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How to determine electric motor speed and horse power?

Tom Kitta

Active Member
#1
I got two 1ph electric motors from industrial blowers. They are ODP enclosure with on/off switch. Shaft is standard 1 1/8 suggesting they are at least 3hp.

Both plates with numbers on these motors are gone - somehow they got ripped off.

My main question is regarding speed - is it 1750 or 3500. Its not that easy to tell from motors running.

Motors are bit old - they have grease fittings for both bearings. They are heavy - 80 to 100+ lbs.

I don't have a tachometer - is there some easy way to tell?
 
#2
If you can physically see the windings of the stator stator you can count them and then divide the total number by 2 giving you the number of poles (e.g. If you have 8 coils you have a 4 pole single phase scim motor, the coil end will look like a "U" at the end of the stator). The motors you have will most likely be 4 pole 1800 rpm (well a bit less as you said 17xx as 1800rpm would be a synchronous speed and there'll be some degree of slip) as they are by far the most common. If the coils cannot be seen easily then the only other easy way is to measure shaft speed...and a cheap non contact tach (~$40) is a handy tool to have (arguably lol).
 

Tom Kitta

Active Member
#3
I guess I can get tachometer even for less:

https://www.aliexpress.com/item/Dig...lgo_pvid=812a7c14-01db-4382-8e79-727168bc09be

same one as on Amazon - https://www.amazon.ca/gp/product/B00850ZCRI/ref=ox_sc_act_title_1?smid=A3TX1GMDW0BT3S&psc=1
but for half price at just $12 CAD with two week shipping.

I try to count the coils today - its ODP so I can try to see stuff - if not then I get the cheap tachometer and have to check it in two weeks.

Any idea as to how to figure out HP?

I have a 440V surface grinder and the hydraulic system needs more power then the 440V motor running at 240V (about 30% of power). So I am thinking of connecting 5HP 3ph motor that is 240V / 480V to a 1ph blower motor and use it as a 480V generator just for the surface grinders two 2hp motors. Not ideal solution as there will be losses all over the place from all these motors but hey, we live in Canada & not only there is no 3ph power at home we also have voltages galore (surface grinder started as 570V but got re-winded by someone to 440V).
 

Johnwa

Active Member
#4
I found by laser tach. It worked the last time I used it but it didn’t turn on. I assume the batteries are dead. If you want to use it I’ll put new batteries in and check it out.
I’m south of the Glenmore reservoir.
 

Tom Kitta

Active Member
#5
I actually just bought one from China so I should have my own in about two weeks. Not a super pressing project. To turn induction motor into a generator is actually not that easy - you need to add "unknown" number of capacitors to energise the coils or you only get 2V-10V from left over magnetism.

Still I want to know what I have.

Thanks for the offer!
 

Johnwa

Active Member
#6
I found this on the HMEM site.
An iPhone stroboscopic tachometer “Strobe Tachometer (RPM Meter)”

I installed it on my phone and measured my lathe spindle speed. With it I measured 655 rpm. My lasertach gave 650. It’s not as easy to use as the laser but would work in a pinch.
 

Janger

(John)
Administrator
Premium Member
#7
I found this on the HMEM site.
An iPhone stroboscopic tachometer “Strobe Tachometer (RPM Meter)”

I installed it on my phone and measured my lathe spindle speed. With it I measured 655 rpm. My lasertach gave 650. It’s not as easy to use as the laser but would work in a pinch.
I have that app too - it does work but it's much easier to tune if you have a good idea of the starting point.

I've bought a 'hall sensor' rpm gauge from aliexpress. This is for permanent mounting - sensor and magnet with LED display. It's cheap and works well. Another option.
 

Attachments

Tom Kitta

Active Member
#11
For now portable one is in the mail - I probably use it to see measure other spindle speeds vs. what the gearing says they should be.

