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I have a circuit that can use 25 A of current max, it is on a 25 A fuse. I am looking to control 4 DC brushless motors using ESC's. The trouble is that the motors are rated at 7A max current. I know that ESC's use a PWM signal to control the speed of the motors, so that means when the voltage is High in the PWM pulse the current will be ~7A for that period of time. I am worried that if I run all 4 motors (even on low speed) all at once, if the PWM signals all go high at the same time then the fuse will blow. Is this idea correct and if so what can I do to avoid this happening?

Further info: I am using these motors and they will be driving a propeller for a ROV. The manual for the esc is here, I believe the PWM frequency is 2Khz (they refer to it as operating frequency so I am not 100% sure).

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  • \$\begingroup\$ They also all need to stall. Max current is at full stall, idle current is at no stall or load. \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 3 '14 at 0:40
  • \$\begingroup\$ You should be careful with the tolerances of the parts. if your circuit can take 25A maximum, maybe you should use a fuse that blows at 22.5A (10% tolerance). That way, your fuse actually protects the circuit, which it doesn't the other way around because it actually takes a (short) time to "activate". \$\endgroup\$ – QuantumFlux Nov 3 '14 at 1:33
  • \$\begingroup\$ Just because a motor is 'rated' for a particular max current doesn't mean that it will draw that much. Which motors do you have, and what are they driving? Also what is the PWM frequency of your ESC's? Do they have current limiting? \$\endgroup\$ – Bruce Abbott Nov 3 '14 at 3:12
  • \$\begingroup\$ I am using these motors and they will be driving a propeller for a ROV. The manual for the esc is here, I believe the PWM frequency is 2Mhz (they refer to it as operating frequency so I am not 100% sure). I can't find anything about current limiting. \$\endgroup\$ – Jabbath Nov 3 '14 at 3:44
  • \$\begingroup\$ To correct my previous comment, the frequency is 2Khz. \$\endgroup\$ – Jabbath Nov 3 '14 at 3:52
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The PWM frequency should be high enough that the motor's inductance has a smoothing effect, so current should not go to max as soon the PWM signal goes high. However the current will ramp up, so you should put plenty of capacitance across the power supply to reduce ripple (good ESC's already have low-ESR caps built in, but it doesn't hurt to add more externally). Most brushless ESC's run a PWM frequency of 8KHz or higher, which is fine for high inductance iron cored motors (coreless motors need 30KHz or higher).

If these two conditions are met then the power supply should 'see' a peak current only slightly higher than the average current draw, even if all PWM signals go high at the same time. Just make sure that the average current doesn't exceed 25A and you should be safe. If the motors experience heavy loading during start up or fast throttle changes then you may need to limit the throttle change rate and/or apply current limiting, to keep below the power supply's current limit.

The power absorbed by a propeller is proportional to rpm squared, so at low speed it will put much less load on the motor, eg. if the full throttle current draw was 7A then at 25% throttle it would drop to about 0.3 Amps. So long as your props are small enough to keep the full throttle current below 6.25A per motor you shouldn't have anything to worry about.

The oscilloscope picture below shows current flowing through the windings of a high power brushless motor at a PWM frequency of 12KHz. You can see the current ramping up to about 32A during the PWM ON time (full throttle current draw was 40A). Current at the power supply would have been much smoother than this, as I had several 220uF low-ESR caps across it. enter image description here

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