Back EMF calculation?

[OCNC]

How does one calculate the back emf that a particular stepper will generate?
Does the value change depending on whether the driver is unipolar or bipolar? Does it change with speed?

Thanks.

Chris

[Mariss Freimanis]

Chris,

You can do it the fun way. Chuck your motor up in a drill-press, set your multimeter to AC vots, connect it to a motor winding and do some science! Let the drill-press rip and measure the voltage. Find out or measure your drill-press RPM. Divide your measured volts by RPM to get the step motor's measured voltage to get that motor's "volts/RPM".

Double the speed of your drill-press and you will read twice the AC volts on your meter. Do the experiment, find out if it's true.

The AC frequecy will be 60Hz at 72RPM, 600Hz at 720RPM and so on. The AC voltage will be 10 times higher at 720RPM than at 72RPM.

Set the drill-press to it's highest RPM, chuck up the motor in a vise, connect nothing to its leads. Go away for 10 minutes, then try to touch the motor. You will probably burn your fingers.

Why is it hot when no electricity has passed thru the coils (remember, nothing was connected to them so no current could flow)?

That is because this is an excellent demonstration of "iron losses". The quickly rotating magnetic rotor induced eddy currents in the "iron" of the stator and heated it.

Science does not require expensive and esoteric equipment. You can use something as simple as a drill-press and a multimeter. That and a pencil and notepad is enough to gather data and come up with reasons why you see what you see.

Mariss

[OCNC]

Originally Posted by Mariss Freimani Chris, You can do it the fun way... Mariss

Thanks for this great method. Just to be certain that I'm understanding you, the back emf will then be the voltage generated at each respective rpm. Since the motors ramp down to a stop it seems that the back emf will be more a characteristic of the ramping rpm than the main feed rpm. Is this correct or is the ramp time interval generally too small to be significant? Also regarding Gerry's comments above does the mass of the load being moved come into play? Since a more massive load draws more current it would seem that the back emf would be higher than for the unloaded motors in the drill press measurement? If so is there a way to include this aspect in your experimental method?

Chris

[Mariss Freimanis]

Things are simpler than you think. The AC voltage and frequency is proportional to motor RPM. Nothing else enters into to this relationship.

Mariss

[ger21]

So when you decelerate, the back emf is whatever voltage is generated from the rpm you decelerate from, using your method above?

[ESjaavik]

Like Mariss said: the BEMF depends on RPM only. Not whether it is accelerating or not, or the method the motor is driven. The same happens when the motor is driven by some electronics. That's why it has to supply a voltage higher than the BEMF.

I've used Mariss's method many times and have just one thing to add: Make very sure the motor leads do not shortcircuit. Otherwise things can get very violent! You will not believe how hard the motor will brake if you do! Yes, I did try something similar. But with a motor driving a rail model with me riding onboard. Let's just say it stopped promptly, I did not! Somebody laughed, I did not.

[arvidb]

Perhaps Chris and Gerry are thinking of the energy supplied by the motor while braking?

Look at it this way: while the motor is driven at a certain RPM by the drive, the drive supplies a voltage to the motor that is greater than the back EMF voltage generated by the motor. This allows a current to flow in the motor which creates torque and thus keeps the motor up to speed despite friction and other "opposing" forces.

When the drive wants the motor to stop, it lowers the voltage to the motor. Until the motor has had time to slow down, the motor's back EMF will be greater then the voltage supplied by the drive. Since there's a voltage difference, current will flow, but in the opposite direction than before - the motor now works as a generator.

So a more massive load or a higher RPM, or a faster ramp down of speed, will give you more energy fed back into the drive from the motor. If you ramp down slowly most of the braking will be taken care of by friction instead. (Note the difference between back EMF and braking energy - back EMF depends only on motor speed; braking energy depends on load, motor speed, and from an electrical point of view, rate of deceleration.)

There are different ways to brake an electrical motor:
* Let it coast to a stop (just disconnect it and let friction do its work).
* Dynamic braking (this is what Einar did by connecting the motor leads. Usually they are connected through a resistor to limit current). The faster the motor spins the stronger the braking effect - as it must be since back EMF (= voltage between motor leads) is proportional to speed, current is proportional to voltage, and torque is proportional to current. So the higher the RPM the more torque is generated to brake the motor.
* "Controlled" braking. Here the drive controls current to give desired braking torque, just like it controls the motor to give desired acceleration torque - the drive doesn't care if it's braking or accelerating or keeping a constant speed. This is what's going on in a servo drive.

Arvid

[Mariss Freimanis]

Actually, the step motor "back EMF" can be many, many times higher than the supply voltage. Don't believe it? Then try this:

Accelerate a step motor to 3,000 RPM using a 24VDC supply. That should be easy to do if the motor is unloaded.

Now, disconnect the motor, put an AC voltmeter across one winding and spin the stepper with a drill-press or other motor to 3,000 RPM.

I just did that using a servo motor to spin the stepper and I measured 127V across the stepper at 3,000 RPM. The "back EMF" was 5.3 times higher than the supply voltage!

How is this possible? It has to do with the driving phase angle. At high speeds, the motor phase lags the drive's phase by the ratio formed by the supply voltage and the motor's "back EMF" voltage.

When you rapidly decel the motor from high speed (or desynchronize it), the motor phase advances and this "hidden" voltage appears on the drive's power supply pins. This phenomena is called "load dump" or "returned energy".

Mariss

[ger21]

Originally Posted by Mariss Freimani When you rapidly decel the motor from high speed (or desynchronize it), the motor phase advances and this "hidden" voltage appears on the drive's power supply pins. This phenomena is called "load dump" or "returned energy". Mariss

And I believe, that the original question was, how do we calculate this?

[Mariss Freimanis]

Sorry.

You don't calculate it because it's almost never specified in any step motor data literature. You have to run your own data to find out what it is for your particular motor.

Mariss

[OCNC]

Continuing with my question, this 'hidden voltage' that Mariss mentions is what I'm concerned about in so far as it can have a damaging effect on the drive circuitry. I know that the Zylotex boards spec. a max voltage of 35v including back emf and I have to assume that all other boards are similarly constrained. So how does one allow for this? Is it sufficient to do the drill press measurement at your max. running rpm and consider the V from this to be the max. BEMF that the drive will see or do other assumptions come into play? Is the max. BEMF as determined empirically indicative of what the drive would normally see or can it be derated in some fashion because of it's AC nature?


Chris

[ger21]

I know the most people consider the Xylotex safe at 28V. That's 20% below max. But, the Xylotex MAX is absolute MAX. There is NO safety margin in that figure. I believe with Geckos, the Max rating is the max you can RUN them at. I think it has some headroom. Mariss?