Running a Linear Actuator at a Higher Voltage

[Mastermime]

Hello everyone,

I've got a bit of an issue.  The power supply (two 12v batteries hooked in series) I'm using is 24v and I'm looking to purchase these actuators
http://www.surpluscenter.com/item.asp?item=5-1759&catname=electric

The problem these are said to operate at 12v.  I have looked into DC-DC Step down converters, however the problem is this actuator draws 24 amps under full load.  When you have to regulate that amount of amperage, converters get pricey (150+). 

So my question is can I run this regulator at 24v?  I know for the majority of times, you don't want to do that often fries components, but I've heard with scooter motors, it doesn't affect them too much, just increases the speed?  I assume there is a correlation between the linear actuator and a motor, but I could be wrong.

If I can't run the actuator at 24v, what other options do I have besides the stepdown converter?

Note: I can't change the supply either.

[jwatte]

You can probably run it at 24V, and it will probably run faster. One thing to worry about is if there's any logic/sensors that may not be happy with 24V in, and need regulated voltage. Hopefully that's broken out to separate supply from the actual motor power if that's the case. Another thing to worry about is lifetime; the lifetime of the motor may go down by a factor of 10 with a doubling of voltage (depends on the motor.)

You should expect it to generate about four times as much heat at 24V than at 12V. This means you have to divide the "on" part of the duty cycle by four, keeping the overall time the same, to have the same heat profile as when run with 12V. If you can keep this in check, you will mitigate some of the lifetime issues.

If you can find a PWM controller that can switch the 24V for you, you can run it at 25% duty cycle, and get the same effect as if you were feeding it 12V. That would probably be better. 30 Amp PWM controllers are also pricey, but probably less than $100.

Btw: What are you using to turn on/off this actuator? Maybe that can already be switched fast enough to work as PWM?

[Mastermime]

Jwatte,

Thanks for the analysis

Btw: What are you using to turn on/off this actuator? Maybe that can already be switched fast enough to work as PWM?

I was originally going to use relays, but now that you mention this, MOSFETs with proper heatsinking may do the trick.

[greywanderer012345]

   Also, putting a capacitor across the leads will smooth out the voltage. I can't remember the math for figuring out what capacitance you want, though. With PWM and the right capacitor, you can get close to any steady voltage lower than your source.

   HOWEVER, since you have 2 12V batteries in series, you can just get the power for these actuators from the positive lead that is also connected to the negative lead of the second battery. This is done in many electric vehicles, and it's one of the advantages of having batteries in series rather than just one of higher voltage.

[Mastermime]

GrayWanderer,

Are you saying I do something just like this (see quick paint drawing below)?  That seems very simple and appears to be the best solution

Thanks for the advice!  I didn't know you could do that with series batteries.

[jwatte]

The device will see zero volts, as you're shorting its input :-)

You want to move the "-" lead to the "-" that's on the bottom-left of the leftmost battery.

Also note that the current draw on that battery will be higher than the draw on the top battery. Thus, it may discharge quicker.

[Mastermime]

The device will see zero volts, as you're shorting its input :-)

You want to move the "-" lead to the "-" that's on the bottom-left of the leftmost battery.

Also note that the current draw on that battery will be higher than the draw on the top battery. Thus, it may discharge quicker.

Oh ok.  Thats what I figured.  Yeah that one battery discharging quicker may cause issues so I think I will go with the PWM route.  MOSFETs will work for that, right? 

[jwatte]

Yes, MOSFETs are great for PWM, assuming you can drive them on/off quickly enough.
I suggest something like a IR2101 driver IC as the driver for a high-side N-channel power MOSFET switch.
You can generate the input to the IR2101 using a microcontroller, or a 555 circuit, depending on how/if you need to vary the duty cycle.

[Pogertt]

Ohms Law says at 12 volts and using 24 amps, the resistance of the motor is .5 ohm.
Volts times amperes = Power in Watts  12 * 24 = 288

The resistance of the motor is not changed by applying different voltages, so 

Doubling the voltage from 12 to 24 volts will increase current from 24 to 48 amps.

It will also increase power in watts from the original 288 to 1152 watts.

This is 4 times the heat generated at 12 volts.

Another spec listed is duty cycle.
It is listed at 25%.
If the motor is ran for 15 seconds, it will require 45 seconds to cool down.

[jwatte]

So, that was already covered above...

Another spec listed is duty cycle.
It is listed at 25%.
If the motor is ran for 15 seconds, it will require 45 seconds to cool down.

As I suggested above: If you run it at 24V, at four times the power (without PWM) then you need to run it at one-quarter the duty cycle: Run for < 4 seconds and then wait > 56 seconds for it to cool down, as you will be generating 4x the heat.

[Mastermime]

Alright thanks for the help everyone.  This problem is solved

[greywanderer012345]

Here's another way to get real 12V to each of your 2 actuators. As long as each actuator is being used similarly, the batteries should drain pretty evenly.

[Mastermime]

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