Stepper motors getting HOT!!!

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Gary Campbell
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Re: Stepper motors getting HOT!!!

Post by Gary Campbell »

Flash wrote: Mon Nov 29, 2021 7:19 am
Miamijerry wrote: Sun Nov 28, 2021 8:03 pm Are there dip switches on the Stepper Motor Drivers?

If so what is the #4 switch set to?
There are no dip switches on these motors.
The question was: are there DIP switches on the motor DRIVES?
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Re: Stepper motors getting HOT!!!

Post by tblough »

So according to that datasheet image, anything above 80°C over ambient would be the limit (~100-105°C i.e. boiling water). What temperature is "HOT!!!"?
Cheers,

Tom
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Re: Stepper motors getting HOT!!!

Post by Richards »

There's some confusion here. In my example, I used an EXAMPLE inductance of 4.5mH. Your motors may have a different inductance. The formula that I posted shows the maximum computed voltage for a motor with 4.5mH of inductance. Be sure that you plug the inductance rating for your motors into the formula.

Simple stepper motors normally don't have switches. The driver may have switch settings to allow the user to choose the maximum amperage that the motor is allowed to draw from the power supply. If the stepper driver does not have switches to limit current, the manual may have a chart or formula that shows how to use a current limiting resistor.

Motors that run too hot for your application will run cooler if you use a power supply with lower voltage and/or if you reduce the amount of current that the motors are allowed to draw. Reducing the current reduces the available torque. Reducing the voltage reduces the top speed. Back when I bought a lot of Oriental Motor stepper motors, (1990s), that company recommended using a 8x8 inch (or larger) quarter-inch aluminum motor mount as both a motor mount and as a heat sink. During that time, I ran some heat/voltage tests. I found that if I reduced the maximum voltage by about 15% that the motors only reached 150-160 degrees F.
-Mike Richards
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Re: Stepper motors getting HOT!!!

Post by drdennis »

All well said. Indeed, stepper motors run HOT by design. They are cheap and offer cheap controls -- pulses -- with no need for feedback (i.e., if all goes well). But there is no free lunch. You will "pay" for the low price elsewhere.
Stepper motors exhibit intrinsic holding torque (detent torque) that makes a motor "snap" into one of its 200 natural equilibrium positions. But what if you want to stop a motor somewhere else? You will need to pump in current in order to fight the detent torque just to keep that position, even in the absence of any external torque.
How do you make sure that you have enough torque if you don't know how much torque you need and if you can't detect when you don't have enough (no feedback)? Well, you pump in [ten times] more current than what you really need in a hope that it will cover your worst case scenario.
You can see that a lot of stepper motor current is wasted, and it is wasted in form of heat.

How hot is hot? 50C is not "really hot" for a human hand, and definitely not too hot for a motor. I have handled components at 60C+. Not pleasant, but definitely possible. I suspect chefs touch 150F pots every day. In my experience the point of "no touching" is about 70C (160F), but obviously different individuals have different tolerance to pain.
But when it comes to motors, as @Muzzer pointed out the standard specs can be as high as 180C (max in class H). I have designed motor stress tests running class B (125C max) motor windings at 170C+ (those were expensive Maxon motors for critical military applications; may not fly with your $15 stepper). As a matter of fact my Maxon motor rep told me they offer motors running at 240C (464F). Now that is hot. :shock:

Back to Earth -- you have a CNC machine to build, your motors may not have a spec'd max winding temperature, and even if they had, you don't plan to invest upward of $500 in a FLIR camera to check the temperature while they are running. My [very unscientific] advice is this:
1. An open-loop stepper system design is too messy, with way too many unknown parameters to consider doing anything analytic -- forget that.
2. Your best bet is to duplicate (or start with) someone else's properly working machine
3. If you have a unique design -- play with the settings until you get to a point where a motor reliably does not [appear to] lose steps
4. Finally, if you can touch a motor with your finger for 1 second or more, you are in a really good place.
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Re: Stepper motors getting HOT!!!

