David Sharpe wrote:
> Thank you for your responses. As an EE I myself I understand the
> issues of
> regen with series motors. Mine is a 9" and no interpoles. My plan
> if I cant
> find ready made equipment is to supply the field with quite high
> currents
> around 200A-300A and convert a semisquare transformerless inverter
> that I
> have from a 50Hz to a 400-500Hz with suitable matching lihjy weight
> transformer thus constructing a DC-DC converter to connect to the
> armature.
> The field will be decoupled from the armature with a contactor. The
> real
> problem with my 144V system is that you cannot generate a high enough
> voltage for a direct connection of the armature, you typically running
> around with 50-80V back emf. The above mentioned power supply will be
> controlled not by varying the field current but by varying the on
> time of
> the inverter. The latter used varied on time to maintain approx
> correct RMS
> volts. Im hopeful that having high field currents will solve or at
> least
> minimise distortion of the NA during regen. D
>
>> On 11/23/2010 11:11 PM, David Sharpe wrote:
>>> I was going to do something like this- applying up to 200A to the
>>> field
>>> then
>>> manipulating the low volts DC produced.
>>
>> OK; that is operating your series motor as a sepex (separately
>> excited)
>> generator. That is possible, but here are the 'danger points'.
>>
>> 1. Interpoles
>>
>> Check to see if your motor has interpoles. Interpoles are a second
>> set
>> of smaller field poles halfway between the main poles, and wired in
>> series with the armature. Interpoles minimize brush arcing when the
>> armature current is reversed (during reverse or regen). Motors
>> intended
>> to also be used as generators have interpoles.
>>
>> But most series motors have no interpoles. Without them, the optimal
>> brush position for minimum arcing changes with armature current. If
>> you
>> optimize brush position for forward driving, arcing gets very bad
>> if you
>> reverse the motor (electrical reverse), or reverse the current
>> (regenerative braking).
>>
>> Without interpoles, your only option is to set the brushes to neutral
>> position, so arcing is equally bad for both motor/generator and
>> forward/reverse operation. This limits the maximum voltage and
>> current
>> you can get.
>>
>> 2. Control
>>
>> A sepex motor adjusts the field current to control the armature
>> voltage
>> (and thus the armature current). But armature voltage is also a
>> function
>> of RPM. If you apply a fixed field current to a series motor, then
>> its
>> armature voltage is directly proportional to RPM. 1000 RPM = 10v,
>> 2000
>> RPM = 20v, 3000 RPM = 30v, etc.
>>
>> So, if you connect the armature directly to your batteries, the fixed
>> battery voltage forces the motor to run at a specific RPM. For
>> example,
>> connecting a 36v pack to this motor forces it to run at 3600 RPM. It
>> will either motor (draw current from the battery) to accelerate to
>> this
>> speed, or regen (charge the battery) to hold the RPM down to this
>> speed.
>> The EV behaves like you have a cruise control set, holding the same
>> speed whether you go up or down hills.
>>
>> To make this practical, you have to measure armature current;
>> otherwise,
>> it heads for *infinity* in a mad attempt to instantly reach that 3600
>> RPM. Current would get so high that something will fail. Therefore,
>> you
>> need to limit or control the field current to keep the armature
>> current
>> within safe limits.
>>
>> To change your speed, you also have to adjust your field current.
>> Basically, this scheme thus requires a very high currrent, very low
>> voltage adjustable power supply for the field.
>>
>> Does this help?
>> --
>> Lee A. Hart | Ring the bells that still can ring
>> 814 8th Ave N | Forget the perfect offering
>> Sartell MN 56377 | There is a crack in everything
>> leeahart earthlink.net | That's how the light gets in -- Leonard
>> Cohen
>>
>>
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