Re: EV Grin Returns

[Lee Hart <leeahart@earthlink.net>]

Todd Hunter wrote:
> ...losses in controllers and batteries due to ripple current. Isn't
> there a way to engineer the controllers to cut these losses, or has
> it just never been an issue?

Step 1 is to become aware of the problem. Old controllers used low
switching frequencies, little or no capacitance across the batteries,
and only the motor's inductance as a filter. The controllers appeared to
be very efficient; the heatsinks of these old controllers were
miraculously small.

But people discovered the range was shorter, and the motor ran hotter
than for an equivalent contactor controller. Why? The motor had a high
ripple current -- often 50% or more -- so 75 amps average was actually a
50-100-50-100 amp sawtooth. This caused higher I2R losses in the motor
windings, thus more motor heating.

Battery ripple current was even worse. It was 0 amps during off time,
and a 50-100 amp ramp during on time. Peukert effects meant the battery
saw the motor as a 100 amp load 1/2 the time, not a 50 amp average. Thus
the amp-hour capacity was the 100-amp rate, not the 50-amp rate. This is
also indirectly a loss.

More modern controllers get around these problems by having capacitors
with an extremely low internal resistance across the battery. The
capacitors handle the ripple current, so the battery sees essentially a
pure DC load. It also helps to use batteries with intrinsically low
internal resistance, like lead-acid AGMs or nicads, so ripple current
bothers them less.

The other solution is to run a higher switching frequency. The higher
switching frequency makes the motor's intrinsic inductance a better
filter. But it also increases magnetic losses, because the motor's core
isn't built for low high-frequency losses. If motor inductance is too
low or too lossy, it helps to add an explicit inductor between motor and
controller.
-- 
Lee A. Hart                Ring the bells that still can ring
814 8th Ave. N.            Forget your perfect offering
Sartell, MN 56377 USA      There is a crack in everything
leeahart_at_earthlink.net  That's how the light gets in - Leonard Cohen