Starting current for squirrel cage prime mover on a cent. pump
Starting current for squirrel cage prime mover on a cent. pump
(OP)
This is probably a bit basic but I was asked a question the other day regarding the procedure for starting up a centifugal pump. The procedure usually involves shutting the discharge valve, turning on the pump, then openiong the discharge valve. I was told the reason for this procedure was to limit starting current on the motor (small motor - no starting resistors). I haven't been able find a good description for why the discharge valve being shut helps.





RE: Starting current for squirrel cage prime mover on a cent. pump
The pump will affect how long that starting current lasts. The more torque the pump draws during startup, the longer it takes the unit to get up to speed and longer the starting current lasts.
For radial flow centrifugal pumps, the least torque is drawn at the lowest flow (valve near closed). That will give the shortest starting time (ensure minimum flow required for cooling/lub is available).
Axial flow pumps draw least torque at high flow (valve full open). From motor standpoint it would be preferable to start these with valve full open (but there may also be fluid considerations such as water hammer).
Mixed flow falls in between these two.
You can see this behavior from the shape of the bhp vs flow curves at:
http://www.gouldspumps.com/cat_technews.ihtml?pid=66&lastcatid=45&step=4
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RE: Starting current for squirrel cage prime mover on a cent. pump
-----------------------------------
Start each new day with a smile.
Get it over with.
RE: Starting current for squirrel cage prime mover on a cent. pump
So I am asking a more difficult question.
How much affect will the type of pump, high Specific Speed (Axial Flow) vs. Low Specific Speed (Radial Flow), really have on starting current?
Assuming Across the Line starting, the radial vs. axial flow affect will not enter until the pump is close to speed.
In theory, the "type of pump effect" could extend the time of higher than operating current, but probably would not increase that current.
PUMPDESIGNER
RE: Starting current for squirrel cage prime mover on a cent. pump
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RE: Starting current for squirrel cage prime mover on a cent. pump
Could you explain the term :”( excluding initial exponentially-decaying component).”
What are the sources and time scale of such a component and what is its peak value?
RE: Starting current for squirrel cage prime mover on a cent. pump
"12.36 INSTANTANEOUS PEAK VALUE OF INRUSH CURRENT
The values in the previous tables are rms symmetrical values, i.e. average of the three phases. There will
be a one-half cycle instantaneous peak value which may range from 1.8 to 2.8 times the above values as
a function of the motor design and switching angle. This is based upon an ambient temperature of 25°C."
The above is easily understood by modeling the motor (per-phase) as a constant impedance ZLRC= R+jwL, with suddenly-applied sinusoidal component.
In the worst case scenario, ZLRC is very close to inductive and closing occurs at the zero of the voltage waveform.
There are two components of the solution:
iSteadyState = sqrt(2)*ILRC*cos(2*Pi*FL*t)
iTransient = sqrt(2)*exp(-t*R/L)
Add them together to get the worst case 2*sqrt(2) which is where the 2.8 in the NEMA quote comes from.
If I have a chance I will try to provide more detailed discussion later.
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RE: Starting current for squirrel cage prime mover on a cent. pump
Thread237-9886
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RE: Starting current for squirrel cage prime mover on a cent. pump
iTransient = sqrt(2)*ILRC*exp(-t*R/L)
and ILRC = VLGrms/sqrt(R^2+<2*Pi*L)>2)
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RE: Starting current for squirrel cage prime mover on a cent. pump
Many thanks for the explanation and graphics. That is a relly interesting subject.
RE: Starting current for squirrel cage prime mover on a cent. pump
http://reliability-magazine.com/pub/ILRC.PDF
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RE: Starting current for squirrel cage prime mover on a cent. pump
http://reliability-magazine.com/pub/ILRC1.PDF
ok, I'm done now.
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RE: Starting current for squirrel cage prime mover on a cent. pump
As Pete has very correctly stated, if you are using a full voltage start or across the line start, the start current is independent of the shaft loading on the motor. The start current under full voltage starting is a function of the motor design and the terminal voltage.
If you reduce the voltage during start, then the start current can also be reduced. The minimum start current that can be achieved by using a reduced voltage starter is a function of the motor design and the shaft torque. The statement about controlling the discharge valve to reduce the start current is only true if you are applying a reduced voltage starter.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Starting current for squirrel cage prime mover on a cent. pump
1. Motor starting current, namely:
The current drawn by the motor during the starting period (a function of speed or slip).
