How To Wire Three Phase Electric Motors

The area highlighted in yellow is the part of the schematic which contains the Three Phase Disconnect Switch and three fuses. If we have a lot of stators, instead of just the two pairs shown here, then we could consider it as a stepper motor: each pulse moves the rotor on to the next pair of actuated poles. In the US, this is the standard household power arrangement with two (Phase A, Phase B) 120V power wires (180 degrees out of phase with one another) like two bicycle pedals and one neutral wire. Once that is understood, a single-phase AC motor is a little easier to explain.

This method of running three phase equipment is commonly attempted with motor loads though it only supplies 2/3 power and can cause the motor loads to run hot and in some cases overheat. Because of higher reluctance ( or low permeability) of air gap the magnetizing current required in motor is 25-40% of rated current of motor where as in transformer it is only 2 -5 % of rated primary current. The more windings of wire, the more slots are required in the stator, and the more slots there are, the bigger the motor must become, hence heavier. If you’re across the alley from a commercial user and there’s 3 phase on the pole – like that.

Based on that diagram and the fact that the motor will be operated at 230 V, I gather that wires 9 and 3, 8 and 2, 7 and 1, and 4 and 5 and 6 all need to be wired together – giving me 4 separate wire bundles. Since a motor has inductance, the breaking of the current is more difficult so the contactor has both a horsepower and current rating that needs to be adhered to. For clarity, it is mentioned here, but to understand how it works, see the diagram and details for 4 wire 3-phase.

This is a single phase transformer and lowers the voltage to a more common value useful when adding lights, timers or remote switches not rated for the higher voltages. Since only the running capacitor (which is of relative low value) is connected in series with the auxiliary winding on starting, the starting torque is greatly reduced. Most groups of houses are fed from a three-phase supply so that individual premises with above-average demand can be fed with a second or third phase connection, although domestic appliances are invariably designed for a single-phase supply.

For the purpose of this article the two wires carrying the single phase 220 VAC power will be called lines 1 and 2. These are connected to terminals 1 and 2 of the idler motor, respectively. Selection of the overload for use with a single phase motor is still based on the motor full load current (FLC). A starter” is another name for the large power relay used to conduct current to the motor lines. Each winding has around 400V across it in UK (that’s 230V times sq root 3). For US,it would be about 190V for a nominal 110V supply. Even large energy consuming heaters and ovens are single phase most of the time.

The total amount of work being done is much greater when added up. Looking at the graphic you can see that one of the three phases is always nearing its peak, thereby generating more consistent power from the motor. However, after the first motor, or Idler Motor, has started, motors below the minimum range may then be started and can usually be left running as the main motor is stopped and started. Commercial vendors of static converters allow using the static converter to start an idler motor so that several motors can be run at the same time.

In the very early days of commercial electric power, some installations used two phase four-wire systems for motors. Following are specific steps to take when identifying leads of both a star connected and a delta connected motor. You connect the first probe of the ohm-meter to lead 1, and then you connect the other probe of the ohm-meter to leads A, B, C, 2, 3 and ground (motor chassis).If the motor has no problem, then you must find infinite resistance between all leads and ground, except from one lead. If you randomly mix-up the L1-L2-L3 phases, the motor will probably spin fine, but there’s a 50% chance that it will spin backwards.


Three phase motors will also vibrate less and hence last longer than single phase motor of the same power used under the same conditions. It is the task of the overload heaters to protect the motor against overcurrent by mimicking the thermal characteristics of the motor itself. With a balanced load, all the phase conductors carry the same current and so can be the same size.

You have to supply enough single-phase current to power the equipment that you are using and to satisfy the parasitic energy consumption of the rotary phase converter(s). The control transformer primary side voltage must match the incoming line voltage and the secondary side must match the coil voltage. Use the slider corresponding to your dominant variable to further narrow your motor selection.

Phases with higher relative loading will experience reduced voltage, and phases with lower relative loading will experience elevated voltage, up to the phase-to-phase voltage. A good way to check for this condition is to use a clamp-on (inductive) ammeter to check line current on all three lines while the starter is energized. One method for using three-phase equipment on a single-phase supply is with a rotary phase converter, essentially a three-phase motor with special starting arrangements and power factor correction that produces balanced three-phase voltages. Three phase has properties that make it very desirable in electric power systems.

All the switches are set to stay closed and maintain the auxiliary winding circuit in operation until the motor starts and accelerates to approximately 80% of full-load speed. Pull-Up Torque: The minimum torque delivered by a motor between zero and the rated RPM, equal to the maximum load a motor can accelerate to rated RPM. In the block diagram one can also see that a diode is connected in reverse bias across each IGBT to protect it from excess voltage spikes that may occur. One such diagnostic check is line current, to detect the presence of an open motor winding. Group the fifth and eighth wires together with a wire nut and attach to the L3 terminal.

The principle is to sample the voltage of the first phase at intervals (as per the normal sketch and library routines). A popular strategy for AC induction motor control is the use of variable frequency drive units, or VFDs. Then, you repeat the same process but this time you connect the first lead of the ohm-meter to lead 2, and the second lead to leads A, B, C, 3 and ground. The Delta configuration has the three phases connected like a triangle, whereas the Wye (or star”) configuration has all three loads connected at a single neutral point.

Using a 3 Phase power arrangement saves on electrical construction costs by reducing the current requirements, the required wire size, and the size of associated electrical devices. The three voltage or current waves follow each other ⅓ cycle apart, and (ideally) if you sum the currents together at any instant, they balance perfectly. Group the fourth and seventh wires together with a wire nut before attaching to the L2 terminal. As it stands, that diagram illustrates the relationship between the 3 voltages.

The high voltage line jolts the motor into starting, a job that the capacitors perform on single phase motors. Undersized wire between the motor and the power source will limit the starting and load carrying abilities of the motor. Split-phase electric power is used when three-phase power is not available and allows double the normal utilization voltage to be supplied for high-power loads. The red indicator light should only come on when the motor is starting, and should go out once the motor has reached operating speed. Connect the green grounding/bonding conductor to the motor controller enclosure.

Safety first, it should go without saying that you can be injured or killed by high voltage electric equipment or you could burn your shop down or damage equipment that you plug into a beast like this. If the voltage is the UK standard 240 V, that is, the voltage between one line and neutral, the length of the arrow represents this. Second you can simply look at the number of electrical leads coming out of the motor. I chose to simply replace the motor on my air compressor with a single-phase motor. Wire the motor for the appropriate voltage to which you are connecting the motor.

Keep in mind that the motor’s current rating is for an individual phase of AC, but if you rectify the motor conversion’s output, then you can get more DC current. That is even though the equivalent circuit of motor and transformer is same rotor of motor rotates where as secondary of transformer do not. It is an elegant concept, and quite practical in real motor control applications.