Induction Motor On No Load

It shouldn't bother the motor, but there may be unbalanced or shock loads to the pump impeller and shaft when it starts drawing a mixture of liquid and air (or vapor).

The motor doesn't draw water from a tank. The motor provides power to a pump, which draws water from a tank.

Depending on the type of pump, the seals may be damaged.

I haven't read the FAQ's lately. Is there a requirement there that original posts be only one sentence long and provide no useful information?

Yes there is. Rule #75. Didn't you get the update?

That seems to be the only rule religiously adhered to by every new poster we see.

As far as just the induction motor only, the motor will continue to draw the same amount of current. The phase angle of this current will significantly change because the motor will no longer be doing significant mechanical work. As far as burning up the motor, it depends on how the motor gets cooled. If the drawn water was the primary cooling agent (a submersible pump) and there are no thermal cutoffs in the design then prolonged dry pumping can burn up a motor. A good engineer will anticipate likely scenarios and prevent a cascade of failures from happening.

Wont the motor draw less current if it slips less due to no load? Thanks.

There will be a slight reduction in the current draw, but not as much as most people think. Many people think of a motor in DC operation. Baldor has a nice white paper on induction motors. From that paper the actual and equivalent circuit diagram.

The variable resistance in (b) increases as one approaches zero slip. What most people focus on is this curve from the same paper.

Particularly the curve to the left and that there's a steep drop in current as one approaches synchronous speed. But there are always additional drags on a motor from internal and external bearings and wind-age that are always mechanically present. So unless the motor has a prime mover driving the shaft to overcome these drags, rarely does a free spinning motor reach the corner of these curves to get the sharp drop in current.

That is an excellent coverage on the motor part of the system. But the reply to the question comes from analysis of the motor & the load, the load in this case being a pump.

There are a number of cases of motor burnout due to long duration dry running. When the pump is dry,

• initial period (15 mins to 45 min depending on various factors related to type of pump & quality of water) the water lubricated parts are still lubricated & the motor runs on 'no-load' with a very slight reduction in current & a low power factor.
• in the next period, the water lubrication effects vanish & the parts such as seals or glands start getting overheated and get damaged. In this phase apart from the damage to the pump components, the motor faces progressively increasing load as a result of increasing friction and at the same time reduced cooling if its cooling system is related to the water pumped - as in submersible pumps. This invariably leads to the motor damage due to overheating.
• The increase in friction in pump can be enormous and very fast if the water being pumped has sand content in it. This happens mostly in tube well submersible pumps.
• Sand or some or the other solid material is always there in most of the pump installations. As such it is always desirable to build in a protection to avoid dry running of a pump, even though the motor technically may not be heading for a a damage by itself.

No, current INCREASES with slip, so if it slips less, it draws less current.

Due to bearing friction, windage, possible cooliing fan losses, and reactive power at light loads, the "no-load" current can be about 30% of full-load current.

Basically the question is meaningless. To many unknown parameters.

Simple answer:

If the motor-Pump is left to run dry for a long enough time, the pump seals will fail, the cooling by the water of the seal area will fail due to the remaining water getting hot and steaming, ---> possible motor shaft seasing and heat transfer to the motor casing if directly coupled with a flange fitting... definitely the motor will eventually burn out for these reasons.

The question leaves many unknowns.

From experience I have noticed the following with centrifugal or fan type pumps:

1) The AC current drops off partially as the discharge stops. No work is being done. However the magnetizing current is still present. Smaller fractional HP motors appear to have a larger % no load current.

2) A submersible pump needs to be submersed for cooling. If it is not submersed it over heats due to lack of cooling.

3) Without water the seals in the pump rapidly dry out and over heat and are damaged. (Even the ceramic ones. The faces are OK but the rubber seal behind gets cooked.)

4) I had one example of a blocked outlet and the pump was left running overnight. It was a 1 HP pump (not submersible). It boiled the water in the pump chamber under about 30 psi pressure. Unfortunately the plumbing was plastic and things started to melt and eventually the plastic pipe at the pump swelled and burst. (That ended the heating effect!)

Suction cavitation

Suction cavitation occurs when the pump suction is under a low-pressure/high-vacuum condition where the liquid turns into a vapor at the eye of the pump impeller. This vapor is carried over to the discharge side of the pump, where it no longer sees vacuum and is compressed back into a liquid by the discharge pressure. This imploding action occurs violently and attacks the face of the impeller. An impeller that has been operating under a suction cavitation condition can have large chunks of material removed from its face or very small bits of material removed, causing the impeller to look spongelike. Both cases will cause premature failure of the pump, often due to bearing failure. Suction cavitation is often identified by a sound like gravel or marbles in the pump casing.

When the Bearings fail, motor will draw heavy current and burn itself.

For more information go to Wikepedia and type Cavitation. There is wealth of information to learn.

wouldn't it be great if the OP of ANY question included ALL the important facts so the question could be answered WITHOUT trying to guess the important points and exactly what it is the OP is trying to find.

Would the motor burn?

Whats missing here?

For more clarification i can say that motor is sub merged nad there is no separate pump is there. There is only a suction - Discharge type of arrangement with motor. Sorry for late reply !!!!!

