Re: [CAD_CAM_EDM_DRO] Re: Power Supply Caps and Xformer

Tim Leech wrote:

>> You should know that at standstill, with
>>50 V across these motors, you can just get 12 A through them. When
>>
>>
>they
>
>
>>are moving, the back-EMF generated by the motor reduces the maximum
>>current. At 900 RPM the back-EMF is 36 V, so you can only get
>>
>>
>(50-36)/4
>
>
>>= 3.5
>>A through the motor. What this all means is that it is IMPOSSIBLE
>>
>>
>for the
>
>
>>Gecko drive to EVER draw 12 A from the power supply, even for an
>>
>>
>instant.
>
>Excuse my ignorance (I've never dealt with servo drives), but why is
>it impossible? I appreciate the back emf bit, but what about stall or
>starting conditions?
>
>
>

The main reason is the high resistance of the Camtronics motors. It is
about 4 Ohms,
plus a bit for the driver. With the motor connected directly to the
power supply,
and stalled, it would pull 12 A. That is an abnormal condition.
Normally, the currents
are less, and the voltage applied to the motor, except during very short
pulses, is
also much less. These PWM amps follow a power-in = power-out law, with
a little
loss in the driver. So, if the current out is 2 A with 4 Ohm
resistance, that drops 8 V
across the motor. Power to motor is therefore 16 W. If the DC supply
is 50 V, the
current from the power supply is 16/50 = 320 mA. This is the normal
condition for
the system. With the motor spinning, then the back-EMF comes into the
equation
and raises the voltage, and therefore the power. Assuming a modest
speed like
450 RPM, the back-EMF would be 18 V. So, now we have 2 A and 8+18 V = 26 V,
for a power of 52 W. 52/50 = 1.04 A. These are reasonable steady-state
values
for such a system. The 20 A peaks are possible when all 3 motors are
accelerating
at the same time, but this will be for a very short duration. (With
these motors, it
BETTER be a short duration, or they will fry.)


Jon