Need advice on what motor flyback diode to use.

I will soon wire up a 36 Volt 80 Amp permanent magnet DC motor to be
run at 48 VDC. It will be used in a homebuilt trike.

I am looking for a flyback diode that will take the full current during
slowdown when the power is turned off by a contactor (no PWM).

I would appreciate knowing whether to use a single diode, series
diodes, a Schottky, a series resistor, etc.

What is the most cost effective solution?


Boyntonstu

 

If you include the time cost of connecting big wires to the diode and
heat sinking it, you may be ahead by selecting something easy to
mount. A DO-5 stud package might be good, but I think a Half-Pak is
better. A 120 amp 100 volt Schottky costs about $17 plus shipping
from Digikey: http://www.irf.com/product-info/datasheets/data/123nq.pdf

 

Thank you John,

I appreciate your response.

BoyntonStu

 

Let us know how long this 36 volt motor lasts at 48 volts with
bang-bang control.

Good Luck!
Rich

 

Let us know how long this 36 volt motor lasts at 48 volts with

Dunno about you guy's but I'd feel safer riding that thing with some kind of
intermediate speed to lessen the strain on the motor/battery/transmission,
or something is likeley to go bang from the startup loads.

Chris

 

http://www.motherearthnews.com/search/

from an 1979 article

With the price of gasoline already out of sight, just about everyone is
scrambling for a way to squeeze the last possible drop of energy from
each precious gallon. However, David Arthurs-of Springdale, Arkan
sas-probably couldn't care less ... because he has designed and built a
car that can travel 75 miles or more on just four quarts of the
expensive liquid!
What's his secret? Well, Dave's Opel CT is a hybrid electric vehicle.
That is, the car is driven by an electric motor . . . but that
powerplant's "juice" is generated with the help of an ordinary,
fuel-stingy lawn-mower engine! Now the fact that the system works isn't
really surprising. What's amazing is that the crossbreed hookup
performs so well! According to David, the Opel has not only a virtually
unlimited range (when driven prudently), but also a top speed of 90
miles per hour . . . and emits a minimum of pollutants as it tools
along the highway. Better yet, the car can-if need be-run on its
batteries alone for short in-town hops . . . and will never be
"stranded" as long as there's fuel in the "on board" generator!
WORLD WAR II TECHNOLOGY
Mr. Arthurs is the first to admit that there's nothing "new" to the
system he's developed . . . in fact, all the technology incorporated
into his design has been available for about 35 years, just waiting for
someone to put two and two together and make the whole thing work. "I
began researching the idea for a hybrid electric auto about a year ago.
There wasn't much information to be found on the subject, so I designed
a system from scratch. In about a month's evening-and-weekend time, I
had the car finished and running."
Surprisingly enough, the project didn't cost a fortune, either. Because
the vehicle's components are either standard "off the shelf" hardware
or available as reasonably priced military surplus, the conversion to
"hybrid drive" only set Dave back about $1,500. By the same token, any
necessary replacement parts are easily obtainable . . . and a good deal
of the equipment can be "scrounged" rather than purchased new.
HOW IT WORKS
In essence, David has utilized a small gas lawn-mower engine to drive a
generator, which-in turn-supplies the vehicle's drive motor with
electricity. To do so, he first removed the Opel's original power-plant
and installed a 400-amp DC motor/ generator (actually a jet engine's
starting motor) in its place. (Since there's no need for a clutch in
Dave's setup, the stock unit was pulled out and the main shaft of the
drive motor was fastened directly to the input shaft of the car's
transmission.) Then, to provide a consistent source of power for this
motor (and to supply an energy storage bank), the engineer installed
four 12-volt, heavy-duty automobile batteries-in series-which are "fed"
by a 100-amp generator that's run off a 5-horsepower lawn-mower engine.
Of course, other components (such as relays, charging diodes,
rectifiers, and an additional motor speed regulation circuit) are
necessary to keep input and output power within optimum limits-and to
allow full control of the vehicle at all speeds-but these are standard
electrical parts which have been available for years.
The engine-driven generator can handle the demand from the main motor
up to speeds of about 50 miles per hour. The "stored" energy in the
batteries comes into play at higher velocities, giving extra kick for
passing and climbing hills. To guarantee that the charging system isn't
overworked . . . Dave has rigged up a regenerative braking circuit
which- in effect-turns the drive motor into a generator, to feed the
batteries when the vehicle is decelerating. (This not only takes
advantage of normally wasted energy, but also saves wear and tear on
the car's conventional braking system.) Since the Opel's "stock"
cooling apparatus has been removed, two small thermostatically
controlled electric fans provide ventilation to the motor and generator
as required ... while the gasoline engine is, of course, air-cooled by
design.
GROWING PAINS
Any project fresh off the drawing board has its share of problems, and
the Opel hybrid was no exception. When David pressed the accelerator
for the first time, he got a 300-amp surge which melted his relays. So
he searched his graduate texts for the answer ... and finally found it
in-of all places-an old high school physics book: A pulser was
necessary to "chop" the current flow and prevent a heavy initial draw
to the drive motor.
As Dave explains it, "The motor will always have full voltage and full
current, but the pulser makes it 'think' the voltage and amperage are
cut down to about 1/4 of what's actually available. With this
gadget-which is simply a combination of a reworked car generator and an
old fan motor-I can keep the draw within limits and effectively control
the car's acceleration . . . without sacrificing the maximum current or
voltage that's necessary for high-speed driving. I could have achieved
the same results with a commercially available FCR control ... but one
of those units would have cut my power slightly, and cost in the
neighborhood of $800! I can build my own device for about $25, and I
can fix it myself if it breaks!"


BoyntonStu

 

Well, motors for A/C, power steering and hydraulic brakes come to mind
rather rapidly.

5 horsepower is 3730 watts at the gasoline engine, 48 volts x100 Amps= 4800
Watts demanded by
the generator. Assuming the 3730 Watts are delivered to the generator,
3730W/48V= 77.71 Amps
at best from the generator. Not all can go toward propulsion.

How efficient is a jet engine starter motor? Aren't these usually mounted
on a truck? What sort of
generator runs the starter when used in its intended role? Such a motor
isn't designed for continuous
duty.

This sound like a fanciful tale.

 

I suggest that you read the article. It is about 25 years old. I
believe that over 15,000 sets of plans have been sold. There was also
a follow up article a few years later.

My only point in this forum is that a mechanical PWM can soft start a
DC motor.

If you think about a brushed DC motor, there is no reason to doubt that
a brushed 'controller' could handle any current the motor required. It
might even be possible to vary the duty cycle for different speed.


BoyntonStu

 

I don't doubt that plans have been sold, more power to the guy for his
marketing prose. I just think the figures need to be revisited. Today,
a solid state PWM would be the speed control choice.

What is needed is an up-to-date rebuild and perhaps a chart plotting the
acceleration. For highway, we need 60 in town and 70 on the intercity
runs & 75 on a few tollways with headroom to accelerate, admittedly a
tall order. Range should not be less than 350 miles though more is nice.

 

"Today, a solid state PWM would be the speed control choice."

I totally agree.

Here's a thought provoking problem:

You are in a "Junkyard Wars" episode and need to control a 500 Amp 48
VDC motor. You are given a $30 and free rein of the junkyard. What
would you do to solve this problem?

BoyntonStu

 

Umm, build a wafer fab?

I guess that's why I find shows like this fascinating!

 

A lot will depend on what "kind" of junk. Military surplus avionics,
or scrap cars? Or something in between, or broader-spectrum? Lawn
furniture? The kitchen sink?

Cheers!
Rich