5 benefits of multi-phase motors and inverters

July 19 2016

Summary

One question our sales representatives often get is related to TM4’s multi-phase approach; what advantages do 6- and 9-phase electric powertrains provide in comparison with more traditional three-phase systems? Let’s take a look at the top 5 benefits multi-phase systems can offer.

TM4 offers a scaled line of multi-phase inverters and electric motors combinations:

  TM4 MOTIVE system - CO150 inverter and HSM20 motor tm4-sumo-md-system  TM4-SUMO
3 phase
MOTIVE series
6 phase
SUMO MD series
9 phase
SUMO HD series

 

1. Increased power

The most common approach to increasing power is the parallelisation of multiple power transistors (IGBT).  The immediate problem is that IGBTs are never perfectly matched and current does not distribute evenly between parallelized IGBTs; this can cause a loss in performance of more than 10% compared to the performance we could expect from the sum of the same number of independent IGBTs.

TM4’s multi-phase modular topology of inverters and motors uses independent IGBTs to drive independent electromagnetic subsets of the motor thereby allowing each IGBT to be used to its fullest potential.

Parralelisation of multiple IGBT modulesTM4 multiphase approach

Parallelisation vs TM4 approach

2. Reduced component costs and DC bus ripple

The industry is always looking for ways to reduce component costs and our multi-phase topology allows to do just that. As the IGBTs are fully independent of each other in our systems, we are able to benefit from interleaved IGBT switching. This spreads the current ripple demand from the DC bus filtering capacitor among the IGBTs allowing to use a smaller capacitor whilst retaining the equivalent DC bus voltage ripple.

The following figures show the values of the DC filtering capacitor current for one, two or three -phase systems using the same capacitor and output current.

Sans titre-3

The end result is that for a defined DC bus voltage ripple requirement, the DC filtering capacitor can be significantly reduced thereby reducing costs as well as product size and weight.

 

3. More efficient use of cable cross-sectional area

At first view, 3 cables with a larger cross-sectional area may seem to be able to carry more current than 6 or 9 several smaller gage cables, but at TM4, we have proven that this is not the case.  As current frequency increases, current tends to flow at the outer edge of the wires while avoiding the center area. The larger the cable and higher the frequency, the more pronounced this effect becomes. This is called the “skin effect”.

Typical electrical systems have maximum fundamental frequencies up to 1 to 1.5 kHz which are sufficient to produce significant skin effect in large cables; this can become significant even at low frequency as presented next for a system at about 10% max speed.

TM4's Multiphase cable reduction advantage
120 mm2 CABLE
78% usable (94 mm2)
3 x 25 mm2 CABLE
99% usable (74 mm2)

Next is a table comparing a single three-phase system with a SUMO™HD 9-phase system (3 x three-phase) at equivalent output current.

  1. Motor @ 1730Nm / 300RPM
  2. Cabling length of 2.5 m
  3. Ambiant temperature of 50°C
  4. Based on HUBER+SUHNER Radox 155 cables

 

This yields an overall reduction of copper surface of 38% and weight of 33%. Although suffering from a slightly higher cable loss, the larger cooling area of the multi-phase cables provides for a cooler cable temperature.

 

4. Easier integration

As shown in the previous table, the bending radius of the smaller gage cable is reduced by 50%; this allows for a tighter and cleaner integration around other vehicle peripherals and brackets.

Smaller and lighter cables also benefit from easier handling and preparation by requiring less cumbersome tools and smaller support brackets

 

5.What about cost?

Typically the more parts you use in your integration,  the higher the costs. However, as smaller gage cables and associated components are more readily available, the overall impact of multi-phase systems yields a small reduction in price as shown in the following table ( prices are for low volumes):

multiphase - benefits in cabling

 

Conclusion

In summary, although seemingly unconventional, the multi-phase topology of TM4’s systems offers multiple benefits both technically and economically with the added bonus of facilitating vehicle integration due to the reduction in component size and bulk.

Please feel free to contact us or more information on our multi-phase products and how we can help you get the most out of your projects.

Eric Toussaint

Eric Toussaint

Manager, Client projects and Contracts at TM4 Inc.
Eric Toussaint received a B.S. degree from Université de Sherbrooke in Canada in 1995 and has been at TM4 since 2006 as senior electrical engineer. He has been involved in the software and systems design of the current motors and inverters technologies. He is currently working with the sales team to handle special project definition.
Eric Toussaint

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