Voltage Stiffness

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Designer69

Senior Member
what would you do in this situation:

We wrote a pump motor purchase spec which had a requirement for the motor to be able to start and come to full speed with an available voltage of 80% at the terminals.

Motor Vendor agreed and build the motor to this spec.

After testing, vendor finds motor cannot actually start the load with 80% voltage at terminals. They are requesting our system voltage to be stiffer (closer to 90%).

would appreciate your input, thanks
 

rcwilson

Senior Member
Location
Redmond, WA
Have the pump vendor comply with the specification by either supplying a better or larger motor or by providing an engineered solution such as a reduced voltage starter or a VFD.

If the 80% voltage requirement is a real possibility, he has to find a way of starting the pump at his cost or pay you to do the same. If it is just a specification requirement that probably will not occur in practice, you might be able to waive the requirement. But still reduce his price to charge him for the extra engineering analysis and any power system changes required. (Deduct an amount at least equal to the difference between his price and the next higher bidder that met spec.)

Also find out more about the test and why it failed. Was it an actual test or just a calculation? Did the motor stall or did it not get up to speed before the allowable start time? Was the terminal voltage held at 80% of nameplate during the full test or was the motor connected to a transformer that was set for 80% volts before the motor was connected and then started? (In that case, an additional voltage drop below 80% will occur during the motor starting.)

What is the worst case voltage profile during actual motor starting at the final location? Does the 80% represent the voltage before the motor is on line or the motor terminal voltage after including the voltage drop caused by the motor starting current?

1. Find out what the real requirement is.
2. Compare that to the "test" procedure.
3. Work with the vendor to find a solution.
4. Make the vendor cover all costs for spec compliance.
 

skeshesh

Senior Member
Location
Los Angeles, Ca
What is your actual question though? It seems like the vendor failed to deliver the product as spec'd and is the responsible for the error. Is there some sort of contract dispute going on?
 

Besoeker

Senior Member
Location
UK
what would you do in this situation:

We wrote a pump motor purchase spec which had a requirement for the motor to be able to start and come to full speed with an available voltage of 80% at the terminals.

Motor Vendor agreed and build the motor to this spec.

After testing, vendor finds motor cannot actually start the load with 80% voltage at terminals. They are requesting our system voltage to be stiffer (closer to 90%).

would appreciate your input, thanks
I assume we are looking at a cage induction motor?

As Skeshesh has pointed out, if the vendor has agreed to comply with the specification then they have not met their side of the contract and, all other things being equal, it ought to be incumbent on them to remedy that and meet the costs of doing so. Often things are not so tidy as rcwilson has noted.

But I think that there are no easy, cheap, or quick fixes if the 80% is cast in tablets of stone. So maybe that is a starting point for technical solutions.

If the 80% requirement is because the starting current of this particular motor would cause such a drop in voltage then a bigger motor won't help.
At 80% voltage the motor will generate about 64% of the torque it would at full voltage. For a centrifugal pump, the load torque varies as the square of the speed so, if the motor is rated to deliver just the pump absorbed power at rated speed, it would get to 80% of full speed or a slip of 0.2. At that point the current would have dropped off some and the voltage drop would be less than that at initial starting. In my experience on pump sets, the motor is usually sized to have some margin over pump absorbed power so the speed might get a bit higher, the current a bit lower and the voltage regulation a bit less. As the others have pointed out, maybe some more work needs to be done that would allow some flexibility with the 80% voltage. Getting up to rated speed and load with 80% voltage would be outside the parameters of most motors.
 

Designer69

Senior Member
thanks for the help guys..

Bob, the situation is that the voltage at the mtr terminals prior to start-up is 100% and during starting, it dips to 80%. That is according to the AC Voltage Study modeled on SKM'S Dapper software.

Besoeker, you say
Getting up to rated speed and load with 80% voltage would be outside the parameters of most motors.

in your experience, can you tell me what a more commone parameter is? I am trying to determine if maybe our requirements were a bit too stringent on the vendor.

