What is Stall Torque

What Even is Stall Torque

Last week I had the opportunity to sit down with Clint Switzer – Product Development Engineer, ICR Services to discuss stall torque and what it means to your production. This is the transcript from our conversation. Happy reading!

Torque is a familiar word to most in the manufacturing sector and especially the engineering profession but there seems to be some confusion about stall torque. What is stall torque and why should I care?

Stall torque is the torque produced by a mechanical or electro-mechanical device whose output rotational speed is zero, whereas torque can be defined as the mechanical work generated by the turning effect produced when force is applied to a rotational axis.

You should care because stall torque is the highest point of failure on a drive unit or a motor. The device is going to draw a lot of current at stall. In fact, it’s going to draw the highest amount of current you’ll ever see while under normal operating conditions.

The keyword there being normal operating conditions. Peak values, as its name would suggest, take top prize as far as amperage and torque readings go.

Can you overcome stall torque and what must take place for this to be successful?

Yes, so that is when the actual moment of movement starts. That’s when the drive and the motor are stressed to their highest capability – exceeding 150% of their normal rating. Once the machine (load) is moving then that demand greatly decreases.

There are a few conditions that must take place in order to overcome stall torque: First, the motor is not moving. Second, there is load applied to that motor that it’s trying to overcome. Lastly, the drive and motor selected are rated to move the load.

Expecting a low power AC drive to have the necessary amperage needed to supply a high-capacity servo so that it can generate the amount of torque required to move its load is just not going to happen.

Production is always under constant pressure to get more and more throughput with what they’ve got. What can go wrong if you’re constantly pushing your drive or motor outside its ratings?

Every industrial motor or AC Drive you purchase is designed to operate above normal current levels. This is especially useful during start-up when a large amount of torque must be generated aka break-away torque. However, this is only allowed for a brief moment of time and is never meant to become the operational norm.

Remember, there’s always the potential for trouble. Brief moments outside the recommended range but not exceeding 150% for more than 2-3 seconds is normal and sometimes even necessary. After all, we couldn’t get passed stall without this. Changes in things like position or direction and speed will cause some fluctuation as well.

The question becomes is this sustainable? It’s only sustainable if your equipment is capable of maintaining that performance level. Things like speed, direction, amperage, and even temperature are all factors here. Make sure you’re within spec. Otherwise, equipment failure is in your future.

What are some problems or warning signs you shouldn’t ignore?

Overheating is a common problem because it’s really obvious but lesser known issues that occur frequently would have to be the gradual degradation of either the inside components of the drive or motor. It’s not obvious at first. You’ll start to see nuance things happen, caused by this continuous influx of abnormally high amounts of current, but nothing is quite to the point of failure yet. Over time things just break or burn out because they weren’t designed to handle such conditions. Motor windings and transistors are classics.

Having the wrong equipment for the job could also be a culprit but most likely it’s people rolling the dice hoping nothing goes wrong and everything holds up because parts need to get out the door.

Oh, and don’t just buy the biggest, baddest drive or motor available. It’s unnecessary and quite expensive. Stay within the range and you shouldn’t have any problems.

There are many different types of torque and it’s not always clear exactly what each one means. What are some misunderstandings of stall torque that you’ve heard through the years?

It’s really easy to simply reduce stall torque as a motor shaft that isn’t moving. This is a great injustice. Well, maybe not that bad… but you get my point. Stall torque is so much more than just an industrial electric motor that isn’t spinning. It’s a motor that’s very much alive. It’s the exact moment when a certain amount of load is applied that causes the output rotational speed to become zero.

If you take away only one thing from this discussion, this is the one.

Is testing for stall torque important when repairing spindle and servo motors?

When getting a servo motor or spindle motor repaired – yes, absolutely. It’s even important for drives and power supplies because you have to look at it as an entire system, not just a standalone piece.

Without testing stall torque your unable to replicate a real-world environment and leave the customer vulnerable to failures.

What’s a nice follow-up piece to this conversation if the reader is interested in learning a little bit more about the industry and stall torque in particular?

It would have to be the anatomy of an industrial servo motor. Go inside to find out what makes them the dependable units of industry we have come to know and love.

The servo motor preventive maintenance checklist is a good tool to have – especially if you’re the one overseeing maintenance or deal directly with them on the daily.

And another for an extra bonus would have to be the 7 questions you should ask your electronic repair vendor. We have a dyno system that just received a new batch of upgrades that can test stall torque among other things and it’s really made a difference to a lot of our customers so definitely ask how they test.

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