Measuring hazardous DC voltages on my new race bike

Discussion in 'General Electronics Chat' started by jonescg, Jan 18, 2013.

  1. jonescg

    Thread Starter New Member

    Jul 24, 2011
    16
    0
    Hi guys!

    It's been a while since I've needed to post here. Last time I posted I was talking about a thermistor circuit for my electric race bike. Well anyway, that bike has been put out to pasture and I have already started work on the next bike. I did pretty good, including winning two races at Winton raceway in 2011. I didn't race in 2012 because of a lack of time, and now I have stacks of time and no money. Oh well.

    Now, about the new bike!

    This bike will be running a very powerful synchronous AC motor and as such, needs lots of power from a big variable frequency drive. This drive in turn needs lots of volts to work the motor to it's full potential. And potential is the word of the day here.

    The battery pack will be charged to a maximum of 700 V DC. It will settle to about 650 V during use, and it's considered spent at 590 V. Don't worry, there are multiple levels of isolation, both physical and electromechanical. But during use, I still need to know what the voltage of my pack is.

    The instrument panel needs four critical diagnostics - AMPS, VOLTS, SPEED (or RPM) and AMP HOURS. Amp hours is a tough one, as most amp hour meters have voltage sensing built into them, and they are not rated to 700 V!

    And here's the rub - I can't find a safe way to measure 700 V DC from the battery pack. Sure, I can run silicone wire inside a HV conduit up to the instrument panel of the bike, and mount a digital panel mounted voltmeter there, but the voltage sense leads are about 2.5 mm apart. That's just too close! Even if I fused it, 70 cm of wire would make an epic plasma ball if it shorted.

    My question is, can I build a voltage divider into the pack and run say, 70.0 V or even 7.00 V up to the instrument panel, and somehow multiply this back up to read 700 V on the display? The decimal place is deliberate here, as I want 6.54 V to read 653 on the display. Most panel mounted meters have dividers built into them. Is there a way I can get around this?

    I have a 75 mV shunt and I will be placing an ammeter on the instrument panel too. At least there's only one HV contact up there.

    Any ideas?
     
  2. Teiknethal

    New Member

    Jan 9, 2013
    9
    0
    Why not have the system controlled via a uC like an Arduino? Not only could you wire the basic outputs (V, A, AH, RPM) into it, but you could also have it store and calculate plenty of useful diagnostics and data. To give an example, you could have it calculate the % efficiency of your complete system at a certain speed. If you go that way then a simple voltage divider down to an Arduino in pin, have that multiply it up and readout. That would be the way I would go.

    If not, you could just drop the voltage down to 7.00 like you said and then just read it like that? 6.53 = 653 volts, nothing complicated about reading. Considering it is a race bike I am going to assume that you're the only one riding it often enough to worry about voltages, and if you know that you have to multiply by 100 whats the problem?
     
    Last edited: Jan 18, 2013
  3. jonescg

    Thread Starter New Member

    Jul 24, 2011
    16
    0
    Hi Teiknethal,

    Yeah, the best device out there is a Cycle Analyst, which does all that and more. It also uses a voltage divider and there is a Vsense setting you can adjust to bump the recorded voltage up to what it needs to be. Unfortunately it stops at 650 V max.

    The other downside is the fact that it requires you to flip through screens to get to what you want to see - not entirely practical on a race bike. Hence the four basic screens.

    I think the 7.00 V reading will be fine for voltage, and the ammeter is only running fro ma 75 mV shunt so while it is connected to the hazardous voltage circuit, it's still fairly well isolated.

    No, a simple panel meter which records amp hours is all that remains. Exceeding 15 Ah would be dangerous, as these LiCo cells don't like it. I think the next innovation here would be a 72 mm x 40 mm panel mounted amp hour meter, which is able to run independently of the system voltage (but still use a shunt or Hall effect sensor). Any ideas how you would do this?

    My electronics knowledge is getting better by the week, but still pretty basic.
     
  4. spankey666

    Member

    Nov 30, 2011
    91
    3
  5. jonescg

    Thread Starter New Member

    Jul 24, 2011
    16
    0
  6. spankey666

    Member

    Nov 30, 2011
    91
    3
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