How to predict KW rating and rated current when there is no nameplate on a three phase induction motor?

[Muhammad Umer ansar]

I came accross the following tasks during my trainee period in an electrical firm. I was provided motor with no nameplate in following conditions:
1) When I can remove the mechanical load from the motor, so that i can run it on no load.
2) When i cant remove the mechanical load from the motor shaft. That is, i need to predict the Motor KW and current rating at loaded condition.
3) When the motor is defective, that is it is not starting( or i cant start it for some reason like overcurrent etc)
My questions are:
1) How to predict(calculate) the KW rating and current rating of motor in above three conditions?
2) If in any case i am unable to predict current rating then how do i know what size of Breaker and protection do i need for safe testing of that motor ?
3) Can i calculate the KW rating using physical size and parameters of a motor?
Umer.

 

[Jraef]

Questions:

1. You cannot, you can only guess, and it will not be an accurate guess. You can put it on a dynamometer and load the motor with a known amount of torque, then monitor the temperature rise of the motor, while guessing at the temperature rating of the insulation to determine when the motor temperature under load is approaching what MIGHT be dangerous for the motor. So maybe +- 50%? Maybe worse.
2. Because of the answer to 1), you cannot safely determine the rating of protective equipment for it.
3. No, you can only narrow the range of your guesses in 1), i.e. there is always a range of power ratings within a frame size, it is never one size for one frame, and it also depends on the number of winding poles which, without having energized it to measure the speed, you will not know from looking at it.

Motors without nameplates are something we call "boat anchors".

 

[Muhammad Umer ansar]

Questions:

1. You cannot, you can only guess, and it will not be an accurate guess. You can put it on a dynamometer and load the motor with a known amount of torque, then monitor the temperature rise of the motor, while guessing at the temperature rating of the insulation to determine when the motor temperature under load is approaching what MIGHT be dangerous for the motor. So maybe +- 50%? Maybe worse.
2. Because of the answer to 1), you cannot safely determine the rating of protective equipment for it.
3. No, you can only narrow the range of your guesses in 1), i.e. there is always a range of power ratings within a frame size, it is never one size for one frame, and it also depends on the number of winding poles which, without having energized it to measure the speed, you will not know from looking at it.

Motors without nameplates are something we call "boat anchors".

Thanks Mate for this

 

[Besoeker3]

I came accross the following tasks during my trainee period in an electrical firm. I was provided motor with no nameplate in following conditions:
1) When I can remove the mechanical load from the motor, so that i can run it on no load.
2) When i cant remove the mechanical load from the motor shaft. That is, i need to predict the Motor KW and current rating at loaded condition.
3) When the motor is defective, that is it is not starting( or i cant start it for some reason like overcurrent etc)
My questions are:
1) How to predict(calculate) the KW rating and current rating of motor in above three conditions?
2) If in any case i am unable to predict current rating then how do i know what size of Breaker and protection do i need for safe testing of that motor ?
3) Can i calculate the KW rating using physical size and parameters of a motor?
Umer.

Cage induction motor?
Measuring the temperature could make a stab at it, perhaps even a reasonable estimate. Rated slip could also be a reasonable measure.

 

[kwired]

Motor already coupled to a load you possibly can find out information on the driven load and get some idea as well. But again it is only a guesstimate.

 

[Hv&Lv]

Just a question..
With no nameplate (and no voltmeter) how do you know the size or the voltage? Or even if it’s DC or AC?

 

[Besoeker3]

Just a question..
With no nameplate (and no voltmeter) how do you know the size or the voltage? Or even if it’s DC or AC?

I did make the comment about possibly being a cage motor. A commutator motor would most likely be DC.

 

[Hv&Lv]

I did make the comment about possibly being a cage motor. A commutator motor would most likely be DC.

True, and I read that..
But if someone just lays a motor at your feet with no label, I agree with what else was stated.. it’s a boat anchor.

 

[Besoeker3]

True, and I read that..
But if someone just lays a motor at your feet with no label, I agree with what else was stated.. it’s a boat anchor.

Well maybe one would look at the picture of a boat anchor and it might not actually resemble an electric.............
And, from the OP: " Can i calculate the KW rating using physical size and parameters of a motor? "
At least the physical size and shape would would be a bit of a clue.

 

[kwired]

Well maybe one would look at the picture of a boat anchor and it might not actually resemble an electric.............
And, from the OP: " Can i calculate the KW rating using physical size and parameters of a motor? "
At least the physical size and shape would would be a bit of a clue.

