4-10-06 Re: AC vs DC; Newbie Question

Hello Osmo and All,

Osmo S. wrote:

DC vs AC … what about sepex, PM, BLDC and whatnot. Do they all belong in the category of DC? Or does DC stand for a brushed series wound motor only. If so, why aren´t the others hardly ever discussed in this group?

When talking EV sized brushed wound field type motors, it is generally assumed that they are series-wound types. They don’t have to be, they could also be shunt wound, sepex, or PM types.

BLDC (brushless DC) motors are as complicated to control as AC motors are, in that they require an inverter or they cannot run. A BLDC is really an AC motor who’s inverter is generally built into the motor case. It’s an AC motor that has no brushes but you feed it raw DC power…hence brushless DC.  Once the power gets into the motor it’s routed through an inverter that converts it to three phase AC. Most are synchronous types where the rotor is a permanent magnet, so they have different characteristics compared to induction type AC motors. Today’s BLDC motors have changed and many now have three feed wires that get power from an external inverter. To me, I call this an AC motor, but the companies that build these still insist on calling them BLDC….go figure! In a nutshell, a BLDC motor is essentially an AC motor, thus an EV BLDC system with EV levels of power have the same high cost as the induction type AC systems. Unique Mobility makes EV sized BLDC systems, but I think they’re priced in the $20k range. I rode in a Humvee powered by four Unique Mobility 100 hp BLDC motors, it was incredible! At $20k, you can see why we don’t talk much about them or use them often.

On a much, much smaller scale, I do own an EV that’s powered by a BLDC motor, it’s my very early model Curie board scooter. When other scooters of the period (pre-2000) were still stuck in 12V land, had a PM motor that ran at 57% efficiency with a binary controller (an on-off switch) and used a friction drive or belt drive, this one ran at 24V, had a chain drive, and employed Curie’s own design BLDC motor at about 95% efficiency with variable throttle control due to the built-in inverter-controller on the backside of the motor. Scooters back then were rated at 15 miles range per charge, but in reality got about 6-7 miles before needing a recharge. My Curie was rated at 12 miles per charge and easily did it. It was also way faster on top end, and off throttle it coasts forever so you only need to blip the throttle once in a while to cruise along. After a few mile son a regular PM 12V scooter, the motor got very hot to the touch, but the Curie BLDC feels barely warm. Today, I have five board type scooters, but my favorite is still my original Curie with its BLDC motor. I just bought a new 36V Curie that has a real disc brake, and full suspension and all, but the disappointing factor is its PM brushed motor. It even has the warning ‘Caution, motor gets hot!’ I assumed when I ordered this scooter, it would be an upgraded version of my trusty Curie …not!

As to sepex and PM, yes, these are considered to be good ‘ol DC motors. There aren’t many large PM motors readily available in road going EV sizes, and a PM of this size would not have the ultimate low end torque of a series wound type, thus, they are not popular. Sepex are also, not readily available and they require more complicated controllers. Randy Holmquist of Canadian EV has rewound Kostovs that are turned into sepex types. Jim Husted could also turn any motor into a sepex type. Sepex have really taken over in the forklift industry and with gear ratios changed to improve low end torque making them on par with series wound types, they work very well, with the added plus of very controllable regen.

Hope this helps…

See Ya….John Wayland

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