Ball Speed is not increasing as per code

I am working on small project , but now I have a problem . The problem is The Player speed (Ball speed) is not increasing as per the code .

In the beginning ball speed increases , but as the time goes the speed becomes constant .

I made Gravity as -50 in Y - Axis .

So here is the C# Script attached to the ball :-

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class Ball_Script : MonoBehaviour 
 {
     // Ball Physics Variables
     private Rigidbody Ball_Rigid;
     public float Ball_Speed = 10f;
     private float Speed_Change_Time = 1.0f ;
 
     // UI Text
     public Text My_Ball_Speed_Text ;
     public Text My_Key ;
 
     // Use this for initialization
     void Start () 
     {
         Ball_Rigid = GetComponent<Rigidbody>();
         My_Ball_Speed_Text.text = Ball_Speed.ToString () + " km/h";
         My_Key.text = "No Key Pressed";
     }
     
     // Update is called once per frame
     void Update () 
     {
         // Restart game
         if(transform.position.y < -7.0f)
         {
             Application.LoadLevel(0);
         }
         
         // Ball Automatic Movement script Starts here
         Speed_Change_Time_Interval ();
         Ball_Rigid.AddForce (new Vector3 (Ball_Speed * -1 * Time.deltaTime, 0, 0));
         // Ball Automatic Movement script Ends here
         
         if(Input.GetKeyDown(KeyCode.LeftArrow))
         {
             Ball_Rigid.MovePosition(new Vector3(transform.position.x , transform.position.y , transform.position.z - 1.0f));
             My_Key.text = "Left Arrow Pressed";
         }
         
         if(Input.GetKeyDown(KeyCode.RightArrow))
         {
             Ball_Rigid.MovePosition(new Vector3(transform.position.x , transform.position.y , transform.position.z + 1.0f));
             My_Key.text = "Right Arrow Pressed";
         }
     }
 
     // This Fixed update stops the ball from bouncing
     void FixedUpdate()
     {
 
         Vector3 currentVelocity = Ball_Rigid.velocity;
         
         if (currentVelocity.y <= 0f) 
             return;
         
         currentVelocity.y = 0f;
         
         Ball_Rigid.velocity = currentVelocity;
     }
 
 
     // This function increases the ball speed in every 1 second 
     void Speed_Change_Time_Interval ()
     {
         Speed_Change_Time = Speed_Change_Time - Time.deltaTime;
         
         if(Speed_Change_Time <= 0)
         {
             Ball_Speed = Ball_Speed + 10;
             Speed_Change_Time = 1.0f;
             print(Ball_Speed);
             My_Ball_Speed_Text.text = Ball_Speed.ToString ()+ " km/h";
         }
     }
 }
 

Here FixedUpdate () stops the ball from bouncing and Speed_Change_Time_Interval () function increases the ball speed in every second . But it works some extent . Then there is no change in speed of the ball . Here is the video of the game ( Recorded with Jing, so file is in .swf format . Sorry for that :( )Link :- http://www.mediafire.com/download/ibk4ffjm3y6i96y/Ball_Vid.swf

File size = 9.28 MB

So what is the problem of this script . If my method is wrong / not good please suggest me a good one :).

Hope you will help me to figure it out .

Thanks , Regards

NB :)