# Direction of rotation, brushless or brushed DC motor

• mjbourquin
In summary, the direction of spin in motors is determined by the commutator on the rotor, which changes the poles just in time. At startup, the direction is initially unknown, but a 3 or more phase brushless motor controller assumes an initial position and then cycles forward until enough speed is reached to switch to using back emf as input. This issue is solved by having 3 or more poles in the motor.
mjbourquin
I still don't understand why these types of motors tend to spin in one direction or another. In theory the direction of spin should be random since when the north pole of the rotor magnet is lined up with the north pole of the electromagnet in the stator the rotor could be repelled clockwise or counterclockwise and so could spin in either direction. Then in another half a cycle the direction is known because of momentum but at the start how do you know?

This is because of the commutator on the rotor. It changes the poles just in time, by sending the current to another coil.

mjbourquin said:
In theory the direction of spin should be random since when the north pole of the rotor magnet is lined up with the north pole of the electromagnet in the stator the rotor could be repelled clockwise or counterclockwise and so could spin in either direction. Then in another half a cycle the direction is known because of momentum but at the start how do you know?
You don't know which direction the motor will initially move. For a 3 (or more) phase brushless motor controller, the start up cycle just assumes the initial rotor will line up with some phase position after 1 or 2 steps using fixed delays between the first steps, then cycles "forwards" at an ever increasing pace until enough cycles and speed has occurred to switch to using back emf as input.

I'm not sure how a DC brushed motor with just two poles avoids getting stuck in a short circuit and/or zero torque state at startup. 3 or more poles solves the issue.

## 1. What is the difference between the direction of rotation for a brushless and brushed DC motor?

The main difference is in the way the motors are powered. Brushless DC motors use electronic commutation to control the direction of rotation, while brushed DC motors use physical brushes and a commutator. This means that the direction of rotation for a brushless DC motor can be easily reversed by changing the polarity of the power supply, while a brushed DC motor requires the physical reversal of the brushes.

## 2. How is the direction of rotation determined for a brushless DC motor?

The direction of rotation for a brushless DC motor is determined by the order in which the electronic commutation switches are activated. By controlling the timing and sequence of these switches, the direction of rotation can be easily reversed.

## 3. Can the direction of rotation be changed for a brushed DC motor?

Yes, the direction of rotation for a brushed DC motor can be changed by physically reversing the brushes. This can be done by either manually switching the wires or using a reversing switch.

## 4. What are the advantages of a brushless DC motor over a brushed DC motor in terms of direction of rotation?

One advantage is that the direction of rotation can be easily controlled and reversed for a brushless DC motor using electronic commutation. This eliminates the need for physical brushes and commutators, which can wear down and require maintenance. Additionally, brushless DC motors tend to have a longer lifespan and higher efficiency compared to brushed DC motors.

## 5. Are there any limitations to changing the direction of rotation for a brushless DC motor?

While the direction of rotation can be easily reversed for a brushless DC motor, the speed and torque may be affected. This is because changing the direction of rotation also changes the direction of the electromagnetic fields, which can impact the motor's performance. Additionally, the electronic commutation system may have limitations on how quickly it can switch the direction of rotation.

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