor is connected to a mechanical load with weight or friction, a larger torque is needed to overcome these resistances, leading to an increase in the starting current.

5. Inductive Effect

Motor windings have inductive properties, which means that sudden changes in current produce a counter-electromotive force (back EMF) that resists the increase in current. However, at startup, since the motor is not yet turning, the back EMF is minimal, allowing the current to rise rapidly to a higher level.

6. Thermal Effects

During startup, the motor may experience a rapid rise in temperature, causing the resistance of the windings to increase. Although the increase in resistance limits the current, during the initial moment of startup, the motor has not fully heated up, so the current can still reach peak levels.

Practical Applications

To protect single-phase motors from damage due to excessive starting currents, starting capacitors, starting resistors, or PTC thermistors are often used to smooth the startup process. Additionally, overload protection devices (such as thermal relays) are employed to prevent the large starting currents from overheating or damaging the motor.

Summary

Single-phase motors require a larger current during startup mainly because they need to overcome static friction, establish a magnetic field, provide sufficient starting torque, and overcome mechanical resistance. Through appropriate design and protective measures, it is possible to ensure that the motor is not damaged during startup and transitions smoothly to normal operation.