Motor Torque Question

[charlie b]

We will be specifying a custom-built induction motor, 3-phase, 480 volts, 5 HP. Our concern for this application is the risk of damaging the load if (1) something jams the system, (2) the locked-rotor torque is excessive, and (3) the overload protection devices don’t act quickly enough. We originally called for the ratio of “locked-rotor torque” to “full load torque” to be limited to 125%. One major motor manufacturer said they could not do it.

Question: Is there a reasonable value for this ratio (i.e., as small as possible, but still can be manufactured)? The higher the ratio, the more we eat into the safety factors that are built into other portions of the system.

 

[gar]

190313-1553 EST

charlie b:

Several possibilities. But you do have to worry about stored energy, inertia, in any case.

Use a current limited DC motor.
Use an electric brake.
Use a slip clutch.
Look at the speed torque curves for induction motors as a function rotor resistance. See a good book on induction motors.

An induction motor can have maximum torque at 0 RPM with a high resistance rotor. But speed regulation is not good.

A slip clutch may provide the best solution with the least inertia.

.

 

[Besoeker3]

We will be specifying a custom-built induction motor, 3-phase, 480 volts, 5 HP. Our concern for this application is the risk of damaging the load if (1) something jams the system, (2) the locked-rotor torque is excessive, and (3) the overload protection devices don’t act quickly enough. We originally called for the ratio of “locked-rotor torque” to “full load torque” to be limited to 125%. One major motor manufacturer said they could not do it.

Question: Is there a reasonable value for this ratio (i.e., as small as possible, but still can be manufactured)? The higher the ratio, the more we eat into the safety factors that are built into other portions of the system.

Peak torque may be more of a concern. Several times FLT at around 80-90% of rated speed.

 

[gar]

190313-1634 EST

charlie b:

Besoeker3 provides another perspective on an aspect that you need to consider.

For a given motor frame and rotor this peak torque remains constant at some value. Its RPM and shape of the curve varies with rotor resistance.

Does your load require a relatively constant RPM with sharp torque limiting?

You might use a motor with a low rotor resistance, good speed regulation, and then when up to speed cut input voltage down to just provide enough power to the load without much likelihood of stalling.

.

 

[Jraef]

Well, they are being a little lazy in that if someone wanted to TOTALLY design an AC induction motor from scratch, it could be done. You may not want to pay the one-off cost for that, but that was (presumably) not what you asked. it would involve custom designing the slot geometry and depth, rotor diameter, windings etc. etc. Then as Besoeker said, Locked Rotor Torque is not the highest torque in the motor, Break Down Torque is much higher and as a gross general rule the steps you take to reduce LRT might increase the BDT.

If the machine can shut down on excess torque, I would suggest a kW monitoring system of some sort. Many newer Solid State Over Load Relays now are capable of that. Unlike monitoring current alone which can vary with voltage swings, kW is always going to relate to actual motor shaft load. So at a fixed speed, a sharp increase in kW means a mechanical issue and the relay can act to shut the motor down, often within fractions of a second if necessary. kW capable OL relays need a voltage feedback in addition to current, because they must be capable of determining phase angles of both values to derive Power Factor. So this type of OL relay is more expensive, but will do the job. There are also stand-alone load monitors that do the same thing, but you also will need an Over Load Relay, so you may as well just combine them into one device (unless you already have the OL relay in place).

If the machine must keep running and torque must be limited, can speed vary? If so, you could use a VFD. Most (although not the cheapest) are capable of torque limiting by tweaking the V/Hz ratio on the fly, essentially making the motor weaker, but some do so by overriding a commanded speed so you have to investigate it carefully first. If that possibility of speed change is not an option, then a mechanical torque limiter / magnetic particle clutch / fluid coupling etc. etc. is going to have the same issue, just arrived at from a different method.

 

[GeorgeB]

While it would be neither inexpensive nor power efficient, this is a typical application for a hydraulic motor where a real torque limit is needed. It is inherent in the design, determined by motor displacement and pressure.

 

[gar]

190313-2439 EST

charlie b:

What are the speed torque requirements and limitations of your load?

.

 

[gar]

190314-0816 EDT

Some example speed torque curves can seen at
https://books.google.com/books?id=r...esnum=4&ved=0CDcQ6AEwAzgK#v=onepage&q&f=false
The date of this book is 1911. It is in its entirety online. See Fig. 40 page 73, and Fig. 107 page 206 in the 1911 book. The author was also the author of a later book that I studied for AC machinery by Bailey and Gault.
Invention of the capacitor run induction motor. http://ece.umich.edu/bicentennial/chairs/bailey.html

An earlier book, 1906,
https://books.google.com/books?id=h...=14&sqi=2&ved=0CFoQ6AEwDQ#v=onepage&q&f=false

By the early 1900s a lot was known about the theory of induction motors.

.

 

[winnie]

Well, they are being a little lazy in that if someone wanted to TOTALLY design an AC induction motor from scratch, it could be done. You may not want to pay the one-off cost for that, but that was (presumably) not what you asked. it would involve custom designing the slot geometry and depth, rotor diameter, windings etc. etc. Then as Besoeker said, Locked Rotor Torque is not the highest torque in the motor, Break Down Torque is much higher and as a gross general rule the steps you take to reduce LRT might increase the BDT.

This.

A motor will have a torque/speed curve.

Most motors are designed to operate fairly far down the torque/speed curve, with 'full load torque' being perhaps 25-40% of peak or 'breakdown' torque.

But if you designed the motor with custom laminations and relatively more copper and less steel, then you could totally do a motor where breakdown torque is 125% of full load torque.

-Jon