Thanks
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
Depending on how your spec is written, it could very well be determined to mean that at T0-, the voltage is at nominal, when the motor attempts to start, at T0+, the terminal voltage drops to 80% of nominal. Keeping in mind that a system terminal voltage of 80% of say 480V is 384V, and that the motor rated voltage is probably 460V, which means a 384V is 83.5%.

According to your spec the motor should start.

Mr. Wilson, in his post, makes many good points on where this starting requirement can be contested. Also, what is modeled in SKM is not necessarily what is going to happen in real life.
 
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Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I think there is also a semantics difference going on here. "Starting at 80% voltage" is different from "no more than a 20% voltage drop at start-up". The SKM modeling is telling you that at 100% voltage at start command, you will experience a 20% VD. As I interpret your needs stated in your follow up post, you just want to be assured the motor will still be capable of accelerating to full speed under that expected condition. This may very well be possible, it's not that tough of a spec to meet.

But the way you initially worded your first post (and maybe your spec?), it would seem to me you are asking that if the voltage is ALREADY at 80% when a start command is given the motor will still accelerate, even with whatever ADDITIONAL VD it may create. That may indeed be a very very difficult if not impossible task. Increasing the motor size may not solve that problem because it will also increase the created secondary voltage drop.
 

Besoeker

Senior Member
Location
UK
thanks for the help guys..

Bob, the situation is that the voltage at the mtr terminals prior to start-up is 100% and during starting, it dips to 80%. That is according to the AC Voltage Study modeled on SKM'S Dapper software.

Besoeker, you say

in your experience, can you tell me what a more commone parameter is? I am trying to determine if maybe our requirements were a bit too stringent on the vendor.

Thanks
I don't know where you are located. I'm in UK where the supply tolerance is +10% and -6%. Much of Europe is +6% -10%. I would thus reasonably expect most motors made for European use to tolerate a +/-10% voltage from nominal.
If, as you say, the supply voltage drop to 80% is on starting then as the motor accelerates the current will drop some and the voltage will rise.

Maybe you need to do a more detailed analysis on the system and the motor parameters.
One final thought. Is the motor manufacturer trying to start the motor against full load torque? In practice your load won't be like that. In many pumping stations where we have drives, start up is against closed valve and that reduces the required torque. It might help in your case.
You must have an interlock such that if the valve doesn't open after a preset time and or flow isn't detected, the pump must be shut down.
Pumps can overheat quite quickly if all they are doing is churning water round.
 

jdsmith

Senior Member
Location
Ohio
How are you modeling motor starting using Dapper? Dapper does steady state load flow analysis but it can be used to analyze voltages during motor starting: If you know the inrush current you can add a fixed load to your model that draws 90-95% of locked rotor current at around 0.15 power factor. The transient motor starting (TMS) module in SKM is difficult to use because it requires data that is not generally available, even with new engineered API 541 motors. I tend to be skeptical of TMS motor starting reports but the method described above using Dapper will work.


In the process industries our typical spec is intended to indicate that the power system will supply at least 80% of nameplate voltage during the entire start. This 80% is based on the nameplate voltage, 460 or 4000V, not the nominal system voltage. The engineering practices of a few major oil companies include a specification that motors must start with less than 8% voltage drop during inrush. This is the design engineer's responsibility to manage the inrush current of their motors and the system stiffness to achieve this goal. One newer trend is the design of large induction motors with 350-450% inrush current to mitigate voltage drop. Special low inrush motors can be more reliable than autotransformer or solid state soft starts and cheaper than upsizing transformers to reduce system impedance.

API 541 4th Edition said:
2.2.3.1 The motor shall start and accelerate the connected load to running speed in accordance with NEMA MG-1 Part 20 with 80% of rated voltage at the motor terminals. A driven-load inertia (referred to the motor shaft speed) in excess of NEMA standard limits shall be specified by the purchaser, or the driven-equipment vendor, on the datasheet.

