# Stepper motor response under varying load

I had a thought that came to mind that made me curious-- Though unfortunately I am not an ME.

I was wondering if someone could assist me in working through my thinking, but you will have to be gentle with me (instead of 'ask me anything', instead lets say 'assume I know nothing').

As a totally 'random and not precise' (thanks DALL-E2) image, I figured this image might elucidate my question a bit better.

Let us just 'imagine' for a moment I was raising up this pail with a pile of blocks in it with a stepper motor.

On its own, reasonably not that hard to figure out-- the force applied would be linear and the weight fixed. Suitable enough for sizing the proper required motor.

However, let us now imagine, as the pail were being raised, I were to slowly, one at a time, remove blocks from the pail and toss them aside, thus with each step, the load becomes lighter.

My naïve thoughts are that 'torque and speed' might alter during this transition, but how ? (i.e. are there formulae for this ?).

Further, since it is a stepper, is it the overall 'power' of the motor that matters most ? (i.e. A man that can bench press 150 lbs can certainly do 20 lbs-- But back to torque and speed, the rate at which they might do so might differ).

Or... it is a stepper motor, not a person, as long as it overpowers the max load, no difference (other than perhaps power consumption occurs during the transition-- Or speed is not at all affected).

Any thoughts ?

• You can think of a stepper motor as a bunch of people in a line passing a bucket. You command them to tell them where to face (let's say left and right). When the first tries to face right, second tries to face left to accept the handoff and vice versa. You command them to turn too fast, the one with the bucket won't be able to keep up due to weight of the bucket; might even end up holding onto the bucket because the one after was facing the wrong way, and pass it back. If they have more than enough power, they handoff fast but end up having to stop the bucket and wait for the next turn
– Abel
Sep 1 at 2:49

You've innocently asked a question with answers that could get complicated. So I'm going to point out right off that I'm going to answer your question for stepper motors as they are normally driven, not how one might drive them if going to absurd lengths to optimize a stepper for things it was not intended to do.

Stepper motors are designed to be easy to drive, and to provide a nice, predictable amount of motion when "stepped" in open loop. For some things that provides a significant value.

However, because they're designed that way, they're horribly inefficient compared to a DC brushed motor or similar, and if you drive them in the normal way while presenting them with a load that's too high they chatter in a way that causes them to stop generating any holding torque.

So imagine your ton of bricks getting about 12 feet off the ground, hearing a loud buzzing sound, and having the whole thing drop as if the chains were cut. I actually worked on a project where this happened in miniature (it was a few ounces of lens, and a few inches of drop). No one and nothing came to harm, but we had to renegotiate some specifications with the customer.

My naïve thoughts are that 'torque and speed' might alter during this transition, but how ? (i.e. are there formulae for this ?).

The load on the stepper will get lighter (so, little danger of the thing dropping to the ground). This means the torque will go down. But when driven normally, steppers are open-loop devices that go at a speed to match the stepped power that's applied to them. So speed wouldn't change.

Further, since it is a stepper, is it the overall 'power' of the motor that matters most ?

Nope. As I mentioned before steppers are horribly inefficient compared to other motors. Most of the power going into a stepper gets "burnt up" in the resistance of the coils (unlike most other electric motors where most of the power goes into making output power, and increasing the torque on the motor means increasing the current driving it).

Or... it is a stepper motor, not a person, as long as it overpowers the max load, no difference.

Again -- horribly inefficient. You want to size your stepper to have a hefty excess of torque available, but once you do that the power input at a given speed is fairly constant.

Just to reiterate: this is nearly the opposite of AC induction motors, AC synchronous motors (if properly driven), wound-field DC or universal motors, brushed DC motors or brushless DC motors. It's even true of a properly driven variable reluctance motor*, which is getting into the esoteric motor category. All of these motors have much higher efficiencies than stepper motors; that means that when the mechanical output power goes up, the electrical input power must go up, because of conservation of energy.

* excluding variable reluctance steppers, of course.

• Ok, Tim, cool. Obviously I am not imagining a stepper to pull a 'load of bricks'. My client has been interested in a very simple application for a 'robot arm'. And of course, usually, a stepper motor would be your last thought for this, you want servos. But like, unfortunately he wants, like everyone, it too be 'super cheap'. So I had another idea in my brain about this, and if it works, I'll just 'open source' the design. Yet I have to work through, 'is it rational', first. Apr 4 at 8:42
• In the end, you gotta do what the client insists on. Apr 4 at 15:35
• @Nevermnd I hope you have encoders at least. And if the steppers are behind a gearbox I imagine it would matter less. Can stepper motors even direct drive a robotic arm? I kinda of doubt it. May 4 at 3:11
• Depends on the type of arm. They can do reasonably well with SCARA style for two joints (with the vertical axis on a screw).
– Abel
Sep 1 at 22:59