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Lesson Six : Starting methods

Starting methods

When starting a motor it’s possible to do so in several ways
  1. Direct on line start
  2. Star delta connection start.
  3. Transformer start
  4. Soft starter
  5. Variable speed drive start.

6.1Direct on line start

When starting a motor direct on line the electrical networks voltage is directly going to the motor
It is the simplest starting method, however the starting current can be as high as 8 times the nominal current as the motor requires additional current while the rotor needs to be magnetized while starting up.
In some scenarios one might want to have a lower starting current and that can be achieved with the use of the other starting methods.

Figure 14. Direct on line start graph
In Figure 14, the dotted line shows the current and the filled line shows the torque. Seeing the starting current is seven times the nominal current for an example motor with direct on line starting method, but as the motor picks up speed the need of magnetizing current decreases down to the nominal current.
At  the  axes  of  the  graph  is  Stands  for  starting  torque, the starting torque is the torque that the motor shaft is able to deliver at zero speed while  Tn
 or nominal torque is the torque the shaft deliver at nominal speed in order to achieve the rated power of the motor.
During the ramp-up of speed, the value of the torque changes. Depending on the rotor slot design and materials used the form of this curve can also vary.
Ts Or starting torque as explained above. In addition to this, there is also minimum torque or pull up torque, meaning the point on the starting torque curve with the lowest value. This point is usually the factor determining if the motor will be able to start when the load from the application is applied.
Also we have the maximum torque or break down torque. This is the highest point on the starting torque curve. As the motor is in continual running you can overload it to the limit of the maximum torque before it stops. This would only be possible for a short time in practice as it will make the motor heat too much if overloaded during a longer time.
Is stands for starting current and is the number of ampere drawn by the motor from the network at the time of the start before the shaft has started to turn. In or the nominal current is then the current drawn from the network at full speed when loaded at its rated power. The starting current curve falls as the speed of the motor is picking up, this is due to the rotor finding its place in the rotating magnetic field in other words the motor is decreasing its slip. During the start the motor needs a lot of current to magnetize the rotor but as the speed picks up the need of magnetizing current decreases.

6.2 Star delta connection start

In some scenarios one might want to have a lower starting current and that can be achieved with the use of a star delta start.
The star delta starter uses the star connection of the motor windings when starting and after a while it switches to delta connection, typically when the full speed has been achieved. This can reduce the starting current so it’s only one third compared to the previous starting current.
Since it gives a lower starting current it also means it gives a lower starting torque which can be useful in some applications, but just as well could it make starting impossible in others so this needs to be checked prior to using such arrangements.
Figure 15 is a graph to compare starting current and starting torque for star delta start and Direct on line start.
The blue curves are for the Direct on line start and the red curves are for the start delta start. The dotted lines represent starting current and filled linesrepresents starting torque. As noticed there is a lower starting current and starting torque with star delta start compared to direct on line start

Figure 15. Star delta connection start graph

6.3Transformer start

With a transformer start, a device called a transformer is used. With the use of the transformer the voltage to the motor can be reduced and thus reducing the starting current and starting torque. It is then possible to switch back to the nominal voltage whenever it’s desired.
The difference between star delta start and transformer start is that the first method is locked to give one third of the Direct on line starting current, while when using transformer start the current depends on the output voltage of the transformer.
Figure 16 shows a graph to compare starting current and starting torque for a transformer start at 80% voltage and Direct on line start. The blue curve is for the Direct on line start and the red curves are for the transformer start. Dotted lines represent starting current and filled lines represents starting torque.
As noticed there is a lower starting current and starting torque with transformer start compared to direct on line start. This means that the transformer start can be more flexible compared to the star delta start.
Figure 16. Transformer start graph

6.4Soft start

A soft starter is a device that can adjust the voltage to the motor to achieve the wanted starting behavior. So if the motor have a low load, the soft starter can feed low voltage to the motor, if the motor have a higher load, the soft starter simply feed more voltage to the motor. With the soft starter, the motor can run with lower the voltage at the start and steadily increase the voltage over time.
The difference between a transformer start and a soft starter is that the transformer switches the voltage between two values while the soft starter goes from zero to nominal voltage over a span time that can be adjusted.

6.5Variable speed drive start

A Variable speed drive is a device that can change both frequency and voltage used by the motor. So like a soft starter but instead of adjusting the voltage only, it also has the possibility to adjust the frequency of the current at the start.
As the variable speed drive can adjust both voltage and frequency it means that it can keep the electromagnet flux inside the motor constant, so it does not lose any of the motor torque during the start. This means that it can use the nominal torque of the motor already from zero speed and at the same time it can limit the starting current of the motor to its nominal value.

This makes the variable speed drive the most versatile of all the starting methods. It also gives an additional feature to control the rotation speed of the motor. (Electrical4u)

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