Series-connected DC motors are practical for electric vehicles
Your reply to the letter from Clifford Wright regarding his ideas on an electric car (“Mitsubishi i-MiEV should have in-wheel motors”, Mailbag, April 2011) leaves me rather puzzled. Firstly, you say “It is not at all clear that having two or more in-wheel motors, with a separate control system for each, is necessarily more efficient . . .” At no point in his letter did Mr Wright suggest using a separate control system for each motor. In fact he says that the increase in efficiency is from removing the need for a differential, and the consequent saving in weight.
Then you ask “. . . how can you connect the motors in series if you need them to run at different speeds when turning corners?” My guess is that you are thinking of AC induction motors. But Mr Wright’s mention of “modern permanent magnet motors” and a “PWM control system” seem to imply the use of DC supply and motors.
As you are probably aware, two DC motors with permanent magnet fields, when connected in series, will have the same current flowing through each, giving approximately equal torque as a first approximation, while the voltage drop across each motor will be roughly proportional to its speed. This seems to meet the ideal characteristic for motors on each end of the driven axle of an electric car.
I do agree that motors within the wheels of an electric vehicle will increase the un-sprung weight to an undesirable amount. There are also the problems of reduced cooling air around the disc brakes, also of providing cooling for the motors and sealing them from the intrusion of water.
But by mounting the motors on the body of the car and using short driving shafts to the wheels, these problems could more easily be overcome. This is the system used in modern front-wheel drive vehicles and in rear-wheel drive vehicles with independent rear suspension.
G. D. MaymanDover Gardens, SA.
Comment: in practice, mainstream electric cars do not use permanent magnet motors; they use some variation of induction, synchronous or electronically commutated motors with variable frequency, variable voltage drive, together with regeneration for braking.
We agree that having permanent magnet DC motors in series may eliminate the need for a differential but that approach does not lend itself easily to regenerative braking, unless you use some sort of boost converter. While it is not our job to defend Mitsubishi, we assume that the big manufacturers have looked closely at a whole range of approaches.
High mains voltage
Thanks for publishing my letter on high mains voltages in the March 2011 issue (Mailbag, page 12).
In reply to my letter, Brendan Falvey of Gundaroo (only a few kilometres from Canberra), in the April 2011 issue, assumes that I was referring to happenings of 30 years ago in Ainslie. Nothing could be further from the truth. The suburb was Forrest. Like Ainslie, it is one of the older burbs in Canberra but the time was only six years ago.
That the problem still exists, at least in the ACT, is evidenced by another organisation in the same geographic area as the church I mentioned recently being told that their voltage woes were their own concern! In other words, “fix it yourselves!” Seems I might have been lucky.
Mr Falvey also mentions Western Australia. In 1982, I was seconded to the ABC “Host Broadcaster” group, at the Commonwealth Games in Brisbane. The ABC, in a rare stroke of brilliance, brought TV OB vans from all over Oz to staff the venues. On arrival in Brisbane, the van from Perth had to re-jig its power systems. Set up for the 260V in WA, the van would not work at all on 240VAC.
The retrospective on ETI was most interesting. Way, way back, I built one of the first of David Tillbrook’s “Black Monolith” amplifiers and a matching control unit. With the exception of one 5534 op amp failing a few months after building, it has performed flawlessly and still gets nice words from visitors on its “sound”; a tribute to really good design. Thank you ETI and its successor, for allowing the Tillbrooks of this world enough free reign to produce good stuff.
Thanks again for an always interesting magazine.