Hi Back,

weber wrote:The slightly higher power density of 2-pole over the 4-pole and hence the 2-pole's greater suitability with a multi-ratio gearbox only applies when the 2 and 4 pole have the same volts-per-hertz winding ratio, e.g. when they are both wound for 230 V 50 Hz.

It was only because we didn't know we could order lower voltage windings from ABB that we went with the 2-pole for the MX-5. The 2-pole was rated at 22 kW while the same size 4-pole was 18.5 kW.

Agreed

It's not easy to get the USA windings in Australia!, but there's always the rewind shop -- or DIY for the adventurous. (after the BMS, this should be a doddle )

In this post I will assume that the motors are wound with windings such that neither the voltage or current are unreasonable. (ignoring this issue)

weber wrote:So T2, we see it is the increased frequency, not the increased rpm, that gives us more power from the same size motor. Someone just needs to figure out how this trend can be continued to 6-pole and higher pole-count induction motors.

This is what ACMotor did when he arrived at a 4-pole option. Perhaps it's time to revisit 6-poles.

Carrying on from my (and your?) motor, I have prepared some data assuming the ABB Frame size M2AA 180 MLA from the "general performance" catalog:

2-pole - 22.0kw @ 2928 RPM, Tn = 072Nm Tmax/Tn=2.8 =202Nm (Pmax=61kw)

4-pole - 18.5kw @ 1465 RPM, Tn = 121Nm Tmax/Tn=3.5 =423Nm (Pmax=65kw)

6-Pole - 15.0kw @ 0968 RPM, Tn = 148Nm Tmax/Tn=3.8 =562Nm (Pmax=57kw)

If we now run these motors at 50, 100, 150 hz respectively to normalise them to 3000 RPM, we get:

2-pole - 22.0kw @ 2928 RPM, Tn = 072Nm Tmax/Tn=2.8 =202Nm (Pmax=61kw) Tmax/Tn is lower

4-pole - 18.5kw @ 2965 RPM, Tn = 121Nm Tmax/Tn=3.5 =423Nm (Pmax=130kw)

6-Pole - 15.0kw @ 2968 RPM, Tn = 148Nm Tmax/Tn=3.8 =562Nm (Pmax=171kw)

I have not made any allowance for rotor slip at TMax or magnetic losses at frequencies other than 50Hz. (The slip is likely to be fairly linear with torque up to near Tmax)

Slip for the motors is quoted as follows:

2-pole 72 rpm at 3000 RPM and Tn (2.4%)

4-pole 35 rpm at 1500 RPM and Tn (2.3%)

6-pole 32 rpm at 1000 RPM and Tn (3.2%)

Slip is a phenomenon of the rotor's interaction with the magnetic field, so is related only to magnetisation and torque, not the field frequency. This makes the 4-pole configuration a lot better than 2-pole at normalised (3000RPM) speed:

2-pole 72 rpm at 3000 RPM and Tn (2.4%)

4-pole 35 rpm at 3000 RPM and Tn (1.2%) - big improvement

6-pole 32 rpm at 3000 RPM and Tn (1.1%) - small additional improvement -

It would appear from the above that there are limits to the performance of the rotor as more induced poles are crowded onto it, resulting in diminished returns for 6-pole.

I see a number of other issues associated with higher pole count implementations:

a) These are industrial motors with laminations that have been optimised for 50Hz operation, so severe a departure from this is likely to result in a a lot of (hysteresis) loss in the field iron, or heating problems.

I am assuming that 150Hz is the limit on the stator, so running the motors at 150Hz produces a result along the lines of:

2-pole - 8928 RPM, Tn=072Nm Pn=66kw, Tmax=202Nm Pmax=183kw

4-pole - 5965 RPM, Tn=121Nm Pn=55kw, Tmax=423Nm Pmax=194kw

6-Pole - 2968 RPM, Tn=148Nm Pn=45kw, Tmax=562Nm Pmax=171kw

b) With the high field frequency (300Hz for the 6-pole), the Windings are going to have to be a lot heavier gauge (or multiple parallel ~ 'multi-filar') to accommodate the V/Hz of the 6-pole. This affects the inductance of the motor, which will be much lower, which affects the switching frequency needed in the controller.

c) It is likely that external inductors will be needed to help regulate the switching current in the motor at the higher switching frequencies.

d) Big power Semiconductors, like IGBTs, have relatively long turn on-off times, so are frequency limited, complicating the design.

My conclusion:

is that 6-pole looks to be feasible as a configuration, but at only lower shaft speeds.

For direct to diff drives, 4-pole IMs look to be the optimum.

Caveat:

The above is based on a popular industrial induction motor and can't be simply extended to other types of motor that have different configurations and/or properties.

It's not the end of the world, but I can see it from here.