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I am trying to turn a disc of 10" of 20kg with a DC motor. I would like to get at least 1200 RPM. The motor is an Ampflow A28-400 and the power supply I am using is an adjustable power supply by Versatile Power called BENCH 30-33 XR.

Although the power supply is rated at 600W, up to 30V and 33A, the V caps off at around 3V at 33A. Can anyone explain to me what this is happening? What is the correlation between the power supply and the load which I am trying to turn? The RPM at 3V and 33A is near or below 150 RPM.

Sun Jang
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  • Comments are not for extended discussion; this conversation has been [moved to chat](http://chat.stackexchange.com/rooms/50634/discussion-on-question-by-sun-jang-right-power-supply-for-dc-motor-with-load). – Dave Tweed Dec 24 '16 at 01:02
  • @Dave Tweed This question is half about interpreting and using DC motor performance specifications and half about basic mechanics. Should it be moved to Engineering? –  Dec 24 '16 at 01:17
  • @CharlesCowie: No, it's fine to leave it here. EE is all about the ratings of motors and matching them to specific applications. – Dave Tweed Dec 24 '16 at 01:23

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The load has an inertia of 0.645 kg-sq meter, and which looks like it is too big to accelerate fast enough for your motor (although I may be mistaken here).

If you can, will you please run your motor at no-load (not connecting anything to the shaft), and see the results (V,I and RPM). Adjust the voltage such that you get around 1200 RPM. From the spec sheet of the motor you provided, 1200 RPM at no-load will run when you apply 12 V. It would help me and others in helping you solve your issue.

Also if you could include the data sheet of the lab bench power supply it would be helpful, as I think that the problem is the internal resistance of the power supply. I tried searching but couldn't find it.

If the internal resistance of the power supply is high, then there will be a large voltage drop across it when you draw high current, and you will not receive the required voltage. That however is just one of the possible causes. Other causes I cannot determine as of now.

Transistor Overlord
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  • If I am not mistaken, the motor will reach 1200 RPM at 6V at no-load. Unfortunately, I won't be near my setup over the holidays to double check. – Sun Jang Dec 24 '16 at 16:26
  • The impedance presented to the load from the power supply is less than 0.001ohm. – Sun Jang Dec 24 '16 at 16:29
  • When you do get to your setup, check the motor resistance. On the datasheet it shows 0.044 ohms. However I calculated from the values you have given and I get 0.068 ohms. Also more importantly, the fact that it is drawing a high current shows that the load torque is very high, which mostly would be friction somewhere in the bearing or the disc itself. The power supply may not be able to produce a high enough voltage at that current of 33 A. Because of lower voltage, the speed is also less. – Transistor Overlord Dec 27 '16 at 19:23