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power supply cutting out


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Hi just registered tried searching through forums for some help and decided to ask my question here.
I have a small electric flywheel motor dyno for radio control car motors, which are normally powered with a 7.2v nimh battery. the dyno is set up to be powered through a battery pack.... here is my problem with that... the results vary as the battery pack decreases.
so i purchased 2 power supplys,
which i have wired parallel each ps is "rated" at 500 watts which should be 138 amps unless im wrong (very possible)
I have no problem running smaller motors but when i get into my larger lower turn modified motors the power supplys cut off due to excessive amperage draw. the total dyno run only takes 5 seconds, but the inital hit is what kills me. I would have to guess they hit 100 amp draw at startup. then once the inital hit is gone im ok. i tested this theory by wiring in a battery pack and lifting the wire as soon as the motor got started.

my ideas are a capacitor of some sort th help with the startup. or somehow put a battery pack in parallel with a diode so i dont charge my pack.  the packs get charged very precisely and i would not want to risk a $80.00 pack.

Thank you in advance any ideas are greatly appreciated.


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Those power supplies have a max output current of only 25A each for a moment. When connected in parallel then the one with the slightly higher voltage supplies 25A and the other one does nothing.
It might be possible to set them to exactly the same voltage then use resistors to make them share about 40A but the voltage would drop in the resistors.

A DC permanent magnet motor draws a very high current when stalled. It is stalled when it starts.

A whole truck load of capacitors might supply 100A for 1 second but the voltage starts dropping immediately.

How about a 6V lead-acid battery? Its fully charged voltage will be 6.9V. A pretty big one can supply 100A.

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That's not necessarily true. The power supply spec's don't mention what kind of current limit the supply has... constant power, foldback, hic-up etc. True, the supply with the highest output voltage would supply ALL the current until it's voltage started to drop. It's voltage drop would stop when it reaches the voltage of the second supply and then the parallel supply would start to supply current as well until it reached current limit and then both supplies would drop together. While the current sharing isn't equal, they are sharing. If the current limit is hic-up or latching, your out of luck.

As for capacitors I=C*dV/dt , so (I*t)/C=V
How much voltage drop can you tolerate?

Even a lead acid batteries voltage will drop when you start to draw current... how much is directly related to how much current you draw.

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Instead of connecting an amp-meter in series with the motor, which reduces the voltage to the motor and doesn't show the very quick peak starting current, just measure the DC resistance of the motor with an accurate ohm-meter that can measure very low resistances. Then use Ohm's Law to calculate its stalled and its starting current from whatever supply voltage you select.

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thanks for the replies. I have both supplies set to 7.5VDC  I had some better luck when i went up to 12ga wire from 14ga and shortened up the length of the leads. they would almost straighten out when i tried to start a motor, gotta love the power!! also i used an older battery pack that could never run the dyno on its own in parallel with both supplies.  so far this is what i have come up with.

I cannot have alot of voltage drop because the dyno will detect a low voltage situation and disregard the run I belive 7.2 v is the absolute minimum.

as far as the cap i found one and they have it locally http://www.circuitcity.com/ssm/Metra-Tsunami-Capacitor-PP1005DM-CAP/sem/rpsm/oid/150798/catOid/-13100/rpem/ccd/productDetail.do I dont know their return policy (just in case) and will the cap work at 7.5 volts ?

as far as the startup current does that not depend on the load? there is a heavy flywheel attached to the output of the motor which the dyno uses to calculate torque of the motor it is also sectioned black an white so the optical sensor can obtain an rpm reading. I can start the motor with no flywheel without a problem using the dyno and only one P/S.

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