# Brushless DC motor runs fine in one direction but not another

We have changed BLDC motor suppliers and are seeing a new problem. In the old system the software commanded the motor to move 5 steps of X hall counts then stop. With exactly the same electronics and software but with the new motor (which may have different hall sensors), the motor moves it's 5 steps but at a much reduced speed. We just got the motors today and don't have the full specs on it yet. Any ideas as to what could be causing the problem? I've attached 3 pics. The first one shows a scope trace on the U phase hall sensor output of the original good motor. The 2nd pic shows the same hall sensor output on the new problematic motor. The third pic shows the circuit diagram. The hall sensors are connected to J9 at the right of the diagram. J10 is the motor connector.

Thanks a great deal for any help, Fred

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Did you happen to notice if it moved in the expected direction? Maybe the winding/sensor phases are scrambled relative to the previous configuration. – JustJeff Dec 5 '11 at 23:04
Yes it does move cw/ccw in response to the commands. The two problems are occasional continuous run-on and what looks like not going to correct amount of hall counts in one direction. – fred basset Dec 5 '11 at 23:19
How is this a electronics problem? Looks like you need to call customer support for the software. – Olin Lathrop Dec 5 '11 at 23:44
The thing is the software has not changed, only the motor and it's integrated hall sensors have changed. – fred basset Dec 5 '11 at 23:48
Why are the waveforms so different? | IF the motor was running at the same speed in each case then you (obviously) have serious differences. It it was not then yo should say so and say what the speeds are. | If you hand rotate the motor and look at the hallsignals OR power rotate the motr and use the hall signals to trigger a stroboscope, are the signals occurring PHYSICALLY at the same separations and phas relationships as before – Russell McMahon Dec 6 '11 at 1:57

If commutation points of Hall sensors have a phase error It will adversely affect control in one direct over the other. Check with low voltage the torque vs Hall transition point, which directs polarity of drivers.

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Check your motor specs. I know you don't have them yet but it could be what is causing your issues is that your motors specs are not the same. Thins to look for:

$$J_m, k_t$$

Your motors moment of inertia could be higher and your motors torqu constant could be lower meaning your motor doesnt have the same power draw and takes longer to spin up.

rember

$$T = k_t*I_m$$ where Im is your motor current... something to check out is how much current is your system drawing if its the same as your previous motor this tells you if there is an efficientcy issue.

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