Sensorless Velocity Startup Fault

[derekcook]

Hello, 

In my sensorless velocity project, I get a startup fault when I set the TRAJVEL Accel limit to 9000, and the Jerk limit to 900000. These are the limits I use in my sensored project so that I get to speed quickly. In the sensorless project with these settings, I get a startup fault. When I lower the accel limit to 1000 and the Jerk limit to 5000, the fault goes away. 

I know the startup fault is thrown when the startup block detects that the calculated flux is less than 50% of the rated flux. Is there also a defined amount of time you have to take to ramp the motor to speed in sensorless control? If so what is that? Also, what are the speed thresholds that the Estimator block takes over control. In the reference manual the estimator block looks to take over at 400 RPM?

Thanks, 

Derek

[linestream-adam]

My guess is that your system is attempting to accelerate so quickly that the sensorless estimator can't lock onto the rotor and handle the transition from open-loop to closed-loop. When you are running using the sensor there is no open-loop period and so the motor is going to have the best possible response.

Have you confirmed using the Software Oscilloscope that your system is actually able to track your velocity profile? If you are trying to accelerate the motor faster than what it is capable of (given the system inertia and friction) you will see issues with the sensorless startup and wouldn't see those same issues when using a sensor.

When performing a sensorless startup you do not have full torque available. KMS will work with up to 50% load, and in some cases more, but it will struggle (like all sensorless estimators) with starting with 100% torque demand to meet a speed ramp.

Is there also a defined amount of time you have to take to ramp the motor to speed in sensorless control? If so what is that?

There is no defined time required to ramp up the motor when in sensorless. The startup failure occurs if the stall error or loss of sync error occurs during or just after (2x Stall Detect Time after transitioning from open to closed loop) the startup period. I would suspect that given your very fast acceleration, you are getting a stall error, but due to the proximity to the startup, it is showing up as a startup failure.

Also, what are the speed thresholds that the Estimator block takes over control. In the reference manual the estimator block looks to take over at 400 RPM?

The estimator will take over control based on the speed set in the Speed Threshold field on the Speed Control tab in Motor Manager. This will default to 10% of your rated speed.

[derekcook]

Hey Adam, 

The startup block not allowing us to startup at 100% torque demand is a big problem for us. 

My motor is rated at 300% current for 30s (I only ever want to run my motor for 15s at a time). Could I set my rated current to 300%, and then the startup block will allow a higher torque demand at startup?

[linestream-adam]

Sure. You should set the rated current to be the maximum amount of current you would like to use to try and control your motor. You can always limit the output of the speed controller later, if needed.

[derekcook]

Thanks Adam, I'll let you know if we run into problems. 

[derekcook]

Thanks Adam, 

This answered all of my questions about the Startup fault. I'll do some testing to make sure I give the estimator enough time to take over.