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Re: Basics for me that has been playing with steppers and Trinamic

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On a serious response, first my thx. Let us go point by point:

You are right, the current limit is the result of the mechanical and electrical parameters of the coils. You are also right to make a difference between the power a motor is fed with and the power torque as a result is a result of the inefficiencies of the motor. So the effective torque a motor offers is lower.

Now the next point is to talk about the stepper motor holding position. This is the moment the stepper motor offers the highest torque, as the inductive voltage, which is always of opposite polarity to the applied voltage. So if we assume nominal values for current and voltage in the holding status, the inductive voltage = ZERO the torque is the maximum under those nominal operating conditions. The heat dissipation for the heat generated by the amount of current flowing through the coils is what limits the amount of current a certain stepper motor can handle. Let us be clear: It is the amount of current that determines the amount of heat generated. The nominal voltage value is the voltage that determines the amount of current flowing through the stepper motor as a result of the resistance the cables in the coil offer to the current flowing and as a consequence how much heat the stepper motor can dissipate.

So what would happen if we apply a voltage twice the nominal value and as consequence generate twice the amount of heat that needs to be dissipated due to the current also doubling. It is what you write, the heat would kill the motor.

But what happens if the Trinamic device uses its PWM to limit the flow of current through the cables of the coil to that amount that flows through them when you apply the nominal value? The motor would generate the same amount of heat as it does when operating at nominal value. The motor is not killed, because the amount flowing through the coils is still the nominal current value! The Trinamic IC generates a high-frequency PWM with a 50% duty cycle. The voltage applied is not the heat-generating parameter, it is the amount of current flowing through a cable that does it. Maybe looking into high-voltage electrostatic voltage is applied? Nearly no current will flow and as a consequence, no heat is generated!

But let us look into the power that results in a capability to generate torque.

Here is where the power equation I always use to explain this.

P = I * U

It always remains limited to its nominal value, that amount of current that the stepper motor is able to dissipate. What changes is the voltage!

U = 12 VDC: P = 1.6A * 12 VDC = 29.2 VA = 29,2 W

U = 24 VDC: P = 1.6A *24 VDC = 38.4 VA = 58.4W

So the torque that our stepper motor can generate if the nominal current is flowing through its coils doubles between these 2 examples, while the heat generated does not change! This is applying it to the stepper motor holding position results in the highest torque a stepper motor can deliver.

Now, let's look into stepper motor doing steps:

What changes is that the polarity of the voltage applied changes. This is simplifying the topic and what I believe helps to understand the issue.

On a coil where the voltage applied changes its value and its polarity, the current induced is proportional to the spread and magnitude the applied voltage changes and it has the opposite polarity of the applied voltage. Let's take a strongly simplified example:

At a certain step rate, the induced voltage has a certain absolute value and its polarity is inverse to the applied voltage. So if I would apply 10 VDC to the coil at s specific step rate the induced absolute voltage value could be i.e. 5VDC

Vt = Va + Vind = 10VDC + (-5VDC) = 5VDC

So at the step rate that induced this 5VDC absolute value the Vt = 5VDC and the torque would just be 50% of the holding torque. This results that at some point the stepper motor is either not able to make available the torque needed to do its job.

But it also implies that if I double the applied voltage at some step rate the induced absolute voltage could also be 5VDC:

Vt = Va 20VDC + Vind = 20VDC + (-5VDC) = 15VDC. So now the stepper motor still delivers 150% torque. So it can either deliver more torque to do its job, or it can take a faster step rate and still deliver enough torque.

This is the whole secret. But Trinamic is even much smarter and experienced. The Trinamic device can register how much torque the stepper motor is exposed to. It does so as it is able to compute this information by the angle by which the rotor of the stepper motor is twisted out of that position it would take if there was no load. This was a specially valuable feature for me as my model sailboat is powered by batteries. So I am interested to save energy as much as possible.

Before I learned this stuff by studying the Trinami8c IC functionality, I had planned to use a mechanical brake to keep the rotor of the stepper motor in its holding position. It is so that a stepper motor in its holding position consumes the maximum power supplied by the batteries. You can find a lot of information here:

[www.trinamic.com]

The Trinamic device can and does reduce the amount of current flowing through the coils of the stepper motor so that it is just able to withstand torque loads it might be exposed in a certain moment. Reducing the amount of current, using its PWM, reduces the heat that the motor needs to dissipate. But, thx to it be possible to operate a stepper motor at a higher voltage than the nominal voltage of the stepper motor its PWM is able and can do so by itself select a duty cycle that increased the amount of current flowing through the stepper motor by up to 20% for a limited time by selecting a duty cycle that allows the current to flow through the stepper motor to a value above the nominal value.

The Trinamic Stepper motor drivers use this feedback it gets from operating the stepper motor to notice when the object moving runs into an obstacle. At the YouTube channel from Trinamic you can see the fancy stuff the Trinamic ICs can offer.

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