Any idea how to figure out motor HP? The only idea I have is to measure amps at no load and compare it to other motors - if it is closer to 3hp then 5hp motors then call 3hp otherwise call it 5hp. I doubt one of them is 7.5hp but we see.
 

PeterT

Ultra Member
Premium Member
#12
I was going to suggest a clamp meter over one of the wires (non-invasive way of determining current). You know panel voltage, therefore you have ingredients to determine watts. But I'm not an electrical guy so this might be a gross simplification. For example I don't know how no-load power equates to capable power under load (thus drawing more current) unless you had the particular motors power curve. But maybe it can get you close. Clamp meters were spendy when I looked at them but I think that was because I required DC. AC was less expensive. They are very handy to have & typically have the banana plugs to act as a regular multimeter with typical wire probes.

https://www.fluke.com/en-ca/learn/b...sics/clamp-meters/determining-load-horsepower
 

Tom Kitta

Active Member
#13
Clamp meter shows 6.4 amps on both 240V lines when cold and dropping to around 6amps on both lines when warmed up after around 10/15 min.

Seeing bank of capacitors implies it is a capacitor start/ capacitor run motor. The 0.4amp is obviously cost of warming up the bearings.

So what is the power factor? With capacitor run it should be something like .9 at full load but at idle? It should be around .25 (people go from .1 to .4)

Here they have a "rule of thumb": https://www.eng-tips.com/viewthread.cfm?qid=131215

All this is a bit off since most of power factor discussion is for 3ph motors not power corrected capacitor run 1ph motor.

Now what is the FLA? Well the "book" in NEC (USA) states 28 amps @ 230V. But more realistic is a bit lower - see https://www.amazon.ca/Baldor-L1430T...qid=1543348604&sr=8-1&keywords=184t+5hp+motor

So 23/ 24 or maybe 25 amps at 230V which translates at at least 1amp less on my measured 240V line. So average of this is 23FLA at 240V - 6amps then is 26% or so... thus it seems to be 5hp.

Doing the same for 3hp... http://www.electricmotorwholesale.com/BALDOR-VL1408T/ seems at 240V the FLA is 15amps ... maybe 16amps or 17 amps for older motors.

Unfortunately 6/16 is 38% which is high but not out of this world ;(

I read others have measured around 6amps plus at idle on 5hp...

I guess I can test the 2nd motor which seems smaller to see what idle amps it takes - if it takes less and given it shaft size I mark it as 3hp and this one as 5hp.
 

PeterT

Ultra Member
Premium Member
#14
I think you are getting close. This explains a few other parameters.
https://www.baldor.com/our-profile/...ail?id={0585DDBC-9DF7-49BC-A1C3-9BDAE434D06A}
https://www.baldor.com/Shared/pdf/SpecGuide.pdf

Measuring current & line voltage is the most direct way of knowing nominal input watts, but idling under low/unknown load I think is only partial information because you cant 'complete the curve' so to speak to nameplate power which is a rpm + torque point on a graph you unfortunately don't have. I Googled a bunch of Baldor motor nameplates & just did the Simple Simon math (HP=V*A/745*eff) It rarely equals nameplate so there are other factors behind it which the PDF 2nd link talks about a bit. You need to build yourself a dyno so you can card these motors & sleep better at night! LOL
 

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Tom Kitta

Active Member
#15
Well, just realised that if these motors are build to NEMA there is some difference in frames. https://www.engineeringtoolbox.com/NEMA-Frame-Assignments-d_1446.html

If I can figure out which frame the motors are and which speed they are (with still in the mail device) I should have more info about HP.

I know the shaft is 1 1/8th thus the frame is either 182T or 184T. The motors don't look like pre-NEMA at least not prior to 1964 when T frames were adopted - thus I don't think 213 / 215 frames are in play (which are not U frames thus even older).

It is very hard to find more info on standard frame sizes for single phase motors - I only found on link (above) - there are like dozens and dozens for 3 phase motors.