2. Motor locked rotor current, namely:
Steady-state current taken from the line with the rotor locked and with rated voltage (and rated frequency in the case of alternating-current motors) applied to the motor.
3. Motor inrush current, namely:
The rapid change of current with respect to time upon motor energization. Inrush current is dependent upon the voltage impressed across the motor terminals and the motor inductance by the relationship V=L*di/dt.
The original posting refers to the starting current as defined in item 1.
The starting current includes inrush current and if the motor happens to stall, the motor locked-rotor current, as defined in item 2, materializes.
If the discharge valve is used properly, there will be two modes of operation of the motor with one motor starting current, namely:
1. The motor starts with relatively low shaft load when the discharge valve is closed. This implies that the motor starting current will be converging faster to the motor steady state current that is lower than the motor running load current (usually designed lower than the motor rated current).
See Reference: IEEE Std 399-1997 Chapter 9 "Motor Starting Studies"
for motor starting time calculations and equations, e.g. Equation 9-18.
2. Discharge valve becomes opened. The motor is loaded as designed by the motor-pump set integrator.
The motor experiences the second part of its motor starting current in terms of its adjustment or increase to its designed motor current value. There may be a motor current transient depending how fast the discharge valve is open. This current increase is not normally referred to as an inrush current even if it may have some parts of inrush current resemblance.
To summarize the advantage. The motor starts faster with the same inrush current, however, the current levels off at lower than the designed current. When discharge valve opens, the motor current adjusts to its running current as designed by the motor-pump set integrator.
Advantage of this concept is in the smaller size of the motor that accomplishes the same function as the larger size of the motor that may be required for the discharge valve kept in open position, and in the starting current that converges to the lower steady state current initially when the motor shaft load is lower.
RE: Starting current for squirrel cage prime mover on a cent. pump
Under full voltage starting, motor locked rotor current and motor starting current are one and the same.
As has already been stated by others closing the discharge valve during starting;
a) will not reduce the starting current if accross the line or full voltage start techniques are adopted. They will however reduce the over-current period.
b) will reduce the starting current if reduced voltage start techniques are adopted eg star/delta, auto-transformer, primary resistance, soft start etc.
Regards,
GGOSS
RE: Starting current for squirrel cage prime mover on a cent. pump
Hi all,
Under full voltage starting, motor locked rotor current and motor starting current are one and the same.
///Please, notice the definition of the locked-rotor current, which refers to steady state current. The motor starting current includes transients. True locked-rotor current is when the motor stalls, e.g. due to high shaft load.\\\
As has already been stated by others closing the discharge valve during starting;
a) will not reduce the starting current if accross the line or full voltage start techniques are adopted. They will however reduce the over-current period.
///Please, notice the difference in definitions of motor starting current and motor inrush current. There is a clear difference between those two according to IEEE Std 100 "Dictionary"\\\
b) will reduce the starting current if reduced voltage start techniques are adopted eg star/delta, auto-transformer, primary resistance, soft start etc.
///Agreed.\\\
Regards,
GGOSS
RE: Starting current for squirrel cage prime mover on a cent. pump
I don't have the time or the patience to argue with you at the momment, so I won't.
Please understand that different terminology is used in different parts of the globe and posts on eng-tips (which has global membership) are likely to reflect that. Also the IEEE dictionary to which you most often refer is only one of many, many references. Are you suggesting all others are incorrect?
Regards,
GGOSS
RE: Starting current for squirrel cage prime mover on a cent. pump
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RE: Starting current for squirrel cage prime mover on a cent. pump
Generally, no one is much concerned with this "inrush" current as they are with the "starting" current. However, technically, there is a difference.
RE: Starting current for squirrel cage prime mover on a cent. pump
How is in-rush significant in this discussion?
Regards,
GGOSS
RE: Starting current for squirrel cage prime mover on a cent. pump
RE: Starting current for squirrel cage prime mover on a cent. pump
By the other hand the motor current from the moment that the contactors are closed to the final steady state operation is changing constantly following a performance defined by the motor reactance and slip, assuming that the voltage remains constant.
I must agree with Jbartos and Electricpete that current taken by a motor is based on the solution to the equation; v=Ldi/dt, where L is changing due to the rotor frequency and saturation.
