Induction Motors

[f350ca]

Dug out an small squirrel cage blower I recycled from the dump, need to ventilate the crawl space under the house.
Made up a widow maker connection to an extension cord to see if the motor worked, ran fine and filled the shop with dust and leaves that blew out of the blower, it obviously works. It has a rather large driving pulley so the fan is running close to motor speed 1740 rpm. I don't need a lot of air movement so thought about putting a smaller pulley on the motor to reduce load and make it cheaper to operate. To make this scientific I thought I'd measure the current so clipped the AC amp meter on a line and it read 6 amps, hum, thats 660 watts, this isn't going to be cheap to operate. Check the plate and its 1/3 hp rated at 5.8 amp.Hum again 1/3 hp is 250 watts, is this thing that inefficient. So I slip the belt off, still draws 6 amp or there about on my analog meter. Hum again. So out to the storage facility and check another motor out of inventory, (pull from the junk pile). Another widow maker and we have it running. This one is 1/4 hp rated at 5 amp, 550 watts. Guess what, thats what my meter says it draws with no load. If I hold a piece of wood against the pull, (proper dynamometer testing) the current goes up well above the rated 5 amp, like 10 or 15 amp. So is my meter not reading correctly? Thought the plate rating was at full load.

[stevec]

Greg, I'll hafta get a couple of motors out of inventory and try them out with my ammeter.
I'll check back here later (don't hold your breath   I sometimes forget ).

Steve

[Sunset Machine]

Did your widow maker connection include a ground? Motor windings can short and still let the motor run if you remove the ground lead. Dangerous, of course.

[Terry S]

There are several variations on "squirrel cage" blowers. If yours is out of a household type of furnace or air conditioner, it is likely a "forward curved" wheel. They are easy to identify - the air foils are  short compared to their length, and have a obvious curve formed into them. The curve is in the "forward" direction of rotation.

These wheels operate with low noise and are compact, but are designed to operate against a minimum range of discharge pressure. If you don't have enough pressure across the blower, the blower will overload the motor.

It seems counter-intuitive, but a centrifugal blower requires the least power when the air flow is totally blocked. The power requirement goes up as the restriction is decreased and reaches a maximum at "unrestricted flow."

Try an experiment. Use a piece of cardboard to restrict the blower flow. You can do it either on the discharge or on the inlet, but the inlet is usually easier. Put your amp meter on the motor and measure amps at several points from no restriction to fully closed. You will find the amps fall off considerably as the blower is restricted.

Your ventilation application might require some restriction to limit the motor amps to a reasonable level while still giving the desired air flow.

Terry S.

[f350ca]

Thanks for the replies.
Would be good to get another set of readings Steve, maybe my meter is out to lunch. Don't worry, if you forget and don't reply I'll probably forget you were going to.

Ground, no had it sitting on the bench. A short to ground could make it draw more I guess but without the ground the draw should be representative.

Thats exactly what it is Terry. Think the same applies with straight blade fans too, used the current draw to fine tune a dust collector system one time. Any pressure drop in the piping would show up as a reduction in current. That one had a 2 hp motor on it. If I get the current draw figured out I'll try reducing the flow to get the electrical draw down, but belt on or off was the same last night.

The two motors I tried are the split phase flavour with no start capacitor, does load not affect current draw at low loads?

[stevec]

Greg, amazingly I did not forget!
I was hoping there would be more replies and I could pretend I forgot.  

I tried 3 motors ( the easiest I could get to)


                                                                 no load          under load
1. Angle grinder, rated 6.0 amps                   3.9                 4.2

2. 1/2hp drill press rated 5.1 amps                3.2                 4.6

3. shop vac rated 9.2                                   6.8                  4.9

As Terry said a blower (or sucker in my test) will draw more power the more air it moves, so, blocking the input
of a shop vac will have it easier the less air it moves ( might be true too of a centrifugal water pump , I have one but it would take a lot more  set-up than my lazy a$$ would care to.)

If you want less air movement so as to consume fewer watts, maybe an on-off timer would do?

Steve

 

[Terry S]

Yep - centrifugal machines all have the same characteristic; less resistance to flow = more power requirement.
Shopvac blowers are a multi-stage centrifugal blower, so minimum motor amps is at no flow. That is why a Shopvac increases in pitch when you block the flow. Works the same with liquid centrifugal pumps.

Induction motors run at a speed slightly less than their "synchronous" speed. In North America we have 60 Hz power, so the synchronous speeds are:
2 pole: 3600 rpm and operate around 3450 rpm
4 pole: 1800 rpm and operate around 1750 rpm
6 pole: 1200 rpm and operate around 1150 rpm

Induction motors run at their highest speed (close to synchronous) with no load.
As load increases, the "slip rate" increases to generate the needed power and the motor slows down.
The motor nameplate rpm will be at the nameplate rated horsepower.

For a motor - blower combination, the blower and motor balance at a speed where blower hp = motor hp.
At this speed, the running amps need to be less (or equal to) the nameplate amps or the motor will overheat.

Terry S.

[f350ca]

Thanks for the responses.
Did a little research on the great goddess google. No representative curves of current vs torque but seams the split phase motor is less efficient than a capacitor start.
Did a little more physical testing with my hi tech dyno (stick) and yes there is a slight change in current with a reasonable load, but less than I expected as Steve saw too. So induction motors are horrible power eaters with no load.
Interesting info on the rpm at rated hp Terry, thanks on that one, didn't know that gem of knowledge. Will now tack the motor with no load and see what it reads, should be higher than the plate rating then.
Was planning on a timer Steve, no point running at night when the air is damp.
Guess I may as well load it and move lots of air if reducing the flow is going to make so little difference. The one paper I read said they were the most efficient at or near rated hp.
I have a 1/4 hp motor so will use it instead of the 1/3 hp and save an amp or 110 wats and belt it to get the motor close to the rated rpm.
Thanks

[stevec]

Greg, maybe a "humidistat" is all you need .  

Steve

[Vinny]

Not sure what your need for ventilating the crawl is, but when I had a place up north (michigan) I needed to due to dampness and a bit of odor. I had suggestions from people on other methods for getting rid of the dampness but what I ended up doing was just putting a small fan ($10 from the local walmart) under there. Don't remember if I set it on a concrete block or suspended it from above, it's been a number of years, but that's all it took. I only needed to move the air around some so it wasn't sitting there stagnant.