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Roots blower - overloaded motor issues

Roots blower - overloaded motor issues

Roots blower - overloaded motor issues

I have a roots blower at a remote site. The blower was supplied as part of a complete package by the vendor, including belt-drive (speed increase set-up) and motor and accesories. Based on our current process/ambient presures and temperatures, and the blower RPM, the vendor's modelling program indicates that the blower requires 7.2 BHP input. The motor on the unit is a WEG 7.5 HP 208-230/460 Volt, 3-Phase, XP motor wired at 208 (We verified 212 Volt site service with all equipment running). The FLA is 18.8, but the motor is consistently drawing 25 amps and up. My field tech tells me alignment and belt tensioning are fine. Have also checked blower lubrication, rotation, and ruled out blockage or restriction upstream and down. I am looking for other troubleshooting suggestions. Any ideas?

RE: Roots blower - overloaded motor issues

i'd check/investigate:
1) when the blower is operating, is there any "resistance" provided to the blower wheel or is the air being dischaged directly to atmosphere?  Where is max. HP located at on the blower hp vs flowrate curve?  Perhaps the flowrate needs to be reduced to operate the blower within the motor hp range.
thoroughly check the curves discharge (or head) vs flow and power vs flow.  Typically, max HP is towards the end of the curve; hence, reason for hi amps.  Check the hp vs. flow curve to be certain.  verify temperatures used in calcs.

2) check the blower mfg data and make certain that the motor is sized correctly.
3) investigate the motor's history - was it tested before the blower mfg purchased it?  is a test certificate available?  Could be defective.

4) motors should be sized to accommodate all blower operating conditions.

RE: Roots blower - overloaded motor issues

sounds like you checked everything, to me it sounds like its moving too much air.  Restricting air only allows the impeller to spin more freeley.  If you increase the mass flow by say, discharging to atmosphere like statede in the previous post, you will run the blower off its curve.  This will impact the motor.  Also, changing the density from design will impact the the mass rate, affecting the motor the same way.

Let us know anything else about the problem that you can...


RE: Roots blower - overloaded motor issues

I am a pump guy so perhaps my question seems stupid to some people but anyway ...

Should not a blower and its motor be non-overloading throughout the blower's operating range?  Assuming there is nothing wrong with this system causing a problem, should excessive air flow cause an overload?

Richard Neff
Irrigation Craft

RE: Roots blower - overloaded motor issues

For a "Roots" blower application rated at 7.2 hp, a 7.5 hp motor doesn't seem like much margin for any variations or uncertainties.  Don't forget, these are positive displacement machines.  Additionally, I always look for a bit more margin with a speed increasing drive.  A 10 hp motor would make much more sense to me for reliability.

A bit of philosophy:  A motor sized with little margin minimizes initial cost and minimizes energy usage if all goes well.  Having to re-do the whole mess to replace it with a bigger motor erases all of the potential first cost savings (plus a whole bunch more), and the energy usage is what it is.

RE: Roots blower - overloaded motor issues

One thing to check would be the density of the air you are pumping as compared to what the manufacturer assumed. If it is colder than it will be more dense and draw more. Also other gases and impurity may effect the density. And worse yet fluid droplets can of course damage it.

Look at flow rate, dp, also suction pressure and discuss it with the manufacturer.  You can also back out bhp by dividing output fluid power by assumed efficiency.

You can measure input power electrically, including voltage, current, and angle between them.  ** There are some electrical problems that can cause you to have high current even if the fan is doing fine.  The main problems to look for would include:
* Low voltage (causes high current)
* Unbalanced voltage (causes high current on some phases)

Less likely, you might find a problem with power factor (possibly due to overvoltage or motor problem), or with heavy harmonics that are contributing to your rms current, but not to you real power output. If you should have a high frequency supply then of course everything will run faster an draw more current than expected.

You will also develop an estimate of shaft power by input power times estimated efficiency.  Yet another estimate of motor shaft out power can be formed by check the speed of the motor shaft speed by strobotach... motor output power roughly proportional to slip.  Compare the three estimates of motor power.  Differences may give you clues where to look. If all three estimates (from slip, from elect input*efficiency, from fluid output divided by efficiency) are the same, then look toward the load creating an actual mechanical overload. If input power*efficiency is high compared to others powers, then look toward problem in motor.

While you have the strobe, you can maybe look at speed of load to see if belts are changing speed as expected (also can be done by measuring sheave diameters).  

An off-the-wall idea, is it possible you have the shaft rotating backwards... I'm not sure what indications expected in that scenario.

