The TB6600 drivers are known to have circuit design issues. There is a thread here all about it. The Toshiba TB6600 chip isn't the problem, but rather the circuit it is built into and many of them have design flaws that can cause problems. I have several of the ones like in the picture I have attached and they work fine, but they also have circuit design flaws.In my case the design flaws found only affect the maximum current and limit it to about 3.1A. In my case I only needed 2.8A, so they are just fine.

As for voltage, The motors will like the higher voltage, but you don't necessarily have to use the ideal calculated voltage if you don't need that level of performance. On the motors I am using with the TB6600, they are pretty high inductance with a calculated ideal voltage of 80V and I run them at 40V. They have all the performance I need in my application so in this case 40V is fine.

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