I wrote a very simple modelplugin. I just want to see how model moves when I use the method "SetAngularAccel" But, nothing happened,"SetAnfularAccel"doesn't work, my model still rests there. Please tell me what's wrong,Thank you!

<?xml version="1.0" ?>
<sdf version="1.4">
  <model name="test">
    <link name="Link">
            <self_collide>true</self_collide>
            <pose>0 0 0.05  0 0 0</pose>
            <inertial>
                <mass> 0.1 </mass>
            </inertial> 
            <collision name="Base_collision">
        <geometry>
                    <box>
                        <size>0.1 2 0.1</size>
                    </box>
        </geometry>
      </collision>
      <visual name="Visual">
        <geometry>
                    <box>
                        <size>0.1 2 0.1</size>
                    </box>
        </geometry>
      </visual>
    </link>
    <plugin name="df" filename="/home/Dropbox/gazebo/torquetest/dist/Debug/GNU-Linux-x86/libtorquetest.so"/>
  </model>    
</sdf>

#include <gazebo/gazebo.hh>
#include <gazebo/physics/physics.hh>
#include <gazebo/common/common.hh>
using namespace gazebo;
class Arm_Control : public ModelPlugin
{
public:~Arm_Control(){}
    public: void Load(physics::ModelPtr _parent, sdf::ElementPtr /*_sdf*/)
    {
        this->model = _parent;
        this->updateConnection = event::Events::ConnectWorldUpdateBegin(boost::bind(&Arm_Control::OnUpdate, this, _1));
    }
public: void OnUpdate(const common::UpdateInfo & /*_info*/)
    {
    model->GetLink("Link")->SetAngularAccel(math::Vector3(0,0,1.5));
    }

private: 
        physics::ModelPtr model;
        event::ConnectionPtr updateConnection;
};

GZ_REGISTER_MODEL_PLUGIN(Arm_Control)