Introduction
Just to be clear from the beginning, a three-phase induction motor is self-starting. When the supply is connected to the stator of such a motor, a rotating magnetic field is produced. This causes the rotor to begin rotating, effectively starting the motor. However, it’s important to note that the starting current at this stage is very high.
The purpose of a starter isn’t just to start the motor. It actually serves two main functions: reducing the heavy starting current and providing overload and under-voltage protection.
In this blog post, we’ll discuss the most common induction motor starting methods, the applications for each, and which methods gives the best bang for buck—among many other things. So buckle up; it’s going to be a wild ride!
The starting methods we will cover are;
- Direct Online
- Star Delta
- Auto Transformer
- Soft Starter
- And Variable Speed Drive starting
Below, I have provided a table with these starting methods and some of the characteristics we will discuss. We will continue referring to this table in our discussion.
Current limitations on motor starting
As previously mentioned, one of the key functions of a motor starter is to limit the inrush currents in motors, thereby helping to maintain system stability and protect equipment.
Various rules of thumb based on the motor’s power rating can be applied to determine this maximum inrush current.
For three phase motors with a power rating of less than 1.5 kW, the starting current should be restricted to no more than 26 Amperes.
For those motors that fall within the range of 1.5 kW to 3.75 kW, the maximum starting current is determined by the following formula.
For three phase motors exceeding 3.75 kW, the maximum starting current is determined by the following formula.
And for a group of motors with total rating exceeding 3.75kW, the maximum starting current is determined by the following formula.
If in any of the cases mentioned above the starting current exceeds that suggested by the equations, then it’s recommended to use a less “stressful” starting method.
For example, if the calculations were based on Direct On Line (DOL), then consider using star-delta or another method depending on your requirements.
It’s worth noting that for single-phase motors, irrespective of their power rating, the starting current should never exceed 45A. If it does, then consider using a three-phase motor
Direct Online Starting
Direct-On-Line (DOL) is arguably the most straightforward method for starting an induction motor.
In this case, switchgear such as a contactor is the only starting equipment required.
While operationally simple, it’s important to note that this method results in a high inrush current, typically between 6 and 9 times the motor’s rated current.
Additionally, the instantaneous torque is quite high about three times the running torque and may result in high mechanical stress on couplings and the load being driven.
Even so, it is the preferred method except when there are special reasons for avoiding it.
DOL starting is sometimes used for small water pumps, compressors, fans, and conveyor belts.
DOL starters are typically employed in areas with a robust electrical grid.
Starting a large motor using DOL can cause voltage dips, which may inconvenience other users connected to the same line.
Typical power and control diagrams for a DOL starter are shown below.
A DOL starter has several pros and cons and these are detailed below.
Advantages
- The design, operation, and maintenance of this system are straightforward and user-friendly.
- This starter is cost-effective and budget-friendly
- Its design requires minimal room for installation.
- It provides 100% of the starting torque.
- Understanding and troubleshooting the system is easier.
Disadvantages
- High starting current: DOL starters apply the full voltage of the power supply directly to the motor, which can cause a high starting current. This can result in voltage drops and other problems in the power supply system.
- Mechanical stress: The high starting current can also cause mechanical stress on the motor, which can reduce its lifespan.
- Not suitable for high torque applications: DOL starters are not suitable for applications that require high starting torque, as the starting torque of the motor is limited by the available starting current.
- Not energy efficient: DOL starters are not energy efficient, as they draw a high starting current that can result in energy wastage. In contrast, other types of motor starters, such as soft starters or variable frequency drives, can provide a smoother and more energy-efficient starting current.
Star Delta Starting
A star-delta starter typically comprises three contactors, an overload relay, and a timer.
This starting method can only be used with motors that are delta connected during continuous operation.
The fundamental concept of a star-delta starter is that during the initial phase of acceleration, the motor windings are connected in a star configuration, resulting in reduced current.
After a predetermined time, the configuration shifts to delta, providing full current and consequently, full torque.
In the delta configuration, the voltage across each motor winding is equal to the network voltage.
The motor current is divided between two parallel windings, reduced by a factor of 1/√3 compared to the line current.
If the impedance in each motor winding is Z, then the combined impedance for the parallel windings is Z/√3.
When the motor is star-connected (Y-connected), the windings are in series. The resulting impedance becomes √3Z, which is three times the impedance when delta connected (since ((√3Z)/(Z/√3) = 3)).
With the voltage level remaining constant, the current in a Y-connection will be one-third of that in a delta connection.
Therefore, using a star-delta start, the star configuration results in a current of 33% compared to a delta-connected motor.
As the main voltage remains constant, the motor perceives the star connection as a voltage reduction, as the voltage across each winding is 1/√3 of the main voltage. This reduced voltage also leads to a decrease in torque.
The torque is diminished to the square of the voltage reduction, [(1/√3) * (1/√3) ≈ 0.33], resulting in about 33% of the torque available when delta connected.
However, this is a theoretical value. A more accurate figure is 25%, considering additional losses and efficiency factors relevant when star connected.
This approach is effective for an unloaded or lightly loaded start, but it may not be suitable for starting heavier applications
There are several advantages and disadvantages to using a star delta starter as opposed to using for example an autotransformer and we will go to through them now.
Advantages
- Star-Delta starters are widely used due to their relatively low price.
- There are no limits to the number of times they can be operated.
- The components require very little space.
- The starting current is reduced to approximately one-third
Disadvantages
- The starter can only be applied to motors where the six leads or terminals can be accessed.
- The supply voltage must be the same as the rated motor voltage for Delta connection.
- Because the starting current is reduced to approximately one-third of the rated current, the starting torque is also reduced to one-third.
