Understanding the High Starting Current in Asynchronous Electric Motors
Introduction
In the realm of electrical engineering, a common question arises regarding the high starting current in an asynchronous electric motor. This phenomenon is often observed when a motor starts from a complete standstill (0 RPM) or from a low RPM. The core understanding of why this occurs lies in the interplay between the motor's back electromotive force (Back EMF) and the initial resistance encountered during startup.
Why is Starting Current High in an Asynchronous Electric Motor?
The starting current of an asynchronous electric motor is significantly higher than its operating current. This is primarily due to the lack of Back EMF at startup and the electrical characteristics of the motor itself.
Initial Analysis: No Back EMF
When a motor is starting from rest (0 RPM), the Back EMF generated by the motor is zero. This is because Back EMF is directly proportional to the speed of the motor. As the motor spins, it generates a Back EMF that opposes the applied voltage, thereby reducing the current that the motor draws from the power supply. At startup, this Back EMF is absent, allowing the motor to draw a much higher current.
Power Demand for Instantaneous Acceleration
At startup, the motor attempts to accelerate from 0 RPM to its natural speed instantaneously. This instantaneous acceleration is practically impossible, but what it means for practical purposes is that the motor must draw a significant amount of current to overcome its resistance. The current drawn by the motor is limited by the resistance in the system, which is typically low.
The voltage supplied to the motor remains constant, so the motor will draw whatever current it needs to operate at the required torque. This high current draw is necessary to achieve the rapid acceleration needed to reach the motor's natural speed. For instance, if a motor is designed to run at 50 Hz with 4 poles, it will have a certain natural speed. The motor must achieve this speed quickly, which requires a large amount of power.
Reducing Starting Current
1. Adding a Temporary Resistor
One method to reduce starting current is to add a temporary resistor in series with the motor. This increases the series resistance, thereby lowering the current drawn by the motor. Once the motor has started and is running at a steady speed, the resistor can be removed.
2. Adjusting Pole Pairs
Another option is to temporarily rearrange the motor to use more pole pairs. This reduces the speed at which the motor will run, decreasing the starting torque and thereby reducing the starting current. Once the motor has started, the pole pairs can be restored to their original configuration.
3. Changing from Star to Delta Configuration
This particularly applies to three-phase motors. If the motor is initially connected in star (Star) configuration, it can be temporarily switched to delta (Delta) configuration to reduce the starting current. This method is often used in industrial settings where large motors are started.
Common Causes of Excessive Starting Current
Excessive starting current can also result from other factors, such as mechanical load. A high mechanical load, which could include faulty bearings, increased weight on a winch, or any other obstruction, can lead to higher starting currents. Additionally, a Locked Rotor Condition where the motor does not reach its normal operating speed, will result in a high starting current that persists.
Locked Rotor Condition
A Locked Rotor Condition is a situation wherein the motor starts but is unable to reach its normal operating speed due to excessive mechanical resistance. This results in an excessive and prolonged starting current, which is not ideal and may cause damage to the motor if left unaddressed.
Identifying and addressing the root cause of the high starting current is crucial for the efficient and safe operation of the motor. Proper maintenance and the use of appropriate techniques to manage the starting process can significantly improve the performance and longevity of the motor.