Multi speed taps in a multi-speed PSC single phase motor - Page 1
Forum section: electronics / repair

author: shockpoint
author_rank: Contributor
timestamp: May 27, 2024, 02:21:51 pm
content: Hi all,I forgot to edit the title.I have some questions about multi-speed taps in a PSC single phase motor.Please refer to the diagramQuestions- i'm not sure if i understand this correct. If you select a medium speed, you are tapping into portion of the run coil that generates enough torque corresponding to the medium speed. This happens because the current enters the coil at a tap (much like a transformer winding) and proceeds through the rest of the circuit. what happens to the current that runs in the other direction, going through the coils that you are not using? Does that contribute to torque at all?- does it matter which winding you tap into for the speed selection? for e.g., i see some tap the run winding, and others tap the start winding.thank you

author: johansen
author_rank: Super Contributor
timestamp: May 27, 2024, 04:25:52 pm
content: Usually the taps are just like an auto transformer and the highspeed tap is the fewest number of turns.So on a 120vacnmotor on highspeed you will see 170 volts between the low speed wire and neutral.

author: Uup
author_rank: Regular Contributor
timestamp: May 28, 2024, 02:18:02 am
content: Quote from: shockpoint on May 27, 2024, 02:21:51 pmThere is induced voltage, as well as some induced current, in the open section(s) of the main winding speed tap. However, for all intents and purposes this interaction is irrelevant, outside of engineering and design.Does it matter which winding you tap into for the speed selection?Yes and no... it depends. Your question lacks specifics in order to give a relevant answer.I presume that the taps that you are referring to are the defined ones for motor usage. The connections to the motor are usually defined as High, Medium, (intermediate, if any), Low and common. Capacitor connection is usually a separate cable or separate wires, to avoid misconnection.It is important to note that only one of the speed taps should be connected, or switched, to line at a time.For example, L1 to one of the speed taps and L2 to common. If L1 were connected to more than one speed tap at the same time, then the motor would draw excessive current and would likely be damaged.Often control circuits for these type of fan motors use SPDT relays for each speed. The NC contact of each relay would supply power to the next speed tap relay common. This is done so that if one relay were to fail (NO contact to speed tap stuck closed) then it would prevent more than one speed tap being powered simultaneously and damage to the motor.

author: shockpoint
author_rank: Contributor
timestamp: May 28, 2024, 02:28:47 pm
content: Quote from: johansen on May 27, 2024, 04:25:52 pmQuote from: johansen on May 27, 2024, 04:25:52 pmThanks for your answerBut according to the diagram as you can see, the wiring of the high speed tap actually results in the largest number of windings.that's the contradiction that I cannot understand.Could it be that this is actually phase control rather than voltage drop?so the speed setting selects how many of the speed coil will be out of phase with respect to the other winding?Quote from: Uup on May 28, 2024, 02:18:02 amApologies, to clarify the question what I meant was does it matter whether you tap the start winding to control the speed, or the run winding to control the speed?I guess it shouldn't matter much if I were to attempt answering it myself, if the PSC motor was a designed such that the run and start windings had the same design specs - same coil pitch, coil span, wire gaugethe only difference between the two would be a phase difference caused by the capacitor, wouldn't it.I had a look ahttps://electronics.stackexchange.com/questions/621588/how-doest-this-psc-multi-speed-motor-circuit-workThis is an interesting findingI wonder if the autotransformer method combined with a capacitor causes one side of the tapped winding (start) to be out of phase and the other side to be in phase with the untapped (run) winding. this would result in different magnetic field strengths (additive) which would then affect the torque of the motor and thus the speed

author: Andy Watson
author_rank: Super Contributor
timestamp: May 28, 2024, 02:56:16 pm
content: Quote from: shockpoint on May 28, 2024, 02:28:47 pmThe "taps" are not exactly the same as a transformer. Remeber there is a mechanical phase difference between the coils. Changing taps can also (depending on how the motor is configured) change the number of poles. Fewer poles = faster.

author: soldar
author_rank: Super Contributor
timestamp: May 28, 2024, 03:51:04 pm
content: This is a bit more complicated than first meets the eye.I would not call then "running" and 'starting" windings because they are both permanently in use.Let's call them "main" and "auxiliary" where the main gets mains voltage directly and "auxiliary" gets it shifted in time by the capacitor.When Speed selector is "high" everything is clear (I hope).When speed selector switch goes to medium the 5th winding is now part of the main winding and not the auxiliary. Now windings 1,2,3,4 and 5 are in series to form the main winding.When the switch goes to low speed now the auxiliary is only formed by windings 7 and 8 while windings 5 and 6 are part of the main.

author: johansen
author_rank: Super Contributor
timestamp: May 28, 2024, 04:47:22 pm
content: Quote from: soldar on May 28, 2024, 03:51:04 pmyep, and it basically works the same way most of them do.low speed is more turns than high speed, which means lower flux density and less torque.it just so happens in this fan motor, the additional windings are on the auxiliary coil at 45 degrees, and by including them in the circuit you are also reducing the effectiveness of the capacitor, which will be passing less current, through fewer turns..

author: shockpoint
author_rank: Contributor
timestamp: May 29, 2024, 03:28:49 am
content: Quote from: soldar on May 28, 2024, 03:51:04 pmThanks for your responseHow does this function then? Is it by modulating the strength of the magnetic field by altering the relative numbers of coils in phase and out of phase by 90 degrees?How does that contribute to motor torque?My understanding is that the main winding at low speed has 6 coils running with it, where as the auxiliary phase winding has only 2. the relative contributions of these magnetic fields is not something I can readily visualise. any clues?Quote from: johansen on May 28, 2024, 04:47:22 pmIt does look like the high speed turns puts more coil turns of the auxiliary winding in line with the capacitor though, doesn't that mean that higher speed settings are driven by a stronger auxiliary field?

author: johansen
author_rank: Super Contributor
timestamp: May 29, 2024, 05:43:54 pm
content: With fewer turns on the capacitor windings, there will be less amp turns, as current is limited by the capacitor. It will also move it off resonance enough to additionally drop the current.The capacitor likely has 1.5 times the nominal line voltage across it, on highspeed