Scheme of controlling resistance in rotor circuit - Page 1
Forum section: electronics / general

author: nForce
author_rank: Frequent Contributor
timestamp: January 03, 2018, 03:44:01 pm
content: Hello, does any one understand this circuit:AM stands for Asynhronous motor (Induction motor), there I think is a rectifier. Rectifier is connected with rotor 3-phase winding. Why is there an inductor, and that standard resistor. Parallel to the reisistor Rd is connected a thyristor.Thank you.

author: duak
author_rank: Super Contributor
timestamp: January 03, 2018, 09:21:37 pm
content: I wonder if it is either a repulsion start motor or a synchronous motor where the extra circuitry is used to control the starting characteristics.Is there any other information available for this motor? Are there any slip rings to bring the rotor winding or circuitry out to terminals?Cheers,

author: Brutte
author_rank: Frequent Contributor
timestamp: January 03, 2018, 09:53:22 pm
content: I suppose this is a symbol ofslip-ring asynchronous induction motor.The thingy on the picture substitutes the starting resistor in some way. Perhaps just limits rpms (in a lossy way)

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 03, 2018, 10:03:49 pm
content: Before the widespread availability of Variable Frequency Drives for AC induction motors, a method of varying an induction motor's speed was by adjusting its slip.Slip (and torque I might add) vary with rotor resistance.Of course with a squirrel cage rotor it is impossible to vary the rotor resistance. The way to do it is wind the rotor, and via slip rings connect it to variable external power resistors.These motors were widely used back then in heavy duty hoists and winches.Your schematic appears to be a SCR which mimics those very large external resistors.

author: Circlotron
author_rank: Super Contributor
timestamp: January 03, 2018, 10:32:03 pm
content: I worked on this exact scheme back in 85 & 86. The inductor and scr form a boost converter. If the scr is not chopping on and off the resistor appears as it’s normal value. As the scr starts chopping at perhaps several hundred Hz, the greater the “on” percentage of the duty cycle, the lower the resistor value appears to the motor because the resistor voltage is boosted so it draws more current. So you can present a variable resistance to the rotor windings. For any given motor speed and therefore slip frequency there is an ideal resistor value that gives maximum torque and (I think) minimum current from the mains. This is when the external resistor ohms equals the rotor reactance ohms. Then the rotor current lags the rotor voltage by 45 degrees. Only used for starting, very wasteful for speed control because lots of power gets dumped in the resistor. Long time ago - getting a bit fuzzy.

author: duak
author_rank: Super Contributor
timestamp: January 04, 2018, 12:13:23 am
content: I found a link to a description:http://www.myelectrical2015.com/2017/08/speed-control-of-three-phase-induction.htmlCheers,

author: nForce
author_rank: Frequent Contributor
timestamp: January 04, 2018, 05:22:31 pm
content: So chopper is a device which uses PWM method?

author: Circlotron
author_rank: Super Contributor
timestamp: January 04, 2018, 11:56:46 pm
content: Quote from: nForce on January 04, 2018, 05:22:31 pmYes. Old technology but very interesting.https://www.google.com.au/search?rlz=1C9BKJA_enAU745AU745&hl=en-GB&ei=0L5OWrStO8SQ8gXEwr34CQ&q=jones+chopper&oq=jones+chopper&gs_l=mobile-gws-serp.3..0l5.5034.7393.0.9094.9.9.0.3.3.0.777.3701.2-1j5j2j0j1.9.0....0...1c.1.64.mobile-gws-serp..2.7.2050...0i46i67k1j46i67k1j0i67k1.0.XDFi8dP2IaY

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 05, 2018, 09:39:55 pm
content: Circlotron:I remember those know! I didn't actually worked on one of them, but actually saw andheardone.SCRs are slow. Even "inverter grade" SCRs could switch no faster than the single-digit kilohertz range. Clearly in the audible range.