Rotoric Starter

[maypot]

Hi,

I need to commission one 250 k W 400 V slip ring motor . The motor was delivered on site with its rotoric starter,a multi-tapped resistance in transfromer oil. However ,some resistances appear to be opened.
The rotor is rated 638 V 238 A . I have one rotoric starter, a liquid one , rated at 515 V and 300 A and would very much like to use it instead. The current rating of the starter do not seem a problem. But could the 638 V rotor voltage be an insulation problem for the 515 V liquid starter ?

Bob

 

[Marke]

Hello maypot

The 638 volts quoted for the rotor voltage is the open circuit rotor voltage and is used to calculate the resistance required in order to design a suitable starter for that machine.
Unless you operate either open circuit or with a very high value of resistance in the rotor circuit, you should never get anywhere near that sort of voltage.

The important issues with rotor resistance starters is that the torque curve is modified by the value of ther resistance. If the final stage resistance is too high, the torque peak will occur at too low a speed and this will result in a high current when this resistance is bridged out.
In your case, your motor has a higher voltage and lower current than the starter that you are wanting to use. The major issue with this is that the initial resistance may be too low and you may find that your initial start current at zero is higher than desirable.
Other than that, in my opinion, you should not have a problem, depending on the load that you are starting.
Remeber, that the secondary starter has to absorb the full speed kinietic energy of the load during start. The starter must, in addition to providing the right resistance values, have sufficient thermal mass to absorb the energy without causing problems.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[maypot]

Hi Marke,

Thank you very much for diligent and intelligent reply.I understand the fact that the added resistance in the rotor circuit is used to adjust the torque/speed characteristic of the motor.
Besides do a liquid starter differ from a standard multi-tapped resitance one.

Rgds.

Bob

 

[Marke]

Hi Bob

Liquid resistance starters can come in a number of different formats.
Some have adjustable probes or levels to control the resistance, while others rely on the fact that as the liquid heats up, it's resistance drops. Some use a caustic solution, and others use a saline solution. The answer really depends on the actual unit that you have.

Best regards,
Mark.


Mark Empson

http://www.lmphotonics.com