How to determine the rating of a capacitor for a specific purpose?

[arkaddas]

I need to determine the capacitance for a capacitor that I will be connecting in series with a DC series motor to control the current drawn from the battery by the motor. How should I proceed?
I have attached the schematic.

 

[NascentOxygen]

Hi arkaddas.

You don't connect a capacitor in series with a DC motor. It won't spin for very long if you do.

 

[Windadct]

Hello Ark... a schematic always helps - I just found another online tool Scheme It ( http://www.digikey.com/schemeit )

 

[arkaddas]

There's a diode in parallel with the capacitor.

Actually I was searching for a way to increase the acceleration of the motor. According to me, the speed controller will determine that but my teammate insisted that capacitors are used to increase the acceleration of motors and he also found out a link that convinced me to give it a thought. I have attached the schematic.

 

[Windadct]

How will you turn the motor on and off? I have a feeling the "diode" you show is not really what you are looking for, the capacitor seems to be more of a snubber (absorbing energy as the motor is turned on and off) and the "diode" a switch?

 

[arkaddas]

I took the schematic from a presentation that was prepared by MIT students so I don't have doubts about it. However I cannot understand the function of this circuitry. Here's the link to the presentation :- http://web.mit.edu/first/kart/everpres.pdf

 

[Windadct]

Well I certainly have doubts about the original schematic. The schematic now - is still really a block diagram and not complete enough to work with. This presentation is about using a Ultra-Cap for bulk energy storage in conjunction with batteries - since we are talking energy storage, typically the more the better, but you pay of it with size and weight. The later schematics - showing the switches for control, are closer but still not close to the whole system. If you look at the photos of their systems there is clearly a lot more going on than in the schematic you first posted.
Sizing the caps comes down to how much energy do you expect to store, in this case from regen braking, so the sizing is almost more of a mechanical calculation: Max Speed, Max Weight, inertia, total kinetic energy that needs to be regenerated back into the cap. They are using a different topology - series vs Parallel connections, and the controller needs to actively regulate the braking effect and current flow INTO the Ultra-Cap.
Also this motor is actively field controlled - another level of complexity - I would probably avoid on a cart.

 

[arkaddas]

I see. Can you help me understand the real purpose of using the capacitors? I was planning to simply control the dc motor with a chopper circuit without introduction of capacitors but the problem that arises is quick drain of the battery.

I have another doubt. Will the motor consume more energy at higher speeds or at lower speeds? From the torque speed characteristics what I could understand is that torque will be less for high speeds so in my opinion power required should be less. But I'm still unsure about this conclusion.

 

[NascentOxygen]

It the motor is driving a load which itself is harder to turn as it rotates faster, such as a fan or wheels, then higher speed demands higher power.

The purpose of the ultracap is to store regenerative energy during dynamic braking, then use that stored energy to assist the battery during powered drive. If your motor is not doing much braking, then the ultracap circuit won't be of any use.

 

[arkaddas]

Thanks for the explanation about the ultracapacitor. It was really helpful. :)

 

[arkaddas]

What can I do to accelerate the motor?

 

[NascentOxygen]

What can I do to accelerate the motor?

Typical DC motors are very robust. You can often overdrive them (i.e., power them above their rated power) for a short time without ill effect, so long as you then run them at reduced drive so they can cool down.

However, there is no magic for getting a continuous 20HP out of a 5HP motor.