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subrao
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Joined: 16-September 07
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Last Seen: 28th August 2010 - 03:06 PM
Local Time: Sep 3 2010, 09:00 PM
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10 Aug 2010
now a days pm ac syn. motors are increasingly used and replacing induction motors.
i have some difficulty in understanding how the motor is started. a conventional synchronous motor is started with the help of damper windings or by driving it with an external motor and exciting the field winding when it attains near synchronous speed. How such thing can be achieved in case of a pm motor as field is permanently produced and there are no damper windings . even damper windings are provided with field on it will not start. subrao
28 Aug 2009
I am told that a vector controlled vfd maintains power factor close to 0.95 even when the motor is loaded lightly because it computes load and reduces excitation as required. this is achieved even when vfd is operated in regenerative mode.
I wonder if it is true ,if so how?
6 Jul 2009
1.many of vfd suppliers as well as motor suppliers claim the following feature for constant hp loads above rated
synchronous speed Up to 60hz voltage is increased linearly with frequncy so that vfd-motor develop constant torque and at 60hz rated hp as well. Above the 60hz the voltage is clamped at rated value and the system can deliver constant hp above rated speed. Unlike a dc motor in an ac motor torque and hence hp varies as square of voltage at a given frequncy. So a 60hz motor if operated at constant volts above 60hz gives torque inverslyvarying with square of speed and hp inversly with speed. out of above two propositions which is correct?
13 Feb 2009
I am commisioning an elevator. Drive used is yaskwa F7 and the motor is 380V,50hz,380V rated 4.2Kw at 50hz. Rated current
of the motor is 8.8A A. I fed rated current of 8.8A as given in the name plate. I struggled but the motor does not start when there were 8 passengers. I changed the motor currnt parameter as 10.5A though name plate current is 8,8 A .Elevator started working. I wonder how and why? I got another doubt If I replace the motor with higher capacity whose rated current is really 10.5 A will it work? which is bettor choice 8.8A motor with feeding of 10.5A or 10.5A motor? What is the mistery behind this?
23 May 2008
what is an ac pm motor? Is it a synchronous motor or induction motor? does it run at constant speed at a given frequency or has a slip?what is its construction like? what are its operating principles?
What are its advantages over conventional ac motors? I understand these motors are increasingly becomming popular even in higher kw ratings |
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Lo-Fi Version | Time is now: 3rd September 2010 - 08:00 PM |
22 Jul 2009 - 13:07