Motor turn down ratio 1

[Krix004]

Hello,

I was looking for a motor with speed turn down ratio of 5:1. Supplier quoted a motor with the following info on the spec:
Rated speed = 1450 RPM
Speed at minimum torque = 210 RPM
Is this value of 210 RPM the indication of minimal allowable speed? (seems lower than 1450/5=290) I cannot find any other confirmation of 5:1 turn down ratio on the provided specification.

Thank you

 

[BrianPetersen]

They gave you a spec for a motor that will do better than what you asked for. What's the problem?

 

[melspuds]

This is time to ask questions with a quote not after. To me, you got better ratio than you asked for.
Dave

 

[waross]

What frequency is the 210 RPM based on and what is the actual torque? Be aware that the torque at 210 RPM when fed from rated frequency will have little relationship to the torque when fed from a VFD.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[melspuds]

Bill, very true with a drive.
Dave

 

[Krix004]

Thank you for replies. I am not a specialist and did not fully understand what was meant by speed at "minimum torque". The purpose for this motor was to regulate the speed of positive displacement pump which requires constant torque as speed varies.
Waross, 210 RPM is based on 60 Hz. Per specification, the Nominal torque is 14.5 N*m and Minimum torque is 5.8 N*m.

 

[jraef]

I'm suspicious of the way they worded that, if it is depicted exactly as stated. Motor suppliers generally depict a turn down ratio, when discussing its use with a VFD, as a simple ratio without qualification as to torque. Indeed, one PURPOSE of a VFD is to MAINTAIN full torque at any speed, but within the motor's ability to keep itself cool. It is the turn down ratio that gives you that lower speed value, beyond which you must provide external cooling or shed load.

By qualifying the lowest speed saying "at minimum torque", they are implying that you have ALREADY been shedding load BEFORE you get to 210RPM. That's why I'm suspicious, it's almost as if they are being purposely cagey, dodging the question with obfuscation.

But, to give them the benefit of the doubt, you may have shared details with them that you left out here with us. For instance if the application is a centrifugal pump or fan and they knew it, then their response might be appropriate for the way they know it will be used. In a centrifugal machine, the load (torque) varies as the cube of the speed change. Generally, it's not necessary to specify a turn down ratio for a motor intended to be used on a centrifugal machine for this reason. So what they are saying is that at the point where your centrifugal load will require minimum torque and still perform useful work, their motor speed can be 210RPM, meaning it will work fine for you. This then may have simply been their own gentle way of telling you that you were asking for a specification that was meaningless in context.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[rhatcher]

I believe they are providing the RPM at which pull-up torque occurs. Specifically, the sine wave (60hz) speed-torque curve can be characterized by describing four points on the curve along with the associated rpm. These points are locked rotor, pull-up (minimum), breakdown (maximum), and rated torque.

 

[jraef]

Well that may be, but it's not what turn down ratio means.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[rhatcher]

jraef.

The OP stated that he has a constant torque load so I do not think this is some unusual way of describing a variable torque speed range. It would be nice to see the motor specification. If we find the rated torque, rated speed, minimum torque, and speed at minimum torque information that the OP provided in the same section as locked rotor torque, maximum torque, and speed at maximum torque, then we are looking at sine wave speed torque data.