# Is it safe to use SMPS instead of LIPO battery to work with BLDC(rated:1400Kv) with ESC(Rated:20A)?

I am planning to build a high speed centrifuge.

Does the discharge rate really matter for low speeds of BLDC(around 50%)?

Can I control the current consumption(strictly less than 10A) by the BLDC motor unit with the help of pwm control to the ESC itself without blowing up my SMPS??

BLDC Motor: 1400Kv
ESC: 20A
SMPS: 12V 10A

• Nothing is safe when megavolt voltages are involved. – Dmitry Grigoryev Jun 7 '17 at 10:12
• @Dmitry The motor used is a BLDC motor rated at 1400kV which means 1400rpm/Volt. We are not dealing with Mega volts here. – Vik Jun 7 '17 at 11:02
• @DmitryGrigoryev It's one of the most retarded units of measurements which has become commonplace/accepted. – winny Jun 8 '17 at 10:53
• @winny Oh, I get it. But it's written backwards then. For consistency, the supply should also be described as providing 10 Currents of ampere. – Dmitry Grigoryev Jun 8 '17 at 12:11
• Just to be clear, "kv" as used here is not a unit of measure. It's really a symbol that represents a parameter, or "constant" (K), of the motor relating its speed to its terminal voltage (sub-V). It would normally be written $K_V$, as in "$K_V$ = 1400 rpm/volt". But hobbyists (and manufacturers) are lazy, and they just write things like "1400kv". – Dave Tweed Jun 8 '17 at 22:03

Yes, it should be safe. However there are a few things to watch out for:-

1. Unlike a battery, power supplies are poor current sinks. During commutation and PWM the motor generates current spikes which can feed back into the supply and cause the voltage to rise. This effect can be reduced by adding a high capacitance low-esr bypass capacitor across the controller's power input terminals. This will also help to suppress voltage spikes caused by inductance in the power wires.

2. If the current would exceed 10A at full throttle (100% PWM) then rather than simply lowering the throttle you should either reduce the load or use a lower Kv motor. The reason for this is that motor current is multiplied by the inverse of PWM ratio. For example at 50% PWM an average power supply current of 10A corresponds to 20A at the motor, with even higher peak currents. This current is drawn from the supply 50% of the time (during PWM 'on' time) so peak power supply current could also be 20A or more.

3. When the motor is accelerating it will draw more current, which might cause the PSU to shut down if it exceeds 10A. To avoid this you should ramp the throttle up slowly so the dynamic torque load is less.

Switch-mode power supplies often have a sensitive current monitor which will shut down the PSU on even brief overloads. This won't cause any harm but is annoying.

• Thanks Mr. Bruce. Could you suggest me a bypass cap value please? I think I am running the motor below its 50% capacity speed. I don't know if the ESC draws current from batt/SMPS in PWM quantities.. Isn't the pwm signal is only to the ESC to control the speed of the motor? – Vik Jun 7 '17 at 9:14
• The 1-2ms servo pulse signal is translated into 0-100% PWM, which turns voltage to the motor fully on and off. Inductance in the stator windings resists current change and causes the current to ramp up and down. Higher PWM frequency smooths the current more, but most ESCs run ~8KHz PWM which does not produce much smoothing. Around 200uF will probably be enough (on my 12V 25A PSU I use 5 x 100uF caps in parallel) rcgroups.com/forums/… – Bruce Abbott Jun 7 '17 at 19:25
• Everything works very well. Thanks for clarification!! – Vik Jun 8 '17 at 5:46
• I tested the setup! And the current consumed by the esc to power the motor is less than 1.8A (@9000rpm without load). I suppose its totally safe. – Vik Jun 8 '17 at 6:14