Asynchronous Motor

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jorge1 Active Contributor Join Date: Jul 2013 Posts: 15	Asynchronous Motor 01/27/2015 9:41 AM
	Hello there, I am checking the brochure from one company which produces motors. So, in the brochure they mention the following for their asynchronous motors: So, I don't really understand the graphic... When having induction motors there is only nominal speed, isnt it? Therefore, if you go above base speed (I guess nominal value), then you will turn it into a generator, unless the difference between base speed and max. elec speed is negligible, which I dont think is the case. Can anyone please explain me these curves? Thanks in advance!
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PWSlack Guru Join Date: Jan 2007 Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on. Posts: 32175 Good Answers: 839	#1 Re: Asynchronous motor 01/27/2015 10:07 AM
	Sure: As the motor accelerates away from rest and before it reaches its base speed, the torque that it is able to supply is limited to some maximum value. As the speed passes the base speed and rises towards the maximum electrical speed, the torque that it is able to supply falls with the reciprocal of the speed, the power being limited to some maximum value. As the speed passes the maximum electrical speed and rises towards the maximum mechanical speed, the torque that it is able to supply falls with the reciprocal of the square of the speed and the power it can supply falls with the reciprocal of the speed. Above the maximum mechanical speed, the motor can be expected to catastrophically self-disassemble in some way. OK? Shaft power is torque multiplied by speed. If the magnetic field in any machine in motoring is generated by the applied supply, then the motor cannot be used as a generator without additional electrical equipment being attached to it to sustain the magnetic field that is used for generation. __________________ "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
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RAMConsult Guru Join Date: May 2007 Location: NYC metropolitan area. Posts: 3230 Good Answers: 444	#2 Re: Asynchronous motor 01/27/2015 10:11 AM
	You have to look at more than "...the graphic..." and read the introduction which states: Translation, the motor is designed to mimic a DC motor of similar rating. __________________ “Tell me and I forget. Teach me and I remember. Involve me and I learn.” Ben Franklin.
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Dobi Member Join Date: Sep 2011 Posts: 5	#3 Re: Asynchronous Motor 01/28/2015 5:05 AM
	From stand-still (zero speed) until base speed, the variable speed drive's output voltage is increasing, in such a way that the relation output voltage over output frequency is kept constant. (V/hz = constant) This results in a constant output torque, and because output power equals to angular velocity times torque, the output power increases lineair. (P=T x N x 2 x pie/60 [W = Nm /s]) Remarque (1) : from zero speed until 1- 2% of the base speed, the output torque will sharply increase from zero until nominal torque, unless the motor is equipped with some sort of speed sensor. Once the variable speed drive reaches its maximum output voltage (mostly equal to thedrive's supply voltage(grid voltage) and the output frequency is still increasing (until Max elec speed), the relation output voltage over output frequency is no longer constant, but decreases. (V/Hz = decreasing) The magnetic field in the motor is decreasing and the generated torque in also decreasing. The torque is decreasing at a rate inversely proportional to the speed. Because output power is angular velocity times torque, and the torque decreases and the speed increases, the output power is constant. Remarque (2) : the slope of the curve T = 1/n will probably be much more sharp down wards.
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7anoter4 Guru Join Date: Oct 2012 Location: 100 miles North from the World Center Posts: 879 Good Answers: 42	#4 Re: Asynchronous Motor 02/09/2015 1:08 AM
	I think this sketch represents 3 [usually] possibilities of load Torque=function[n]. In the first if T=Trated=constant then the rated power will be at the steady operation point and here n=nrated[nN]. The second curve represents the case in which load power is constant. If Pload=~0 then the maximum velocity will be n=nsynchronous[approx.]=n1. The third curve it represents [usually for centrifugal pump or ventilator] the quadratic variation of the torque. In this case the extreme point will be the maximum torque-from this point down the motor loss the stability and goes to stop. If the torque is maximum the speed is minimum possibly. I think it has to be:"Max.mech.[power]speed" __________________ Julius
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