Author Topic: How do you measure the power going into an AC induction motor?  (Read 731 times)

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Offline Robomeds

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How do you measure the power going into an AC induction motor?
« on: October 19, 2018, 11:36:48 am »
So I have a home repair question and thought people here might offer some suggestions.  So recently I've been working to adjust the speed of the blower fan used as part of my home furnace/air conditioner. 

This furnace lets you adjust the blower fan speed by changing the diameter of a pulley on the motor. 
See this informative video here!


I started the project thinking, hey, this will be easy.  I've got an amp clamp, I'll just measure the current in, I know I can't exceed the 6.5 A motor rating, all will be good.  Then I measured the motor... oh 6.5A.  Well I hate to think it was already at max power output since I really want more airflow out of this thing.  As a sanity check I measured the current into the motor with no belt... wait, still 6.5A.

So the current was the same in both the loaded and unloaded case.  So now this power factor thing just got real to me...
https://en.wikipedia.org/wiki/Power_factor

So here is my question, without a power clamp meter, how do I use things like multimeters to measure power into an AC motor where my power factor isn't always 1?
 

Online IanB

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Re: How do you measure the power going into an AC induction motor?
« Reply #1 on: October 19, 2018, 11:45:15 am »
Something doesn't seem right. If it is truly an AC induction motor running approximately at a fixed speed, then the current with no load should be far less than the current when driving the fan. Regardless of power factor, the current should be less with no load.

As to your main question, the best option would be to use a device like a Kill-A-Watt. That will tell you voltage, current, power and power factor, accurately and without fuss. A DMM is not the right tool for this job.
I'm not an EE--what am I doing here?
 

Offline Robomeds

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Re: How do you measure the power going into an AC induction motor?
« Reply #2 on: October 19, 2018, 12:12:52 pm »
Something doesn't seem right. If it is truly an AC induction motor running approximately at a fixed speed, then the current with no load should be far less than the current when driving the fan. Regardless of power factor, the current should be less with no load.

As to your main question, the best option would be to use a device like a Kill-A-Watt. That will tell you voltage, current, power and power factor, accurately and without fuss. A DMM is not the right tool for this job.

That's the catch with the reactive load of the AC motor.  What happens is the current and voltage sine waves are out of phase.  With a simple resistor the voltage and current sine waves are perfectly in phase.  With an AC motor they aren't.  So normally you would say the instant power into the motor RMS voltage*RMS current or peak voltage*peak current * .707 (conversion from peak to RMS assuming pure sine waves).  But with voltage and current out of phase the peak current might occur when voltage is low. 
https://demonstrations.wolfram.com/ACPowerFactorPrinciple/
In theory the peak current draw could occur when voltage is 0.  This is why it's confusing.  As far as I can tell I either need to know the phase difference between current and voltage and use that to calculate power or I need to have a simultaneous voltage and current measurement.  This is a one time setup so I don't really want to buy a power meter for this job. 
 

Online IanB

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Re: How do you measure the power going into an AC induction motor?
« Reply #3 on: October 19, 2018, 01:10:38 pm »
That's the catch with the reactive load of the AC motor.  What happens is the current and voltage sine waves are out of phase.  With a simple resistor the voltage and current sine waves are perfectly in phase.  With an AC motor they aren't.  So normally you would say the instant power into the motor RMS voltage*RMS current or peak voltage*peak current * .707 (conversion from peak to RMS assuming pure sine waves).  But with voltage and current out of phase the peak current might occur when voltage is low. 
https://demonstrations.wolfram.com/ACPowerFactorPrinciple/
In theory the peak current draw could occur when voltage is 0.  This is why it's confusing.  As far as I can tell I either need to know the phase difference between current and voltage and use that to calculate power or I need to have a simultaneous voltage and current measurement.  This is a one time setup so I don't really want to buy a power meter for this job.

I agree about reactive loads. But it's similar to transformers. If there is no load on the secondary, the primary draws little current (reactive or otherwise). Some with induction motors. If the motor is unloaded there is little current flowing, unless it is the world's crappiest motor. Forget the power factor. Even though the power factor may be bad, the actual load is tiny so the reactive current is tiny.

A power meter costs only a few bucks, like the price of a couple of sandwiches, and it has many uses beyond this one. Regardless of whether you want to buy one or not, you can't make this measurement without one. Your choice. Either measure or don't measure  :-//
I'm not an EE--what am I doing here?
 

