BLDC motor phases

[abhishek.2138]

Hi,
Generally, there are 3 phases in BLDC motors.
What defines the no. of phases in BLDC motor. For multi-phase BLDC motor, are odd phases preferred like 3, 5, 7 phases etc.
What will happen if even phases are wound like 2, 4, 6 etc.?

 

[KlausST]

Hi,

the windings determine the count of phases.

Practically AC synchron motors as well as stepper motors are not much different from a standard BLDC motor. They have different winding geometry (and magnet geometry) and count. Like 2 phases.

****

The 3 phase AC (sine) system has some benefits:
* adding the voltage of all three phases equals zero - all the time. Thus the sent out electric field (cable) is minimized.
* the same is true for the current and the magnetic field
* if you square each current (or voltage) and add all three signals, then it gives a flat line. This means the drawn power is constant ... and thus also the torque is (almost) constant.

For sure we don´t control BLDC with clean sine, but still the three phase method has benefits over 2 phase or 4 phase.

If you go higher with phases ... it means a lot of production effort and thus the motor becomes more expensive.


Klaus

 

[abhishek.2138]

Hi,

the windings determine the count of phases.

Practically AC synchron motors as well as stepper motors are not much different from a standard BLDC motor. They have different winding geometry (and magnet geometry) and count. Like 2 phases.

****

The 3 phase AC (sine) system has some benefits:
* adding the voltage of all three phases equals zero - all the time. Thus the sent out electric field (cable) is minimized.
* the same is true for the current and the magnetic field
* if you square each current (or voltage) and add all three signals, then it gives a flat line. This means the drawn power is constant ... and thus also the torque is (almost) constant.

For sure we don´t control BLDC with clean sine, but still the three phase method has benefits over 2 phase or 4 phase.

If you go higher with phases ... it means a lot of production effort and thus the motor becomes more expensive.


Klaus

But, higher the no. of phases, will increase the motor power and RPM.
This is one advantage.
Second thing is if any 1 phase is failed, magnetic field remains to rotate the motor. It becomes fail safe.

So, why don't even phases are wound. Like 4,6 etc. Odd no. of phases are popular like 3, 5 etc.

 

[KlausST]

But, higher the no. of phases, will increase the motor power and RPM.

definitely not.

Higher number of phases
* decreases RPM.
* may increase the torque
* does not increase power ... for the same size
* does increase motor production cost
* does increase cost of control / drive electronics

Second thing is if any 1 phase is failed, magnetic field remains to rotate the motor. It becomes fail safe.

Definitely not.

It reduces torque, increases the voltage in the remaining windings, in most cases it causes the core to saturate and thus the currents to extremely increase.
This is a very well known problem that causes the motor to burn!

So, why don't even phases are wound. Like 4,6 etc. Odd no. of phases are popular like 3, 5 etc.

Explained in post#2.
Popular? Never seen a 5 phase AC motor.
There are documents that say 5-phase BLDC motors have advantages over 3-phase BLDC motors, but I have never seen any evidence to support this claim.
(This does not mean the evidence does not exist)

I can imagine there are specialized/optimized motors for dedicated applications to offer features like: to improve holding torque, increase RPM to torque relation, to increase efficiency at some operation point, and so on.

Klaus