I have flat-towed EVs without disconnecting the drive shaft or motor leads, but the one that I towed over 10 miles on the freeway (at night and staying below 50 MPH) had an induction motor, so it would not have any magnetic field while turning and thus no losses in the motor or risk of controller-blowing short circuit currents. I do not know about the iMiev motor but the Leaf has a PMAC (Permanent Magnet AC) motor, just like the Prius. (The Prius actually has two, but that is not relevant here) I have flat-towed the Prius, but I made sure to stay below 35-40 MPH since the Prius' motor will generate a (high) voltage while turning and I did not want to blow the controller from overvoltage (and I did not want to disconnect the leads of the two motors). I know that the Prius uses Field Weakening by controlling the stator current in a way that counter-acts the magnetic field of the rotor magnets, in order to achieve the highest RPMs. Since the controller is off while towing, the output voltage of the motors would actually be higher than when in operation, so I wanted to stay away from the redline of the motors, as that would definitely send too high voltage to the controller.
The Leaf's motor redline is calculated to be at 94 MPH so I suspect that flat-towing at (much) lower speed may be safe, but as others have pointed out: the motor might still warm up from Eddy current losses since the PM motor's losses are always-on. Then, because the controller is not on, it might have a funny reaction to the motor developing a (high) voltage. In theory the motor voltage could be rectified by the output transistors and start to half-power the controller, which is in an undefined state and this could cause the controller to drive output transistors so that they form a short-circuit and since the towing vehicle may not be able to stop quickly enough when noticing (or not) that something is dragging, this could lead to inverter-destroying large short circuit current. I see two ways to avoid this - either use a tow dolly and put the driven wheels on it (cost of a tow dolly is not outrageous) or you make sure the 12V aux battery in the EV is strong and you tow it a limited distance in "Neutral" so the controller is on and running and the coolant pump is on and running, but the motor is not dragging from regen braking. I have towed a Leaf on a tow dolly from San Diego to Bay Area (over 500 miles) and I would not think about flat-towing it anywhere close to that distance. A few miles, OK. Also, because I flat-tow with a (shortish) rope so ~35 is the highest speed that you can safely go and not be sure that the tow will hit the towing vehicle before the driver in the towed vehicle has a chance to respond (brake) in an emergency. (Note that the rope must be short enough that it is impossible for a car to merge between the two vehiles). Cor van de Water Chief Scientist Proxim Wireless office +1 408 383 7626 Skype: cor_van_de_water XoIP +31 87 784 1130 private: cvandewater.info http://www.proxim.com This email message (including any attachments) contains confidential and proprietary information of Proxim Wireless Corporation. If you received this message in error, please delete it and notify the sender. Any unauthorized use, disclosure, distribution, or copying of any part of this message is prohibited. -----Original Message----- From: EV [mailto:[email protected]] On Behalf Of Mark Hanson via EV Sent: Saturday, November 26, 2016 4:47 AM To: Bill Dube Cc: Electric Vehicle Discussion List Subject: Re: [EVDL] Towing 4 wheels down Spark EV or Leaf Thanks Bill I'm used to towing my conversions over the years to shows further away than range and also occasionally home to fix when something burps. I don't feel comfortable not having that option with factory EV s. It sounds like even if you were able to disconnect the motor wires there could be overheating on the motor Best regards Mark Sent from my iPhone > On Nov 25, 2016, at 10:34 PM, Bill Dube <[email protected]> wrote: > > I'm going to take a guess that the coolant pump should be on and circulating coolant to the motor and controller. When you spin a PM motor, even when it is off, it still generates some amount heat due to eddy current losses. This would be especially true at sustained highway speeds. > > When you put the car in "neutral" you aren't really disconnecting anything mechanical. (The motor still spins.) You are just signalling the motor controller not to do anything, like use the generated power to charge the battery (regen). > > However, if you can actually turn on the vehicle, and the wheels would roll correctly, why would you need to have it towed? > > Bill D. > >> On 11/25/2016 5:51 PM, Mark Hanson via EV wrote: >> I know the operator manuals on my Leaf and Spark EV say they can't be towed with 4 wheels down but why not can't you just put it in neutral or disconnect the motor wires? On all my conversions I've had no problems towing wheels down. Both say to use a tow dolly >> Thanks Mark >> >> Sent from my iPhone >> _______________________________________________ >> UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub >> http://lists.evdl.org/listinfo.cgi/ev-evdl.org >> Read EVAngel's EV News at http://evdl.org/evln/ >> Please discuss EV drag racing at 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 Read EVAngel's EV News at http://evdl.org/evln/ Please discuss EV drag racing at 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 Read EVAngel's EV News at http://evdl.org/evln/ Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA)