Online IanB

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Re: How do you measure the power going into an AC induction motor?
« Reply #4 on: October 19, 2018, 01:15:25 pm »
A power meter costs only a few bucks, like the price of a couple of sandwiches, and it has many uses beyond this one. Regardless of whether you want to buy one or not, you can't make this measurement without one. Your choice. Either measure or don't measure  :-//

Actually, I lied. You can make this measurement with an oscilloscope and some differential probes and use of math functions. The cost of those items will set you back way more than the price of a lunch or two.
I'm not an EE--what am I doing here?
 

Offline Robomeds

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Re: How do you measure the power going into an AC induction motor?
« Reply #5 on: October 19, 2018, 01:35:45 pm »
Yeah, that seems to be the problem.  I used to have a Flukemeter which did have relatively high voltage, isolated inputs.  My current Agilent scope isn't isolated and buying isolation probes seems more expensive than an amp clamp with power readings.  I was just hoping there was some clever trick to this or calculating the phase difference. 

Anyway, the only power meters I've found are ~$200 and up.  I do have one of the Kill-A-Watt type things but I suspect it just multiplies RMS voltage*RMS current.  I doubt it actually measures true power (instant voltage* instant current) into a non-sine wave RMS conversion. 
 

Online IanB

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Re: How do you measure the power going into an AC induction motor?
« Reply #6 on: October 19, 2018, 01:43:25 pm »
Anyway, the only power meters I've found are ~$200 and up.  I do have one of the Kill-A-Watt type things but I suspect it just multiplies RMS voltage*RMS current.  I doubt it actually measures true power (instant voltage* instant current) into a non-sine wave RMS conversion.

No, the Kill-A-Watt meters do a proper measurement and indicate an accurate power factor (at least, the genuine ones do). They sample the waveforms many times per second and do the math.
I'm not an EE--what am I doing here?
 

Offline schmitt trigger

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Re: How do you measure the power going into an AC induction motor?
« Reply #7 on: October 19, 2018, 01:59:39 pm »
Blower motors are prime candidates for split-phase induction motors.

With an unloaded motor ......
As the motor gathers speed, can you listen to a sharp “click” while simultaneously the current drops?

If not, the centrifugal switch could be defective.
 


Offline Robomeds

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Re: How do you measure the power going into an AC induction motor?
« Reply #9 on: October 19, 2018, 02:04:22 pm »
Blower motors are prime candidates for split-phase induction motors.

With an unloaded motor ......
As the motor gathers speed, can you listen to a sharp “click” while simultaneously the current drops?

If not, the centrifugal switch could be defective.

The motor is relatively new and nothing wrong with it.  I think the source of my problem is a near 40 year old pulley that has worn in groves.  I'm going to replace it when I get the chance.  Since I have the option to change the fan speed (my new pulley can be set to a faster speed vs the old one, I just want to make sure I don't overload the motor. 
 

Online IanB

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Re: How do you measure the power going into an AC induction motor?
« Reply #10 on: October 19, 2018, 02:22:06 pm »
I'll have to try mine.  I think this is the model Belkin I have
https://www.wiredathome.com/belkin-f7c005-conserve-insight-energy-use-monitor-free-shipping/?ref=lexity&_vs=google&_vm=productsearch&gclid=EAIaIQobChMI7tiNhMGR3gIVl0wNCh0-LA1iEAQYASABEgIx-vD_BwE

I've no idea, that thing looks like a $150 paperweight. Actual, functional meters cost about $20. They display voltage, current, true power (W), apparent power (VA), frequency, power factor and integrated kWh.
I'm not an EE--what am I doing here?
 

Offline Robomeds

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Re: How do you measure the power going into an AC induction motor?
« Reply #11 on: October 19, 2018, 02:30:35 pm »
I'll have to try mine.  I think this is the model Belkin I have
https://www.wiredathome.com/belkin-f7c005-conserve-insight-energy-use-monitor-free-shipping/?ref=lexity&_vs=google&_vm=productsearch&gclid=EAIaIQobChMI7tiNhMGR3gIVl0wNCh0-LA1iEAQYASABEgIx-vD_BwE

I've no idea, that thing looks like a $150 paperweight. Actual, functional meters cost about $20. They display voltage, current, true power (W), apparent power (VA), frequency, power factor and integrated kWh.

I got it on ebay for about the same price as the Kill-A-Watt units but this is the first time I've ever actually had to deal with this sort of issue. 
 

Online IanB

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Re: How do you measure the power going into an AC induction motor?
« Reply #12 on: October 19, 2018, 02:36:10 pm »
I got it on ebay for about the same price as the Kill-A-Watt units but this is the first time I've ever actually had to deal with this sort of issue.

Throw it away and get a Kill-A-Watt. It looks like junk. (Most or all Belkin stuff is junk.)
I'm not an EE--what am I doing here?
 

