Power in 3 Phase Electric Motors 2

[rdrago]

How do I measure power in a 3 phase electric motor? I am trying to justify the installation of a drive on a motor based on the power savings obtained through the use of a drive. The motor is running a fan load. The problem lies in the fact that the power factor at different motor speeds varies, thereby varying the reactive componet of the power. Although I can certainly measure the resistive power used by the motor, I don't know how to measure the reactive power as the motor speed varies. The power factor listed on the nameplate of the motor is the power factor obtained at the full speed of the motor only. Since the power factor varies at lower speeds, it cannot be used to calculate the reactive power. Can anyone help me with this?

 

[rbulsara]

Doesn't the drive have kW metering?

Take readings at regular intervals and plot a graph..

Or else you need a wattmeter and two CTs.

 

[pablo02]

Since you know the resistive power at a given load (w), measure the amps and the volts at the same load points... multiply the volts (V) * amps (A) * 1.732 (Sqr3) -- this gives you VA (3ø)

square each value, subtract W^2 (squared) from VA^2 (squared) -- this should be your vAR's^2 (squared) -- take the Sqr of this remaining value (vAR^2)-- this should be the reactive power (vAR).. (set up a spreadsheet to perform the calculations and the costs associated with the loads)

the efficiency of the motor will need to be accounted for -- without spending a lot of time reviewing my formulas, I think it should cancel out...

If you are looking at justifying the use of a drive, you'll need to know (or give good estimates) of the % the fan runs at different loads -- obviously, the more time spent at lower loads, the more justification you'll have..

best of wishes

 

[cbarn24050]

Hi, I assume you mean an inverter drive. The reactive current in the motor circulates throught the motor windings and the inverter power switches and not through the main inlet. Therefore you can measure the actual power comsumption by using a current meter in the mains input to the drive. Remeber that this input current is not a sine wave so you need a meter that measures true rms.

 

[rbulsara]

One question:

In order to find the 'energy savings' you need to know the kW cosumed by a fisxed speed motor. And compare it to kW consumption of the drive over same period of time, at variable speed (that is still performing the same task as before..without giving up final effec..for example, getting same or better level of comfort if it is a air handling unit's fan. Saving energy at expense of the performance is no 'saving'.)

I am wondering why do u need to know reactive power? It has nothing to do with energy saving, which is only measured in kW.

 

[rdrago]

Thank you all for your response.

To rbulsara: The factory pays for the reactive power even though it is returned to the source. Therefore, my thought was to measure the real and reactive power that is delivered to the motor in order to report the true savings in dollars. In my application, the motor currently runs at full load and we control the air flow through a duct with a damper valve. I will measure the motor power at the fixed speed and then measure the power (using an inverter) at a motor speed that produces the same typical air flow in the duct without the use of the damper. The motor runs 24/7.

To cbarn24050: I don't completely understand your response. Are you saying that measuring the input current to drive will allow me to calculate the the real and reactive power delivered to the motor? Others have told the same thing and I did'nt understand them either, so don't feel bad.

To pablo02: An interesting technique. I quess that I can measure watts at the motor and calculate the reactive power with your method. I'll try it.

Thanks again. It's my first time on the forum. Its nice to know that there are knowledgable people that are willing to help.

rdrago

 

[cbarn24050]

Hi, yes measuring the input drive current will give you the real power.

 

[rbulsara]

Measuring the inut to the drive will give yoy 'apparent power' = VA as mentioned by pablo02. If you know W you can find VAR as indicated by pablo02.

rdrago:

I guess you mean you pay for 'reactive' power as part of demand charges or power factor penalty. Howelse would you measure and pay for reactive power?

Demand charges will not change by much as at somepoint you will run the motor at full speed and it will incurr the almost same Maximum demand (only differing by p.f. margin).
It would be interesting to allocate 'savings' of power factor correction of a single motor to overall plant!
Unless this single motor is a large part of the total load or you have a bunch of similar motors.

