hingeJoint.motor.targetVelocity = 5;

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Awake() {
hingeJoint.motor.targetVelocity = 5;
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Awake():
hingeJoint.motor.targetVelocity = 5