shermaine Posted March 25, 2004 Report Share Posted March 25, 2004 Any one can advise me on that.(1) Why is the no-load input current and power as a percentage of their full(rated)values recorded are considerablyhigher than those of the transformer? (2) Estimate the starting current of the motor(as a percentage of its rated value) when it is started at rated voltage.How can the starting current be reduced to an acceptable value in practice. Link to comment Share on other sites More sharing options...
marke Posted March 25, 2004 Report Share Posted March 25, 2004 Hello Shermaine Although there are similarities between the equivilent circuit and a motor, the two are not the same.1) The induction motor behaves a little bit like a transformer, but the secondary is short circuited. The frequency of the current flowing in the rotor is proportional to the rotor slip. This greatly influences tha current and power flowing into the motor under all conditions. An induction motor, even under no load conditions, has rotational losses, frictional and windage as well as rotor losses. These losses do not occur in a transformer. 2) When an induction motor is started at full voltage, it will initially draw locked rotor current. This high current will slowly reduce as the motor accelerates, but will only reduce significantly when the motor is approaching full speed. The actual current and the speed/current curve of the motor are a function of the motor design.The Locked rotor current is typically between 550% and 900% of the rated cuirrent of the motor. Best regards, Mark Empson | administratorSkype Contact = markempson | phone +64 274 363 067LMPForum | Power Factor | L M Photonics Ltd | Empson family | Advanced Motor Control Ltd | Pressure Transducers | Smart Relay | GSM Control | Mark Empson Website | AuCom | Soft Starters Link to comment Share on other sites More sharing options...
shermaine Posted March 29, 2004 Author Report Share Posted March 29, 2004 For No-Load Testing Any one knows why the no-load current and power of the motor expressed as percentage of their full-load values is considerably higher than those of a transformer? For Blocked Rotor Test Can the starting current be reduced to an acceptable value in practice? Are there many methods? Pls advise. Thanks Link to comment Share on other sites More sharing options...
marke Posted March 29, 2004 Report Share Posted March 29, 2004 Hello shermaineAs referred to previously, the induction motor is like a transformer with a shorted circuited secondary. I also has additional losses that do not occur with a transformer, hence the current is higher. 2 The Locked rotor current can be reduced by reducing the applied voltage. This is done in industry and is referred to as reduced voltage starting. If you look on the web pages for this site you will find examples of reduced voltage starters and their characteristics. Best regards, Mark Empson | administratorSkype Contact = markempson | phone +64 274 363 067LMPForum | Power Factor | L M Photonics Ltd | Empson family | Advanced Motor Control Ltd | Pressure Transducers | Smart Relay | GSM Control | Mark Empson Website | AuCom | Soft Starters Link to comment Share on other sites More sharing options...
shermaine Posted March 30, 2004 Author Report Share Posted March 30, 2004 Hello, Any great website to recommend in reducing the starting current of the motor? I can't seem to find more detail information like how it is been done to reduce the starting current?Pls advise.Thanks. Link to comment Share on other sites More sharing options...
marke Posted March 30, 2004 Report Share Posted March 30, 2004 Hello Shermaine You could try this website for a start. There is a page on motor starters showin how they operate and reduce start current. http://www.lmphotonics.com/m_start.htmThere is also a page on soft starters. Best regards, Mark Empson | administratorSkype Contact = markempson | phone +64 274 363 067LMPForum | Power Factor | L M Photonics Ltd | Empson family | Advanced Motor Control Ltd | Pressure Transducers | Smart Relay | GSM Control | Mark Empson Website | AuCom | Soft Starters Link to comment Share on other sites More sharing options...
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