The solution for the current “i” as function of the time “t”, has a exponential component and a variable component proportional the the inductance change until finally becomes a steady state component when the load-speed equilibrium is reached.
RE: Starting current for squirrel cage prime mover on a cent. pump
I don't think the consideration of water hammer has been forgotten. My very first post:
"From motor standpoint it would be preferable to start these [axial flow pumps] with valve full open (but there may also be fluid considerations such as water hammer)."
I don't think it is reasonable to suggest that the selection of valve closed start is always driven by water hammer consideration and never driven by acceleration time/motor heating/voltage sag duration considerations. Either one may be important or unimportant depending on the application. Yes we have talked about the motor considerations more. This is after all a motor forum.
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RE: Starting current for squirrel cage prime mover on a cent. pump
If theta = Pi/2,
i(t) = (1/L)*integral V0sin(w*t-Pi/2) dtau from tau=0 to t
i(t) = -V0/(L*w)*sin(w*t)=-I0*sin(w*t)
(no dc component)
If theta =0,
i(t) = (1/L)*integral V0sin(w*t) dtau from tau=0 to t
i(t) = V0/(L*w)*(1-cos(w*t))=I0*(1-cos(w*t))
(maximum dc component)
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RE: Starting current for squirrel cage prime mover on a cent. pump
The original question was in regard to the effect of the discharge valve on the starting current of the motor.
Firstly, the closed discharge valve will reduce the shaft torque presented by the pump during start.
If you use a full voltage starting method, the reduction in starting torque will serve to reduce the starting time, but will not alter the start current. Under full voltage starting conditions, the start current is a function of the motor design, the applied voltage and the rotor speed.
The start current will initially be the Locked rotor current and will reduce gradually as the motor accelerates and will only fall quickly as the motor reaches full speed.
At the instance of the contactor closure, there is an inrush current that will last for a couple of cycles only.
The locked rotor current of the motor is typically in the range of 550% to 900% of the rated current of the motor. The inrush current could be as high as twice this.
Secondly, if you use a reduced voltage starting method, then the lowest start current is dependent on the motor design and the shaft load. By reducing the starting torque required by the pump, you will reduce the minimum start current required to start the pump. This can only be achieved provided that an appropriately engineered starting system is employed.
To summarise, the statement "limit starting current on the motor" could be true if an appropriately engineered reduced voltage starter is used, otherwise, the start current will be the same, but for a shorter period of time.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Starting current for squirrel cage prime mover on a cent. pump
RE: Starting current for squirrel cage prime mover on a cent. pump
The minimum start current required, is a fucntion of the required load torque, the motor speed torque curves and the motor speed current curves. It is easy to determine what the lowest start current is provided that this information is available. At a guess, I would exect that you would need a start current in the order of 400% to start this motor unless it has particularly good high slip curves. You can reduce the minimu start current requirements by modifying the start torque requirements of the driven load. In the case of a pump, you can close the outlet valve to reduce the start torque requirements.
If the motor is not yet in existance, you do have the option of using a multistage secondary resistance starter. This will allow you to dramaticaly reduce the start current requirements with careful starter design. You are however faced with brushes and swithcgear that need continuous maintantence, and of course, a slip ring motor.
The other way to minimise the start current, is to use an inverter drive system, but in this application, the costs would be prohibitive.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Starting current for squirrel cage prime mover on a cent. pump
Is the problem:
A - Motor trips due to long start time
or
B - Unacceptable voltage drop in the system
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RE: Starting current for squirrel cage prime mover on a cent. pump
RE: Starting current for squirrel cage prime mover on a cent. pump
Your option is optimize the starting Amperes vs Torque.
The best motor to convert inrush current into torque is the IWRM (induction wound rotor motor).
Vector drivers can attempt a similar performance but the most cost effective way to get torque is modifying the rotor circuit resistance.
RE: Starting current for squirrel cage prime mover on a cent. pump
There are some readily available; however, for smaller size motors, e.g.
http://www.magnadrive.com
RE: Starting current for squirrel cage prime mover on a cent. pump
RE: Starting current for squirrel cage prime mover on a cent. pump
RE: Starting current for squirrel cage prime mover on a cent. pump
RE: Starting current for squirrel cage prime mover on a cent. pump
The use of a fluid coupling, or a magnetic clutch coupling will not directly reduce the starting current. It may result in a reduced start time.
