Starting three-phase asynchronous motors



This article deals with the starting processes of three-phase asynchronous motors, the connection of the motor, the advantages and disadvantages of each process.
Apart from direct starting, the basic objective of the various starting processes is to limit the absorbed intensity while maintaining the mechanical performance of the " motor-driven machine "assembly in accordance with the specifications.
The processes that we will deal with are as follows:
  • Direct start
  • Start star-triangle
  • Start by stator resistors
  • Starting with rotor resistors
  • Start with electronic starter

Direct start :


To achieve a correct engine start, the following functions must be performed :

Isolating is the role of the dis-connector.
Protect the power from short circuits, for this purpose fuse cartridges are used .
Controlling the flow of energy to the motor is the role of the contractor.
Protect the motor from overloads, function provided by the thermal relay or (a motor circuit breaker).
Diagram :

Curve :

Benefit:

  • Simplicity of the apparatus.
  • Important Couple.
  • Short start-up time.

Disadvantage:

  • Important current call.
  • Brutal start.

Direct start with 2 direction of rotation:

To change the direction of rotation of a three-phase asynchronous motor, it is necessary to reverse two of the three phases of the supply circuit (L1 and L2 or L1 and L3 or L2 and L3).

Diagram:

Start star-triangle:

The principle of Star start triangle is to underfeed the engine for almost the entire duration of the start by coupling it to star.
It is therefore necessary to use a motor normally coupled in a triangle and all the winding ends are output on the terminal plate.
Example: on a 230/400 V network it is therefore necessary to use a 400/660 V motor.
Using triangle Star start: vacuum starting Machine :

Diagram:

Curve :

It is found that the torque and intensity at startup are reduced by about 3 times compared to a direct start.
Due to the significant decrease in starting torque the motor cannot start under load.
In there is power cut between the star and triangle positions.

Benefit :

  • Reduction of starting current
  • Relatively cheap

Disadvantage :

  • very low torque
  • Power cut-off during Star-triangle switching
  • Start-up time + high

Start by stator resistors

The principle of stator start-up, such as the triangle Star start-up, is to underfeed the engine for almost the entire duration of the start-up by putting it in series with resistors.
Use of stator start :it is suitable for machines whose starting torque is lower than Cn (nominal torque). Ex: woodworking machine fan …

Diagram:

Curve :

The advantage in this boot mode is that there is no power outage during boot.
The torque characteristic is substantially identical to that obtained with a Star start triangle. On the other hand, the current at start-up remains high.

Benefit :

  • Possibility to choose the starting torque
  • Selecting the starting current accurately
  • Switching between start-up phases without power interruption

Disadvantage :

  • If the current is divided by 3 then the torque is divided by 9 !

Starting with rotor resistors

The principle of the rotor start is to limit the rotoriccirculating currents in the armature.
As the motor behaves like a transformer, the line Current will also be limited.
For this starter, it is imperative a motor with coil rotor.
It is generally used for machines of Power > 100 kW. For example: Rotary piston compressors, pumps…

Diagram :

With the starter selected in the example below, the engine starts in three strokes. There are three contactors: KM1: the line contactor, KM11 and KM12 (which short-circuit the two sets of three rotor resistors).

Curve  :

The current absorbed is substantially proportional to the torque supplied or very little higher.
Ex: for a starting torque Cd = 2.5 Cn.
The intensity will be substantially 2 in Id< 2.5 in.
Starting pairs equal to 2.5 times Cn are frequently obtained without excessive overcurrent.
One can still reduce overcurrent by increasing the number of starts. Cd < 2,5 Cn.

Benefit:

  • The current call is for a given starting torque the lowest compared to all other starting modes.
  • Possibility to choose by construction, torque and the number of start-up times.

Disadvantage:

  • Need for a coiled rotor motor.
  • More expensive equipment.

Start with electronic starter

The start-up is done gradually, which avoids the current spike in direct start-up.
The voltage changes from 0 to one (the nominal voltage) according to a programmable ramp.
The start is done at constant current and the ratios t/Tn = (U/A)2 = I / in are
check.

Diagram :

Benefit :

  • Smooth start
  • Current Limitation.

Disadvantage :

  • Higher price