3
\$\begingroup\$

I'm working with a custom 24V BLDC and I'm trying to find the right power supply to use for it.

The motor is rated for 2A continuous and in normal operation we will be operating under that, however there will be times when it needs high torque instantaneously and will draw 5A very briefly.

When selecting a power supply, I've heard you are supposed to use the nominal current (so we would need a power supply of 24V, >2A output) and to rely on capacitors to provide the high current at short bursts (stall current). Is this true and if so why ? I'm assuming it's a money issue (if you can get away with a cheaper power supply then you should do that), but is there an engineering reason ?

Using a power supply rated for the nominal current is also mentioned in this question here but I'm looking for a more in depth explanation of why: Proper power supply for BLDC motor

\$\endgroup\$
1
  • 1
    \$\begingroup\$ 5A for how long? 5A for 1 second = 5 Coulombs : this and the definition of capacitance dQ = c* dV allows you to calculate the capacitance you need to supply the required charge at the acceptable voltage drop. Try it. You may decide it's better to rate the supply at 5A. Or you may find power supplies with a surge current rating designed for motors. \$\endgroup\$ – user_1818839 Sep 10 '20 at 10:30
3
\$\begingroup\$

If you are starting up under load with no soft start, I would use stall rating since stall current is what your motor starts up with, even if you never really plan on actually running near stall. Something that starts up under load is a wheel or conveyor belt. Something that does not start up under load is a fan.

What capacitors do is smooth out power dips the motor causes for other things sharing the power supply with the motor, and to provide the high frequency current demands of the BLDC driver (since the power supply won't react fast enough due to inductances). Those last two things I mentioned are actually the same thing.

They probably are not big enough to support the motor itself if the motor truly exceeds the limits of the power supply for any noticeable period of time.

\$\endgroup\$
1
\$\begingroup\$

You should not think of the power supply for a BLDC motor as simply a power supply. It is a motor controller or electronic speed controller ESC. It should be capable of limiting the current to a level that the motor and ESC can safely tolerate. It should be able to limit the motor current for both continuous operation and for short term operation. If you know that the motor can tolerate 250% of rated current for X seconds, you should have a current limiting scheme that will impose that limit. That can be done electronically or you can depend on the capacitor and the circuit impedance including the internal resistance of the battery. Better overall performance can be achieved electronically.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.