I have a 24V battery bank and would like to discharge it in order to test the performance of different solar charge controllers. Eventually the batteries will supply a 2hp (1500W) DC motor and it is this that I would like to simulate.

Can I use a resistive load to approximate the current draw of the motor in lock conditions? How do i calculate this? A pot or a rheostat seems like the way to go but I am unsure of how to select one.

Following Ohm's law I expect the current draw from the motor to be near to 58A when the motor is locked.

  • This doesn't make sense. If the motor is rated at 1500W from 24V that implies a current about 60A when running. When locked if you mean stalled, you can expect much higher currents - 5-10x higher. Find the motor winding resistance, it'll probably be well below 0.1 ohms. – Brian Drummond Jan 5 '15 at 18:43
  • Why can't you use the motor as the load to discharge the battery bank? As Phil F. says below you'll need to dissipate a lot of power. (More than 1 kW.) Maybe some (thick) nichrome heater wire and a fan. – George Herold Jan 5 '15 at 20:26

Yes, you can use a resistor to simulate a locked DC motor. A slightly better simulation is an inductor, but unless you are doing PWM or something like that, the inductance of the motor shouldn't be significant.

In fact, when the rotor is locked, the current is just limited by the resistance of the motor windings. If you don't care about the inductance of the windings, than a resistor is exactly right. You just need to pick a resistor with a resistance equal to the motor's winding resistance. I can infer that your winding resistance must be (by Ohm's law):

$$ { 24\:\mathrm V \over 58\:\mathrm A } = 414\:\mathrm m \Omega $$

Also be sure to select a resistor that can handle all that power. 58A at 24V is:

$$ 58\:\mathrm A \cdot 24\:\mathrm V = 1392\:\mathrm W $$

That's a pretty hefty resistor. Some simple reasoning would suggest that it's at least as hefty as your motor. If you don't intend on running this test for very long, you can get away with a somewhat smaller resistor. Just be sure to shut it down before the resistor melts.

  • Notice that this power is less than the rated 2hp so cannot be realistic for a stall condition. – Brian Drummond Jan 5 '15 at 18:44
  • @BrianDrummond Is "2hp" the maximum heat the motor can dissipate, or its maximum mechanical power when driven by the rated voltage and mechanically loaded for maximum power transfer? (that is, half-way between maximum speed and no torque, and maximum torque and no speed) – Phil Frost Jan 5 '15 at 18:55
  • Thank you for the quick answer. When you say " not very long" what kind of time are you thinking of? Ideally i would like to run it for 3 minuets as this will be the time the motor would be running in a real life situation. – AdamC Jan 5 '15 at 19:03
  • @AdamC Well, the full answer depends on a lot of thermal calculations, but from experience and intuition I can tell you that 3 minutes is effectively "forever" in this case. "Not long" would be "less than 30 seconds". – Phil Frost Jan 5 '15 at 19:08
  • @BrianDrummond the 2 Hp is the maximum mechanical power when under full load (rated nameplate performance of the motor). Currently the required Hp is unknown but initial calculation indicate it will be in this range. This is part of the reason why i am not concerned with being to accurate with simulating the current draw – AdamC Jan 5 '15 at 19:15

absolutely you can use a resistor to simulate the motor,

at these power levels the resistors you need behave like space heaters...

58A is nowhere near what the current into a locked 24V 2HP motor will draw, expect closer to 500A that's about 0.05 ohms. I'd think about constructing such a resistor from steel fencing wire. stall current is usually brief and the resistance of the steel wire will increase as it heats up, which is probably not a bad thing.

under its rated mechanical a load the motor will draw 58A, 1.5KW is the sort of power used in a hairdryer

thinner steel wire (eg mig welding wire) or thick nichrmome wire could work for you here..

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