This is from API 541, which is the premier spec applied to engineered motors. Note that it requires the purchaser to provide the motor manufacturer with the load inertia and speed vs. torque curve - without this information the motor cannot be properly sized.
 
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I don't know where you are located. I'm in UK where the supply tolerance is +10% and -6%. Much of Europe is +6% -10%. I would thus reasonably expect most motors made for European use to tolerate a +/-10% voltage from nominal.
If, as you say, the supply voltage drop to 80% is on starting then as the motor accelerates the current will drop some and the voltage will rise.

Maybe you need to do a more detailed analysis on the system and the motor parameters.
One final thought. Is the motor manufacturer trying to start the motor against full load torque? In practice your load won't be like that. In many pumping stations where we have drives, start up is against closed valve and that reduces the required torque. It might help in your case.
You must have an interlock such that if the valve doesn't open after a preset time and or flow isn't detected, the pump must be shut down.
Pumps can overheat quite quickly if all they are doing is churning water round.

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NEMA MG-1

20.14.1 Running​
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Induction machines shall operate successfully under running conditions at rated load with a variation in the voltage or the frequency up to the following:
a. Plus or minus 10 percent of rated voltage, with rated frequency
b. Plus or minus 5 percent of rated frequency, with rated voltage
c. A combined variation in voltage and frequency of 10 percent (sum of absolute values) of the rated values, provided the frequency variation does not exceed plus or minus 5 percent of rated frequency.
...............​
[FONT=Arial,Bold][FONT=Arial,Bold]
20.14.2 Starting
20.14.2.1 Standard​
Induction machines shall start and accelerate to running speed a load which has a torque characteristic not exceeding that listed in 20.10 and an inertia value not exceeding that listed in 20.11 with the voltage and frequency variations specified in 20.14.1.
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20.14.2.2 Low Voltage Option​
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When low voltage starting is specified, induction machines shall start and accelerate to running speed a load which has a torque characteristic not exceeding that listed in 20.10 and an inertia value not exceeding that listed in 20.11 with the following voltage and frequency variations:
a. -15 percent of rated voltage with rated frequency
b.​
?5 percent of rated frequency, with rated voltage
c. A combined variation in voltage and frequency of 15 percent (sum of absolute values) of the rated values, provided the frequency variation does not exceed
?5 percent of rated frequency.
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20.14.2.3 Other​
For loads with other characteristics, the starting voltage and frequency limits may be different. The limiting values of voltage and frequency under which an induction machine will successfully start and accelerate to running speed depend on the margin between the speed-torque curve of the induction machine at rated voltage and frequency and the speed-torque curve of the load under starting conditions.
Since the torque developed by the induction machine at any speed is approximately proportional to the square of the voltage and inversely proportional to the square of the frequency it is generally desirable to determine what voltage and frequency variations will actually occur at each installation, taking into account any voltage drop resulting from the starting current drawn by the machine. This information and the torque requirements of the driven (or driving) machine define the machine speed-torque curve, at rated voltage and frequency, which is adequate for the application.
-----------------------------------
API STANDARD 541
Form-Wound Squirrel​
Cage Induction Motors-
250 Horsepower and Larger

E.1.2.28 Data on Page 1, Right Column, Line 3​
?Voltage at Locked Rotor?-The motor is normally designed to
be capable of accelerating its shaft load with 80 percent of its rated voltage applied. This is usually a conservative approach, and 80 percent should be specified unlesst he ?stiffness? of the power system ensures the higher voltage of 85 percent or 90 percent. This should first be verified by a motor starting voltage drop calculation with the minimum capability from the supply transformers, lines, utility company, or generation including thee ffects of the starting method. For captive transformer applications, the voltage available may be less than 80 percent, typically 70 percent to 75 percent for normal transformer ratingisn relation to the motor rating. For autotransformer starting, an 80 percent autotransformer start may actually result in7 5 percent voltage availableto the motor, depending on the power system and autotransformer impedance.​
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