Offline schmitt trigger

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Re: How do you measure the power going into an AC induction motor?
« Reply #13 on: October 20, 2018, 02:02:29 am »
Blower motors are prime candidates for split-phase induction motors.

With an unloaded motor ......
As the motor gathers speed, can you listen to a sharp “click” while simultaneously the current drops?

If not, the centrifugal switch could be defective.

The motor is relatively new and nothing wrong with it.  I think the source of my problem is a near 40 year old pulley that has worn in groves.  I'm going to replace it when I get the chance.  Since I have the option to change the fan speed (my new pulley can be set to a faster speed vs the old one, I just want to make sure I don't overload the motor.

Didn't know it was new, it was just a comment on possible failure mechanisms..

Anyways, split-phase motors are notoriously inefficient. But they are the cheapest too.

The best way to determine whether you are overloading a motor is to measure its actual shaft rotational speed, with load.
As simple as painting a pair of white stripes on the pulley and utilizing a photoreflective diode-sensor combination as a sensor. If your DMM has a Hz function, you are all set up.

If the supply voltage is nominal, and the loaded speed is below the plate's stated RPM, you may be overloading the motor.
 

Offline David Hess

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Re: How do you measure the power going into an AC induction motor?
« Reply #14 on: October 20, 2018, 03:18:30 am »
A cheap Kill-A-Watt will measure the current, VA, watts, and power factor.

However I have made this same measurement using just an RMS current meter and current varied considerably with load on an induction motor.
 

Offline Robomeds

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Re: How do you measure the power going into an AC induction motor?
« Reply #15 on: October 20, 2018, 10:15:18 am »
Well I found at least one reference that claims the Belkin meter does account for power factor.  I will try it. 

schmitt, I had thought about rigging up an IR couple to make a tach using my meter.  I've been thinking about it for a while so it's probably worth doing regardless of the particular project. 
 

Offline Robomeds

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Re: How do you measure the power going into an AC induction motor?
« Reply #16 on: October 20, 2018, 12:17:09 pm »
Well the Belkin was able to measure true watts.  Unloaded the motor used half the power it used when turning the blower.  Oddly, it didn't change power consumption much as I increased the fan load.  I guess it was just a 6% increase in pulley ratios.
 

Offline james_s

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Re: How do you measure the power going into an AC induction motor?
« Reply #17 on: October 21, 2018, 09:34:31 am »
Those Kill A Watt meters are surprisingly accurate. Several years ago I compared one with a proper benchtop power analyzer and found it to be nearly bang on, well within the specified accuracy.
 

Offline tecman

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Re: How do you measure the power going into an AC induction motor?
« Reply #18 on: October 23, 2018, 05:00:53 am »
Watts versus VA

Larger induction motors can draw 40% to 60% of FLA when unloaded.  Watts, however are low.

paul
 

Offline schmitt trigger

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Re: How do you measure the power going into an AC induction motor?
« Reply #19 on: October 23, 2018, 06:34:10 am »


schmitt, I had thought about rigging up an IR couple to make a tach using my meter.  I've been thinking about it for a while so it's probably worth doing regardless of the particular project.

If you work around motors, it definitively is a worthwhile project. And not too difficult. I say, Go for it!

Doing it with a microcontroller is quite simple. The way I did it though, was with a LM331, because I wanted the output to drive an analog meter. The analog meter naturally filters any jitter and it provides a quick visual interpretation whether the speed is increasing or decreasing.

Of course, you can combine the best of both worlds.  ;)
 

Offline Robomeds

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Re: How do you measure the power going into an AC induction motor?
« Reply #20 on: October 23, 2018, 03:08:42 pm »
I've created quite a few IR couple type circuits over the years.  With an op-amp and a schmitt trigger it's easy to make one with adjustable sensitivity.  A while back I set up this sort of circuit connected to a counter to verify some of our cycle test equipment actually cycled the device under test as many times as we requested.  I probably have spare parts from that build. 

I've also done things like use a loop of wire around a spark plug wire to measure engine RPM (well engine frequency in hz).  I was thinking I might make two and give one to a mechanic friend so he would have something he could use to measure RPM on random projects. 

It's a quick, fun project... but since my power meter gave me the answer I needed I'm probably going to be a bit slow to make the thing.  I've got a few other projects in front of it. 
 

Offline schmitt trigger

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Re: How do you measure the power going into an AC induction motor?
« Reply #21 on: October 24, 2018, 12:54:47 am »
I know, I know the feeling!
More project ideas than time (or money) available.

Plus the Missus always has some projects of her own!
« Last Edit: October 24, 2018, 12:56:28 am by schmitt trigger »
 


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