This will require a new rule. Rule #75 does not cover totally useless and misleading information.

I think this statement, " i can say that motor is sub merged nad there is no separate pump" demands a new rule.

I suggest, "ignorance is bliss" be the name of the latest rule.

That should make the OP happy, if not ecstatic.

Well, that more or less destroys my earlier reply. I work mostly with nonsubmersible pumps, and my reply matched those. As several others have pointed out, running out of liquid will soon burn out a typical submersible pump motor.

The motor won't burn if it has an overload protection device installed and the device is set to the motor's full load current as stated on its rating plate. If the tank is empty, though, the current will drop as the motor is no longer doing any work.

The pump impeller and the seals might suffer, though.

If it is a self protected motor, one that has O/L protection that senses the actual motor temperature then it has a chance. External O/L's that only sense line current will not protect the motor from cooling loss. This is true for both submersible and separately ventilated motors (or self ventilated with blocked cooling openings). No matter what, if you remove the cooling, external O/L protection is useless.

(I am an advocate of industrial motors wound with RTD temperature feedback!)

Commonly in submersible pumps they are designed to work at certain load against the water pressure and running freely in air it is working over and above its design load which is known as pump cavitation which will cause the pump to cavitate in other words cause damage to controls and instrumentation.

Ronald Naidoo

Electrical Engineer

Running freely in air it cannot cavitate!

GA

Nicely done.

I would only add "centrifugal pumps don't suck much". If they CANNOT suck at all, how does the water enter the impeller?

One more thing; If the application is a sewage lift station tank (again, MORE INFORMATION FROM THE OP WOULD HAVE BEEN NICE), some submersibles for that purpose, Flygt pumps for example, are designed not to sustain significant damage from running dry. These are not the same as ESPs mentioned above.

the clue is the word "submersible"

ESP's are in the fluid, the design will keep the dymanic fluid level at approx 1000ft ABOVE the intake.. therefore keeping the pump "submerged"

Dymanic fluid level.. the level the fluid reaches when the production rate of flow from the pump matches the inflow from the well.

Static fluid level.. the level the fluid reaches when the pump is not working.

GA

The only things I wish to add is that both an underload and an overload condition will not instantly (milliseconds) cause damage to the motor and pump. Also one must consider that briefly both current conditions will normally occur in the operation of the pump. Overcurrent will happen during startup and undercurrent will happen prior to operation and with a large bubble. So care and consideration should be used in deciding how to respond to both of these failure modes and how quickly one should respond to them. One should also consider the possible risk from a restart. It might be tempting to use a PTC resettable fuse to handle both trip conditions but this can easily become worse than nothing. The PTC will work perfectly for a motor that has a shorted winding, but think about what can happen with running a pump dry. A pump that continuously cycles on and off will not really be protected.

All this is great.

But if this is the cheap 1/4 Hp off the shelf submersible sump pump all the extra protection far exceeds the cost of the pump.

Those $90 pumps don't last long anyways. Water leaks in and ground faults, bearings wear out, plastic impeller seperates from shaft, casing rusts out, etc.

There is a reason for a one year limited warranty.

Very good, another important critical factor in deciding if protection is needed. But in making this cost/benefit calculation you shouldn't only worry about the cost of the pump replacement. One should also consider the possible cost to what is getting flooded and the possible down time.

2 pumps and float switches!

Hi Redfred

You are right, the U/L will not occur immediately, that would depend on the time delay set on the motor controller. However the O/L will trip immediatly if the timer settings are set to do so.

To explain: A submersible pump motor controller (MC) is not like anything in use anywhere else, it has so many functions, all to protect the motor. The typical functions are under & over loads plus the time delays on those functions, the over & under voltage plus time delays, Hz trips, automatic starts, restart timers, cumulative restart timers, lock out settings, back spin timers and lock outs, plus digital and analog inputs, plus a host of other functions that would make this reply way to long to read.

With a ESP if it trips, it may not be a failure, under-loads are caused by gas, blocked intake, NOT cavitation (lets not go there please) and several other things. Over load trips:.... heavy oils thro pump, sand, grit, stones, temporary drop in volts, and a host more.

Typically I set both trips to operate after a few seconds, however in the case of O/L's then the controllers measures the O/L and dependent on the value will either ignore the value until the timer has expired or if its over a preset higher value it then trips. Same for start-up (inrush current), it also has a short circuit value.

Underload trips can be restarted automatically by the controller if programmed. Typical restart is 30minutes after the shutdown (to allow motor cooling) and providing the pump/motor is not back spinning. That is measured by what we call a, "Back spin relay". Motor rotating in reverse generates a small current, this device detects this and and prevents the MC from starting.... don't want to brake shafts or couplings!

The auto restart, again all programmable, for a underload trip this will only happen three times with a time interval of minimum 30 mins between starts, then the MC locks out, never to start until the technician investigate the problem.

Over loads... a one time only trip that requires technical intervention

So not all trips are failures, just something the MC does not like happening down-hole, as for pump cycling.. not a good idea, reduces the "run life" of the pump. A re-design should be considered if consistent cycling is happening