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Running a PWM controlled Motor off a Step Up Converter

rcx_6000

Dec 31, 2015
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I have a model gas turbine that originally used a 12v ecu package. After the ecu died, I replaced it with a new autostart ecu package but it runs on 7.4v(2S Lipo). The starter motor is run using PWM at battery voltage by the ECU. The motor is struggling to spool the engine up and I was wondering if a step up converter set to boost the 7.4v-8.4v from the lipo to 12v would work. On a fully charged battery, the starter pulls ~3.5A max during startup. I'm not sure if the PWM on the input side would effect it and thought I'd ask before trying it and frying something. Thanks.
 

Harald Kapp

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I'm not sure if the PWM on the input side would effect it
I couldn't tell either as this depends on the design of the step up converter.
A suitable way would be to first step up the voltage to 12V and then apply the PWM to it.
 

BobK

Jan 5, 2010
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If PWM was used, that would indicate that the starter motor was designed to run off less than 12V. Did you try running it at full duty cycle (or just skipping the PWM) from the 7.4V battery?

Bob
 

rcx_6000

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The pmw is used to prevent burning out the clutch and controlling the speed until the propane is lit. It's 100% duty cycle once it starts ramping up to idle after the warmup cycle. Direct to battery makes no difference for rpm. I basically want to put a voltage step up between the ecu and motor and set it for 12 volts but I think that the pwm load control wouldn't work properly with it. Basically a DC version of using a transformer on a ac circuit to up the voltage is what I am looking for. I am also trying to find a 22/30 brushed motor that will spin at 28000 rpm no load on 7.4v and just replace the motor instead.
 

BobK

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Easier would be to provide a 12V supply and use the 7.4V PWM signal to control the 12V power to the load. All that is needed is a MOSFET.

Bob
 

rcx_6000

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Good idea Bob, but would I be able to use the same battery 2S Lipo split with a step up as the 12V? I can't see it being a problem since an electric RC vehicle using a BEC would be the same thing, just stepped down instead of up. I'm sure it would work fine but thought I'd ask for a second opinion before I go fry $400 worth of electronics lol. I will have to do a manual start with a 12v Battery to see how much amperage I'll need from the stepup before I order a stepup and mosfet.
 

rcx_6000

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Also if the source and gate are +12V and +7.4V respectively, I would want a N-Channel MOSFET?
 
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Harald Kapp

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Also if the source and gate are +12V and +7.4V respectively, I would want a N-Channel MOSFET?
When the souce is positive with respect to drain you use a P-channel MOSFET, when the source is negative with respect to drain, you use an N--channel MOSFET.
You then take care that the G-S voltage matches the selected type of transistor.


With Vs=12V and Vg=7.4V, Vgs is negative wich calls for a P-MOSFET.
 

rcx_6000

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Could you clarify your statement Harald? You mention Source to Drain and then Source to Gate. From your first statement, I get from it that Vs>Vd = P-Channel and Vs<Vd = N-channel but your second statement states Vs>Vg = P-Channel and Vs<Vg = N-channel. I'd imagine the Source to Gate relation is the correct one to follow?
 

Harald Kapp

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You mention Source to Drain and then Source to Gate.
Where do I mention Source to Gate? Not in the same context as Source to Drain. In my 1st sentence in post #8 I refer both times to Vds.

but your second statement states Vs>Vg = P-Channel
Why "but"? There is no contradiction:
Vgs = Vg-Vs
With Vs=12V and Vg=7.4V Vgs = 7.4V-12V=-4.6V and therefore a P-channel MOSFET is appropriate.

As you state it Vs>Vg is equivalent to Vs=Vg+x (x being any positive voltage >0V) and therefore Vgs=Vg-Vs = Vg-(Vg+x) = -x and from the prerequisite in parentheses "-x" is a negative voltage and therefore Vgs < 0V. Consequently a P-channel MOSFET is in order.
 

rcx_6000

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Just didn't catch the relation right away, I saw your response while busy at work so I didn't really think about it properly before responding. Wasn't questioning what you were saying. Now that I have stopped to think about it, I understand it now. Thanks again for the help. Sometimes I wonder if I have dyslexia, I occasionally flip things backwards in my head for no good reason lol.
 

rcx_6000

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wiringmosfet.png

Got a irf9z34ns p-channel mosfet which is rated for 55v 19a. Here's the problem, I hook up the motor's red wire to the drain, the ecu wire(+8.4 volts) to the gate and the 12v battery positive to the source pin and connect the 12v battery negative to common ground. The motor kicks in with or without the gate wire being connected and on top of that, the motor only gets around 8 volts. I'm not even gonna attempt to turn on the ecu. I get 4.3 megaohms on the source to drain when its by itself. Am I doing something wrong? Should this have been an n-channel or is it wired wrong? The starter is directly attached to the engine and shares a common ground with the ignition so this is the only way I can even see this working. I really don't want to have to use a relay for this.
 

rcx_6000

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Hmm well P-channel MOSFET's apparently are inverted so they are closed when the ecu signal is off which is the opposite of what I need. I was right in wanting to use a n-channel but I think a Solid State Relay would be a better choice anyways. Do they handle PWM well?
 
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