Using Ultracapacitors to Power Electric Motors

When I first moved to California from England in 1987, this is precisely what I observed - endless traffic lights every few hundred yards, with very long red-light periods. None of them appeared to be linked so that you could drive at a constant speed without stopping. Time wasted, fuel wasted. This problem was solved in Britain right from the early days of the motorcar by using roundabouts. These allow traffic to drive through major junctions without stopping, much of the time. When they get really large and busy, traffic lights situated at various points, are brought into play, particularly at rush hour.

A friend of mine worked on a city council. The cities lights were linked to use more fuel not less because the city gets a percentage of the taxes from the sale of fuel there, thus creating more revenue for the city.

A perverse incentive.

Having other dealings with this burb of Seattle, they are quite polite but preditory.

Think of the capacitor as a reservour of electrons. When more amperage is required, or more electrons, you have a great big lake to call on for them. Just like when everyone flushes the toilet during Super Bowl half time.

There are also considerations for balancing the inductive load on induction type motors. Especially if they are large and many of them.

Essentially, ultracapacitors and batteries serve the same purpose of supplying DC power to a motor controller unit like this unit.

Batteries can store (or more correctly charge and discharge electricity via a chemical means) large amount of electricity, but they do not give it up very quickly, enter the ultracapcitors. Ultracaps can discharge large amounts of current very quickly and they can also be charged very quickly since the electricity is stored in layers of foil sandwiched with a insulating dielectric. This eliminates the chemical conversion process required by a battery.

Ultracaps are more costly and have a lower energy density than batteries.

The advantage of ultracaps is that you can get huge amounts of instantaneous power when doing heavy acceleration and you can also store large amounts of current when regenerative braking. Typically ultracaps are used to augment a battery packs.

The efficiency of the system is primarily limited to the motor controller and motor. Ultracaps are 95 to 98 percent efficient

Thanks, but as an ex-electrochemist, I know all that. My question is what electronics are required to accept a continuously falling voltage yet give an output that can drive an electric motor at whatever speed the operator choses. For instance, suppose we have an ultracapacitor that can be charged to 1,000 v. and it's purpose is to drive a DC electric motor at a nominal 120v. Eventually the voltage on the ultracapacitor will fall below 120v., yet will still be required to deliver 120v. to the motor. I assume therefore that the DC-DC converter, referred to above, must be capable of stepping the voltage up as well as down. Is there an AC stage in the converter that does this?

The big difference between electrochemical batteries and ultracapacitors is that in the former case, the energy is stored extensively, (like a tank full of fuel) while in the latter, it is intensively (like a wound up rubber band, spring, or cylinder of compressed air). The latter case is much less convenient.

This is where the motor controller comes in. The motor controller will usually use a voltage doubler and regulator (in a sense). The controller type actually depends whether it is a DC motor or AC motor and translates the input voltage to the correct voltage levels.

That means that the capacitors are either charged to a higher voltage than needed and/or the controller converts the input voltage to the needed output..

The circuitry that allows an ultra-capacitor to be used is basically a buck-boost converter. Additional steering circuitry is around the converter to allow for the back EMF of dynamic braking from the motor to recharge the capacitor. There will also be the variable reference (throttle pedal) that controlls the actual output voltage to the motor.

I've seen these setups controlled with IGBT(insulated gate bipolar transistor) and buck-boost converters...The capacitor is allowed to discharge only about 2/3 capacity...

Here is an answer:

You can use a electronic chopper circuit if you want to reduce the losses. (a series regulator needs to absorb heat - a well designed chopper almost nothing (creates an interval like an electronic dimmer does)

The trick is to switch the DC voltage on-off with a different assymmetric block pulse at a convenient frequency. (pulse width modulation).

When the frequency is high enough your (DC) motor will spin around, overlapping the steps you create. At lower frequency a buffer capacitor can be used parallel to the motor. Your motor will work on the average content of the energy, built up by all the pulses. A thyristor can do the work.

Thanks for your excellent answer - I am still digesting it.

A switching power supply that will tolerate a declining power supply input voltage is quite common. Nowadays the small switchers for chargers and printers etc are intrinsic 120 to 240 volt units.

Consult with a switching supply maker via his online catalogs and see what he has as a regulated output and what will the input voltage be. You may find one standard unit will work. If not, you will have to engage in conversation to make a unit, and may have to pay high for a one-off, or promise him volume sales to induce his free work.

With the current rage of supercapacitors, you may well be able to find a standard unit.

Supercapacitor power supply design aspects

GA AH Just a few details and implications you left out that I'm certain you know but probably need clarifying for the OP.

An ultra-capacitor is very efficient at energy storage and is capable of storing and releasing that energy in very brief periods of time. So the power delivery of an ultra-capacitor is potentially very good. Careful engineering of a wide input range DC/DC power supply allows for the voltage droop on the ultra-capacitor to not be a problem in using this energy. The drawback of an ultra-capacitor is the energy density (energy per unit volume) is not as dense as most rechargeable batteries. All rechargeable batteries loose energy in their charge cycle and cannot be fully charged quickly. So a hybrid or electric vehicle designer makes a design choice of using an ultra-capacitor and/or rechargeable battery in their vehicle. Just like every other engineering design choice this is always a compromise decision.

ultracaps are used to multiply voltage since V is proportional to motor rpm by some sort of switching.

you need to step-up dc voltage to accelerate, if you are using a dc power from a battery supply.

Unlike, ac, you just need transformers to raise or lower the voltage.

You should be able to use the same technique used in switching power supplies - turn the circuit on and off with the appropriate duty cycle to maintain the required output voltage and then filter the pulsating output.

Also, I recall seeing a circuit that switches the connections between ultracapacitors from parallel to series configuration as the voltage drops - e.g. 8 parallel -> 4 parallel in series with 4 parallel -> series connection of 4 sets of 2 parallel -> 8 wired in series.

Interesting idea.

That can work but while simple in concept, you need many isolated switches that increase cost and losses.

In general, it is more cost effective to use a simple buck/boost converter that will supply the load with the voltage needed from the discharging or charging capacitors.

It is actually easy to boost voltage to a regulated level or bring it down once you have a pair of transistor switches (IGBT, MOSFET...), diodes, and an inductor. Couple those to a microcontroller through a gate drivers and your problem is solved... All you need is a few line of code. Easy!

Joke aside, the converter technology can be procured off the shelves for most applications.