Control HDD spindle motor

Lee Marshall

hi, i want to control a hdd spindle motor with a propeller.
the motor is(i believe) delta configuration with 4 connections:
phase 1
phase 2
phase 3
center/common

my design would involve connecting the center connection to V+(the motor voltage), and have the phases each connected to their own mosfet drain. all the sources would be connected to ground.
the mosfet gates would be connected to prop pins.

i would then control them with 3 shifting registers:
phase 1: 111000111000
phase 2: 001110001110
phase 3: 100011100011

what voltage should i run the motor on?
what current rating should the mosfets have?
what current rating should the protection diodes have?

the attached pic shows my circuit

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
I hear and I forget. I see and I remember. I do and I understand
-Confucius

mcstar

I believe HDD motors are driven by 12V (hence the need for a 12V connection).· However, your design will not work because the propeller cannot drive the Mosfets to full conduction since it can only drive 3.3V. You'll find that the o/p of your mosfet will never go over 3.3V with that config.· You'll need a mosfet driver that can operate with inputs down to 3.3V and can drive its outputs over the 12V source.· These drivers connect to the 12V source and (via a couple capacitors) they provide the brief current spike needed to drive the capacitive load on the Mosfet.· Check digikey (TC4426), I bought some of these recently, but haven't hooked them up yet.· They should work.

Lee Marshall

I never considered the mosfet ON-level(and probably never would have). that's gonna save me some time/money. thanks.

but i still need to know how much current these mosfets need to sink(and the diodes).

is it just as simple as going I = V/R?

7.06 = 12 / 1.7(the resistance of each coil measured with an ohm-meter)

i doubt it due to the whole induction thing(which i havent really looked into).
7.06 amps per phase? where the hell do i get that kind of current?
im making a wall clock here, i dont want to have to power it with a car battery.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
I hear and I forget. I see and I remember. I do and I understand
-Confucius

Post Edited (Lee Marshall) : 3/12/2008 6:44:39 PM GMT

mcstar

Mosfet's have a very low resistance when they are fully on ( < 1ohm) so, your current will be primarily controlled by the load.· Mosfets are rated by current and most have·quite high current capacities.· 7 amps may be within the capacity of your mosfet, check the datasheet.· However, the average current in a motor is not determined by it's resistance.· Since heat is·the enemy, current that spikes·quickly and returns to low·levels is not a concern.· It's long periods of high current that must be protected against.· Inductors (motor windings) are opposed to current changes.· So, they start off·extremely opposed to increasing current , but as their magnetic fields build the current quickly increases.· Once the field is built up, the inductor will appear like a short·plus the·wire's resistance, but since it begins to spin and you're only sending a small pulse·anyway, this mean the high current state is only happing for a small slice of time.··This is true as long as the motor is free spinning.··Add a load, and the average current consumed by the motor will start to increase.· IF you held the motor shaft stationary, then you'd get the situation where the mosfet is sourcing a large amount of average current.· This will cause it to overheat.· A sink will help and/or you can parrallel your mosfets to get greater current capacity.· IF you don't need to push a large load, a single mosfet will most likely work just fine.· The other thing Mosfets don't like is staying in the quasi-on state.· They like to be On or Off.· The longer it takes them to switch, the more heat is generated.· This is why a Mosfet driver is highly perferred.· In high speed switching (PWM) scenarios, you want the switch to happen very fast.· This keeps the mosfet cool

Post Edited (mcstar) : 3/12/2008 8:49:21 PM GMT

Ale

Those are (I pressume) brushless motors. Get yourself a prebuilt brushless controllers. The hdds I have have 4 leads to the motor, so I always assumed they were two windings steppers. i.e. easier to control.