This tutorial will explore one of the many functions of Reactor in 3DS Max, which is a powerful tool that lets us simulate various real-life physics so that we can, for example, simulate physical behaviours such as wind, wheels and motors. We will learn in this tutorial how to create a simulation of a fan with the help of the Reactor Motor, Point Constraint, CSolver, and the Rigid Body Collection. I will show you how to use these reactors to create an animation similar to the one you see below. This is an intermediate level tutorial, make sure that you know the 3DS Max basics before attempting this tutorial.
Motor is a reactor helper that allows you to apply a turning force to any non-fixed rigid body in the scene. You can specify the target angular velocity and the maximum angular impulse the motor can apply to achieve this velocity.
This tutorial will only cover the animation aspect and will not involve any modelling, you can download the fan model for this tutorial from this link.

Once you have the model ready in the scene, access the reactor panel and click on the Rigid Body Collection button as illustrated in the image below.

The Rigid Body Collection is a reactor helper object that acts as a container for rigid bodies. Rigid bodies are the basic building blocks of reactor simulations. For more info please refer to the the help file that comes with 3DS Max.
Click anywhere in the scene to create the Rigid Body Collection, this should make the properties rollout appear, we will add our rigid bodies, so click on on Add and then click All to select our blade and fan, click on the Select button when you're done. Check the image below for more details.

This should set the blade and the fan as rigid bodies, check that both of them are visible in the RB Collection panel as illustrated below before moving to the next step.

We will now create the 'Point Constraint' to our fan blades. Simply click on the specific icon in the Reactor Panel as illustrated in the image below.

A point-to-point constraint lets you attach two objects together. It forces its objects to try to share a common point in space. The objects can rotate freely relative to each other, but always have the attachment point in common
While 'Point Constraint' is sill selected, look for its Properties rollout in the modify tab. Click the none button next to Child and select the fan blades. This should assign the target to Point Constraint.

Go back to the Reactor panel, we will now use the Reactor Motor, access the Reactor Panel and click on the icon highlighted below. The Reactor Motor will add a spinning rotation effect to our fan blades.

Motor is a reactor helper that allows you to apply a turning force to any non-fixed rigid body in the scene. You can specify the target angular velocity and the maximum angular impulse the motor can apply to achieve this velocity.
Create the Reactor Motor anywhere on the scene. Access its properties and click on the button labeled None next to Rigid Body. Click on the Fan Blade and then set the Ang Speed to 20, the Gain to 4 and the Rotation Axis to Z. You can alter these values to have your own customised effect.

Access the Reactor Panel once more and this time click on the CSolver icon as illustrated in the image below.

The Constraint Solver (CSolver) acts as a container for the cooperative constraints in a particular rigid body collection, and performs all the calculations necessary for the constraints to work together.
Create this one anywhere on the scene. Access its properties through the modify tab, click on the RB Collection button and then choose Rigid Body Collection as illustrated in the image below. The button should say 'None' at the start, click it to select Rigid Body Collection.

It's now time to add a Point Constraint to our solver. To do this you will simply need to click on the Add button and then click on All and Select as illustrated in the image below.

This should add the Point Constraint to our Solver list as illustrated in the image below.

We will now have to set the various physical properties of our fan blades such as the mass. Start off by selecting the fan and then access the Reactor Panel and click on the Open Property Editor button to open the Rigid Body properties ******** Set the Mass to 5 and then set the Simulation Geometry to Mesh Convex Hull as illustrated in the image below. The mass affects the speed of the rotation as heavier masses would make the fan spin slower, while lighter masses would make the fan spin faster.

The Simulation Geometry rollout tells Reactor which geometry (physical representation of your object) is to be use for the rigid body. Mesh Convex Hull is the default option where the object’s geometry is passed through an algorithm to create a convex geometry using the geometry’s vertices and which completely encloses the geometry’s vertices.
Everything is now set, click on the button highlighted below to test the animation. This should open up a test window, simply hit P on your keyboard to play the animation.

We are now ready, to pass our simulation to a timeline as an animation, simply click the button highlighted in the image below. This process should not take more than several seconds. When it finishes, click on the Play button to watch the animation.

I hope that you learnt something helpful about reactors in this tutorial