# Size a power supply for a DC motor?

I'm wondering how to size a power supply for DC motors.

I have a 70W 24V brushed DC motor [Linix 63zy24-70-A I picked up from a surplus store]. I intend to control this using a PWM/MOSFET. If 10 bucks is 10 bucks, 70 watts is 70 watts. However, thinking of the inrush current, I measured a resistance of 6Ω, so with 24V/6Ω = 4A, and therefore I might need a 96W supply. I haven't worked with DC motors enough to know if that makes sense (seems low, but maybe not). My bench supply has a current limiter that tops out at 600mA, so I can't test it.

I am more familiar with sizing AC motors to the NEC, which involve an inrush current of 6 (or more) times the full load current, and any motors which can't be run at a locked rotor current are usually given an overload protection of 115-125% of the FLC.

I would imagine that using an obscenely large DC power supply would work, but I'm hoping more experienced folk can point me in a more reasoned direction.

As an example, I'm looking into something like this: https://www.digikey.com/product-detail/en/LRS-150-24/1866-3321-ND/7705015/?itemSeq=298448846

Edit:
Thanks for all your answers. I wasn't realizing how much effect the controller can have on the power supply requirements.

Below is what I am planning on making. The PWM signal will be from a 555, ATtiny or Arduino. The BJT is there to (hopefully) make that choice irrelevant.

More application details:
The motor is magnetically coupled to a pump, so high-torque is not reasonably possible. Flow rate is currently controlled manually using an ball valve on the output of the pump attached to a single phase shaded pole motor. I'm looking to change that.

It depends what you want the motor to do.

If you want a rapid start, as from a low impedance supply, then you want a supply that will give you the full stall current.

If you are happy for a slow start, then you only need a supply that will limit without foldback at the running current. You'll get full running torque, just not stall torque.

You choose.

For the sake of the power supply, the commutator and brushes, and the PWM switching devices, you should design the controller to limit the current to about 150% of the rated motor current. The controller should measure the motor current and use the measure value as a feedback signal to keep the current continuously under control. The speed command can be an armature voltage command. The current command is the error signal obtained by comparing the speed command with the armature voltage.

The motor needs enough current to meet the inrush current spec. If the only thing in the circuit is power supply, then it needs to source enough current to meet the inrush current spec.

Inrush current can also be limited by series resistance or an NTC to limit the effect the motor has on the power supply.

If you measure 6Ω, then the stall and starting current is as you calculated it 4A,

If you have a PWM controller turining it down to 72% will reduce the current to 2.88 A which will work fine with your 70W power supply.

Yet another option -- if you're designing an H-bridge driver for the motor, then limit the input current to the driver to what the supply can handle, and let the motor current rise as it will. You'll get something in between the stall current and the running current of the motor.