disadvantages of electric motors

The general way of starting an electric motor

1. Full pressure direct start

When both the grid capacity and load allow for full voltage direct starting, full voltage direct starting can be considered.

The advantages are convenient operation and control, simple maintenance, and relatively economical. Mainly used for starting low-power motors, from the perspective of energy conservation, this method is not suitable for motors larger than 11kw.

2. Self coupling decompression starting

The use of multi tap voltage reduction of autotransformers can not only meet the needs of starting different loads, but also obtain greater starting torque. It is a commonly used voltage reduction starting method for starting larger capacity motors.

Its biggest advantage is that it has a large starting torque. When the winding tap is at 80%, the starting torque can reach 64% of the direct starting torque. And the starting torque can be adjusted through tapping. It is still widely used today.

3. Y - Δ Start

For squirrel cage asynchronous motors with a delta connection of stator winding during normal operation, if the stator winding is connected in a star shape during startup and then in a delta shape after startup, the starting current can be reduced and its impact on the power grid can be alleviated. This starting method is called star delta depressurization starting, or simply star delta starting (Y - Δ starting).

When using star delta starting, the starting current is only one-third of that when starting directly using the delta connection. If the starting current during direct starting is calculated as 6-7 Ie, then during star delta starting, the starting current is only 2-2.3 times. This means that when using star delta starting, the starting torque is also reduced to one-third of the original direct starting torque when using the delta connection.

Suitable for situations where there is no load or light load starting. And compared to any other pressure reducing starter, it has the simplest structure and the cheapest price. In addition, the star delta starting method has another advantage, which is that when the load is light, the motor can operate in a star connection. At this point, the rated torque can be matched with the load, which can improve the efficiency of the motor and thus save power consumption.

4. Soft starter

This utilizes the phase-shifting voltage regulation principle of thyristors to achieve voltage regulation starting of electric motors, mainly used for starting control of electric motors. The starting effect is good but the cost is high. Due to the use of thyristor components, there is significant harmonic interference during thyristor operation, which has a certain impact on the power grid. In addition, fluctuations in the power grid can also affect the conductivity of thyristor components, especially when there are multiple thyristor devices in the same power grid. Therefore, the failure rate of thyristor components is relatively high, and due to the involvement of power electronics technology, there are also high requirements for maintenance technicians.

5. Variable frequency drive

The frequency converter is the most technologically advanced, comprehensive, and effective motor control device in the field of modern motor control. It adjusts the speed and torque of the motor by changing the frequency of the power grid. Due to the involvement of power electronics technology and microcomputer technology, the cost is high and the requirements for maintenance technicians are also high. Therefore, it is mainly used in fields that require speed regulation and high speed control requirements.

Comparison of the advantages and disadvantages of pressure reducing start, soft start, and variable frequency start

Reduced pressure starting, commonly known as star delta starting, has the disadvantage of low starting torque and is only suitable for no-load or light load starting. The advantage is that the price is cheap.

Soft start, can set the starting time and initial starting torque to achieve soft start and soft stop of the equipment, and can limit the starting current, with a moderate price.

Variable frequency starting can smoothly start according to the set time and keep the equipment running at the set frequency, which is relatively expensive.

Comparison of performance principles of soft start, frequency converter, and reduced pressure start

1. Soft starter is a combination of thyristor AC voltage regulation technology and power factor control technology, which achieves motor soft start and soft stop through thyristor voltage regulation and does not have speed regulation function.

2. A frequency converter is a motor control (speed regulation) device that uses the on-off function of power semiconductor devices to convert the power frequency power supply to another frequency. Controlling motor operation through frequency conversion (voltage also varies with frequency, such as constant v/f) is a truly efficient speed regulation method with high efficiency. The frequency converter can achieve true soft start, soft stop, and efficient speed regulation.

3. The commonly used methods for pressure reduction starting are autotransformer pressure reduction starting and Y - Δ starting. The biggest advantage of autotransformer pressure reduction starting is that it has a large starting torque. When the winding tap is at 80%, the starting torque can reach 64% of the direct starting torque. And the starting torque can be adjusted through tapping. It is still widely used today.

Y - Δ is suitable for situations where there is no load or light load starting. And compared to any other pressure reducing starter, it has the simplest structure and the cheapest price. In addition, the star delta starting method has another advantage, which is that when the load is light, the motor can operate in a star connection. At this point, the rated torque can be matched with the load, which can improve the efficiency of the motor and thus save power consumption.