Electric Motor Question

[Banister Stairwell]

I have an air compressor that is driven by a Doerr (model LR22132) 2hp,

3450rpm single phase electric motor. It's rated at 15 amps at 115 volts. I want to configure this motor to operate on 230/240 volts. It appears that all I need to do is remove a small access plate on the back of the motor and reconnect the wires to the appropriate higher-voltage terminals. Aside from changing the plug, is this all I need to do, or do I need to change the start and run capacitors as well?

To power the compressor at the higher voltage, I plan to install a 240 volt receptacle on a dedicated 20 amp circuit. Since this circuit is intended for a motorized appliance, the circuit must be run directly to the outdoor service-rated panel, not an indoor sub-panel. Is this correct?

Thanks in advance.

[jim]

i got an old sears 1 hp compressor and its hooked up to 115 volts.. you can change it over the 230 volts by taking off the cover off the side of the motor(probably like yours) and there is a diagram inside the cover telling you how to move the wires around for getting the 230 volts set up... it makes no mention of changing any capacitors(i have one on this motor)

[The Masked Marvel]

If it's dual voltage you should just have to rearrange the wires at its terminal plate.

[Joseph Meehan]

I believe all you need to is to change the wires and the supply voltage. BTW it will then draw about half the amps it did before and many have a slight increase in performance.

[Banister Stairwell]

It's on a 20 amp circuit now and the motor draws every bit of it's rated 15 amps.

I haven't had a problem with the breaker tripping, since it's currently plugged into the one and only 20 amp dedicated circuit receptacle in the garage. Unfortunately, this receptale happens to be in a *very* inconvenient location and I don't want to run the compressor with an extension chord, nor tie up this one receptacle for the air compressor. I figure, since I need to intall another receptical specifically for the compressor anyway, I might as well make it a dedicated 240 volt circuit and convert the motor to operate on 240 volts.

Besides, the motor won't draw as much current and (hopefully) won't have to work as hard.

[I-zheet M'drurz]

Power is constant. Higher volts * lower current still = same amount of power. Workload stays the same, as does the effect on your electric meter. Anybody told you different, they lied.

[Banister Stairwell]

You're correct, of course.

My thoughts in this case were that perhaps the motor may have been originally designed as a 240 volt motor, with a built-in wiring setup to accomodate 120 volts. In other words, running it at 240 volts may be better in that (perhaps) the motor would be operating more within it's original design parameters. Of course, this is just speculation on my part. I may post this question to someone at Emerson Electric Co. and see what they say (Doerr is a division of Emerson, I believe).

After looking more closely at the information plate on the motor, it does show that the motor is rated at 7.5 amps at 230 volts. Assuming no difference in actual motor performance from 120 to 240 volts, and other than the new 240 volt 20 amp dedicated circuit not being maxed out amperage-wise, are there any other compelling reasons for converting to 240 volts for this particular application?

Thanks.

[Jeff Prevett]

Technically this is incorrect. The amount of power delivered to the load is the same (P=V*I) so the motor will draw 1/2 the current at 240V than it would at 120V. But when it comes to the 'wasted power', the 240V setup wastes less power than the 120V setup. Because the 240V setup uses 1/2 the current, 4 times more power is wasted when using 120V (P=I^2*R). This 'wasted power' is in the form of heat in the wiring all the way back to the transformer on the street. Also, the windings in the motor will be producing heat. You ARE paying for the wasted power - it does show up on your meter. However, this wasted power is so small relative to the power consumed by the motor, that it can be neglegable.

This is where 240V helps the efficiency of the motor. As the windings temperature increases, the resistance increases, therefore consuming more power (therefore more power 'wasted'). Also, the life of any motor is increased by running the motor cooler.

It would be better to run this motor at 240V because you also don't have to worry about the motor unbalancing the electrical system. This occurs when you have too many heavy demands on one phase (I use this term to indicate

180 deg out of phase, not 120 deg as in 3-phase) and light demands on another phase. This causes a lower voltage on the 120V phase (and thus, more current consumption) that has the heavy demands and a higher voltage on the other phase.

