If you want to install Ferro Fluid in your large direct-drive hubmotor, this is a pictorial to help you do that. Ferro Fluid (FF) doesn’t help geared hubmotors, but for the large double D’s (direct drive), a dose of FF can dramatically improve the amount of temporary peak amps you can shove into it.
Dis-assemble motor, or drill a hole?
The are two ways to get FF inside your hubmotor. You can just drill a hole into the sideplate and squirt it in, or…you can disassemble the motor. If you decide you are going to pull the hubmotor apart, mark the sideplate so you can re-assemble it in exactly the orientation that it came apart. It can be something as simple as a scratch made across the joint where the sideplate and the rim meet, or…(as shown below) I like to drill two shallow dimples next to each other.
If you are going to be taking your hubmotor up to the maximum power limit that it can survive, you may want to install a temperature probe (even if it takes years before you actually hook it up to a digital thermometer). In that case, you will likely be taking the motor apart.
The stator of the motor will have a power cable exiting the hub-case somewhere near the axle on one side or the other. Loosen the bolts holding the core into the hubmotor on the cable side (seen below). All the large brushless hubmotors I have seen have no physical connection between the stator and the rim, so when I remove the sideplate bolts and then push down on the rim, the central core pops out.
Be very careful. The steel in the stator core and the rim-magnets are very strongly attracted to each other, and you can have your fingertips cut off if they slam back together. Just lift the core out by the central axle.
Once you get the stator out of the rim, it’s pretty safe, just don’t put your credit card or ATM card next to the magnets…they are very strong!
This is not as hard as it may seem, just use the right technique, and exercise caution.
One important thing to realize is that…notice in the pic above that the FF is not dripping. It works because it is drawn to the magnetic fields in the magnets. This is the feature that will be important to understand, when you read the section below.
Notice in the pic above, the rim of the wheel is resting on two chairs so there is nothing interfering with the stator and axle sliding right into the rotor. Hold it by the central axle, and you will live to an old age with all of your fingertips intact. Check for the alignment mark you made earlier, and slap the two parts together.
No disassembly, just drill a hole?
In this pic below, I am drilling a hole into the sideplate so I can install a vent. When the motor gets hot (and brother, I will definitely be getting it HOT), the air pressure inside will rise, and that can affect the motor in a variety of bad ways. One way is that when it cools off, it can draw in humidity, which accelerates corrosion inside the motor. Another problem is that…if there is no air-pressure equalization vent, a build up of interior pressure can force FF or liquid coolant to leak out of every joint..
I’m using a 1/4 inch diameter drill bit because I want to epoxy-in a threaded brass vent, but…it really only needs a pinhole for air to escape when it gets hot.
In the pic above, I located a position for the hole I wanted to drill that was nearer enough to the axle enough that…I couldn’t accidentally drill into the copper windings of the stator (near the rim).
If you aren’t going to dis-assemble the motor to install FF and a temp sensor, there is one caution I recommend if you are simply going to drill into the sideplate of a DD hubmotor. Use some type of “drill stop”. In the pic above, the left side shows a professional drill stop that you tighten onto the bit.
When I was young, I was …lets just call it…”on a tight budget”? So…I sympathize with enthusiasts who don’t have a lot of the “proper” tools. On the right is a pic of a 1/4-inch drill bit that has been shoved inside the plastic sleeve taken from an ink pen. This is a cheap and easy way to make sure you can only drill down the exact amount needed…and no more.
Neither one of these drill stops will prevent you from getting a few aluminum shavings inside the motor, but…you definitely don’t want to drill into the copper windings. I have even heard of hot rodders who laid down and held the hubmotor above them , and drilled in an upwards direction to ensure the shavings “fell out” of the hole when drilling (wear glasses, and swim goggles would be even better).
I had a natural desire to disassemble the motor and gently apply the FF to the entire magnet array one at a time. However, once I did that?…I can assure you that you can simply dump it in though a small hole and the FF will spread out evenly on its own. In fact you couldn’t prevent it from spreading out onto the magnets if you tried. The magnets and FF are drawn to each other like two dogs in heat (Bow chicka bow wow!)
Air is a poor heat-transfer medium, and there is normally a thin layer of air between the hot stator and the aluminum sideplates. FF is a great heat transfer medium that is attracted to the magnets in the rim (no drips!), and it fills that air-gap…providing a heat-bridge from the core to the outside. By increasing your hubmotors ability to shed heat, you can double the amount of watts you can feed it for those first few seconds of acceleration after the light turns green.
FF doesn’t do a whole lot for the long term continuous heat-shedding of the hubmotor (long uphills), but…on relatively flat land? FF can dramatically improve how much power you have to accelerate, without needing to swap to a larger motor.
Written by Ron/spinningmagnets, February 2018