And also being 24/7 load, kWh will be the predominant charge, way dominant.

 

[pablo02]

I've had some second thoughts about your post and my prior comment -- I guess I need to ask about how the fan output is controlled now? Is it by speed or by dampers?

The drive unit (I'm assuming a ASD), regardless of the output speed, should not vary the PF as seen by the utility, it should be near unity -- so I guess I'm asking what do you mean by the statement: "The problem lies in the fact that the power factor at different motor speeds varies, thereby varying the reactive componet of the power."

The savings for a drive unit is the energy needed to run a fan at full speed, taking the pressure drop across the dampers vs. that of the drive unit... hence, the more the fan runs at a lower load, the greater the savings..

If you are paying for kVA vs demand (kW), then there might be some savings since the drive should run near unity PF -- but as rbulsara implies, you may need to look at the whole plant -- maybe you want to consider another option to PF correction ???

 

[jbartos]

Suggestion to rdrago (Electrical) Nov 8, 2003 marked ///\\How do I measure power in a 3 phase electric motor?
///Pwatt by wattmeter and Pva by ammeter and voltmeter. Some modern circuit breaker (e.g. electronic circuit breakers) electronic trip unit display, Watts, VAs, etc.\\ I am trying to justify the installation of a drive on a motor based on the power savings obtained through the use of a drive. The motor is running a fan load. The problem lies in the fact that the power factor at different motor speeds varies, thereby varying the reactive component of the power.
///Watt or Watthour recorder may be used to justify the savings.\\ Although I can certainly measure the resistive power used by the motor, I don't know how to measure the reactive power as the motor speed varies.
///There are VAR meters. Visit

http://www.powertekuk.com/Pwr3ph.htm

for suitable VAR or VARhour meter.\\ The power factor listed on the nameplate of the motor is the power factor obtained at the full speed of the motor only. Since the power factor varies at lower speeds, it cannot be used to calculate the reactive power.
///Yes, PF may be measured or calculated at different speeds. Alternately, a PF curve versus speed may be available from the motor manufacturer.\\ Can anyone help me with this?

 

[rdrago]

Thanks again to everyone for the responses.

To pablo02: Doesn't the reactive power delivered to the motor go through the drive? How can the utility see unity power factor? In response to your other question, we have 50 of these motors that need inverter drives.

To rbulsara: Am I trying to be too precise in attempting to measure reactive power when real power is dominant in the motor? Is the dollars savings from reactive power insignificant as compared to real power dollar savings?

To jbartos: I will buy a meter that measures watts and vars. Thanks for website. I plan to measure the output of the drive (inverter to motor connection). Is this correct or should I measure the mains input to the drive to obtain valid readings?

Thanks again,
rdrago

 

[rbulsara]

rdrago:

Well, I think so.

Also measuring output of drive is of no use imho, what utility sees and meters (what you are paying for)is what you feed into the drive! unless you utility meter on load side of the drive!

Primary gain of a drive is energy saving, if the speed is in fact varied on regular basis. Improved power factor (on line side of drive) a side benefit.

 

[wwilson]

We use a Fluke 41B or a Fluke 39 power meter. They are designed specifically for measuring power and provide information on harmonic content. I've saw places on the web that where you can rent them.
wwilson

 

[pablo02]

I agree with rbulsara about trying to measure the drive outputs -- (I'm lost on your effort to measure the reactive power on, I assume, the load side of the drive)

how are the fans controlled now?

is this a new installation or a retrofit? again, how are the fans (output) controlled and what is the need for varying the speed? (50 drives aren't cheap)

what is your approach for justifying the drives?

 

[rdrago]

To wwilson: Renting is a really good idea. I'll try it. Thank you.

To pablo02: The motor shaft is connected to a fan that pulls air through a duct. The motors are not controlled. They run at rated speed and FLA. We regulate the amount of air that is pulled through the duct with a butterfly valve that is installed in the duct. We can completely close the valve to have zero air flow in the duct or we can totally open the valve to provide maximum air flow. So, we control the amount of air that flows in the duct but we do not control the speed of the motor or fan.