Work closely with the vendor if possible and don't take anything for granted.

RE: Roots blower - overloaded motor issues


good post, but were you asking a question when you were refering to mass flow and overloading?


RE: Roots blower - overloaded motor issues

Perhaps I should clarify my thoughts and comments a bit.

"Roots" type blowers have a couple of dominant operating characteristics:

1-  The torque required by the blower is determined primarily by its displacement and the pressure differential that it experiences at its suction and discharge ports.  Fluid densities, etc. are mainly issues involving the flows in the suction and discharge piping systems with their effects on the pressures experienced at the suction and discharge ports.

2-  The temperature rise across the blower can be expected to approximate that of isentropic compression of the gases or vapors passing through it.  (These machines do not provide an isentropic compression process--only a similarity in the temperature rise characteristic.)

If there are no devices such as vacuum breakers or pressure relief valves the pressure differential can result in some serious overloading of the motor and drive system while introducing a nasty spike in the discharge temperature.  Sometimes very simple, inexpensive devices serve well in blower-related systems.

Regarding the belt drive, is it a v-belt or cogged belt type?  Cogged belts can provide better efficiency and lighter bearing loads.  One point that I missed initially was the statement that the blower required 7.2 hp.  If that did not include allowance for the efficiency of the belt drive, the 0.3 hp apparent margin would be more than gone.

RE: Roots blower - overloaded motor issues


Yes I was asking a question.
The posts following mine cleared up this issue for me.  I did not understand that a Roots blower is positive displacement.

I think ccfowler's stated philosophy was precise and excellent.  Close matching of motor and load can obtain high efficiencies, but when you get in trouble you lose it all quickly, and then some.  Unknown and unforseen site conditions require some slack in the design.

Richard Neff
Irrigation Craft

RE: Roots blower - overloaded motor issues

A couple of additional thoughts have come to mind:

RV, you have mentioned concern for the motor overload problem but not for process-related performance problems.  Is it possible that the blower is "comfortably over-sized?"  This is very common, and it may permit a "low-bucks fix" for you.

When operating with a significant overload, the motor would have to be operating at a greater than rated slip (lower than rated speed).  This implies some modest reduction from the rated volumetric flow rate combined with decreased motor efficiency.

Based on everything that you know about your process with the now-apparent degraded blower performance, could the motor and blower be re-fit with a cogged belt drive with a lesser (but still adequate) ratio?  There may be enough "margin" available in the original design flow rate to permit reducing the blower load enough to permit the existing motor to continue in service.  This would obviously be much less costly than going the route of bigger motor, bigger switchgear, and likely bigger supply wiring.

Second, is there a filter (or other device) in the suction piping system that could be loading up and causing a significant reduction in the suction port pressure?  If the blower is operating in a situation where the discharge pressure remains stable but the suction pressure is reduced, the differential pressure increases, but the pressure ratio increases more severely.  Since the pressure ratio controls the temperature rise, the discharge temperature can become troublesome through distortion of the blower housing and/or increased flow velocity in the discharge piping system.  (I would expect this to be only a secondary issue to the fundamental motor and drive issues.  A reduction in the flow rate would probably help with any possible pressure ratio issues.)

RE: Roots blower - overloaded motor issues

I agree with all the Electricpete has said, as a pump & electrical man, i would definately look for any problems that could be associated with the "power supply".
My experiance is usually the problem lies with something electrical when it comes to new installation ( not always 100% i would point out).
I would as a first step before i do anything else, is check the power supply is arriving at the motor terminal connection is corrrect off load and under load.Then check the direction of rotation the motor is correct, even on single phase motors, on a three phase i've forgotten the number of times my own engineers / client / consultants / everbody and his uncle has told me the motor was going the right way ?? guess what, time after time it wasn't.
Having checked it yourselve " you then know its ok" then you can start looking at the voltage balance etc.

Good luck, let us all know how you get on, as we have many many queries on the board but not many answers as to how anyone gets on with their problem and would be good to know what kind of success our offers of help have been.

RE: Roots blower - overloaded motor issues

Hope I can help with this late post. One thing which is not mentioned is elevation. You have indicated that this is in a remote location. If the elevation is high, you will draw a lot more HP! If you know the flow and pressure, I can do the power consumption calculation for you. Also, I would suggest sticking with your v-belts. I have found that cog belts transmit too much vibration to the drive bearing.
Should you have any questions, do not hesitate to contact me.
Mark Caudrey

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