Re: For you, AC drive fans - Lamborghini EV sample

[Lee Hart <leeahart@earthlink.net>]

Otmar wrote:
> I'm thinking that the slip is required in order to transfer the
> energy to the rotor.

Right.

> But I'm curious how much higher the losses are compared to a wound
> field motor.

The field energy for the induction motor is transformer-coupled into the
rotor. Transformers are 90-99% efficient, depending on how hard you try.
This transformer loss is in addition to other losses; there is no
equivalent for a DC motor.

Then, you have the eddy and hysteresis losses in the rotor iron. When
slip is low, the frequency in the rotor is low, so these losses are
small; probably no more than 1-2%.

Then you have resistive losses in the rotor windings. They are
comparable to the resistive losses in the wound field motor, so the two
types come out the same here. However, one cheap way to make an
induction rotor is to substitute aluminum for copper; but this increases
resistive losses. Another is to deliberately undersize the rotor
conductors, to produce a high resistance rotor which increases starting
torque. Neither of these techniques can be used for an EV motor; the
rotor would quickly fry!

So, I'd say that the induction motor has a 2% to 12% efficiency penalty
over a wound-field motor.

> Concerning motor selection: I've seen diagrams of cross sections of
> induction rotors used for line frequency applications. In those, as I
> understand it, the rotor bars (conductors) are somehow shaped in such
> a way that they can still provide torque even when there is a lot of
> slip. There are compromises made for that.

The fun games they play with the shape of rotor bars mainly apply to
induction motors that must run on fixed-frequency AC power. They want X
amount of starting torque even with full voltage 60 Hz applied to the
motor, for example. You wouldn't do this for an EV motor, because the
inverter could just be turned down to a suitable voltage and frequency.
The EV motor never has to run at high slip except from a dead stop.

I believe the EV1 motor has rotor bars that are basically square, with a
thinner tang on one side. The thin tang provides a high-resistance rotor
action when the slip frequency is very high, just for the instant that
it takes to get it started. From then on it always runs at low slip,
with the large cross section of the bars carrying all the current.

Remember too that it is fairly easy to change rotors on induction
motors. All you need is one of the correct diameter. You could even
replace it with a wound rotor, or permanent magnet rotor.

> Speaking of inductance, people building DC controllers often think
> they just need to deal with the inductance of a normal motor such as
> the 120 micro henries that is typical of the motors most of us use.
> At low currents this is true. The rude awakening comes when the
> currents rise above 500 to 600 Amps. At higher currents I've measured
> inductance closer to 18 micro henries. It sure requires a much faster
> response from the current limit, especially when the bus voltage is
> 300 volts!

Absolutely! The drop-off in torque per amp means that the iron is
saturating, which also means its inductance is going away!
-- 
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