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I am trying to lift a platform hydraulically with a mini power unit that has a 24VDC motor. We are using a converter box to convert 110VAC to the DC of the motor. These converters are only rayed to 40 amps. We have a roughly 200 amp, 24VDC inrush current that lasts between 0.5-1 second and it is too much for the motor at start up. I am desperately looking for a solution. I have looked at capacitors with no success. Someone suggested “soft start”/voltage regulators, integrated load switches, or thermistors. I have no knowledge or experience with any of these. I am not an engineer. I'm searching for solutions for my friend. Any help or suggestions would be greatly appreciated. GXC

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You need an inrush current limiter. With the details you provided (voltage, current) you should have no trouble picking out one that will work for your application.

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Just use a DC Drive. They inherently provide ramping, and current limiting. There are several available for 24VDC motors, with various supplies, the most common being 24VAC, which can be provided by a simple transformer.

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Using an NTC (Negative Temperature Coefficient) Thermistor as Adam suggests as Inrush Current Limiter is possibly a good solution as long as you are not planning to start and stop repeatedly.

An NTC in series in the circuit works as follows:

  1. Initial State : NTC is "cold" --> "High" resistance
  2. Apply Voltage on circuit --> You essentially get a voltage divider between the motor resistance and the NTC so less voltage on the motor windings --> less current.
  3. As current flows through the circuit, the NTC heats up, being a temperature dependent resistor, its resistance decreases --> more voltage is applied to the motor.
  4. In steady state operation the NTC remains "hot" providing a supposedly "negligable" resistance to the circuit and all voltage is applied to the motor.

Durring steps 2 and 3 you assume that the motor windings develop enough torque to overcome the initial startup requirements and the motor starts turning so that a Counter EMF is generated.

Depending on when that happens the NTC selected will be more or less effective on limiting the current to start the motor.

If you stop the motor, the startup conditions of the motor will be similar to the first startup, but the NTC will need to cool down before being able to provide the same initial conditions. How much will depend on the NTC parameters, ambient temperature and the motor startup torque requirements amoung others...

If you plan to start / stop frequently , you could:

  • to reduce the time to cool down I would suggest increasing the surface area of dissipation of the NTCs by using more NTCs (series/parallel) that satisfy the circuit requirements.
  • You could incorporate another switch to bypass the NTC once the current requirements enter within the range of your converter, thereby allowing the NTC to start cooling down earlier.

Incorporating a bypass switch will also allow you to extend the solution to use plain power resistors too, on their own (perhaps with multiple stages) or in conjunction with NTCs.

Cheers

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