Post by drdennis »

Flash wrote: Mon Nov 29, 2021 7:22 am
Richards wrote: Sun Nov 28, 2021 8:04 pm Stepper motors run hot, especially when stopped. Many stepper drivers reduce current to the motors about one second after the motor stops. The insulation on the stepper motors that I have is designed to handle a temperature rise of 80-degrees C over ambient. That's hot enough to blister.

Also, check to make sure that your power supply voltage matches the needs of your motors. The formula is SquareRoot(inductance) X 32 = MAXIMUM voltage. For instance, if the motor is rated at 4.5mH, then SquareRoot(4.5) X 32 = 67.8 Vdc. A 60VDC power supply could be used with that motor.
Thanks for the input, I just wanted to be sure. I'm positive that the power supplies are the proper ones for these motors. I double and triple checked my stats before I even ordered them. So what you're saying is even though the motor is rated for 60 volts DC, based off that calculation they can accept 67.8vdc? And the temperature the motors get while in standby is all normal?
Well, first of, the formula above based on the motor inductance is some sort of empirical guess. Unless someone can explain what's behind it, I would say you can ignore it.

In general, the power supply voltage targets the motor controller, not the motor behind the controller.
Your motor is not rated for 60V; that is not how it works.
The key stepper motor parameter is the torque rating which in turn determines the motor current rating. A stepper motor voltage rating, if you ever find one, is simply a product of the motor resistance and the motor current and as such it is a completely useless number. It reflects a DC condition, that is, a steady condition when the motor is not turning. I will not go into the EE fundamentals of inductors, but here is a slightly simplified but accurate model to consider.
What matters is motor current because it represents torque. Motor current cannot instantaneously jump (because a motor is an inductor and an inductor current cannot jump due to fundamental principles of physics). So the motor current is ramping up slowly and the instantaneous value is proportional to the voltage applied (i = V/R * (1-exp(-t/tau))). Without going into the math details, you can see that higher voltage will produce higher current, so if the motor controller has enough voltage to play with it can perform a bit of math magic and get the current (torque) to rise more quickly. That is why we need the high DC voltage on the controller.
A bit to remember is that the higher the voltage is on the controller, the faster the controller can ramp up the motor. But as I just showed the voltage is there for the motor controller, not for the motor. You can have 50V on the controller for a motor [uselessly] rated at 3V!

The bottom line:
1. Ignore the motor voltage rating -- it is a completely useless number
2. It is the current (and thus torque and heat) you control (via the dip switches)
3. The motor current rating is a good current to set on the controller
4. The supply voltage must not exceed the max controller voltage (that is the only parameter you MUST obey)
5. For motor temperature see my post above

Cheers
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Re: Stepper motors getting HOT!!!

Post by Nigelo »

Mike Richards has shown a formula above which is identical to Gecko Drive's own recommended equation 32 * SQRT (Inductance in Mh) = VMAX for calculating the safe max voltage for stepper motor operation. This is shown in Section 6 of their article https://www.geckodrive.com/support/step ... asics.html

I use this formula (as do many others) provided the result does not exceed 20 x stepper nominal voltage.

I answered the OP on the unofficial FB group based on his motor spec sheet that 60v is 25% below max 80.5v calculated on above formula based on inductance of 6.336 mh (i.e. 7.92mh less 20% variance for safety). Max 80.5v is marginally less than 20 x nominal voltage of 4.2v

Dr D may disagree and with no disrespect, I prefer to be guided by a reputable driver manufacturer whose guidance is used by so many without issue

hope this helps
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Hope this helps
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Re: Stepper motors getting HOT!!!

Post by martyscncgarage »

Nigelo wrote: Tue Nov 30, 2021 10:14 am Mike Richards has shown a formula above which is identical to Gecko Drive's own recommended equation 32 * SQRT (Inductance in Mh) = VMAX for calculating the safe max voltage for stepper motor operation. This is shown in Section 6 of their article https://www.geckodrive.com/support/step ... asics.html

I use this formula (as do many others) provided the result does not exceed 20 x stepper nominal voltage.

