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andreas
11-26-2006, 04:28 PM
Greetings all!

I have a 3/4 hp electric motor that I'm planning to use to make a belt grinder.

The information plate on the motor states that the motor can be wired for 115 volts or 230 volts. I can wire it either way as I have 230 volts in my shop. Question is, is there any advantage to wiring the motor for 230 volts?
Does it change the RPM or give you higher torque or something?

thanks

runchman
11-26-2006, 04:38 PM
Greetings all!

I have a 3/4 hp electric motor that I'm planning to use to make a belt grinder.

The information plate on the motor states that the motor can be wired for 115 volts or 230 volts. I can wire it either way as I have 230 volts in my shop. Question is, is there any advantage to wiring the motor for 230 volts?
Does it change the RPM or give you higher torque or something?

thanks

It cuts the current draw in half, which can be nice for something like a tablesaw that sucks a lot of power at start-up; can keep your lights from dimming. Other than that, can't think of any advantage.

And no it isn't cheaper to run with the current draw cut in half, because you pay for power which is voltage*current. Double voltage, halve current, no difference in power. Not that you asked that!

- John

mla2ofus
11-26-2006, 10:17 PM
Higher voltage will give you a lot more torque. I don't know if you really need more for a belt sander.
Mike

awright
11-27-2006, 04:21 AM
I beg to differ, mla2ofus. 230 will NOT result in higher torque unless the wiring of the 115 volt circuit is inadequate and is starving the motor. A typical 3/4 HP motor draws about 9 to 12 amps and 115 volts and half that at 230 volts, the actual value depending upon design details like how much copper and iron the manufacturer put into the motor. A properly wired 20 amp, 115 volt branch circuit can easily supply this current.

A 115/230 volt motor basically has two 115 volt windings. If you connect them in parallel, you run each winding at 115 volts and the total current to the motor is twice the current to a single winding. If you connect the two windings in series, you run the pair at 230 volts, or 115 volts on each winding, and the current to the motor is equal the the current through a single winding (except it flows through both windings in turn). Provided you have adequate capacity in your circuit to supply 115 volts at double the current per winding, the motor can't tell the difference between the 115 volt configuration and the 230 volt configuration.

If you have both 115 volts and 230 volts available you can configure the motor for either. However, if you have both options available, I think it makes more sense to wire at the higher voltage because the heavy current draw at startup is shared between your two conductors to the subpanel, while at 115 volts double the current passes through only one of the conductors to the subpanel (and the neutral). However, the practical effect is minor unless you have marginal power to your shop.

awright

mla2ofus
11-27-2006, 07:40 AM
Not being an electrician, I can't argue your point. All I can say is before my garage was built I had a 115 v receptacle attached to the bottom of the main panel of the house w/ 10 ga wire approx 18 inches long connected to a 30 amp breaker.With comp plugged directly into this receptacle,no ext cord,
and the ambient temp was below 35*F., the 3 1/2 HP compressor motor wouldn't start it up unless I slipped the belt to allow it to loosen up the oil resistance in compressor. After garage was built I converted comp motor to 230 v and it will start up w/ no trouble at temps below 0*F. This is w/ approx 20 ft of 8 ga from main to sub panel in garage and approx 35 ft of 12 ga from sub to comp receptacle. The motor cord was not changed,only put a 20 amp 230 volt plug on the end. I would appeciate an explanation of the difference in starting at cold temps.
Thanks,
Mike

awright
11-27-2006, 02:33 PM
I am not an electrician or an electrical engineer, but I have been "messing" with motors since having an Erector set 60 years ago.

What you have when your compressor balks at starting under pressure is a "locked rotor" condition. Even when starting it's load normally, the LRA condition exists for a very short period of time, and some motor/load combinations will draw fairly high currents for an extended period (several seconds) until the load comes up to speed. This is what I was referring to when I mentioned "...heavy current draw at startup...," and is the reason I advised using the higher voltage configuration when available.

Some motors list the "LRA" (Locked Rotor Amps) on their specification sheets. The LRA can be from about 150% to 500 % of the nominal running current, depending upon motor design and intended application. Split-phase motors have the worst LRA to running amps ratio because they have poor starting torque, but you will never find a split-phase motor on an air compressor due to the high starting torque requirement of a compressor.

So your compressor is drawing, let's say, twice its running current during starting. This can be a serious thermal load on the motor and a serious load on the conductors supplying the motor. If the locked rotor condition is sustained for more than a few seconds you will either pop the circuit breaker, open the starter overload sensor (if you have a more sophisticated starter than a plain switch), or burn out the motor.

For a common 230 volt supply to sub-panel in the garage, you have two hot conductors selected for the expected load on the sub-panel. Those conductors supply 230 volts between them, or 115 volts each to neutral. For a motor wired for 115 volts, all the LRA for the two windings (simplistically speaking) are drawn through only one of the hot conductors (plus return on the neutral), so you will have double the voltage drop in that conductor compared with drawing half that LRA through both hot conductors with the same motor wired for 230 volts. If your motor starting torque is marginal for the compressor under load and your wiring is less than very robust, the motor may not be able to start the compressor until something gives - not a good thing. That is the condition you had with your compressor on 115 volts.

As an aside, you can ease the starting condition on your motor and wiring by providing an unloading valve on the compressor outlet to the tank. This device has a check valve to hold tank pressure, but relieves the pressure on the compressor outlet line when the compressor is off. Thus, the compressor is not required to start up against full tank pressure. This will extend the life of your equipment. Look at the Grainger Load Genie 4X996-999 and 5A703-706. I'm not positive of the application of these valves, but I think they are intended for just this application. Check with the manufacturer or Grainger. Some better quality compressor controllers have this unloading function built in. That is the hiss you hear for a few seconds when the compressor stops and starts. (Actually, since I'm talking about them, perhaps I'll install these valves on my compressors.)

In my first post I was thinking of ideal conditions with fully adequate wiring for the LRA and running current draw by the motor. What we are discussing now is the non-ideal condition with less than ideal wiring.

Hope this is helpful.

awright

runchman
11-27-2006, 02:51 PM
Also the voltage drop from the panel to your device is proportional to current, (voltage = current * resistance), so at 110v you'll have twice the voltage drop from the panel to your device than you will at 220v (since you have twice the current at 110 as at 220).

If you're wiring is good and it isn't a long run, perhaps this isn't a factor.

- John

mla2ofus
11-27-2006, 06:18 PM
Thank you for the detailed explanation,I appreciate it very much. My compressor came w/ factory unloader valve. This however,doesn't work as well as the ones for compressors w/ force fed oiling as those kind will not close until oil pressure closes it,thereby allowing compressor to come up to full speed.
Mike

awright
11-28-2006, 04:12 AM
If your present unloader valve is not doing it's job (or if you don't have one), you could easily (but perhaps not cheaply unless you have a good junk collection) make your own. All you need is a check valve at the tank inlet and a solenoid valve connected to the side branch of a tee in the line from your compressor to the check valve. You can then control the solenoid valve by whatever means pleases you to allow the compressor to come up to speed without load. Control could be by a time-delay relay that opens the valve when the power to the motor is off and for a few (programmable) seconds after the power is applied to the motor or by means of a motor current-sensing relay. The outlet of the valve should be vented to the atmosphere through a sintered bronze or polymer noise suppressor to save your ears.

awright