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'[EE] Motor Starting Current and power supplies'
2008\06\22@190916 by

Hi Piclist,

I am looking for a power supply to purchase. I realized that supplies that
can drive higher currents tend to cost more and I want to keep costs to a
minimum.

It needs to drive a Maxon EC 45 flat, 12v motor. The motor data sheet says
that the "starting current" is 10.0 A. If I am not putting any load on the
motor, can I get away with a power supply rated at the "Nominal Current"
(2.14 A) or even the "No load Current" (151 mA) without a fuse blowing? Is
there a typical percentage that a power supply should be rated above what it
is driving, just in case? Thanks.

--Zach
Zach, some thoughts

Starting current is basically the load of the DC resistance of the
rotor coil(s) when the motor is stationary. eg if the motor was
jammed, the PSU is just pumping energy into a piece of wire. A
technique I've seen but not used is to switch a resistance in series
with the coil to lower starting current. The resistance is switched
out once the motor is turning. Adding a resistance does limit the
available torque, torque which may be needed to overcome
static forces (eg a load) at start-up

A large capacitance on the PSU o/p would supply that extra
ooomph needed to kick the motor into motion. It must be
allowed to charge up, meaning that the start switch would need
to be between the PSU and the motor, not simply turning on at
the wall

Or a higher voltage at a lower final duty cycle ? eg 24V @ 50%

Another thing to bear in mind is that most power supplies have an
active current limit. So, if you try to draw too much current from the
output, no fuse will blow, it will just lower the voltage to force no
more than the maximum rated current to flow. This would mean that your
motor would start up more slowly but once it reached operating speed,
it should work normally.

Jinx, I don't think that output capacitance could make up for the
starting transient. Assuming that the motor draws an average of 5 amps
during startup (half of the max of 10), and that startup lasts 50
milliseconds (probably an under-estimate), then your caps would need
to supply 0.25 Coulomb. For a 1 V voltage sag, this is a 250,000 uF
capacitor. Pretty huge!

Sean

On Sun, Jun 22, 2008 at 8:40 PM, Jinx <joecolquittclear.net.nz> wrote:
{Quote hidden}

> -
On 6/22/08, Zachary Noyes <znoyesgmail.com> wrote:
> Hi Piclist,
>
> I am looking for a power supply to purchase. I realized that supplies that
> can drive higher currents tend to cost more and I want to keep costs to a
> minimum.
>
> It needs to drive a Maxon EC 45 flat, 12v motor. The motor data sheet says
> that the "starting current" is 10.0 A.

If data sheet says so, you need a power supply which is able to supply
a pulse of 10A for the shortest time requested by the motor start.
For example almost any \$10 PC desktop power suply will do it if the
working current is lower than 8A.
> .... this is a 250,000 uF capacitor. Pretty huge!

I do recall saying 'large' ;-))

0.25F seems intuitively over-large to kick a motor with a run
current of just 151mA, but maybe not

Running the numbers through this calculator

http://www.welwyn-tt.co.uk/CalcTools.asp

using 250000uF, 12V, 11V, 1.2 (approx implied DC R of 10A
12V motor) -> a lot of power dumped in a very short time

One other possibility, if this was a motor to be started frequently,
might be to have a small 12V SLA as a buffer to take the majority
of the start-up current strain

How the hell are you charging a 250.000uF without a special charging circuit ?
The initial charging spike is much larger  than that silly 10A spike
required by the motor.

On 6/23/08, Jinx <joecolquittclear.net.nz> wrote:
{Quote hidden}

> -
> How the hell are you charging a 250.000uF without a
> special charging circuit ?

OK, well, here's my idea, you start another motor, and this
one is a generator. See. Then you take that current to ....

I think a big capacitor is being ruled out as a starter. Literally
and figuratively

On 6/23/08, Jinx <joecolquittclear.net.nz> wrote:
> > How the hell are you charging a 250.000uF without a
> > special charging circuit ?
>
> OK, well, here's my idea, you start another motor, and this
> one is a generator.

I have transformed a school laboratory into a plain engine (ward
leonard DC-DC generator without excitation become an assembly running
at 30.000 rpm).
en.wikipedia.org/wiki/Harry_Ward_Leonard
Vasile Surducan wrote:

>> I am looking for a power supply to purchase. I realized that supplies
>> that can drive higher currents tend to cost more and I want to keep
>> costs to a minimum.
>>
>> It needs to drive a Maxon EC 45 flat, 12v motor. The motor data sheet
>> says that the "starting current" is 10.0 A.
>
> If data sheet says so, you need a power supply which is able to supply a
> pulse of 10A for the shortest time requested by the motor start.

Not necessarily. It depends on the starting torque you need and how the
power supply reacts to overload. If you don't need the full torque and the
power supply limits it output current by reducing the output voltage, you
can drive your motor from just about any power supply. It provides a "soft
start" in this case :)

Look at a typical DC motor torque/speed/current diagram to get a handle on
how the three are related.

Gerhard

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