Hi,
Test setup
- DRV10983
- BLDC motor with an impeller on its axis
- Diameter of impeller 13 millimeters, depth 7 millimeters, made of plastics
- Application: air flow generator in a measuring device
- Ambient temperature 25...125 °C
- Extremly small motor, almost no load on motor => very little power dissipation on driver
Test scenario
- Test starts at room temperature
- Set point are incremented in fixed time intervals and fixed step size from 0 to 500 and back to 0 and so on
- Set point values are sent via I2C
Problem:
- At some point around T > 80...90 °C the motor speed does not follow the motor set point when decreasing the set point
- Motor keeps running at some random high rpm value although set point is decreased, eventually the motor stops at a set point of 0
- The behavior is fully repeatable
- Profile of speed signal response in function of ambient temperature, first two runs are pass, third and fourth runs actual speed stays randomly high until setpoint = 0
Aalysis:
- At any tested temperatures there are no thermal shutdowns observed. DRV10983 is not over heated.
- The read speed signal from the register does match the actual speed from the motor even when the driver does not alter the motor speed at elevated temperatures. This is verified with a differential pressure sensor that is connected to input and output of air flow generator
- We started out with setting the set point value via analog signal. We first thought it might have something to do with a reference voltage comparator in the analog path of the set point that has a drift at elevated temperatures. To prove that analog path has no influence we started setting values via I2C and observed the same behavior. Therefore, the misbehavior/problem is not related to the way of a setpoint is applied.
- We kept away the driver from the heater so its ambient temperature was around room temperature and the motor was heated up to 125 °C. Same behavior.
- We kept away the motor from the heater so its ambient temperature was around room temperature and the driver DRV10983 was heated up to 125 °C. The behavior did not occur anymore.
- We used to have a motor that had a coil to center tab resistance of 18 Ohms where we did not observer the problem. Now we have around 13 Ohms. We cannot use this motor anymore because of some other drawbacks. Inductance value of both motors are the same.
- We believe it must have something to do the way the DRV10983 calculates the actual speed from the motor.
Question:
- Which parameter should be changed to overcome the problem?
