AC Induction Motors burning out under no load?

You could use a restrictor to keep the air flow so that the motor is below its max current draw.

Size of the inflatable is irrelevant.

At the simplest level, you could use a properly chosen fuse! A more elegant solution would be to use a proper motor starter with overload protection. Look at the Square D Class 2510, it is a combined switch and overload relay with the same footprint as a wall-mounted toggle switch. Available with melting alloy overload to fit your motor.

Have you investigated the possibility of using a thermally protected motor, and/or adding some kind of external thermal protection equipment for the current motor? Or, have you verified that the motor isn't just marginally undersized for the blower that it is used with?

One possible thermal protection scheme could be just a simple bimetallic switch that opens when it gets too hot and closes after it eventually cools off enough. If in sufficiently close thermal contact with the motor, and if it were controlling the power delivered to the motor, this could potentially save your motors. Something to keep in mind with this self resetting scheme is safety. If say the motor overheats and shuts itself off, then the user might be inclined to poke around with it trying to figure out why it stopped working. In doing so they might stick their grubby paws where they don't belong, and then the motor just then cools off enough to reactivate while their fingers are in the wrong spot. If this is a potential problem you might want to invest in some kind of latching thermal protection that must be reset by the user before the motor will reactivate.

An electronic approach could be used that perhaps includes a thermistor, voltage reference, comparator with hysteresis, a power relay, and perhaps some other kind of electronic circuit to provide a latching function with human user reset switch.

Note this is a different Mark then the OP

you may be able to get a different impeller design that does not increase load as much when the hose is removed. I think it depends on which way the blades are curved. Most vacumm cleaners speed up when you block the air. I agree with Bob, the most reliable solution is to use a motor that is sized to operate continuously with the hose removed. i.e under the most heavy load.

Mark

There is also the possibility that these are intermittent duty motors, and should not be left running regardless of load.

Tam

Not exactly electronics but most probably the best place to ask. I am looking into the design of a blower unit to inflate bouncy castles etc. The normally use a 1.5HP, capacitor start AC Induction motor with an impeller housed in a Snail shell type housing. One comment that has come up during my research into a design, is that users of these things are saying that if a blower is left running but not inflating anything, they burn out after a while under no load. Is this a motor problem or is it a cooling problem, any idea's.

snipped-for-privacy@markXscotford.com (remove the X to reply)

Thanks for the info. anybody got ideas on how we can protect these units from this problem, remember blowers will be used for different sizes of inflatables!

If by "impeller" you mean a centrifugal blower, then it means bad design. Motor loading is nearly constant with any back pressure for a centrifugal.

Why does it have to shut?

If the motor is burning up, the motor is under powered for the application, period. Blowers use a little more power moving air than they do when the air is static. If they are operating on the margin, they may overheat under a flowing load as you have described. By rights and to avoid law suits and the possibility of burning down someones house or business, you should use a larger motor that can take any load that's likely to occur and that includes being unhooked from the inflateable. Murphy's law says if it's possible to happen, it will happen. I would not micky mouse around with flaps or other contrivances intended to unload the motor. That's a sure way to get a lawsuit should a fire start. Get the right motor and be done with it. Also be sure any motor used in this application around flammables and kids is properly grounded, thermally protected and operated on a GFI, ground fault breaker. Bob

Fuses would'nt work, operators of these things tend to not be the brightest, so will in the first instance of the fuse blowing, will replace with higher rating to stop fuse blowing again. Motor starter too expensive for this product.

I think it just might be a case of saying fit a sticker to the blower saying "do not operate without inflatable attached", although this does not help the situation where the operator has left one of the (deflation) zips open, nor does it help us where the operator has run the fan with no load, burnt it out, and swears blind he did not, just gets another one under warranty anyway.

I was hoping for a more elegant, innovative idea, but perhaps what I am asking for is impossible.

Anyway the original question has been answered, thanks for everybody's help.

One day Mark got dressed and committed to text

Why not try a flap on the output of the blower, it would be held shut with a light spring. Thus there will always be some restriction on output. This principle is used on many centifugal water pumps for the same reason.

-- Regards ..... Rheilly Phoull

So the Inflatable is shut down every half an hour?

elegant

relay

And from the mechanical viewpoint, the hose attachment should have a flap which closes to restrict/seal airflow when disconnected from the 'load'.

Ken

Its never under 'no load' conditon if the impeller is still turning, its just not doign any usefull work, whats worse is that the air flow probably cools the motor so its losing its cooling, also if it is not pushing any air it is probably working harder due to the back pressure and generating more heat than it can get rid of, also dont forget that all the power going into the motor ends up as heat not just the winding resistance etc but just churning the air up with the fan takes power and this leads to heating up the air also, if this air isnt moved away it wil heat up the motor.

I would sugest ensuring a minimum airflow by a low pressure/high flow releif valve, or just a certain leak somewhere, just recirculating the same air around the motor isnt a good idea. you might be able to ensure the motor has its own seperate cooling airflow. you might also consider a thermal cutoff/control.

it would be quite easy to make a releif valve, just a hinged plate over a reasonably large hole with a spring or weight holding it closed.

have fun bouncing !!!

Colin =^.^=

Seems to me that you would want some kind of pressure sensing switch that controls the whole thing.

In line with some of the previous discussion, I would measure the current, and if possible, power drawn by the motor under various conditions. Beats philosophizing about it.

Tam

Consider asking your blower supplier (or another blower supplier) for a "non-overloading" design, rated for continuous duty with no back pressure. This will eliminate your burnout problem at the cost of longer inflation time (with the same size motor), and should not increase the price of the blower much if at all.

The previously suggested restrictor plate could do the same thing, but would be removable and probably incur more of an inflation time penalty.

Thanks for all the answers so far, I will speak with the impeller (Scirroco fan) manufacturers and motor people on Monday. It should be noted that we have to avoid as far as possible, any form of protection that turns the motor off completely, some of these inflatable's are some 10-12 metres high, its not a good idea for the motor to switch off unnecessarily, when we may have children on the inflatable and the thing starts deflating around/on them (but perhaps that may just be a matter of opinion)

You wouldn't by any chance be slowing down the Async motor by some electrical means that you are not telling us about? to slow down the common joe AC motor (not the universal type), you need to alter the frequency! this requires an Inverter drive other wise, any other way that can think of would simply heat the motor up when you attempt to slow it down by means of lowering the voltage. simply put, you need enough voltage verses the shaft load to keep the shaft rotating with not much lower than 85% of the cycle rate to the pole count. other wise know as not allowing the motor to enter slip most to much.. most async motors ( 4 pole types) are rated to reach 1750 RPM at 60 Hz, 1800 RPM is the real number but because rotor is not energized ( Sync mode types with Slip rings), the rotor will never reach 1800 RPM. in the case of yours, you may be using a 2 pole unit that should generate around 3500 RPM's. if you have a universal motor how ever (series type with brushes), you can then apply DC or AC to the motor and vary the voltage to adjust the speed. also these motors can really wind up fast but can eat up brushes and bearings, so you lose on that end! etc..