Just my $0.02 Jeff

[zxcvbob]

If the motor can be converted to 240v, there will be a wiring diagram under the access plate that tells you what wires to swap. When you are all done, make sure the motor does not run backwards! The compressor should still work going backwards, but its oil pump won't work (if it has one) and the fan on the flywheel will not blow the right direction. If the motor runs backwards, usually you just have to reverse the two wires you moved. Not all motors are reversable.

An indoor subpanel is fine. Maybe I don't understand your question.

Best regards, Bob

[Chris Lewis]

According to Banister Stairwell :

Running a convertible motor at 240V is always better than running it at 120V, but the differences are often so small to not be worth doing anything about.

It's all in startup and at high load where the factor-of-four reduction in power loss in the wiring at 240V over 120V will result in lower voltage drop (dips) to the motor, easier starting, and longer life.

Compressors are almost always drawing very near full rated amps, and they are often hard-starting. So this is particularly important for compressors.

The difference gets larger the closer the 120V circuit is to max amps, and the longer the circuit is. If that compressor was 10A, and the total circuit was only 20', I'd say, don't bother.

But, your compressor maxes out a 120V 15A circuit, and it's probably significantly longer than 20'.

If it were me, I'd definately go to 240V. But I would consider leaving it a 15A circuit on 14ga because the compressor is only half of that load. But if the circuit is 100' or more, I'd go to 12ga (perhaps even leaving the breakers at 15A).

[twiedeman]

Why would you change it? Is it popping your breaker? If your current recepticle is only 15 amp, do you have another recepticle(20 amp) nearby.

[Chris Lewis]

According to Chris Lewis :

I just reread part of this thread, and realize that the existing circuit is 20A/120V. I'd still switch to 240V. 15A breakers, unless you intend on sharing this circuit with other 240V equipment ("one man shop" rules assume you'll be only running one tool at a time. Give provision for, say, a 3HP tablesaw, and go to 20A if you think you might need it for something else too.)

12ga if you're leaving the circuit at 15A and the circuit is going to be longer than about 40-50'.

[twiedeman]

Sounds like good advice from some of the other posters, I would have to agree that 240v dedicated seems like the way to go. Concerning your question about the source, your sub-panel is as good a source as any,especially if it is closer.Is it a 100a sub? Make sure it isn't maxed out, & that you can add the extra dedicated circuit. Todd

[Robert]

replying to Banister Stairwell, Robert wrote: ////I need to know what size capacitor is used on this . I have the doerr 2hp

120/240v 20.4a/10.2a 3470rpm motor l22132 on an old 2 post lift , however the cap is damaged and un readable....any help is greatly appreciated

[gfretwell]

If you google around a bit on the specs for that motor you should find something. Are you sure it is not on the motor label? If there is only one capacitor it is the start capacitor and they are not really that critical. I would try something around 500-600 on a

120v motor and 130-140 on a 240v motor. Run capacitors have to be more closely matched.

[Pasieka]

think reactive power and power factors all come into the picture, but i cannot say I remember enough to comment, but have a feeling the Inductive load of the motor is more favorably billed in terms of realized power when you have it wired across two legs of a single phase instead of a one leg... i believe the neutral current measured actually approaches zero in the 240 wired config and is significantly more in the 120 configuration... the neutral gets connected to ground at the main bonding jumper and goes back to the electric company with no credit to the home owner....

any chance any of you guys have a 5 HP does motor that matches mine? need your help with RUN capacitor value

[bud--]

An 18 and 4 year old thread. Utility meters accurately measure actual power. They are not confused by inductance (reactive 'power'), neutrals or 120 vs. 240 connection.

Neutral:

conductors: H N H N-1 conductors: H H

Note that a start cap can't be used as a run cap. A run cap is much higher quality, larger, and should be lower value.