DRV10983EVM: Motor velocity constant

Hi Anders,

Our experts have been notified and should reply soon.

Hi Anders,

Do you see any problem running motor with load ? You can use same velocity constant as long as you are able to support load and spin motor.

Regards,
Krushal

I can run  the motor with the load so the empirical found constant can be used to spin the motor with the load I have.

It seems as I the back emf constant from the motor suppliers and the constant that is needed to drive the motors are not the same.

To not do time consuming empirical studies to find an optimum setting across different batches of the same motortype I would like to learn how the motor constant is calculated in your program and how you measure.

Can you supply this information?

Then I can also learn from the motor suppliers about their way to measure this point.

Since this constant is important in if I can start or not I think it would be good to know how to translate between motor spec and driver spec.

Is the motor constant also involved in the closed loop regulation?

If so in what way is it important??( acceleration, deacceleration, load changes).

When starting up the motor I first get an increasing sinusiodal current which after a while reached the

Open to Closed Loop Threshold and then continues at that level until the closed loop kicks in. When closed loop the current drops significatly.

From the SLOU395C I understand that the curves shown for the open loop behaviour does not show the cross over to the loaded case.

At a fixed load should the aim be that the condition at crossover between the open loop and the closed loop conditions are almost equal?

Any news on my questions?

Hi Anders,

Sorry for delayed response.

Comment about BEMF constant calculation :
DRV10983 has built in BEMF observer and it estimates BEMF internally. It calculates BEMF constant by integrating estimated BEMF.
My experience has been that Motor Datasheet always are not always straight forward and accurate enough, can you measure it on bench while spinning motor manually.

BEMF constant is used in closed loop for abnormal Kt lock and calculating voltage limit based on current limit using formula Ulimit = (Acceleration_Current_limit) * motor resistance + speed * motor kt

Comment about open loop drive:

Generally for BEMF based sensorless solution, motor is driven blindly in open loop i.e. commutation is based on open loop parameters without any feedback from BEMF observer. Whereas in close loop it is driven based on calculated BEMF. There will be difference in current because the commutation in open loop is inefficient because phase current and BEMF are out of phase (drive angle is not optimized).

Regards,
Krushal

Hi Krushal!
Thanks for your reply.
In the section motor parameters Phase to Phase Kt found in DRV10983 EVM GUI.
My understanding then is that this figure then should come from the motor spec or or by spinning motor manually.
Then based on your information I come to the following conclusion. The provided BMF (Kt) is only valid for the open loop.
Is this correct understood?

At the first instant of the closed loop.
Is the inital BEMF(kt) first estimate of the observer the same as the one that is given in the GUI?
From the information about closed loop then the BEMF is a calculated value not a constant.

My understanding then is that the critical parameter from my point of view is to make sure that the open loop BEMF is accurate in a way so it can start all possible motor individuals of the same motor type.
The problem here is what you know and what I recently have learned. The BEMF is not very defined from the motor supplier.
This in turn can then lead to that we have non starting motors.
Do you have any suggestions on how to cop with this?
Should I spin 100rds of motors manually?
This is impractical.
How should I state the requirements to the motor manufacturer so that I know that the motors will start with the DRV10983 device?

Kindest regards Anders