Rotor induced voltage frequency with wound rotor slip ring motor - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: August 22, 2023, 03:26:29 am
content: Say we have a 2-pole wound rotor induction motor. Apply 50Hz to the stator and if the rotor is not moving we would expect to be able to measure a 50Hz voltage at the slip rings because the vector direction of the stator magnetism is rotating at 50 revs per second. Now we pull that rotor out and put it in a 4-pole motor and power up the stator, again with 50Hz. Seeing the stator magnetism is now going at 25 rps would we not see 25Hz on the slip rings? Or 16-2/3Hz if it was a 6-pole motor? I'm sure frequency division can't be that easy.

author: moffy
author_rank: Super Contributor
timestamp: August 22, 2023, 05:25:34 am
content: Quote from: Circlotron on August 22, 2023, 03:26:29 amSorry I am slightly confused as induction motors don't have slip rings but DC motors do.

author: Circlotron
author_rank: Super Contributor
timestamp: August 22, 2023, 06:01:29 am
content: Quote from: moffy on August 22, 2023, 05:25:34 amhttps://www.pthglobal.com.au/what-is-a-wound-round-motor

author: moffy
author_rank: Super Contributor
timestamp: August 22, 2023, 06:04:42 am
content: Thanks for the clarification, I had never heard of slip ring induction motors before, just the basic squirrel cage.

author: jonpaul
author_rank: Super Contributor
timestamp: August 23, 2023, 01:28:32 am
content: see original Nikola Tesla (NOT THE CAR!!!) induction motor patetns and designs of 1880sMany types exist, most small fraction HP are indeed squrrel cage shaded pole.Large machines are slip ring.The difference betwee synchronous RPM (#polesx mains frwq x 60, eg 1800 RPM) and actual RPM is called slip and is a nonlinear function of torque, approaching zero at no lod.See any text on "Rotating electrical machinery"I took the courses and lab at CCNY in 1965..1971.Jon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 23, 2023, 02:18:29 am
content: Most likely you get zero output, because the rotor's salient pole is spread across a pair of complementary poles, summing to zero, give or take harmonics of the rotor (i.e. its response to an applied field is not perfectly \$\hat{B} \cos(\theta_\textrm{rotor} - \angle B)\$.Otherwise, the effect would be getting four nodes and four (+/-) antinodes around the circle, with respect to rotor voltage versus rotor angle. It could be more than this of course, being that rotor harmonics matter.If the rotor is made four-pole as well, of course the expected result is had, where rotation is effectively doubled, and 50Hz looks like 1500RPM.Tim

author: moffy
author_rank: Super Contributor
timestamp: August 23, 2023, 03:14:23 am
content: Quote from: jonpaul on August 23, 2023, 01:28:32 amThanks Jonpaul, I don't think we ever covered slip ring induction motors when I was at Uni, just the standard squirrel cage, though they might have snuck it in while I was dozing.Great thing about this forum is the exposure to such a wide array of technologies, even old ones and the experts to help.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 23, 2023, 10:00:15 am
content: Buy a text book on rotating machines and suppose no more

author: jonpaul
author_rank: Super Contributor
timestamp: August 23, 2023, 01:56:22 pm
content: Tesla Induction motors with slip ringshttps://www.teslasociety.com/hall_of_fame.htmhttp://ffden-2.phys.uaf.edu/211_fall2013.web.dir/Ryker_Dial/The_Current_Wars/AC_Motor.htmlJon

author: jonpaul
author_rank: Super Contributor
timestamp: August 24, 2023, 01:11:11 pm
content: My Motor Memoire...At CCNY EE courses, I elected the LAST "Rotating Electrical Machiner" course and lab.Huge motors, 5..50 HP, generrators, prony brakes (HP) Patch cables 15 mm dia with #4 welding wire...Lab assignment was to take data on a 3 ph synchronous motor generator mains synchronization with 3 lamps.When generator was in sync to the mains (3 ph 480V), I threw the contactor ON , WOW!!! a ball of sparks and fire erupted from the slip rings...YIKES!!I instantly flew to the lab main 500A circuit breaker (you crank it up to start, filled a 19" rack 1m high) and hit the emergency disconnect button.As the lab fell quiet and the machines wound down, The class was amazed, and the professor congratulated me...we were 180 deg out of phase!This was between 1966 and 1971.Walked away from another one....Your thoughts?Jon

author: moffy
author_rank: Super Contributor
timestamp: August 24, 2023, 03:26:29 pm
content: In a power lab at university there were large metal cabinets filled with power switch gear and of course safety interlocked if one of the doors was opened. We were watching a demo of some machine when there was a bang and all the lights went out. Everyone in the class including the professor turned around and we saw one of the students standing guiltily with his hand on the door of an opened cabinet. The safety interlocks certainly worked.