If you were able to decouple the load from the motor shaft and use a reduced voltage starter to start them motor under zero load conditions, and then apply the load to the motor at full speed, then there may be an opportunity. Is this what you meant??
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Starting current for squirrel cage prime mover on a cent. pump
I fully agree that use of fluid coupling wil reduce only the start up time and not the starting inrush current. I had assumed that the motor tripped due to longer start up time than due to lower voltage caused by inrush current. My mistake.
RE: Starting current for squirrel cage prime mover on a cent. pump
Regardless of what you do to the pump, you will initially have a starting current ~5 - 7 times full load amps (excluding initial exponentially-decaying component).
///Yes, unless there is a different motor applied with the lower NEMA Code Letter, e.g. e.g. A, which implies lower starting currents\\\
RE: Starting current for squirrel cage prime mover on a cent. pump
RE: Starting current for squirrel cage prime mover on a cent. pump
How does this happen in that the pump will still lift water and impress it against a shut valve?
RE: Starting current for squirrel cage prime mover on a cent. pump
The pump will cavitate. That is, it will spin the fluid within the housing, but none can exit from the housing due to the closed valve.
There will be cavitation losses in the pump, but there is no fluid moving so no "work done"
If the pump was a positive displacement pump, this would not be true.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Starting current for squirrel cage prime mover on a cent. pump
What you said about 'no flow, no work' is certainly true, and a pump working against a shut off valve is, in fact only circulating fluid in the pump housing, but that is not cavitation.
Cavitation, and there are lots of threads in other places in this forum, is a condition of flow, not the lack thereof.
Rodmcm is correct in his assumption that discharge head is produced against the shut off valve. It is called 'shut off pressure' in pump parlance. And, if left in that condition too long, the electrical work done on the pump at no flow will just heat up the fluid in the pump, and in the case of water, will after a finite time, boil or flash it, vapor locking the pump. While often confused with cavitation, it is not that.
A pump always draws some horsepower, if no more than just to overcome bearing and seal friction. It draws the least amount of horsepower possible at 'shut off', and the most horsepower at wide open flow.
Apart from water hammer , which is an problem in and of itself, getting to the point at which the hammer occurs, or filling an empty pipe to the point of hammer, is a wide open flow situation, and the pump is requiring maximum HP from the motor, all while the motor is trying to start and come to speed. It is the worst of all worlds, hence, the procedure of shutting the discharge valve to start the pump.
rmw
RE: Starting current for squirrel cage prime mover on a cent. pump
Best yet, tell the utility that thaey are failing to me ansi c c84.7 voltage drop requirements, it is their problem now.
RE: Starting current for squirrel cage prime mover on a cent. pump
The pump will affect how long that starting current lasts.
///True.\\\
The more torque the pump draws during startup, the longer it takes the unit to get up to speed and longer the starting current lasts.
///It depends. If the motor HP is proportionately increased and the correct NEMA Design Letter is applied, the larger motor-pump set may start faster than some smaller motor-pump set with a relatively small HP motor for the pump.\\\
RE: Starting current for squirrel cage prime mover on a cent. pump
Yes I agree, the fluid spins within the housing, but I am led to believe that there is also cavitation withing the pump and this can cause damage to the impeller over a period of time. - I am not a pump expert, so this information may be wrong, but the description given by many is that it is this cavitation loss that is reduced when a pump with restricted flow is slowed down. Is this a reasonable explanation, or are the losses elsewhere?
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Starting current for squirrel cage prime mover on a cent. pump
Pumps are variable torque loads.
The power curve for a pump defines how the pump loads the motor at any given flow rate. This power curve is supplied by the pump manufacturer.
However, lacking manufacturer's power curve, Pump Specific Speed (Ns in the US) is an indicator of the relationship of the power curve to flow rate. In general the following applies based on Pump Specific Speed:
Low Ns pumps (< 2500)draw least power at low flow rates, while high specific speed pumps (> 9000) draw maximum power at low flow rates. When Ns is between those two values the power curve is less dependant upon flow rate.
In review.
