Re: Flyback Diode


"John Popelish" <jpopelish@xxxxxxxx> wrote in message
news:TfWdnSRLVJ9fzEvYnZ2dnUVZ_vamnZ2d@xxxxxxxxxxxxxx
Michael wrote:
"John Popelish" <jpopelish@xxxxxxxx> wrote in message
news:o6-dnV7dFte71EvYnZ2dnUVZ_umlnZ2d@xxxxxxxxxxxxxx
Michael wrote:

I was told by an electrical engineer that when the nameplate on the
motor states "Voltage 36V Current 50A" this is the stall current.
I think those ratings would be the normal load, full speed voltage and
current ratings.

Motors driven by constant voltage (at name plate voltage) sources have
stall currents that may be 8 to 10 times rated current, which would not
be a safe stall current for very long, and may even degauss a permanent
magnet motor's magnets, almost instantaneously.

But if you are driving the motor with a PWM drive, it may have a built
in current limit function that drops the average voltage to the motor to
limit the current if it exceeds some settable value, and a good current
to set that limit might be the full rated current.

Do you know if your PWM drive includes a current limit?

Could you please explain what you mean by 'normal load'?

I am referring to the full rated load (the torque the motor can produce at
name plate current) assuming that it is getting enough cooling to keep it
at a safe temperature. The torque a permanent magnet or shunt wound motor
produces is proportional (roughly) to the armature current.

I would have thought that could be just about anything......

The normal range is between no load current (the current it takes to spin
the motor with no external load) and full rated current that drives full
rated torque.

No it doesn't - I'm building the drive :-)

Lets hope you decide to add this feature. It protects not only the motor,
but the PWM components, battery and wiring, as well. The current sense
might be based on the voltage drop across a current shunt (a low value
series resistor) in series with the motor, or some magnetic field sensing
mechanism (like a hall effect sensor) that reacts to the magnetic field
around a motor lead, produced by the motor current.

Thanks John,

I'm thinking about using a shunt resistor, with a voltage divider either
side fed into a 741 setup as a diff amp. Would that cause any problems?

Cheers,

Michael


.

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