I have this problem when the U,V,W terminals of the motor are open or not connected to anything i can rotate the motor with hand and it is free but when i connect the terminals to my hardware U,V,W terminals, the motor rotation becomes hard and i can’t rotate with hand very easily and kind of jerks. When i disconnect it becomes smooth again. Why does this happen?
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The problem is I am not running any algorithm with my hardware powered off and U , V, W terminals connected the motor becomes hard to move with hand.
This is likely because of the BEMF generated by the magnets moving through the coils.
For example, if you take your motor, and short the phases together, you will notice it’s hard to move as well.
Do you have some BEMF protection on your PCB that could be causing them to short together? What board are you using? With the hardware off they really should be isolated via the high and low side of the half-bridge although it could be possible with some configuration that they are becoming electrically connected when back-driving.
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It is custom board tomorrow I will check the continuity between the U, V, W boards. So according to my understanding based on your inputs they are shall be resistance between the terminals of U, V, W.
The resistance you measure (with motor connected to board) should be just the phase resistance (1/2 of what you measure if you measure from any two terminals with multimeter, if you have a Wye type BLDC). If it’s less than this then your board might have a layout problem.
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When power is off, the transistors are in an undefined state. In my experience, they will usually float in the “on” state for a few minutes after disconnecting power. That allows induced current to circulate in the coils which causes a braking effect. Once the transistors settle to “off” state, then it goes back to turning freely. Nothing to worry about, just an interesting aspect of brushless motor behavior 
The braking effect can be used intentionally by activating all transistors of one polarity. It’s better to brake by applying voltage in opposition to the motor motion, and either feed the generated energy back into the battery or send it through a brake resistor to be converted to heat. But if you’re using a non-rechargeable power supply and don’t have the circuitry to use a brake resistor, then the short circuit braking method is useful to protect the power supply by effectively using the motor coils as a brake resistor. Not sure if SimpleFOC has a function for it… if not, it might be something good to add.
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