- If the motor does not reach at least 90% of its rated speed at the time of switching from Star to Delta the current peak will be as high as in a D.O.L. start, thus causing harmful effects to the contacts of the contactors and the connection system brings no advantage to the electrical system.
Auto-transformer starting
In a motor starting connection, an autotransformer is used to reduce the voltage applied to the motor during startup.
This reduced voltage is supplied to the motor until it reaches its maximum expected running speed.
Once this speed is achieved, the motor is reconnected to the full voltage line from the mains.
The autotransformer utilized in this process has a single copper winding that functions as both the input and the output.
An autotransformer is characterized by a significant number of taps that act as outputs.
Typically, these output taps are connected to provide a range of voltages from a minimum of 50% to a maximum of 80% of the full voltage.
These taps allow for different voltage levels to be used for motor starting.
The operation of an autotransformer is simple and economical, offering a lower weight density compared to other types of transformers.
However, an autotransformer starter requires complex cabling because it has three terminals that need to be connected to the motor.
The primary purpose of an autotransformer motor starter is to reduce the initial starting current of the electric motor.
This reduction is proportional to the square of the transformer’s voltage ratio.
Advantage
- On the 65% tapping the line current is approximately equal tp that of a Star-Delta starter, however, at the time of switching from reduced voltage to the full supply voltage, the motor is not disconnected so that the second peak is very much reduced since the transformer is converted into reactance for a short time.
- It is possible to vary the tapping from 65% to 80% or even up to 90% of the supply voltage in order to ensure that the motor starts satisfactorily.
Disadvantage
- One of its great disadvantages is the limitation of its operation frequency. It is always necessary to know the operation frequency in order to determine a suitably rated auto-transformer.
- This starter is much more expensive than a Star-Delta starter due to the auto-transformer.
- Due to its size, much larger control panels are required which increases the price.
Variable Speed Drive Starting
While there are several variations in Variable Speed Drive (VSD) design, they all provide the same fundamental functionality: converting the incoming electrical supply of fixed frequency and voltage into a variable frequency and voltage output.
This output is then delivered to the motor, resulting in corresponding changes in motor speed and torque. The motor speed can be varied from zero RPM to typically 100-120% of its full rated speed, while achieving up to 150% of the rated torque at reduced speeds.
The motor may be controlled in either direction. VSDs applied to AC induction motors are the most common. Referring to the diagram below, their basic design consists of four elements:
- Rectifier: This component converts the incoming alternating current (AC) supply to direct current (DC). Various designs are available, chosen based on the VSD’s required performance. The rectifier’s design influences the degree of electrical harmonics induced on the incoming supply and can also control the direction of power flow.
- Intermediate Circuit: The rectified DC supply is conditioned in the intermediate circuit, typically using a combination of inductors and capacitors. Most drives in the current market use a fixed-voltage DC link.
- Inverter: This converts the rectified and conditioned DC back into an AC supply of variable frequency and voltage. This is usually achieved by generating a high-frequency pulse width modulated signal of variable frequency and effective voltage. Semiconductor switches, commonly Insulated Gate Bipolar Transistors (IGBT), are used to create the output.
- Control Unit: This unit oversees the entire operation of the VSD. It monitors and controls the rectifier, intermediate circuit, and inverter to deliver the correct output in response to an external control signal.
VSDs are typically 92-98% efficient, with 2-8% losses due to additional heat dissipation from the high-frequency electrical switching and the power required by the electronic components.
Similarly, motors connected to VSDs may experience additional losses due to heating caused by the high-frequency electrical switching.
Typical power and control diagrams for a VSD starter are shown below.
Soft starting
A softstarter, unlike a drive, does not alter the frequency or speed. Instead, it gradually increases the voltage applied to the motor, starting from an initial low voltage and ramping up to the full voltage.
Initially, the voltage supplied to the motor is so low that it only serves to adjust the play between gear wheels or stretch driving belts, etc., thereby preventing sudden jerks during startup.
Gradually, as the voltage and torque increase, the machinery begins to accelerate. One advantage of this starting method is the ability to finely tune the torque to meet specific needs, regardless of whether the application is loaded or not.
Using a softstarter reduces the starting current, which helps avoid voltage drops in the electrical network. It also lowers the starting torque and mechanical stress on the equipment, leading to a reduced need for service and maintenance.
Similar to a drive, a softstarter can facilitate a soft stop. This function is especially beneficial in preventing water hammering and pressure surges in pumping systems, and it helps avoid damage to fragile materials on conveyor belts. Typical wiring diagram for a soft starter are shown below.
💭 Deciding between a Variable Speed Drive and a Soft Starter? It's a complex decision that depends on the specific application, system requirements, and budget considerations. Let's dissect these factors. #SparkyCalc
Which one to use, VSD or Softstarter?
VSDs and soft starters are quite similar, and it can sometimes be confusing to decide which one to use.
The choice between a VSD or a soft-starter depends on the application type, the mechanical system’s requirements, and the costs of installation and maintenance.
Soft-starters, usually the most cost-effective option for applications requiring a soft start, protect the motor and load from mechanical shocks by controlling motor voltage.
This reduces the starting current and enables gradual acceleration to rated speed.
Conversely, VSDs control both torque and speed.
In summary, while soft-starters manage voltage and/or current during start/stop phases without affecting load speed, VSDs control the start/stop ramp and continuously manage speed and torque.
Rounding Up
In conclusion, understanding the various starting methods for AC induction motors is crucial for optimal motor performance and longevity. From Direct Online to Variable Speed Drives, each method offers unique benefits and limitations.
This exploration helps in making informed decisions based on specific application needs, balancing factors such as starting torque, current reduction, and mechanical stress.
The choice of a starting method significantly impacts the efficiency, cost, and maintenance requirements of motor operations, underscoring the importance of careful selection in electrical engineering applications.
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