I answered the OP on the unofficial FB group based on his motor spec sheet that 60v is 25% below max 80.5v calculated on above formula based on inductance of 6.336 mh (i.e. 7.92mh less 20% variance for safety). Max 80.5v is marginally less than 20 x nominal voltage of 4.2v

Dr D may disagree and with no disrespect, I prefer to be guided by a reputable driver manufacturer whose guidance is used by so many without issue

hope this helps
And it is spot on.
The spec sheet says the inductance rating of your motors is 7.9. Square root of 7.9 is 2.81 * 32= 89VDC I would run them just under that voltage to have optimal speed and torque.

Provide the make and model of the stepper DRIVES along with the manual please....you probably have inexpensive stepper drives with no Current Reduction when the drive is idle (no step and direction commands being received)
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Re: Stepper motors getting HOT!!!

Post by drdennis »

Dr D may disagree and with no disrespect, I prefer to be guided by a reputable driver manufacturer whose guidance is used by so many without issue
No offense taken. Please note that the very reputable manufacturer states (right below the formula):
An acceptable way of determining your power supply voltage if this is the only information you have is to multiply this number by any number between 4 and 20.
Note the magnitude of the spread. That should give you an idea why so many people don't have any issue.
The spec sheet says the inductance rating of your motors is 7.9. Square root of 7.9 is 2.81 * 32= 89VDC I would run them just under that voltage to have optimal speed and torque.
The torque is determined by the current which is in turn controlled by the choppers (think PWM) inside the motor controller. The internal processor reads the dip switch desired current setting and in turn controls the choppers by a closed loop on the motor current. Thus as long as the loop is closed the supply voltage does not matter.
The speed is determined by the frequency of the pulse train coming from Acorn, i.e., it is determined by software which does not even know what voltage is being used.

Cheers

To be clear, the power supply voltage does matter. After all without the voltage there is no current. My point is that the voltage is not what controls the behavior; it just has to be sufficient to allow the motor controller to perform the requisite controls.
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Re: Stepper motors getting HOT!!!

Post by Gary Campbell »

Mike Richards has shown a formula above which is identical to Gecko Drive's own recommended equation 32 * SQRT (Inductance in Mh) = VMAX for calculating the safe max voltage for stepper motor operation. This is shown in Section 6 of their article https://www.geckodrive.com/support/step ... asics.html

I use this formula (as do many others) provided the result does not exceed 20 x stepper nominal voltage
All true. That said, has anyone considered that formula, while correct and follows good design principles, may not apply to drives not designed by Gecko?

I have seldom followed that rule, because for the most part I won't use Gecko products unless absolutely necessary to accommodate high inductance motors. Mostly because of the heat and noise they generate and the fact that they cost half again to double of what you can get a cooler running, quieter, better performing drive for.
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Re: Stepper motors getting HOT!!!

Post by martyscncgarage »

Gary Campbell wrote: Tue Nov 30, 2021 12:20 pm
Mike Richards has shown a formula above which is identical to Gecko Drive's own recommended equation 32 * SQRT (Inductance in Mh) = VMAX for calculating the safe max voltage for stepper motor operation. This is shown in Section 6 of their article https://www.geckodrive.com/support/step ... asics.html

I use this formula (as do many others) provided the result does not exceed 20 x stepper nominal voltage
All true. That said, has anyone considered that formula, while correct and follows good design principles, may not apply to drives not designed by Gecko?

I have seldom followed that rule, because for the most part I won't use Gecko products unless absolutely necessary to accommodate high inductance motors. Mostly because of the heat and noise they generate and the fact that they cost half again to double of what you can get a cooler running, quieter, better performing drive for.
Gary, I have had good success following the formula on stepper drives other than Gecko. I've always had good luck with Geckodrive and good support from them.
I've not experienced overheating problems and do follow their recommendations for heat sinking their drives when current runs above 3amps or so, and setting the drives properly so that they idle the current down.

Some like Chevys some like Ford's. 😁
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