Servo vs. Induction Motor Rotor Tq

[bgbbgb246]

I have an application which requires a motor to rotate a cylinder. The cylinder will be up to 24 in. in diameter weighing up to 250 lbs. Density will be equal throughout. Generally for this type of application, I would see an induction motor but a wide speed range that includes very low rpms (0.31rpm) is pushing me to, at least look into, a servo. Upon attempting to cross reference a 3 phase induction motor to a servo, I noticed a pretty large discrepancy in rotor inertia between a servo and induction motor of comparable size. For example, using 2 Automation direct products comparable in terms of kW rating.

1. Marathon Black Max, Y547, is a 10HP or approximately 7.5 kW motor. Its full load torque is 29.5 lb ft. Its rotor inertia is 1.3 lb ft^2

2. SureServo2 High Inertia Servo Motor, SV2H-475N, is a 7.5kW motor. Rated torque 35.21 lb ft. Its rotor inertia is 142.7 x 10^-4 kg m^2 or 0.3389 lb ft^2 if my conversion is correct.

My concern is the huge discrepancy in rotor inertia that would seemingly suggest that I would need a servo possibly 4 times the size as the one with a comparable kW rating if sizing by inertia ratios. Can anyone provide some insight on what I’m running into here? I don’t spec out motors very often so I’m assuming that my inexperience is misleading me.

Servo Data sheet https://cdn.automationdirect.com/static/specs/ss2motors.pdf
Induction motor data sheet https://cdn.automationdirect.com/static/specs/motorsblackmax.pdf

 

[GoldDigger]

What leads you to believe that high rotor inertia is a desirable characteristic, required to handle high inertia loads?
The total load resisting the motor torque is the sum of the load (cylinder) inertia and the rotor inertia plus the contribution from any other mechanical parts in the drive train. The cylinder inertia is far greater than the motor rotor inertia, making its contribution relatively insignificant and in any case favoring a low rotor inertia.
In the case of very light load (low load inertia), the acceleration possible from the motor is determined by the torque divided by the rotor inertia, so the rotor inertia slows load response but also somewhat damps the effect of varying load acceleration from drive pulses in the case of a synchro.

 

[zbang]

If I have this straight- 250 pound load, lowest speed .31 RPM... what's the highest speed and application? Is there a gearbox involved? If so, a standard 1-1.5HP 3-phase motor and VFD might be the ticket.

Oh, and how much $$$ do you have to spend?

 

[Jraef]

One big DIFFERENCE between a servo and a standard induction motor is precisely what you discovered; the servo rotor inertia is decidedly lower, so that the response to a speed / position move command can be faster and more precise in a smaller package. The trade off is cost and that a servo always requires a drive. If all you are doing is rotating a 250lb. drum, I seriously doubt that you would want a servo. If you need to VARY the speed, use an induction motor with a VFD, but attain the lowest speed via gearing or belt drive. If it's a fixed speed all the time, just use the gearing / belt drive.

 

[bgbbgb246]

What leads you to believe that high rotor inertia is a desirable characteristic, required to handle high inertia loads?
The total load resisting the motor torque is the sum of the load (cylinder) inertia and the rotor inertia plus the contribution from any other mechanical parts in the drive train. The cylinder inertia is far greater than the motor rotor inertia, making its contribution relatively insignificant and in any case favoring a low rotor inertia.
In the case of very light load (low load inertia), the acceleration possible from the motor is determined by the torque divided by the rotor inertia, so the rotor inertia slows load response but also somewhat damps the effect of varying load acceleration from drive pulses in the case of a synchro.

What I was attempting to do if calculate an inertia ratio (Load Inertia/Motor Inertia). I think you have just identified my issue in that rotor inertia is not the motor inertia but instead just the motors contribution to the load inertia. Rotor inertia would obviously not be the figure to use as Motor Inertia in an inertia ratio? How do I find the motor inertia for such a calculation?

 

[Jraef]

What I was attempting to do if calculate an inertia ratio (Load Inertia/Motor Inertia). I think you have just identified my issue in that rotor inertia is not the motor inertia but instead just the motors contribution to the load inertia. Rotor inertia would obviously not be the figure to use as Motor Inertia in an inertia ratio? How do I find the motor inertia for such a calculation?

The rotor is the only moving part of a motor, so rotor inertia IS the "motor inertia"...

 

[GoldDigger]

The rotor is the only moving part of a motor, so rotor inertia IS the "motor inertia"...

And the OP's response also still seems to indicate that a low "inertia ratio" is thought to be a good thing.
Perhaps in some situations, it is a useful performance parameter, but only if it is reduced by reducing the load inertia rather than increasing the motor inertia.

In the OP's case, the directly applicable measure for performance in moving the load and changing load speed is just the torque.

 

[bgbbgb246]

The rotor is the only moving part of a motor, so rotor inertia IS the "motor inertia"...

Okay thank you

And the OP's response also still seems to indicate that a low "inertia ratio" is thought to be a good thing.
Perhaps in some situations, it is a useful performance parameter, but only if it is reduced by reducing the load inertia rather than increasing the motor inertia.

In the OP's case, the directly applicable measure for performance in moving the load and changing load speed is just the torque.

And yes this was my assumption. While researching the proper way to size a motor in this scenario, it seemed that a lot of importance was placed on inertia ratio. Now having a better understanding of what the inertia ratio actually is, im struggling to understand why the inertia of the motor shaft would have much of a bearing over the motors ability to control the load. Ive seen several instructional documents on motor sizing that say to never go under an inertia ratio of 30:1 but if the motor has the torque to control the load, it seems like this is all that matters.

One big DIFFERENCE between a servo and a standard induction motor is precisely what you discovered; the servo rotor inertia is decidedly lower, so that the response to a speed / position move command can be faster and more precise in a smaller package. The trade off is cost and that a servo always requires a drive. If all you are doing is rotating a 250lb. drum, I seriously doubt that you would want a servo. If you need to VARY the speed, use an induction motor with a VFD, but attain the lowest speed via gearing or belt drive. If it's a fixed speed all the time, just use the gearing / belt drive.

Finally to respond to the several posts that mention that an induction motor may still be the best choice, I don't disagree with you yet. I just wanted to to look into whether the a servo was a logical hardware choice that would provide some advantages. Servos are cheap these days. The servo I've thrown out as an example above + the drive to pair with it comes out to a cost equal to or less than the example induction motor above + a VFD. The biggest hurdle in the project is that the load will need to spin faster when the cylinder is much smaller, maybe 1200rpm? There will ultimately be some type of pully reduction between the motor and the load to give the motor more of an advantage over the load but while keeping the motor in its usable rpm rage. A reduction of motor revs to load revs somewhere between 1.5 to 1 and 3 to 1 would ultimately exist.