# Help picking a power supply for DC motor

Hello there,

I want to avoid making the mistake I did last time when buying a motor, so please advise me in buying a power supply for this DC motor.

Here are its specs:

• Cooling fan: Yes
• Rated voltage: 24 V
• Current Rating: 0.16 A
• Rated Speed: 7000 RPM
• Different voltage parameters:
• DC 12V, current 0.14A, speed 3500RPM
• DC 18V, current 0.15A, speed 4500RPM
• DC 24V, current 0.16A, speed 7000RPM
• DC 30V, current 0.17A, speed 8100RPM
• DC 36V, current 0.20A, speed 9000RPM

From what I read, calculating the stall current should amount to 0.16A x 6 x 24V = 23.04W. This should be the minimum power output of a 24V power supply? 23 Watts?

So theoretically, something like the MeanWell RSP-75-24 with constant current limiting should work well?

Something you have to understand about fans, the change in power varies with the cube of the change in speed, if the speed is doubled, the power increases by a factor of 8. So 1.5 Watts at 3500 RPM would be 12 Watts at 7000.
Check this link on the "affinity laws".

From what I read, calculating the stall current should amount to 0.16A x 6 x 24V = 23.04W.

The stall current is measured in Amperes and can be estimated from (motor supply voltage)/(winding resistance) if it is not in the motor data sheet. Measure the winding resistance with your multimeter.

The power supply must be capable of supplying the stall current.

@edgemoron - I thought that the fan current draw was already calculated into the final current draw at different speeds I have listed?

@jremington - Any good tutorials on how to measure the winding resistance? I found a bunch of stuff on google, but not sure if there’s a difference between measuring “winding resistance” or just "resistance’ - though they all involve applying current to test it, and I’m still not sure how much current to apply. What I originally read was that a good estimate of stall current in Amperes is the running current x 6.

Set your multimeter to the Ohms scale, and connect the test leads to the motor terminals (after the motor is disconnected from everything else). You should also check the zero reading, by connecting the test leads to each other, and subtract that from the motor reading.

What I originally read was that a good estimate of stall current in Amperes is the running current x 6.

No, that is not a good way to estimate the stall current. The running current depends on the motor load.

Never mind, I ass-oomed you were driving a fan.
The stall current is only drawn for a short time after start when the motor is accelerating up to full speed or if the mechanism jams, then an overload device such as a fuse or circuit breaker should open the circuit to prevent damage, I would think a 10W PS would be fine for normal operation.

lacomir:
@edgemoron - I thought that the fan current draw was already calculated into the final current draw at different speeds I have listed?

Power to the fan blades is not current. Its not even voltage times current. Some current is used in losses
in the motor and to overcome mechanical friction. So the actual electrical power used will not be just
the cube-law power needed to spin the blades in the absense of mechnical losses.

And real fans can experience aerodynamic stall, which greatly reduces the power needed to spin at a given speed (the
air spins with the fan rather than being pushed through it).

Anyway you haven't said what this fan is - model and datasheet are how you explain what a piece
of hardware is, not a brief description in words!

It may well be a BLDC for instance, which are more somewhat more complicated to calculate current
for.