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subrao
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Joined: 16-September 07
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Last Seen: 6th March 2010 - 05:12 AM
Local Time: Mar 18 2010, 04:08 AM
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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
21 Dec 2007
I am somewhat mixed up with use of VFD drives in case of double cage motors.
1. In case of some VFDs we are required to feed equivalent circuit parameters of the motor. In case of a double cage motor rotor resistance and reactance are different at starting and running. These values are usually given at slip =1 and at slip =0. The question is which values to feed? To my mind it appears that because with VFD, slip =1 condition is not encountered we should consider values at slip =0. Am I correct? 2. A double cage motor is used to have high rotor resistance at start and low rotor resistance at running.This incidentally gives more rotor reactance at running than at start giving some what lower power factor. When we use VFD, slip =1 is never encountered as frequency is adjusted as per speed. So why to go in for a double cage motor for VFD drives ? It may unnecessarily reduce power factor of motor. Is my analysis on right track? |
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Lo-Fi Version | Time is now: 18th March 2010 - 04:08 AM |
22 Jul 2009 - 13:07