Alternative To Liquid Starter

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LMPForum > Motor Control > Soft Starters

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Alternative To Liquid Starter, Soft start

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bob View Member Profile	May 27 2008, 07:27 AM Post #1
Member Group: Full Member Posts: 187 Joined: 16-August 05 Member No.: 1,186	Hi, I understand that by using a soft start on a slip ring motor, the lock rotor torque is very poor and this is not a convenient way to start high inertia load even by adding extra resistance to the rotor circuit. I have heard that some soft starters are able to produce 400-500 % starting torque. How far this is true ? Regards. Bob

marke View Member Profile	May 27 2008, 06:46 PM Post #2
Posting Freak Group: Moderator Posts: 2,336 Joined: 24-April 02 From: Christchurch, New Zealand Member No.: 1	Hello Bob The major advantage of the secondary resistance starter, is that you can alter the slip of maximum torque by altering the rorotr resistance. By doing so in a number of steps, you can achieve a high torque from zero speed to full speed at a relatively low starting torque. With sufficient well placed steps, you can have in the order of 200% torque at less than 250% current right through to full speed. The only other way that you can achieve this, is by using a shorted rotor and an appropriate variable frequency drive. Note: Most drives will not produce the high torque at less than 5 - 10 Hz! If you use a soft starter and a single stage of resistance, you essentially make the slip ring motor emulate a standard cage motor and so you have a high start current for a nominal start torque, for example 400% current and perhaps 100% torque. I do not know of any way that a soft starter can give you 400% start torque. This is higher than the motor will produce DOL or with a secondary resistance starter. Best regards, -------------------- Mark Empson administrator Skype Contact = markempson \| phone +64 274 363 067 LMPForum \| LMP Tech Warehouse \| L M Photonics Ltd \| Empson family \| Advanced Motor Control Ltd \| LMP Software \| Mark Empson Website

bob View Member Profile	May 29 2008, 09:40 AM Post #3
Member Group: Full Member Posts: 187 Joined: 16-August 05 Member No.: 1,186	Thanks Mark. This subject is very well explained in the soft starter topic on your site. Kind regards. Bob

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