Since High Ns pumps (called axial flow pumps) draw maximum power at shut off, those pumps should not be started against a restricted output or the motor will be heavily loaded,
while,
Low Ns pumps (Radial Flow and lower value Francis Vane impellers/pumps) draw lowest power at shut off or low flow rates, so those pumps are often started against maximum resistance. In reality though, for Low Ns pumps less than 50 hp, starting against closed valve or open valve usually does not matter because the starting time for these pumps is so short that the effects of load on starting characteristic is very low, perhaps extending the duration of the high inrush by a small amount I would guess. For High Ns pumps I would think big trouble could occur if started against a closed valve or high resistance, inrush current might only fall to locked rotor value at most.
PUMPDESIGNER
RE: Starting current for squirrel cage prime mover on a cent. pump
RE: Starting current for squirrel cage prime mover on a cent. pump
Another reason why starting a centrifugal pump against a closed discharge valve is good practice: It's entirely possible that the pump is already running, backwards.
All it takes is a check valve that's failed open, and a downstream pressure source like an elevated reservoir or a parallel pump; it happens.
-Mike-
RE: Starting current for squirrel cage prime mover on a cent. pump
http://www.irrigationcraft.com/power_curve.htm
I am not an electrical expert, and I am not sure how starting characteristics relate to the power curves of pumps. Starting is a fast event when going direct across the line, and pumps are variable torque loads, so the effects of power curves on starting are unknown to me.
Much earlier in this thread I asked electricpete (who is an expert in my opinion) about the affects of pump type on starting current and he agreed that start duration time may be affected by pump type.
PUMPDESIGNER
RE: Starting current for squirrel cage prime mover on a cent. pump
RE: Starting current for squirrel cage prime mover on a cent. pump
The power characteristic is the power required to maintain the pump at a constant speed against varying degrees of resistance.
If the pump is held at a constant speed, and the flow rate is then taken from zero flow to maximum flow while power input is measured at points through the flow range, then the power characteristic is produced as a line on a graph with head and flow rate as the comparison.
Then, to obtain the power at varying speeds the Pump Affinity Laws are used. Of course you could just test the pump that way, start the pump at 1 rpm then ramp it up through the flow range, measuring power draw at specific points to produce the curve. Most pumps are not tested that way however, the manufacturer assumes you will derive the power characteristic in the form that you require by the Affinity Laws. The Affinity Laws are not always dead accurate however because they assume efficiency remains unchanged. But the laws are used extensively and are pretty reliable. Only testing can reveal the dead accurate power curve.
I am not a whiz at typing formulas as text, but here are the various forms of the affinity laws, where:
Q = Volumetric Flow Rate
N = Rotative Speed (RPM)
P = Power or BHP
D = Diameter of the impeller
H = Head
Q1/Q2 = N1/N2 (Solving for flow rate vs. rpm). Where the flow rate and rpm are known under condition 1, then under condition 2 either flow rate or rpm is known and the other can be solved for.
H1/H2 = N1squared/N2Squared (Solving for head vs. rpm). Notice that head varies directly as the square of the speed.
BHP1/BHP2 = N1cubed/N2cubed (Solving for power vs rpm) Where power is known at rpm1, power required for rpm2 is then solved for.
Notice that power varies directly as the cube of the speed.
I have not taken the time to correct something erroneous I have seen on this thread. It is incorrect to state that a centrifugal pump should be started against a closed valve. The reason is that there are different types of centrifugal pumps. Specific Speed is an indicator of the power characteristic and the operating range of a pump, and thus the reason why a pump should be started against either a closed valve or an open valve. However the pump manufacturer's published information is more accurate and must be used. Some centrifugal pumps would either not start or would damage themselves quickly if started against a closed valve, some centrifugal pumps don't really care much if valve is open or closed, and some pumps prefer to start against a closed valve. Specific Speed and the pump manufacturer's published data are the sources that tell us how wide the recommended operating range of the pump is, and how the power characteristic relates to flow rate.
The operating range is both a Subjective and Objective determination by the pump manufacturer. The range takes into account the pump's responses to operationg off BEP including power, vibration, Recirculation Cavitation, and materials of construction including shaft stiffness or R3D4. Bottom line here is that some pumps cannot tolerate shut off for even short periods, while others work well at shut off.
I am throwing out what I think you can use without trying to tailor it to your abilities because I do not know what you may or may not understand.
I have the time now to pay attention to this thread, so ask on if you want.
PUMPDESIGNER
RE: Starting current for squirrel cage prime mover on a cent. pump