Re: EV airconditioning

["Bill Dube'" <bdube@BOULDER.NIST.GOV>]

>     When I had the Solectria Force for the weekend, I took a look at the
>     air-conditioning.  They use a small 144V, 1.5 hp brush motor mounted
>     next to the original air-conditioning compressor.  It drove the
>     compressor directly with a cogged belt, but no clutch.  All of the
>     original refrigerant and control stuff was left as is, with the only
>     change being that when the system needed the compressor, instead of
>     kicking on the clutch, it would run the 1.5 hp motor.  The system
>     worked well, and had the advantage of full air-conditioning regardless
>     of vehicle speed.  I would guess that in order to handle the startup
>     torque, it was a series DC motor.

    A PM motor will likely have the needed starting torque. It doesn't have
the enourmous strating torque of a series motor, but it should be OK for an
A/C compressor.

    The starting torque for the compressor is not that bad. Generally, the
difference in pressure across it is quite low when it starts. Typically, the
compressor comes on when the evaporator is warming up and the pressure has
pretty much equilibated between the condenser and the evaporator.
Occasionally, however, you could manually cycle the A/C control on and off
quickly or flip the keyswitch on and off. Under these circumstances, the
compressor will start under full load. If the motor will actually start the
compressor under these circumstances, it is unlikely that damage will occur
because it happens only rarely. If, on the other hand, the motor does not
have the starting torque and stalls under these circumstances, you will
toast the motor for sure even if it happens just once.

    You could put in an unloading valve to guarantee that the starting
torque was always low if you liked. You could also put a slight delay
between the motor start-up and the clutch engagement to use the inertia of
the spinning motor to swing-over the ocasionally fully-loaded compressor. A
high-side pressure switch could be used to switch in the delay circuit. You
could also use a high-side pressure switch to completely lock out full-load
starts if you are using a direct drive without the clutch on the compressor.
             _   /|
             \'o.O'              Bill Dube'
           =(___)=           bdube@boulder.nist.gov
              U

Re: EV airconditioning