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Motor starters


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#1 porky

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Posted 24 June 2005 - 05:50 PM

Please advise on why some motors are limited to several starts per hour....

Is it due to fear of Overheating the windings due to high inrush current or is it ti prevent mechanical damage to motor and or pumps.

#2 marke

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Posted 24 June 2005 - 06:54 PM

Hi porky

There is a thermal limit on the motor that determines the life expectancy.
During start, the kinetic energy of the load at full speed is disipated in the rotor, plus there is a high dissipation in the stator. The temperature rise in the rotor is dependant on the load kinetic energy at full speed. The end temperature of the rotor is dependent on the prestart temperature and the temperature rise during start. It takes a long time for the rotor to cool, so if you start too regularly, you will over heat the rotor. If you start a machine with too much inertia, you will also overheat the rotor.

Strictly speaking, the starts per hour should be quoted at a particular load energy. If you start a lighter duty load, you should be able to start more often. Additionally, the end temperature is going to be affected by the ambient temperature, so there is more thermal capacity if you are starting with an ambient temperature of 0 degress than if you are starting with and ambient temperature of 50 degrees.

Best regards,

#3 porky

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Posted 25 June 2005 - 10:13 AM

Just a quick thanks for the info...

#4 saleem

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Posted 20 July 2005 - 09:41 AM

explain the physical phenmenom in rotar circuit during starting.

#5 marke

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Posted 20 July 2005 - 10:30 AM

Hello saleem

I am not sure what you are looking for. There is no change in the rotor during start, there is power dissipated in the rotor bars and this causes the rotor bars to heat up.

Can you please explain your question in more detail.
Best regards,




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