If freeSpin is enabled the motor will only accelerate but never slow down.

	hingeJoint.motor.freeSpin = false;

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Example() {
        hingeJoint.motor.freeSpin = false;
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Example() as void:
		hingeJoint.motor.freeSpin = false