This is a retrofit of an existing system.

My approach to justifying 50 drives is as follows:

1. Measure the power consumed by a single motor in the current arrangement.

2. Measure the air flow in the duct at various butterfly valve positions starting 0% (full closed) to 100% (full open) at 10% intervals.

3. Connect a drive to one motor. Measure the power consumed by the motor at a motor speed that causes a similar amount of air flow in the duct as the historical average position of the valve does. The difference in power consumed at reduced speed verses the rated speed is the savings. Since I know the average historical positions of the 50 butterfly valves, I can calculate the savings for all the drives in this manner.

My concern with measuring only real power is that I want to provide an accurate dollar savings number as possible to my managers. Therefore, I thought that measuring reactive power would also be of value. However, I agree with rbulsara that the reactive power charge may be a moot point if we are on a power factor penalty charge from the utility. I need to research our charges further.

Thanks for your help.
rdrago

 

[Skogsgurra]

rdrago,

I think that you have got all answers you need. So I ask some questions instead:

1 What power range are you talking about?

2 Single phase or three phase?

3 Are all fans fed from one transformer or are they spread out over a wide area with separete transformers?

4 Is it an industrial or domestic application?

The answers to these questions may have some impact on the outcome of a cost/benefit analysis.

 

[jbartos]

Suggestion: Please, notice that many VFD or ASD are converters consisting of AC-DC-AC conversions. The one, which counts for the utility in terms of power factor and harmonics, is the input side conversion, namely, AC-DC. Therefore, the input measurements or recording of AC-DC inputs should not be ruled out. Ultimately, the power factor and harmonics measurements performed as close as possible to the utility property line box are most significant.

 

[wwilson]

rdrago,

I would recommend doing some measurements prior to committing to buying 50 drives. I think power factor of the drives may vary depending on the current requirements - load/speed. As I recall, looking at the input current to the drive, will look like a switching power supply - current is taken around the peak of the voltage sine wave(s). One way to help this is to add line reactors to the input mains for the drive - improves power factor / harmonics. We've had problems when retrofitting 5hp, 3ph, contactor operated motors with VSD's. In short, the way the VSD's draw current can cause distortion of the voltage wave-form (flat-topping) and harmonics problems.
Other items:
1. I'm not sure what type of improvement you are hoping for, but drives are probably 70-90% efficient due to the AC-DC-AC conversions (they generate heat) which you may want to take into account.
2. They can be a major point of failure if you do not get a good drive.
3. Not all motor windings are well suited for the type of voltage waveform generated by the drive - check with the motor manufacturer.
4. With the drives your power factor may be more consistent but not necessarily good. You may need to add line reactors to improve PF.
That said I generally support using a VSD for our new designs. But do try to take into account the above list.
wwilson

 

[jbartos]

Suggestion: So-called Active Front End (AFE) VFDs tend to have very small harmonic content on their input side. Also, the Power Factor can be very high, near unity.

 

[wwilson]

I have not found any power factor corrected VFD's in 1-5hp (our typical size range), but I feel that active PFC is an option to look for. Smaller drive mfg's do not tend to worry about harmonics, but when you get 100+ drives in a building taking current at the same time, you see very high peak currents.
To give rdrago an idea of the data he can get using a power meter, I pulled up some recent measurements:
For a 3ph 5hp blower motor; measuring the power/harmonics (Fluke43) into the drive we have:
just motor and drive:
0.65PF, 1.0DPF, 0.98kw, 1.51kVA, 1.15kVAR, 75.5THD%r, 12.72ARMS
adding line reactors, everything else the same:
0.92PF, 0.96DPF, 0.99kw, 1.08kVA, 0.43kVAR, 27.7THD%r, 9.24ARMS
wwilson