Power Supply Question 1

[nornrich]

Hey All,

I am working on the mechanical redesign of a power transmission unit. The unit is a DC motor coupled to a gearbox. The motor is a standard 3/4 HP, 12V motor that is powered during normal operation through a power supply that is converting 110VAC into 12VDC. The goal is to get the whole power transmission unit built for under $800USD. The main stopper is that the current power supply is around $500USD. This doesn't leave a lot for other things. We are considering developing our own circuit board for control and diagnostic functions of the power unit. I am wondering if it would be feasible to also develop a power supply circuit as well. The motor draws about 84 amps under steady load and I'm not sure about the in rush current on start up. We might also consider soft start/stop options for the control of the motor. Any suggestions or advice would be helpful.

Regards,

Rich......

Richard Nornhold, PE

 

[itsmoked]

That's pretty cheap for that!

Can you get out of the box to fix this?

I would instead find a less expensive motor that is three phase and then use an inexpensive VFD to create 3phase 240V power from single phase 120VAC. This would provide a boatload of other nifty features and probably cut half the cost out of your whole picture while increasing the reliability. It would ditch a bunch of big wire too.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[nornrich]

itsmoked,

I've never gone from 120V, 1phs to 240V, 3phs. I didn't even know that there were inverters on the market that did this. Of course that's why I'm an ME. The one thing I did forget to mention is that there is also a battery backup for this unit. This unit is being used for a wheel chair ramp in residential applications and in the event of a power outage we still need functionality. What say yea now? Thanks for the help.

Regards,

Rich.....

Richard Nornhold, PE

 

[Noway2]

One of the first stages of the inverter would be to rectify the 120V into DC. From there, a control circuit could handle the firing timing of three power circuits, generating the three phase. VERY BIG UPS systems do this all the time, generating 3 phase 480V to power entire buildings with racks of 12V lead acid batteries as a backup power source. You could do the same.
You will also find that most likely the system neeed to create the 3phase power is similar enough to VFD that once you solve how to do one, you also prety much have the other. If you would be looking for a starting point, take a look at TI's website in their DSP section and application notes. The C2000 family even has starter libraries to provide the rudiments of VFD systems.

 

[rtronics]

Gidday Rich.
Two things, you state motor current draw is 84 amps @ 12 volts steady state. This is pretty close to 1000 watts, one HP is 746 watts, so you are overloading that motor somewhat.
And if you have to have a battery for backup, why not use it for motor power, and just keep it charged up, from the mains.
ItSmoked has just the charger for you!!

Regards, Ray.

 

[Noway2]

Yeah, ItSmoked does have the charger for you.

I heard somewhere, too, that his charger may even become available for sale as a standalone product.

 

[nornrich]

Now I'm curious about the itsmoked charger. I was thinking about using battery power to normally cycle the unit, but wasn't sure what that would do to the SLA battery life? Right now we get about 30 cycles out of the two 12V batteries. If we run power through the batteries for normal operation what does this do to long term battery life?

Rich....

Richard Nornhold, PE

 

[itsmoked]

Hmmm maybe I do have a charger for you.
It's been thru a kind of Hell recently too, and I'm told survived!

You say it gets about 30 cycles on two batteries? Tell me you mean 30 cycles between needing to be charged to do another cycle!!??!?

If you do manage 30 between charges then there is absolutely NO reason you should be trying to provide the power directly from the line. That would be crazy.

All you need then is a charger that will keep your batteries floated at their correct float voltage and withstand being nearly shorted when the motor is activated.

A better charger will vary the float voltage to the immediate ambient temperature. This can add years to the battery life.

A good number to know for all this is the amp hours it takes for a cycle and how often these occur so you can make sure that in normal use you don't take the batteries down below (for your app) about %70 charge in the course of a non-exceptional but busy day. I also imagine up and down as being very different in the required amp-hrs/cycle.


SLA life... The life of an SLA as with flooded is a direct function of the depth of discharge cycles it's subjected to. If it never gets discharged more than about %85 I'd say there would be little impact on the batteries life beyond the normal float life of all SLAs. As you get down below that level you will start to whittle life out of them. You get down to %50, (though I can't imagine how), then you will see the minimum life time of about 500 cycles....


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com