2
\$\begingroup\$

This is my first submission/question on StackExchange so please let me know how I can improve it.

A machine module at my customer site uses a DC power supply (230VAC -> 12VDC) to test run two small DC motors simultaneously (the motors are part of a steering column assembly). This current assembly type seems to work fine with the existing power supply, but we are worried that the motors of the new assembly type will have current spikes which are greater than the current rating of both the power supply and the 1mm2 wiring in the switch cabinet

The power supply being used is a Siemens SITOP Power Flexi 6EP1353-2BA00 which has a 10A maximum current rating. The two motors are expected to have a current spike at start up, when the movement runs against the hard stop, and again when it changes direction and comes off of the hard stop. We expect that the spikes could be as much as 20A from each motor. The power supply has a switch-on current limiting maximum of 32A, but I don't think that would apply to current spikes during normal operation (I assume that's only for the start-up of the power supply).

I checked the Siemens website for the technical specs (https://support.industry.siemens.com/cs/pd/462616?pdti=td&dl=en&lc=en-WW) but there doesn't seem to be much info on handling current spikes during normal operation, and my phonecall to their technical support line was less than helpful. Google searches regarding the current spike capacity of the 1mm2 wiring was equally unhelpful, as the answer seemed to vary wildly depending on the source.

What do you all think? Let me know what additional info you would need, thanks!

\$\endgroup\$
7
  • 2
    \$\begingroup\$ A very warm welcome to the site. Thank you for clearly putting a lot of effort into your question. \$\endgroup\$
    – TonyM
    Commented Jan 26, 2022 at 14:46
  • \$\begingroup\$ The output capacitors of the DC supply will probably handle spikes but, how long is the spike and what is the true shut-down characteristic of the dc supply. This information is missing. \$\endgroup\$
    – Andy aka
    Commented Jan 26, 2022 at 14:49
  • \$\begingroup\$ So have you measured the current spikes from the motors in the conditions you mention? If not, then you should. \$\endgroup\$
    – Solar Mike
    Commented Jan 26, 2022 at 14:54
  • 2
    \$\begingroup\$ "Output characteristics" graph shows supply will limit current at 10A (or adjustable lower limit) instead of folding back or shutting off. So at startup, end stops etc the torque will be reduced to match the current limit. If that torque is enough for starting and acceleration under load, you should be good. You can use the current limit <= 10A to limit damage to end stops and protect the wiring. \$\endgroup\$
    – user16324
    Commented Jan 26, 2022 at 15:13
  • 1
    \$\begingroup\$ As a total hack, you could add right out of the power supply, a 100Ω/5W resistor with a switch across it, to a large capacitor (say 22000µF/35V.) Open the switch, turn on the supply, and the cap will start charging through the resistor. After 15 seconds, close the switch. Now the cap will supply large current spikes for a short time, reducing the demand from the supply ("bypassing" it.) If one cap helps but is not enough, could try adding more caps in parallel but not recommended. Careful with large, charged caps - accidentally shorting them will result in a spark at least. \$\endgroup\$
    – rdtsc
    Commented Jan 26, 2022 at 15:24

2 Answers 2

Reset to default
1
\$\begingroup\$

Thanks for supplying the precise Type of power supply. This helps a lot.

The limitation to 32A is the inrush-limitation on the primary side of the power supply. So this does not apply to your problem here.

As I read the datasheet it seems that this PS has a hard current limiter (which can be set from 2A to 10A with a potentiometer). It's not defined in the datasheet (at least from what I've seen) how it reacts to overcurrent situations. But it probably won't just supply more current (even for short amounts of time). So it will either just switch off its output or it will reduce the output voltage until the current is below the set limit.

There are power supplies which have defined time windows on how long you can draw more current that nominal. But this supply does not seem to fall into that class. So either find ways to limit the current draw or get a power supply that is able to handle the situation gracefully.

\$\endgroup\$
1
  • \$\begingroup\$ I've seen this answer a few times and it has been enormously helpful. I misinterpreted the current limiter as a current rating but it explains so much. I had a hard time believing throughout this process that these new motors were suddenly so large as to cause a problem when the existing ones don't. I'd bet money that the existing motors have just been having their torque reduced, but the gear ratios keep it from being an issue. Thanks so much for your help! \$\endgroup\$
    – William
    Commented Jan 27, 2022 at 12:59
0
\$\begingroup\$

The Siemens power supply has an electronic current limiting function that serves as short-circuit protection. That indicated that it is sufficiently responsive to impose a hard limit of 10 amps on the motors. That makes it unlikely that the power supply will be damaged by the testing operation. If the ultimate use of the motors does not impose such a current limit, that would mean that you will not be testing the motors under conditions comparable to the intended use.

You should calculate the wiring voltage drop and determine if that will interfere with the testing operation. Consider also if the wiring of the final installation is comparable.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.