week 14 - Transverse flux permanent magnet machines

what is transverse flux?

The combination of the axial flux path (from the stator coils) and the radial flux path (from the permanent magnets)

what is the pole pitch in a machine?

The angular distance between two close poles of the machine. In a four pole machine it would be 90 degrees

What are teh advantages of a small pole pitch?

Reduced iron losses from shorter magnetic flux path in the core

reduced saturation as lower flux per pole

compact magnetic structure and reduced overall material cost

what does it mean if in a transverse permanent magnetic machine it is decoupled?

The electric path is decoupled from the magnetic path. So you can control magnetic field separately from electric field

what is the assembly of a transverse permanent magnet machine stator?

Stacked C shaped silicon steel plates

the middl espace of the c shaped stator core is where the stator coild passes

the nich of the c shaped stator core is where the rotor pole passes (If it was a full loop this is the break in the full loop)

Each coild is called a pole

There are no slots like RFPMs as the stator is formed only by magnetic cores

Which of these is a radial air gap?

the first one is

Which of these is an axiam air gap?

the second one is

what are the 3 configurations of transverse flux machines

(a) surface mounted (SM)

(b) Flux-concentrating (FC)

(c ) axially magnetised (AM)

What are the names of each of these configurations?

A surface mounted

B Flux concentrating

C axially magnetised

what are the 6 core types of a transverse machines?

U core

U core with I bridge

C core

CP core

E core

Z core

what are the advantages of the transverse flux machine?

Higher torque density

decoupled magnetic and electrical loading

no end winding structure - les losses and copper mass

independent phases or modular design (easier scaling of the number of phases)

Fault tolerance operation (due to modular design, phases can be made to produce power even if other phases fail)

thermal advantages - better thermal conductivity so cools more

What are the benefits of having higher torque density in a transverse flux machine?

Higher specific tangential force

magenetic path ahs less restrictions in design, small pole pitch is possinle without affecting the coil cross section

Torque can be increased by increasing the number of poles (without effecting flux or current)

E is the peak EMF, $\omega$ is the electric speed, p is the pole number, $\phi$ is the pole pair flux linkage produced by PMs

what happens if you increase the number of poles in a RFPM or an AFPM?

The pole area is halved so Torque is not changed

what happens if you increase the number of poles in a TFPM?

The p can be doubled without changing the pole area and thus T will be doubled too. So the torque and volumetric torque density can change with the pole number.

why is the electric and magnetic circuits decouples in TFPM?

Because of its structure, where the magnetic flux is perpendicular (transverse) to the rotation plane

what is the impace of decouples magnetic and electrical loading in a TFPM

They can be controlled independently.

The megnetic circuit can be designed purely for hgih flux and low reactance

the electrical circuit can be designed purely for high current capacity and better cooling

What is the benefit of a TFPM not having an end windings structure?

This improves resistive losses

reduces copper mass

higher winding slot fill factor is achieved

conductor length of circumferential dimension is reduced

What are the benefits of independent phases (allowing modular design) of a TFPM?

There is easier scaling of the number of phases

The spatially seperated phases reduce the risk of a short circuit them compared to a radial flux PM with distributed windings

What does it mean if there is fault tolerance in a TFPM?

Multiple independent phases can be condigures (with power electronics) to produce power even if other phases experience a failure

this is important for reliability (like in aerospace)

What are the thermal advantages of the TFPM?

due to the more tightly wound tangential stator coils, higher surface contact is achieved which improves thermal heat conductivity

The stator teeth can act as cooling vanes, enabling an advantageous air cooling

What are the challenges of the transverse flux permanent-magnet machine?

1. Three dimensional flux path means more complicated machine

2. pole pitch

3. modelling and simulation difficulties

4. noise and torque ripples

5. magnetic insulation between the phases

6. low power factor

What is the challenge of the three dimensional flux path in a TFPM?

The machine has a complex structure so manufacturing and assembly is elaborate and cost intensive

What are the effects of the small pole pitch of the TFPM

Due to a small distance between the centres of two adjacent poles,there are many pole pairs and small angular misalignments ahve a strong effect in machine operation. This is a problem for small machines

What is the challenge of modelling and simulation difficulties in a TFPM

Due to the 3d flux path the flux distribution characteristic has to be solved for a 3d geometry in simulations which leads to a longer calculation time compared to 2D geometry.

what is the challenge of noise and torque ripples in a TFPM?

There is a large derivation of the magnetic reluctance during one rotation. So these machines have a high cogging torque as well as a large torque ripple.

what is cogging torque?

Cogging torque is the no-load torque ripple caused by the interaction between rotor permanent magnets and stator slotting, due to variations in magnetic reluctance as the rotor moves.
(The undesired torque ripple in a permanent magnet machine caused by the interaction between rotor permanent magnets and stator slots/teeth. This exists even when no current is flowing.)

what are the challenges of magnetic insulation between the phases of a TFPM

To get mangetic decoupling there must be magnetic insulation between the single phases. This is usually implemented with additional air gap in the axial direction

Why is there a low power factor in a TFPM?

due to:

• high armature inductance there is high synchronous reactance.

• stator excited flux leakage due to the segmented nature of the stator and fringing between the stator pieces

• improper utilisation of PMS causing leakage between poles

• armature flux dominated saturation

equation for the power factor of a TFPM

I is the phase current

X is the phase reactance

E is the back EMF

why are TFPMs food for direct drive WTs?

This is due to their high torque density and allowing direct coupling with the prime mover without a gearbox.

What are the benefits of using a TFPM for a direct drive WT?

Cost improvement (maintenance)

increased reliability

Increased efficiency

What kind of topology, air gap and diameter are good for TFPMS?

The TFPM has a higher dependency on the air gap, so needs a small air gap and a large diameter for mass and cost in wind power applications

how muh higher torque density does a TFPM have than a typical induction machine?

5 times higher torque denisty

What are key improvements for a TFPM

The copping torque, the efficiency, the low power factor and the manufacturing