3 Phase motor starter

[gravi]

Hi,

I am interested in understanding how 3 phase starter works, According to my understanding one 3 pole contactor and switch to control contactor coil would be sufficient to start or stop a 3 Phase Motor. Am I correct ? if so why don't we use 3 pole MCB to switch ON/OFF a 3 phase motor.

Thanking You,
Ravi

 

[ArNvC]

Any 3 phase starter's primary job is to reduce the very high starting current (due to its obvious consequences), also sometimes direct starting online can produce unnecessary high starting torque, even when not required by the load, thereby increased mechanical stress on the mechanical systems such as rotor shaft, bearings, gearbox, coupling, chain drive, connected equipments, etc. leading to premature failure and plant downtimes...
To do away with the limitations, motor starters are used.

Now, 3 phase motor (presuming Induction motor) can have different starters (Star-delta, Autotransformer, etc) which you need to study from a book about their detailed principle and working, but all these do the same job of limiting the starting current of the m/c.

Well, 3 pole and even 4 pole MCB's are there but it will still be directly on line. So you need some kind of assisted starting.

 

[gravi]

Dear Arunava,

Thank you, I do accept with you. I hope DOL Starters can be replaced with MCB which could even trip during over loads, but need to switched Manually and switched Off manually where as DOL starters switchs off during power failure.

Thanking You,
Ravi

 

[FvM]

You'll notice that motor circuit breakers are listed as "manual motor starters" in many catalogs. This should already answer your question.

 

[gravi]

Hi,

Can we use capacitor on each phase and switching them OFF after 5 secs i.e as soon as motor is started, using micro controllers to minimize start up currents.

Thanking You,
Ravi

 

[FvM]

That's an unusual method. Power resistors would be a better idea.

 

[ArNvC]

I agree, that instead of capacitors, power resistors would be better. Resistors connected to the stator of the motor increase the overall impedance of the circuit and limit the starting current. In effect the voltage drop across the resistors gives a reduced voltage to the motor terminals.

 

[hareeshravi]

Hi,

There are some electrical and mechanical parameters to be looked into before selecting a starter.

- Voltage drop at the panel should not drop below 85% of the rated voltage, as it will affect other systems.
- The drop in voltage (which reduces motor torque) should not reduce the motor torque less than the required driven load torque.
- Heavy starting torque should be within mechanical drive-train capability.

An overview of starter properties:
1. DOL - Starting torque depends on voltage drop, usualy moderate. Suitable for loads for which torque requrement increases with speed.
Heavy starting current.

2. Star-delta - Moderate starting current and torque.

3. Electronic V/F controlled : High starting torques can be generated. Very low starting current.
Rated torque can be achieved at any speed desired (less than rated)


So depending on the situation, we have to select an appropriate starter. For eg;

1. In a fire pump, which may be located away from other loads, we can use DOL.
2. For a conveyor motor, starting torque is very high. Smooth starting is preferred. Electronic Soft starters (V/F control) may be used.
3. For a A/C plant compressor, reducing high starting current is usually required. Star-Delta starter can be used.
4. For platform operations, Electronic soft starters or Slip-ring motors with additional starting rotor resistances can be introduced.
5. For small motors,

While selecting, obviously cost actor comes into consideration, for which DOL < Star-delta < Soft starter

regards,
hareesh

 

[Tipu606]

without mentioning the HP(horse power) rating of the 3 phase motor, it is useless to discuss motor starters

 

[hareeshravi]

yes you are completely right.
But it eventually converges to limitations due to Voltage drop / torque .

 

[gravi]

Dear Tipu,

Starter for 5 HP Motor.

Thanking You,
Ravi