When is three-phase power -- the right choice?

A primer on three-phase power, and when it's appropriate to make the switch from single-phase. June 14, 2000

I am getting ready to buy a band saw and radial arm saw in the 3 to 5 HP range, but I'm not sure how motor phase affects performance.

Forum Responses
Single-phase power is not smoothly delivered.

The sine wave of our voltage goes from V+ to zero to V- to zero, and back to V+ sixty times a second. In this case the power (P) delivered equals the voltage (V) times the current (I) or P=VxI.

Each time the voltage goes to zero the power goes to zero. So single-phase delivers a torque to the shaft of a motor that pulses from full power to zero - 120 times a second! With small motors (less than 5 HP) you don't notice it and you really don't care.

Three-phase power uses three sine waves that are separated by 120 degrees (or 1/180th of a second). When any one sine wave is at zero the other two are still delivering power to the motor shaft. In fact, the SUM of the power delivered by the three waves is perfectly CONSTANT, so power to the motor shaft is constant and smooth.

On big motors this helps keep vibration down and makes the bearings last longer. This smoothness or constant torque also applies to power generators. This smooth power delivery is the only reason people mess with three-phase. The power delivered to a three-phase motor is P=1.73 x V x I, where V and I are the voltage and current on any ONE phase of the motor.

Some folks have stated that the current is less on a three-phase motor but that is really a factor of different voltages and having additional wires that carry current.

But before you get too excited about long bearing life don't forget that hanging a 120-tooth blade on your saw results in 120 power surges per revolution. So much for smooth, vibration free motor shafts.

If you're going to use three-phase you're going to need three-phase in your building, or a phase converter. I don't think the addition of a radial arm saw and a band saw would warrant the cost of adding three-phase if you don't already have it.

The main differnce between the two, and I'm no electrician even thought I play one sometimes, is three-phase motors have no capacitors, they rely on magnetic starters. Three-phase motors are cheaper to buy, and seem to last longer than single-phase but you do have the added cost of a starter.

Another point: We have 208v three-phase in our building, and our motors pull about half the amps of a like-sized single-phase motor. If you were to go to 440 volts, again, you'd pull half the amps of a like-sized 208 voltage. Smaller amps equals smaller wire and controls, equals less money in wiring, equals less electricity used, equals lower monthly electric bill.

This is a brief synopsis; I'm sure you could contact an electrician and probably get a better answer.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
A previous poster says:
"Smaller amps equals smaller wire and controls,"

Thinner wire: true. Smaller controls: only where they can be made smaller from using thinner wire.

"equals less money in wiring,"

Thin wire is cheaper, although wire's awfully cheap to start with.

"equals less electricity used, equals lower monthly electric bill"

100% false.

The amount of work a motor can do is usually measured in horsepower. Watts (like calories or BTUs) are alternative units for (electrically speaking) the same thing: power, or work. (1 horsepower = 746 watts)

P=IE. Power = Amps (I) x Voltage (E) (I don't know why the formula uses I and E, but it's a first-year electrical formula).

The power company doesn't bill you for the number of amps you use; they bill for the number of kilowatthours. If you run a 4 horsepower motor (3000 watts) for 20 minutes, you've used one kilowatthour of power. Doesn't matter if it's a 25 amp 120V single-phase motor, a 12.5 amp 240V single-phase motor, or any other combination of amps, volts, and phases: 4 horses @ 20 minutes = 1kWh.

[However, a standard 'house' meter doesn't actually measure kWh directly. It measures power as if the house were entirely 240V. Normally, you'll have lights and whatnot at 120, but they're divided between the two sides of the 240V house main. A really big 120V appliance would throw it out of balance, and get billed as if it were 240. That's one reason why ovens, clothes dryers, and heaters tend to be 240V, to avoid fooling the meter into overcharging. Also for the reasons mentioned above by the previous poster.]

Comment from contributor H:
There is some truth to both of the previous contributors, but rated motor horsepower does not match energy used. There are many factors involved, but motor loading is not necessarily constant, and hence, not the same horsepower all of the time. Even though the motor may say 5 HP, the energy (and horsepower) used is determined by the amount of load. The loading, for instance, may vary in using a 5 HP table saw by the thickness of the wood, the wood type, or the amount of force used to push the wood through. If the current goes too high (and the correlating horsepower), the overload device will kick out, and the motor will stop. Depending on the setting of the overload device, the horsepower (and current) may be double for a certain amount of time. All of this is regardless to the number of phases involved.