Measuring Motor Inrush Current

[ndzied1]

Let me try this again with the plots rotated.


The clamp on current probe puts out 1mV/Amp. Each plot shows the starting of the same 125 HP / 460VAC / 60Hz Motor. The motor is a double shaft with one shaft driving two hydraulic pumps and the other shaft driving a single hydraulic pump. All pumps are unloaded during startup.

The first plot starts on a down cycle and peaks around 1500A at the bottom. The second plot starts on a an up cycle and peaks around 2200A. There were no changes in the system between startups. I guess there can be some variability in startup current?...

The RMS reading on the right is a function available in the scope and is the RMS value for the entire plot. I thought it might be a good way to compare plots from different motors. Does this seem like the best way to compare startup currents or should I just look at the first peak?

There is a function in the scope called CycleRMS which is supposed to measure the RMS of the first cycle but it was giving me an error. This might be because of the 50msec pre-trigger in the plot. I will check that next week.

In both plots, the current settles down in the first 350 msec after starting.

Next step is to put together some of the 25HP units in question and start them up with different motors and see what the plots look like.

 

[rsdoran]

Norm now that you can do this what exactly will it be used for? I could have sworn you had a post that stated some of it but not seeing it now.

My point is that the oscope shows exactly what the "rule of thumb" does about inrush current being up to 10 times load current. I am not sure how this aspect would be used when sizing a motor or drive.

Just curious.

 

[Leadfoot]

Norm,

When you want to capture only one signal, turn off all the other channels. It makes it easier to read the scaling and identify the ZERO reference point.

Your use of the RMS is a nice touch. Look at the other measurements that are available. They can help with the analysis. I think you need to set the time base for a faster time, less cycles displayed, to get cycleRMS to work right. I seem to remember you cannot capture more than 1-1/2 or 2 cycles to get an accurate measurement.

All motors have a RANGE of in-rush current. The KVA code on the name plate gives you the code's letter. You get to look up the code in the most any reference manual. After you have started a cold motor and it warms up a bit, the resistance changes. This will impact starting current.

As you have seen some REAL in-rush currents, you now know why soft starts and drives have gotten more popular. Soft starts reduce the inrush currents by at least half and a drive limits it to 150% of name plate value.

 

[ndzied1]

Ron,

The information will not be used to size systems but to diagnose a problem one of our customers is having with one of the power units they purchase from us.

They have reported systems that will not start and also excessive noise during startup. I will be testing two of these systems next week with the standard (low cost) motor and also with a more reputable motor. They also didn't want an unloading valve on the system when it was originally designed so the system starts with the pump dead headed. I am also going to show them what using an unloading valve on the system will do to the startup current.

The plots posted above are from a system we had happend to have in our test area at the time and is starting up just as I would have expected.

 

[jwh]

Norm
Nice readings. I have THS720 myself and i also found that under volts/div menu, probescaling you can scale your reading from mV to Amps. I love my TekTool

Jack

 

[ndzied1]

You get what you pay for!

Finally a little time to post data. There are 4 charts below.

The first is the production motor with the pump outlet free to flow to tank on startup. The second is the higher cost reference motor also with the flow free to tank.

Next the motors are show with the pump outlet blocked.

The production motor (a low cost option) draws much more current during startup than the reference motor. Both draw more when starting with the pump outlet blocked but the effect on the production motor is more pronounced.

The pump is a pressure compensated varriable displacement piston pump.

Almost all of the motor nameplate specs are identical except

• the reference motor sf=1.25 while the production motor sf=1.15
• the reference motor is rated 50/60Hz while the production motor is only rated 60Hz.

Other nameplate specs are identical, including the locked rotor kVa code.

 

[Leadfoot]

Nice charts. Initial currents do not look much different but the higher $$ motor gets down to normal amps quicker. That does cut cost to operate. However, I do not see sufficient reduction in Peak draw.

What probe(s) did you actually use to capture these waveforms?

 

[ndzied1]

What probe(s) did you actually use to capture these waveforms?

I used the Fluke i1010 I mentioned earlier in the thread. I had a 10X Tektronix probe attached to the scope reading the 1mV/A signal from the Fluke.

Whole problem was not with high power costs for inrush, it was that when end users power fluxuated, the current draw would go up and cause the motors to stall and start blowing breakers or require much larger transformers than should be required.

I don't have a multitap transformer to simulate the voltage variations at the end users but my customer (the OEM) does. When the system gets to the low end (-10%) on voltage, the motor is not able to start the pump. He did his testing with the lower cost motor and now we've given him the better motor to test and see if it's reduced current inrush will allow them to start the motor.

