<TABLE id=Table19 cellSpacing=0 cellPadding=2 width="100%"><TBODY><TR><TD vAlign=top>Is there any way to reduce the power output of a motor (or change the power output from its rated power)?

If a 5 HP motor is used to drive a load but when the load drops to a point where only 2 HP is needed. Is there a way to reduce the motor output to 2 HP? Or does the motor still runs at 5HP regardless of load and extra energy simply get wasted?(in resistor or other ways)

I have no knowledge of electric motors but common sense dictates that it is probable that if only 2hp is used then only slightly more than 2hp of input is consumed. Maybe I have got this wrong and will soon be told so if that is the case.

Someone told me that "alternating current motors generally will draw only sufficient current to match the load. An AC motor under no load draws very little current
regardless of its rated horsepower. It will increase its draw as the load increases. There is such a thing as over-motoring a piece of equipment with an AC motor. I'm not sure how it works, but there is an inefficiency
in say, attaching a ten HP motor where a 1 hp would be adequate. Direct current motors will draw a fixed amount of current regardless of the load."

Just want to get things straight once and for all.

Thanks.

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I'm not 100% sure of this...but just a thought. It makes sense to me that if you cut down on the current by means of a rheostat or resistor, the HP would decrease.

 

Hmm, trying to remember thru a forest of gray hairs , my schooling on this one Yes, the full amp load rating on a AC motor will be reduced if it does not have a heavy enough load on it. I dont think that the description of "small amount of current would apply totally though, on a severely reduced load AC motor the F.L.A. ( full line amps) draw will probaly be 50% of full rating. Industrial and commercial AC motor applications are now utilizing what they call VFD's ( variable frequency drives) to compensate for the fluctuating loads on AC motors. By the use of solid state circuitry and other electronic alien gizmoes and stuff to me , they change the frequency to slow the motors power comsumption ,resulting in lower power output and shaft rpm's. . Some people will use even lower HP motors for their loads, and use gear reduction units to get more torque with less HP...


In large plants that utilize a lot of AC motors the engineers remain attentive to the loads on their AC motors. If a plant full of AC motors are not loaded to rating, they will affect what is called "power factor" on the utility electrical distribution , and if it is out of the utility companies parameters, the consumer can be penalized for it in their billing. That is why when plants or factories are electrically engineered in the design phase, they are careful to work closely with the mechanical engineering division to get the proper horsepower rating for the proposed load. A oversize motor wastes power in both power factor and size of unit cost. And by keeping them sized properly, it dwindles that old Power factor bug, and keeps the total costs of the project down . jmo

 

If the load in amperes is within the rating of a common household lighting dimmer switch, I suppose you could adapt one to whatever the specific device is. I'm not sure what the ampere rating of a five horse single phase AC motor would be, but it might be within. Mount it in a project box somewhere on the equipment, and connect the wiring accordingly. It could also serve as an additional emergency shutdown switch if its the "Slap on and Off" type. Heck - paint it red!

But I don't know how much power those dimmer switches consume.

I DO think that they have an overall effect of reducing the peak-to-peak sine wave excursion of the AC voltage going through them (In Plain English - they reduce the voltage).

An adjustable "current limiting circuit" might work - this would be effective with DC motors. It would pretty much have to be solid state, a shunt type would still use all of the available power, but a series type should do the trick.

Motors under full load, or when starting up, or when a load suddenly increases tend to pull a lot more amps, and this is when most fuses and breakers blow. Its a response to instantaneous current flow increases. As more current is required, the devices draw more.

If I have a twelve volt source, with so many amps available, and I begin adding 12V lights to the circuit (in parallel) eventually the total amount of current is enough to exceed the amount of electrical energy on the line (amps x volts = power), and this is seen as a decrease in voltage.

In restricting the amount of power applied, there are two approaches.
1) Reduce the voltage.
2) Reduce the current (amps).

Transformers reduce voltage, but the trade off is an increase in amperage that is equal and opposite - so a simple transformer is not the solution.

A rheostat is in essense a large variable resistor, and will at the same time it is reducing the power flow, convert electrical energy into heat - thereby wasting energy.

I think an AC or DC current limiter circuit is the ideal answer.

 

On edit : Not a motor expert here, but I meant to add that when trying to control the speeds of AC motors, a person needs to be careful of what type and HP of AC motor they have . Shaded pole motors like sewing machine motors can be put on a resistance or bridgestone type of rheostats. Not many other style AC motors can have their voltage supply tampered with, without damaging them. Most require a stable voltage supply within 10-15 % of their stated operating voltage. ..

These small shaded pole variable speed motors are usually fractional HP motors. AC motors as large as a 5 hp would take a VFD tye control unit to make them variable rpm/ power output. Whenever anyone is ever in question of how they can control a motor speed/HP output, it would pay for them to stop by their local motor repair shop and ask the pros. That will help keep the motor and installer safer, good luck. jmo

 

I recommend a search on "Motor Controllers"

I did some looking around last night, but the sites I went to were way to in-depth for most people. http://www.discovercircuits.com was one of them - and they are a great site for advanced electronic/electrical people.

 

If an electric motor is oversized, the power factor goes down. Usually the power company bills you extra for every % below 85%. Plus, you pay more for the bigger motor. You can correct the power factor with capacitors, but that's a band aid. So mainly it's lost efficiency when the motor is too big. Not a huge deal when the motors are 2 hp or 5 hp, but a big deal when the hp is 800 when you really need a 600 (dealt with this last year on a big fan). Service factor is another consideration (typically 1.15).

If the load is a pump, vfd's (variable frequency drive) will help. Otherwise, you may be throttling with a discharge valve if the motor and pump are too big. It's like setting your engine's throttle at wide open and regulating your speed with only the brake.

 

spikedog, if capacitors are only a "band-aid" they are sure working on my welder, 131 amp input without caps, 101 amps with caps. Unless there is another way to change the PF without caps the "band-aid" will have to do.
I'll shut up now.
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I'm not an expert on motors, or AC power distribution, but I believe Power Factor relates to the phase difference between the voltage and current. If they are off 90 deg, the 'instananeous' power at the max voltage (or max current) would be 0 since the current (or voltage) would be 0 (power = volts * amps). It would seem to make sense that the load would cause changes in load factor, as the inductance of the windings would change - I think. . . [Now I'll have to ask the geniuses at work for the full story. . .]

 

Post at 11 news. I believe your correct to get the current and voltage close together to improve the power factor.
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