Aquaticus ROV

Homebuilt underwater robot

Brushless DC motor

Introduction

A brushless DC motor is electric engine where the stator is a classic 3 phase stator and the rotor has surface mounted permanent magnets. It is equivalent to a reversed DC commutator motor, in which the magnet rotates while the conductors remain stationary. BLDC motor has no commutator and brushes, it requires electronic control for operation. They often have external position sensors used by control unit, but sensorless control is also possible.

Where to find?

Very popular source of small BLDC motors are old CD-ROMs, floppy drives and streamers. To get more power from these small engines many people replace original magnets with strong neodyme magnets and rewind the winding. You should look for motors where housing and magnets rotate (they are easer to dismantle).

Old CD-ROMs: Cheap source of brushless motors $3 types of BLDC motors: 5¼ floppy (biggest), streamer and CD-ROM (smallest)$ 3 types of BLDC motors: 5¼ floppy (biggest), streamer and CD-ROM (smallest)

BLDC in underwater environment

In theory, you could put BLDC motor into water and it should work (assuming you isolate wire connections). Wires and stator are isolated. But in case of a mechanical damage, you can easily destroy isolation of wires (it is in fact an isolation paint). A better isolation is needed. The easiest solution is to put stator into epoxy. Compared to classic DC motor you only need to isolate stator! No o-rings nor magnetic couplers!

Summary

Advantages

Magnetic coupler like isolation
No gearbox, speed controlled by electronic unit
Good size/power ratio
Water cooling

Disadvantages

Modifing CD-ROM/floppy motors can be tricky
Need dedicated BLDC controller

My motor

My plan is to build complete engine for underwater wehicle based on modified CD-ROM motors.

Propeller, stator and rotor: Incomplete motor with propeller

Motor

Replace original magnets with strong neodyme magnets
Rewind
Isolate stator
new rotor and bearings (optional)

Controller

After examing BLDC projects aviable on the net I decided to build my own controller. See BLDC controller page for more details.

Step by step instruction

I did not build any working engine, so this description is not yet complete. Check this page time to time to find more details.

Preparation

Below is the list of items you need to modify CD-ROM motor:

Old CD-ROM/DVD drives, cost 0€
Small neodyme magnets, cost 1€ for 40
Two types of glue:
1. “Superglue” like, to quickly and temporary attach magnets to rotor, cost about 1€
2. Epoxy glue, to permanently attach magnets to rotor, cost about 2.5€
Optional lathe, to make own rotor or new bearings
Hammer, vice, drill, screwdriver, etc.

Epoxy glue: 2 components glue, permanently attaches magnets to rotor Superglue: To temporary and fast attach magnets to rotor

Removing motor from CD-ROM

That is easy part. Use screwdriver and remove motor that spins the disk. It is always located exactly in the centre of the box. All parts can be dissasembled using only screwdriver, you do not need to use hammer or things like that.

CD-ROM without casing: First step to get the motor from CD-ROM drive

CD-ROM parts: Motors and lasers from 3 different types of CD-ROMs

3 motors: BLDC motors from different CD-ROMs

Motors bottom view: BLDC motors bottom view

Dissasembling motor

This is the moment you can use the hammer and vice. Motor has no servicable parts, so brutal force is the only method that can be used. The best method is to use vice and additional tools to slowly separate parts. If you use hammer, use it “softly” in a way you do not damage bearings or stator. It is very important that you do not damage stator, it is the only part that can not be replaced.

Motor in a vice: The best method to dissasemble motor.

Motor PCB: Base PCB with stator (rotor removed). There are 3 small Hall sensords on the right, close to stator.

BLDC motor in two parts: Separated rotor and stator from CD-ROM drive

Motor without PCB: PCB was separated form stator

All parts of motor: Separate parts of CD-ROM motor.

Usefull BLDC parts: Separate BLDC parts, stator without wires.

Usefull parts

Streamer BLDC: Dissasembled streamer motor

Replacing magnets

First remove old magnet from rotor. Original magnet is gum like substance, so it is easy to remove it with screwdriver. In place of original magnet, place a numer of strong neodyme magnets. You must select size of magnets to fit properly empty place. Air gap should be minimal. I use 5x5x1mm magnets. Number of used magnets must be choosen carefully, see link sections to read more.