Re: CVTs instead of motor controllers (was Re: Lynch motors, UK)

[Colin Dedman <cjd121@rsphy1.anu.edu.au>]

re field control of speed vs. variable ratio tranny, Lee Hart wrote:

>
> Aha, now I see. If maximum field strength is 1.5T, then it sets another
> limit. This is what is limiting the field-controlled motor's power in
> your example.

For a meaningful comparison between the field controlled motor, and the
constant speed motor with tranny, you must obviously assume the same maximum
field strength in both cases. This max field strength limits the power of
the field controlled (FC) motor AND the PM motor.


>
> However, since you rewound the armature of motor D with 3x the number of
> turns, couldn't you then apply 3 times the voltage? This allows full
> field strength to get 6000 rpm, and thus the full 60 kw power output is
> maintained. In other words, you've changed the basic motor D voltage by
> rewinding it, so it is a 180v motor being run on 60v.

Yes, but then you would not be able to use field control to reduce RPM down
to 2000RPM. Remember, for the purposes of our comparison, the FC motor must
be able to operate from 2000RPM up to 6000RPM.


>
> Suppose you left the armature exactly as-is for motor D. At full field,
> you still have 60 kw at 6000 rpm. Suppose you *increase* the field
> strength to 3.5T to slow it to 2000 rpm. Now you still have 60 kw at
> 2000 rpm.
>
> Ahh, but can we increase the field strength this much? The iron will
> saturate at some point, but it's a soft transition. For example, we
> might need 10 times the field current to get 3 times the field strength.
>
> The drawback is efficiency. Field power is normally around 1% of
> armature power. If 3 times the current produced 3 times the field,
> that's 9 times the field power, and we've lowered efficiency 8% -- not
> bad. But if we need 10 times the current, that's 100 times the field
> power, and efficiency would drop to under 50% (Which, by the way, is
> what happens with series motors being run at currents far over their
> ratings for racing).

It may or may not be practical to get more than 1.5 Tesla field strength,
but that is irrelevant to the comparison. In practice, iron saturates very
hard above 2.2 Tesla or so and no amount of field current will give you more
flux density than that. But it is irrelevant anyway. We were comparing 2
motors identical in all respects, except that one uses field weakening to
vary the speed, while the other runs at contant speed into a variable ratio
tranny.



>
> So the picture is still not entirely black-and-white. It depends on the
> relative field strenths achievable with PM vs. wound fields.

A comparison of PM vs. wound fields is a separate topic, and I could as well
have chosen a constant-speed shunt wound motor coupled to the tranny to
completely remove this distraction. As far as I can judge, the advantages I
claimed for the tranny vs field control of speed are entirely "black and
white", at least as a 1st order theory. In a nutshell, the field-controlled
system has the advantage of simplicity and ease of use (no gear changing),
at the expense of a physically larger motor. Broadly speaking, this
conclusion is also true for a series wound motor, as the higher motor speeds
are still obtained by weakning the field. E-motors are NOT a direct
substitute for a variable ratio transmission.

Regards,  Colin Dedman