I am new to BLDC motors. I am working on a project where I need to spin a BLDC motor from speeds of 10k to 40k RPM using sensorless back EMF as speed feedback.
The application requires changing the speed of the motor suddenly, i.e. increase from 10k to 30k RPM and decrease from 30k to 10k RPM with a ramp time of less than 100 ms. My motor driver is able to achieve the change in speed, but I am facing an issue with my power supply.
The problem is, when I decrease the speed suddenly from 30k to 10k RPM the supply voltage rises to very high levels. My nominal supply voltage is 12-16 V but it rises to about 40 V and gradually ramps down.
Below is a oscilloscope plot for the same. Sometimes the voltage rises to the max. upper limit and the driver stops due to an overvoltage condition.
When I decrease the speed gradually, the supply voltage does not increase or increases by 5 to 10 V only.
My questions are:
- Is this normal for BLDC motors?
- Why is this happening?
- What can I do to prevent this?
Below is one of the output phase of the motor.