Re: VSI Fed IM

[Lee Hart <leeahart@earthlink.net>]

Ihsan Bahri wrote:
> ... my current supervisor wants me to do this project on EV
> specifically on VSI fed IM. So I'm giving it a go, it is interesting.

VSI = Voltage Source Inverter (i.e. imitates an ordinary AC powerline)
IM = Induction Motor (i.e. a perfectly ordinary AC motor)

> I have this IM which has the following parameters:
> Nom. power at 2000rpm : 14kW
> Max. Power: 40kW
> Terminal Voltage: 180V
> Nom. Torque: 30Nm
> Max. Torque: 91Nm
> Nom. Speed: 3500rpm
> Max. Speed: 9000rpm
> DELTA-connected.
> 
> >From these parameters, what I understand is that the
> VSI must supply 180V AC (no more) to the IM. But what
> about the line current flowing into IM?

First, this motor is specified for inverter use, i.e. at something other
than plain old 60 Hz AC.

Nominal speed = 3500 rpm is probably specified at 60 Hz, which makes it
a low-slip rotor (what you want). 30nM torque at 3500 rpm = 14.75 HP =
11kw output. Too bad the nominal power input is given at 2000 rpm, or
you could figure out its efficiency. If we assume it's the same 14kw at
3500 rpm, then efficiency is 11kw / 14kw = 78%. This is a bit low; I'd
expect more like 85%.

If the maximum phase voltage is 180vac, then it's probably wound for
120vac at 60 Hz. 14kw input at 120vac requires 117 amps. Because it is
3-phase, the line currents are 117 / 2 = 58 amps each. But because it is
an induction motor, its power factor is less than 1; assume 0.8, so
individual line currents are 58 / 0.8 = 73 amps each.

Now, that is the current requirement at nominal load. "Max" load is
somewhat arbitrary; how much you get depends on how long you want to get
it. Since torque is roughly related to current, to get 3x the torque
(91nM vs. 30nM) will require 3x the current (73 x 3 = 219 amps per
line).

That should give you some idea of what the inverter is going to have to
deliver.
-- 
Lee A. Hart                     Ring the bells that you can ring
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leeahart@earthlink.net                  Leonard Cohen