# Brushless DC Motor Formula

Hello to all.
I want to make a simple speed driver for BLDC motor, But I don't know so much about Brushless DC motor.
I want to control speed of motor under nominal speed by control DC link voltage.What formulas I need for Brushless DC motor to control the speed of motor under nominal speed.
Thank you.

What do you mean by "make"?
Sounds like you think it's a software problem?
Look up Electronic Speed Controls or digital to analog convertors. These are hardware components you need.

Can you say what you are trying to do ?

Else where you have posted a fan speed of 12000 RPM and a power of 350 watts.

Are you trying to build a jet engine ?

Brushless and brushed permanent-magnet motors basically behave the same, no-load speed proportional
to voltage, torque (less friction) proportional to current. With a brushless motor you need correct
commutation for this to be a good approoximation.

MarkT:
Brushless and brushed permanent-magnet motors basically behave the same, no-load speed proportional
to voltage, torque (less friction) proportional to current. With a brushless motor you need correct
commutation for this to be a good approoximation.

Isn't a brushless motor one that is effectively a 3-phase motor and is therefore dependant upon the frequency of the power supplied. This itself may be a simple function of a DC signal applied to the DC-AC driver.

Brushless DC motors with inbuilt frequency generators, such as fans, may well be supply voltage dependant but there will be quite a range of non-linearity at the lower end of the supply voltage since the electronics will require a minimum voltage to activate.

A brushless motor can be driven with an open-loop control system as jackrae mentioned but without the controller knowing the rotor position everything will break down when the load changes. It will spin but lose sync as soon as any force is applied.

As soon as anything were to slow down the motor ( mechanical load ) the "frequency generator" would have to compensate.

Even simple computer fans have either hall sensors or use back-emf to determine rotor position for proper commutation. Normally the driver circuitry is on a small pcb in the hub of the fan itself.

jackrae:
Isn't a brushless motor one that is effectively a 3-phase motor and is therefore dependant upon the frequency of the power supplied. This itself may be a simple function of a DC signal applied to the DC-AC driver.

No, because you are commutating. Hence my comment about correct commutation above. The power
supplied is DC to the motor driver anyway!

Commutating is the terminology for switching the DC supply into discrete phases (generally 3 on an RC out-runner motor) to excite the stator poles in turn. The PM rotor then rotates endeavouring to sync with this "rotating" flux field.

3-phase BLDCs have 6 phases, not 3. DC brushed motors commutate mechanically.