To some extent yes. single and three phase induction motors are about same size for same output rating. You might only be able to say it is likely fractional HP, or say it is either 1-1.5, maybe even 2 hp. 3-5 are similar sized quite often and so is 7.5 and 10. then 15, 20, 25 sort of similar but each one in same brand/ series is slightly bigger, 30 sort of can be confused with 25 or 40. 40 and 50 somewhat same and 60 and 75 somewhat same as you get bigger they become a little more predictible if you know a little bit about them and know their approximate age.

Voltage wise, and this applies more to general purpose motors than some OEM's, single phase 1.5 hp and occasionally even 2 hp and less is usually dual voltage 115/230, 3 HP and up usually just 230 volt. Three phase most motors 60 HP and below are dual voltage 230/460. could be wye or delta connected, but you can determine that with an ohmmeter if there is no nameplate.

If you have equipment that came from Canada it may be 575 volt instead of 480.

If you not sure what you have apply 208 or 240 instead of 480 first, it won't destroy it as easily as applying 480 then finding out it was only 200-230 volt rated. If you don't use it at all because you don't know what it is, then it is a boat anchor. If you proceed carefully you may get some use out of it, otherwise worst case it is still a boat anchor.

 

[Hv&Lv]

Well maybe one would look at the picture of a boat anchor and it might not actually resemble an electric.............
And, from the OP: " Can i calculate the KW rating using physical size and parameters of a motor? "
At least the physical size and shape would would be a bit of a clue.

Would you be able to tell DC vs AC, and voltage?

 

[kwired]

Would you be able to tell DC vs AC, and voltage?

9 lead motor - majority of time dual voltage three phase induction motor.

If there are capacitors hanging off side of motor - single phase, 6 leads probably dual voltage, 4 leads probably single voltage. (not counting any leads on internal thermal protector or jumpers between capacitors.

AC motor leads usually numbered 1 up to 8 or 9 or even 12 occasionally.

DC motor leads often will at least have an F1 F2, or maybe + and - on the field windings instead of just numbers only.

Motor with no nameplate often will already have leads still connected for whatever configuration it was last used for, if removed from service maybe with the input leads either disconnected or with old supply conductors still attached but cut very close to connection method. That can sometimes give you clues to what it was last used as.

 

[Hv&Lv]

9 lead motor - majority of time dual voltage three phase induction motor.

If there are capacitors hanging off side of motor - single phase, 6 leads probably dual voltage, 4 leads probably single voltage. (not counting any leads on internal thermal protector or jumpers between capacitors.

AC motor leads usually numbered 1 up to 8 or 9 or even 12 occasionally.

DC motor leads often will at least have an F1 F2, or maybe + and - on the field windings instead of just numbers only.

Motor with no nameplate often will already have leads still connected for whatever configuration it was last used for, if removed from service maybe with the input leads either disconnected or with old supply conductors still attached but cut very close to connection method. That can sometimes give you clues to what it was last used as.

Too many probably’s...
like I said, no nameplate, it’s a crapshoot

 

[paulengr]

I came accross the following tasks during my trainee period in an electrical firm. I was provided motor with no nameplate in following conditions:
1) When I can remove the mechanical load from the motor, so that i can run it on no load.
2) When i cant remove the mechanical load from the motor shaft. That is, i need to predict the Motor KW and current rating at loaded condition.
3) When the motor is defective, that is it is not starting( or i cant start it for some reason like overcurrent etc)
My questions are:
1) How to predict(calculate) the KW rating and current rating of motor in above three conditions?
2) If in any case i am unable to predict current rating then how do i know what size of Breaker and protection do i need for safe testing of that motor ?
3) Can i calculate the KW rating using physical size and parameters of a motor?
Umer.

Item 3: generally no. If you know it’s NEMA and age (T frame, U frame, or older U frame) and can use measurements to find the frame size AND know the number of poles you might be able to narrow down then HP, but that’s a lot of unknowns.

 

[kwired]

Too many probably’s...
like I said, no nameplate, it’s a crapshoot

The more you work with them the more you can just tell what you have without ever looking at a nameplate. If there is a little uncertainty an ohmmeter may help narrow things down.

Again much what I said there is for general purpose motors. OEM or definite purpose motors sometimes can throw you way off.

Earlier I mentioned motor size being a clue, but still had many that sort of overlap one another. That is where one guy may not be certain, but someone that sees a lot of motors will be more certain which one it is just looking at it. Kind of like some car gurus can tell you what year and make many cars are without any names or logos in view.

 

[Besoeker3]

Would you be able to tell DC vs AC, and voltage?

Yes, for the vast majority of them. Most of the AC motors are induction machines and most of those are cage motors. DC motors are, by and large, commutator machines. There are some special machines. One of the divisions in our company made so called brushless DC motors. These were in the typically 20,000 rpm at 56kW. High performance stuff. As far as I was concerned they were permanent magnet synchronous machines.