Yes that was the concept however Im wondering if I really need the extra torque??
Sent from my iPhone On Feb 7, 2013, at 5:54 PM, "Cor van de Water" <[email protected]> wrote: Of course, with the two motors in series the expected max speed is half (37 MPH) so this sounds like an interesting city/freeway drive setup where the City setting allows the full current through both motors, giving twice the max torque at lower speeds, while the freeway setting gives higher voltage (higher speed) at the cost of lower torque (current divided between the two motors if the wheels turn at the same speed) Cor van de Water Chief Scientist Proxim Wireless Corporation http://www.proxim.com Email: [email protected] Private: http://www.cvandewater.info Skype: cor_van_de_water Tel: +1 408 383 7626 -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Ds2inc Sent: Thursday, February 07, 2013 5:37 PM To: Electric Vehicle Discussion List Subject: Re: [EVDL] Series and parallel switching Lee thank you again so very much for the clarification on the two wiring effects and on you confirming my math re: speed at rpm! As a newbie I am not always sure I'm applying the data and formulas correctly!! Sent from my iPhone On Feb 7, 2013, at 5:24 PM, Lee Hart <[email protected]> wrote: On 2/7/2013 1:44 PM, Ds2inc wrote: > Also if I read this correctly with pm motors there is no need to run > them in parallel to achieve higher hp I just need more voltage in > each parallel? Am I missing then concept here? Mechanical power (horsepower) is torque x RPM. Electrical power (watts) is volts x amps. With a PM DC motor, volts = RPM, and amps = torque. The relationships aren't perfect (they assume the motor is 100% efficient). But in practice, a motor can easily be 90% efficient, so this relationship is accurate within about 10%. A PM motor tries as hard as it can to run at the RPM demanded by the voltage. With the numbers you gave (72V 6.75HP 4200 RPM), you can expect something like: no load: 72v, 10 amps, 0hp, 4300rpm full load: 72v, 80 amps, 6.75hp, 4200rpm double load: 72v, 160amps, 13.5hp, 4100rpm If you increased the load, the motor would just keep trying. The RPM would barely fall, but the current would go up to supply the horsepower demanded, until the motor burned up from excessive current. Two motors in parallel would supply exactly the same RPM. But each motor draws half the current and supplies half the torque (thus half the power in, and power out). The main advantage of two motors is that they can supply power for twice as long before burning up. With PM motors, RPM is *directly* controlled by voltage. Double the voltage, double the RPM, etc. It doesn't matter how many motors you parallel; they *all* run at the same RPM. You said your motors produce 4200 RPM at 72v, and your gear ratio is 4:1. That lets us precisely calculate your maximum speed. Your wheel will be turning at 4200/4 = 1050 RPM. A 24" diameter tire moves 3.14 x 24" = 75.36" per revolution. 75.36"/rev x 1050 RPM = 79128"/minute = 6594 feet/minute = 1.249 miles/minute = 75 miles an hour. If your motors were connected straight to a 72v pack, the car would go 75 mph, uphill or downhill, drawing as much current from the batteries, or regenerating as much current into the batteries as it could to stay at this speed, as if you had a "cruise control" engaged. -- The principal defect in a storage battery is its modesty. It does not spark, creak, groan, nor slow down under overload. It does not rotate. It works where it is, and will silently work up to the point of destruction without making any audible or visible signs of distress. -- Electrical Review, 1902 -- Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm _______________________________________________ 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) _______________________________________________ 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) _______________________________________________ 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) _______________________________________________ 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)
