function rotatad(iframe_id)
{
    var random_time = (Math.random())*120000;
    random_time = Math.floor(random_time);
    var fixed_time = 30000;
    var waiting_time = random_time + fixed_time;
    if(waiting_time<30000)
    {
        waiting_time=30000;
    }
    var javascrexpr = "exec_rotatad('" + iframe_id + "')";
    var t = setTimeout(javascrexpr,waiting_time);
}
function exec_rotatad(iframe_id)
{
    var thiselement = document.getElementById(iframe_id);
    if(thiselement!=null)
    {
        var thecurrent_src = document.getElementById(iframe_id).src;
        //reassign source
        document.getElementById(iframe_id).src = thecurrent_src;
        //call another turn
        rotatad(iframe_id);
    }
}