It does not. It is simply a segway we took when discussing the fact that coupled PM motors have a tendency to act as if they are mechanically coupled and then the selsyn was brought up not as a clarification for the PM motor paralleling but as an example of other devices that connect electrically and track each other's position. I believe these devices are also used in antenna rotors, so that the indoor control unit will show the actual direction of the antenna outdoor, because the device in the rotor will feed back its signal indoor. Though it can also be achieved with simple potmeters, where a small motor turns the indoor indicator until its potmeter output is identical to the potmeter coupled to the outdoor rotor.
Anyhow - as I said, I see little problem with PM motors in parallel, since they will only be switched like that when the vehicle is traveling fast. At speed, you never have large difference in speed between the two wheels and the times that you do have speed differences are a very small part of the typical travel time unless your commute consists of hairpins that can be taken at high speed. For a typical good motor efficiency of 90%, you would need to have approx 20% speed difference between the two wheels before the outside motor is feeding all its power to the inside motor, doubling its instantaneous power (the outside motor is then getting zero current & torque while the inside motor gets double) but it is unrealistic to have anywhere close to 20% speed difference at speeds of 40+ MPH, because that would mean for a typical 5 ft wide tire track that you are making a turn with a 25 ft radius, which is about the smallest turn radius that a typical car can achieve (wheels full lock) so this is not going to happen at 40+ MPH. Therefor, there will always only be a fraction of the power pushed to one or the other motor when making turns at speed and motors are pretty good in dealing with short bursts of power alternated with periods of moderate or even lower power when a turn the other way is made. Normal freeway driving should give 50/50% load division and a healthy turn at speed may cause the power to be divided 40/60 or in extreme (uncomfortable tight turn) cases, you may see something like a 30/70 power division. But the best thing is to measure it - put the two motors you expect to use in parallel on a test jig or the real car and run it through some of the driving you do. Measure the currents of the two motors and you will know. YMMV, Cor. Cor van de Water Chief Scientist Proxim Wireless Corporation http://www.proxim.com Email: [email protected] Private: http://www.cvandewater.com Skype: cor_van_de_water XoIP: +31877841130 Tel: +1 408 383 7626 Tel: +91 (040)23117400 x203 -----Original Message----- From: [email protected] on behalf of Ds2inc Sent: Sun 2/10/2013 8:32 AM To: Electric Vehicle Discussion List Subject: Re: [EVDL] Series and parallel switching I thank you for the link am I have read it twice and as a non scientist I don't understand how it applies to the below information suggesting that pmdc motors should not be run in parallel. I apologize for my ignorance. Sent from my iPhone On Feb 9, 2013, at 6:52 PM, "Cor van de Water" <[email protected]> wrote: http://en.wikipedia.org/wiki/Selsyn Cor van de Water Chief Scientist Proxim Wireless Corporation http://www.proxim.com Email: [email protected] Private: http://www.cvandewater.com Skype: cor_van_de_water XoIP: +31877841130 Tel: +1 408 383 7626 Tel: +91 (040)23117400 x203 -----Original Message----- From: [email protected] on behalf of Ds2inc Sent: Sun 2/10/2013 8:08 AM To: Electric Vehicle Discussion List Subject: Re: [EVDL] Series and parallel switching I don't really understand this principal and can't find much reference to it could you explain in more detail if possible please as tho is all new learning for me! Thanks so much!! Sent from my iPhone On Feb 8, 2013, at 5:22 PM, Jeffrey Jenkins <[email protected]> wrote: Lee Hart wrote > ... > If you run PM motors in parallel, their voltage is the same, and they > both run at the same speed. They behave like a limited-slip or locking > differential. Nice in snow or on a drag strip; but it leads to extra > losses in normal driving. Going around a curve, the outside motor is > forced to turn faster, so it becomes a generator, and is actually > dragging the wheel backward (negative torque). The current it generates > drives the inside motor harder, trying to make it turn faster... It perhaps should be emphasized a bit more strongly that PM motors should not be connected in parallel because there will be always be some current circulating between them depending on the RPM/Volt (Kv) ratio of each motor, the total resistance of the motor interconnection loop and the difference in RPM (if the shafts aren't locked together). The magnitude of the circulating current can be surprisingly high, and effectively amounts to unequal sharing of the load between the two motors. TL;DR version: don't parallel PM DC motors. -- View this message in context: http://electric-vehicle-discussion-list.413529.n4.nabble.com/Series-and-parallel-switching-tp4661043p4661139.html Sent from the Electric Vehicle Discussion List mailing list archive at Nabble.com. _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
