FLA OK but low rpm

[Tmoose]

Over the years I've measured dozens of 3 phase motors and come to trust nameplate rpm and FLA data. If the speed is lower than nameplate I expect to find excessive current draw. I have an older ~ 1/2 HP single phase direct drive (motor mounted). The FLA are less than nameplate, but the rpm is 20 rpm lower/slower than nameplate. I don't recall the actual values for amps and rpm right now.

Is this within "normal" tolerance for fractional HP motors, or should I be calibrating my 30 year old analog ammeter?

thanks

Dan T

 

[itsmoked]

I could easily believe a 20rpm variation.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

I agree with Keith.

Let's say you had motor at full load with voltage 5% below nameplate.
Let's also say your full load at rated nameplate conditions is 100rpm (you didn't tell us this number, but I have certainly seen 2-pole 60hz tiny motors with nameplate speeed 3500rpm or even less).
(by the way, give us more complete nameplate information including speed gives a better picture to discuss).

As a first approximation, slip s ~ P * R2 / V^2 where P is airgap power, R2 is rotor resistance, V is supply voltage.

5% low voltage would mean 10% low slip.
If you full load rated nameplate slip was 100 then your slip at 5% low voltage would be 10%*100rpm ~ 10 rpm lower.

If I remember right, NEMA MG-1 says something about 5rpm deviation from nameplate rpm allowed at rated conditions (load, voltage, frequency, maybe temperature). That means there can be a 5 rpm error built into the nameplate even if you consider exact rated conditions.

Combine all the above, it's not hard to imagine reading 20 rpm below what we think we should be based on currrent. (although at low voltage, current may read a little higher than FLA at full mechanical load).

Also do you really know your grid voltage. In US it is stable, but other places not so much.

More importantly, how accurately are you reading the speed. Picking it off of a spectrum... consider the bin width.

A few less likely possibilities: high rotor temperature, rotor degradation. I definitely would NOT jump to these conclusions without further investigation. Current signature analysis is a good test for rotor degradation. Again these are not likely possibilities compared to potential errors discussed above.


=====================================
(2B)+(2B)' ?

 

[electricpete]

I left out a word:
"Let's also say your full load at rated nameplate conditions is 100rpm "
should've been
"Let's also say your full load slip speed at rated nameplate conditions is 100rpm "

=====================================
(2B)+(2B)' ?

 

[electricpete]

Should've proofread. Here is correction to the entire post (basically typo's whose meaning should be obvious):

I agree with Keith.

Let's say you had motor at full load with voltage 5% below nameplate.
Let's also say your full load slip speed at rated nameplate conditions is 100rpm (you didn't tell us this number, but I have certainly seen 2-pole 60hz tiny motors with nameplate speeed 3500rpm or even less).
(by the way, give us more complete nameplate information including speed gives a better picture to discuss).

As a first approximation, slip s ~ P * R2 / V^2 where P is airgap power, R2 is rotor resistance, V is supply voltage.

5% low voltage would mean 10% high slip.
If you full load rated nameplate slip was 100 then your slip at 5% low voltage would be 10%*100rpm ~ 10 rpm higher.

If I remember right, NEMA MG-1 says something about 5rpm deviation from nameplate rpm allowed at rated conditions (load, voltage, frequency, maybe temperature). That means there can be a 5 rpm error built into the nameplate even if you consider exact rated conditions.

Combine all the above, it's not hard to imagine reading 20 rpm below what we think we should be based on current. (although at low voltage, current may read a little higher than FLA at full mechanical load).

Also do you really know your grid voltage. In US it is stable, but other places not so much.

More importantly, how accurately are you reading the speed. Picking it off of a spectrum... consider the bin width.

A few less likely possibilities: high rotor temperature, rotor degradation. I definitely would NOT jump to these conclusions without further investigation. Current signature analysis is a good test for rotor degradation. Again these are not likely possibilities compared to potential errors discussed above


=====================================
(2B)+(2B)' ?

 

[OperaHouse]

There has to be a backstory here. What would even prompt you to look at a motor that closely?

 

[Tmoose]

I was shooting my mouth off (again) in front of a co-worker about how useful rpm is as an indicator of how close to full load an induction motor is. So I had bought this cute industrial fan for $5 at a yardsale, and dug out the Monarch digital strobestrobe and clamp on ammeter ( the small yellow ones that were everywhere a few decades ago.)

Fortunately no beers had been bet, but one of the few tenets I cling to was damaged for the moment.

thanks

Dan T

 

[YuriB]

The first that has come to mind:
1. Lowered than nominal voltage(the load is fortunately low too - at the nominal load the current would be higher than FLA, the slip higher, rpm of course yet furhter slower).
2. Motor 60 Hz supply 50 Hz.
3. The "strobostrobe" itself needs calibration.

 

[ScottyUK]

Don't assume the grid is a pefect 50Hz or 60Hz. It's usually pretty good but it does slow down and speed up a little. Sometimes it experiences bigger excursions if a major event occurs on the system, although these are usually fairly short-lived. If the grid frequency moves, so does synchronous speed.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[OperaHouse]

We built a monitor for an oil filtration system to identify when the filter needed to be replaced. I was sent to investigate when there were a lot of false indications. The difference between normal load current and filter replacement was pretty small. The motor mfg had changed and that caused the problem. I thought looking at the phase angle between voltage and current would be a better indicator of load, this was a fractional hp motor. This system was microprocessor based so it would be easy to do. It wasn't my project and we soon lost the contract so I didn't investigate further.

 

[waross]

I'll bet this is a calibration issue.



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

 

[itsmoked]

How's a strobe need calibration? Seems sort of like a digital process to me. Either you get every strobe flash or you don't get any.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

should I be calibrating my 30 year old analog ammeter? ?????
And;
For a 1760 RPM motor the slip is 40 RPM. 20 RPM more slip is quite a bit. (+50%)
For a 1650 RPM motor the slip is 150 RPM and an extra 20 RPM slip is more acceptable. (+13%)
Given the ratio of the permeability of iron to air and most other materials, it doesn't take much dirt or corrosion on the mating surfaces of the clamp meter to lower the reading considerably.

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