Three Phase Frequently Asked Questions

If you’re into serious woodworking machinery, you’ll encounter equipment that was built with motors requiring 3 phase power. Single-phase loads may be connected to a three-phase system, either by connecting across two live conductors (a phase-to-phase connection), or by connecting between a phase conductor and the system neutral, which is either connected to the center of the Y (star) secondary winding of the supply transformer, or is connected to the center of one winding of a delta transformer (Highleg Delta system).

This is usually done if the motor is small (smaller than 3.5 KWatts) or if the motor is driven by an inverter or some kind of electronic driver. Always make certain that the replacement motor has a maximum HP rating (rated HP x SF) equal to or higher than that which it replaces. The motor windings functions as a rotary transformer, or generator, and running unloaded, consume very little power. Another option, but usually more expensive, for running a three phase motor from single phase power is the purchase of a rotary phase converter. You should expect very unbalanced power (kW) readings and varying reactive power from phase to phase.

The higher speed might produce slightly better phase angles, but the lower speed is generally easier to start. The illustrated, animated schematic to the right displays a Three Phase Motor Starter with start stop button control. The contactor is specifically designed for motor control, but can be used for other purposes such as resistive and lighting loads.

If you have eight reasonably thick cables connecting to the meter, four of which go to your consumer unit or fuse-board, and your main circuit breaker has 3 or 4 sections with one operating lever working all three or four – known as a 3-pole or 4-pole linked MCB, then you have a three-phase supply. The terminology used to described the phase converter parts needs clarification. The single phase load is connected between one of the three phases & the neutral.

There are two basic types of phase converters on the market which will allow 3 phase motors to run using single phase input to the converter. The horsepower of the first motor to start, or Idler Motor if used, must fall within the minimum and maximum HP range on the converter. It is not uncommon for the original equipment manufacturer (OEM) to load the motor to its maximum load capability (service factor). The type of starter I am familiar with generally has a start and stop button incorporated.

The X2 terminal is bounded to a ground making this terminal neutral (No voltage potential to ground). A 3 phase 240V/480V motor nameplate means just that, depending on how you connect the windings it can be connected to either a 240V or 480V supply. The permanent split capacitor motors, a single-phase induction motor, is defined as a capacitor motor with the same value of capacitance used for both starting and running operations. Voltage to ground on a delta system by the way is also irrelevant, other than as the reference for your neutral tap between A and B.

As already explained by Jon, care should be taken to put on a balanced amount of single phase loads on all three phases in order to ensure that the overall system is balanced. For the simplest converter, without a separate start circuit, using 25-30 microfarads per idler horsepower between one of the input lines and the third (generated) line will provide an acceptable phase converter.

The windings are arranged such that the currents vary sinusoidally at the same frequency but with the peaks and troughs of their wave forms offset to provide three complementary currents with a phase separation of one-third cycle ( 120° or 2π⁄3 radians ). The generator frequency is typically 50 or 60 Hz , varying by country. Example 3-pole or 3-phase breaker would snap over 3 hot busbars, and then blue, red and black wires connected to breaker.


The coil, split ring, brushes and magnet are exactly the same hardware as the motor above, but the coil is being turned, which generates an emf. The edgebander was the most complicated machine to match a converter because it runs on single phase (while the glue heats up), so the wiring had to be done in such a way as to bypass the third leg while heating the glue. In that case, a motor with a speed just under 1000 RPM has 6 poles (6000/1000=6).

This method adds another 3 phase motor – an idler – to the static converter providing current in all three phases and while not perfect will allow your motor to run at all or nearly all its rated power. For easy reference, standard NEMA service factors for various horsepower motors and motor speeds are shown in this table. Well now, let’s keep it nice and simple, a 3 phase motor with these voltage ratings would indicate that depending on how it is to be connected (either star or delta) would dictate the voltage rating. Another reason for voltage imbalance is a worn or badly pitted set of contacts in the motor starting contactor.

VFDs work by converting the supply voltage to DC and then converting the DC to a suitable three-phase source for the motor. When the S” and Y” contacts are all closed, the autotransformers form a three-phase Y” connection, with line voltage (L1, L2, and L3) applied to the tips” of the Y,” and a reduced motor voltage tapped off a portion of each autotransformer winding. The name commonly given to the NC contacts is interlock, because each one locks out” the other starter from being energized.

The VFD will maintain a volts-per-Hertz ratio (V/Hz) to ensure that the motor has sufficient power to provide torque to respond to changes in the load. The power and other measurements on the first phase (the one we measured the voltage of) will be accurate (to within the normal limits). In star connection, there is four wire, three wires are phase wire and fourth is neutral which is taken from the star point. Also, if using a single phase source you may want to oversize the inverter slightly.

The Four Percent Rule requires you to determine the 4% value of the lowest voltage (which is 230 volts). The best simple description I’ve found is at It goes like this: 110 / 220 volt single phase current is like a big strong guy driving a tent stake himself. During the current balancing tests the 3 phase motor was only turning the spindle on the lathe, no metal was being cut.

This is a realistic scenario, where the only type of switch you have available is a SPDT, but the wiring diagram calls for something different. On all service problems where the motor protector was energized, the integrity of the windings’ insulation should always be questioned. Relay coil M1” is energized by this switch, and actuates three normally-open contacts (also labeled M1”) to send three-phase power to the motor. The output terminals of the VFD provide a place to connect the three motor leads.

I understand that one could power up the motor and check the current being pulled by each phase, if there is a serios unbalance on a single winding, then turn that winding around, BUT I would like clarity on how to sort out the windings start and end before power up. The self starting phase converter uses capacitance between only one phase (1-3) instead of using 2 sets as recommended here.

It is extremely important to perform all the tests, especially if you suspect 3 wire w/grounded hot leg and will be using only two legs or you may create a dead short with your connections. The three phasors are 120° apart, and the three voltage waveforms are 120° apart — 1 complete cycle being 360°. This very much comes down to the particular appliance or hardware you’re using and you should check the voltage and power needs of the gear carefully before making any assumptions. Connecting an electrical circuit from one phase to the neutral generally supplies the country’s standard single phase voltage (120 VAC or 230 VAC) to the circuit.

In the end I just (sort of) guessed, but I took some extra precautions to prevent destroying a perfectly good motor. The method assumes the voltages will be relatively close to each other, and the phase disturbances small – though as we’ve seen above, neither of these are necessarily the case. This motor-generator combination can provide a frequency changer function as well as phase conversion, but requires two machines with all their expense and losses.

To spin up the big motor, he just pulled the board with the small motor until the belt was tight, got the big motor turning, then applied the main power and dropped the board (and turned of the aux motor). Note that always connect the earth wire to motor, and adjust the overload relay ampere by using the adjustment switch. The advantage of a 4 wire system is that when a fault occurs, there is more flexibility to keep the system running since it may only affect one of the phases. It has a single moving coil that is permanently but flexibly wired to the voltage source, so there are no brushes.