Pls Help: is there any way to calculate the surge in voltage upon an load removal?

Discussion in 'General Electronics Chat' started by Mentis, Dec 28, 2008.

  1. Mentis

    Thread Starter New Member

    Oct 27, 2006
    Good morning to all:
    I was troubled by a problem,
    if i have a 40 KVA load, upon application to a 115V 400hz power supply
    is there anyway i could calculate the drop of voltage upon application of the load or the "surge" in voltage upon removal of load.

    And is there a possibility to also find out the duration for the voltage to be stabilize after the removal or application of load ?
  2. kubeek


    Sep 20, 2005
    You can guess, or measure. Unless you know all parameters of the supply, like resistance, inductance and capacitance at the delivery point, you will have hard time calculating anything.
    Also you need to know ho does your load behave. Is it a huge motor, or some Dr. Frankenstein´s stacked capacitors? :)
  3. Mentis

    Thread Starter New Member

    Oct 27, 2006
    Thanks for your reply.
    Basically I am trying to find suitable power supply to support 40KVA load. Of course we can just double up the spec to 80KVA, but it will not be financially prudent to do so.

    Is there a baseline computation i.e. an norminal percentage that the surge or drop in voltages will be ?
  4. awright

    AAC Fanatic!

    Jul 5, 2006
    As stated by kubeek, you have to know much more about the detailed characteristics of the load AND the supply to even guess at the transients. Simply doubling the KVA rating of the supply is not even close to an appropriate answer.

    The "40KVA" rating of your load is merely an average current x average voltage rating and conveys no information that will allow estimates of current surges or voltage transients to be calculated or guessed at.

    The 400 Hz frequency implies an aircraft or mobile military application which implies a solid-state inverter or rotary inverter, in which case the characteristics of the power source will have a significant influence on how it handles load changes. A solid-state inverter may burn out or shut itself down if a large motor is switched across it. On the other hand, a solid state VFD (variable frequency drive) with speed ramped up and down at a controlled rate can start and stop a large motor load with minimal starting surge current or stopping transient. A motor-generator set may simply ride out the sudden application of load without a whimper because it has rotary inertia to help it carry over the load application and because most electric motors can produce much more than rated torque for a short period. A "service factor" rating above 1.00 indicates an enhanced capability to handle short-term overload, but even an SF 1.00 motor can handle some short-term overload. An engine-generator set, on the other hand, may stall or slow down severely on a sudden application of load near its rated power because an engine operating at rated power doesn't have much reserve torque.

    The type of load is significant because its characteristics will strongly influence what happens when it is switched. A motor will demand very much higher starting currents than running currents, the magnitude and duration being strongly influenced by the type or motor and inertia of load. Different types of motors have radically different starting currents. In fact, the motor type is sometimes selected for just that property.

    A large resistance heater bank that does not reach very high temperatures - say, a large water heater - might have a relatively low ratio of starting to running current because the resistance of the heater will not change much upon startup. Conversely, a large incandescent lighting load will draw a very large current for a few cycles upon starting because the cold filament has a small fraction of the resistance of the same filament at incandescent temperatures.

    A large capacitive load looks like a dead short circuit if switched across the supply at a voltage peak.

    A large inductive load like a motor or transformer can generate a large voltage spike of short duration upon disconnection from the supply.

    So, can you tell us any more about the load and the power supply?

    Alternatively, simply measure the surge current and transient voltage (many times to be sure you catch a peak) and optimize the transient suppression.

    Last edited: Dec 29, 2008
  5. Mentis

    Thread Starter New Member

    Oct 27, 2006
    Thank you very much Awright.
    I guess there is no easy solution to this. I will do more research on it.
    Perform the actual measurement if needed.
    Thanks again to all for your help.