Yes, the end customer should be responsible for there voltage and rail on their utility to make it right but the world is an imperfect place. I also think that the OEM's instance that all system be wired for 480VAC is a bit bull headed. This forces the end users to have step up transformers. With the voltage drop across the transformer the situation gets worse.

I recommended a soft start but the guys there think it will make things worse. I can't buy that. His thinking comes from him looking at his plots at increased voltage and seeing the motor startup right away. He thinks a soft start would be the same as starting under reduced voltage.

Does a soft start vary the frequency like a VFD? If so I think is should definately help.

 

[Vic]

Norm

He thinks a soft start would be the same as starting under reduced voltage.

That is exactly what a Soft Start does. Another name for a Soft Start is "Reduced Voltage Solid State Starter". It has the ability to apply an infinitely variable voltage to the motor when starting. It does not change the frequency as does a VFD.

If the motor will not start "Across The Line" then a Soft Start will only make things worse. However, the motor will be getting a reduced voltage start now if the transformer is not sized to supply the starting current of the motor.

 

[Leadfoot]

A softstarter/reduced voltage starter is definately NOT the answer here. A VFD might actually be the better choice. Due to the bus caps ability to absorb some line variations and the processors ability to modify the PWM out put to compensate, a VFD might just work.

The VFD will allow 150% rated amps for 1 minute at reduced volts and frequency. A VFD truly does soft starting in high torque loads by ramping up the torque and maintaining it with out overloading things. If the motor does not get moving in that 1 minute to normal levels, they need a bigger motor from the get go.

 

[ndzied1]

This confirms what I was thinking. Not sure if this makes sense but in my Mechanical Engineer's brain I have been looking at it like this.

With a reduced voltage starter (or voltage drop across a transformer) the frequency remains the same 60Hz. Therefore, it seems that the motor WANTS to go at it's nameplate speed immediately. However with the voltage reduced, it's can't develop the power to get the load moving. Since a VFD can vary the frequency to the drive, the motor won't try to turn at full speed immediately and the application won't demand full power immediagely so it should help.

Does this seem right?
Thanks,

 

[kamenges]

Take a look at the first page of the PDF here:

http://www.enm.com/training/siemenscourses/acmotors2.pdf


You most likely have a NEMA Design B motor. Not only will a soft start reduce the available starting torque due to decreased voltage it also operates at a disadvantagous location on the torque curve.

Keep in mind that this graph assumes across the line starting. A good VFD will keep you to the right of the breakdown torque at all frequencies, giving you the best chance of using the available motor torque.

I don't know enough about hydraulic pump load profiles to make an intelligent comment here, but I'll try anyway. Would it make more sense to go with one of the other motor design typles, like a C or D?

Keith

 

[Leadfoot]

Norm,

You are thinking along the right lines. kamenges explained the torque curve right on the money.

Also, if they won't do a VFD then you need to look at a nema D motor. Unfortunately, Nema D motors draw even higher levels of starting current. The D motor probably will start into a dead head. It will pull 3 to 4 times the amps you are currently drawing. Which I do believe is the "current" problem.

One other thing about using the VFD, as I said it will apply the torque smoothly and continuously. This reduces the mechanical shock load effects tremendously. Couplings, bearings and all wear parts last a bit longer as a result. The inrush and peak loads from the utility also drop. The drive can also be used to help regulate the pump output.

 

[JohnW]

I expect you have been told this already but here it is anyway.
Larger supply cables will reduce the volt drop on starting. The motor fault you are experiencing is a common symptom of under-sized supply conductors. Unless of course the supply transformer is under rated as well.

I would expect the 50/60 hz rated motor to develop more power at 60 hz than a 60 hz rated motor with the same nameplate value. If sf=1.25 means the slip frequency then the reference motor develops more torque than the other motor at the cost of a slight reduction in speed.

If you use a VFD it must be overrated by one or two sizes for high torque starting, we have a lot of problems with starting progressive cavity pumps. These require a very high starting torque and soft starts and star-delta do not work, we have either use a VFD or DOL starting.

One of my customers uses DOL starting on pumps up to 110 kW that gives us some headaches with inrush current.

 

[kamenges]

Originally posted by JohnW:

If sf=1.25 means the slip frequency then the reference motor develops more torque than the other motor at the cost of a slight reduction in speed.

sf in this case means service factor.

Keith