Single-phase and three-phase AC motors
The three-phase asynchronous motor is the workhorse of all industry: a stator with three windings, a cast-aluminum rotor and nothing else. No brushes, no commutator, no electronics. Understanding why it turns means understanding the rotating field.
01The rotating field
Three windings placed 120° apart in the stator, fed by the three voltages of a three-phase system, produce a magnetic field whose direction rotates at constant speed, even though nothing moves. That is Tesla's discovery and the basis of this whole topic.
The rotating field cuts the rotor bars and induces currents in them (Faraday's law). Those currents, within the same field, produce a force (the Laplace force) that drags the rotor along.
But if the rotor reached the speed of the field, there would be no relative motion, nothing would be induced and the force would vanish. That is why it always turns slightly slower: that difference is the slip, and it is what gives the motor its name, asynchronous.
| Poles | Synchronous speed at 50 Hz | Typical actual speed | Common use |
|---|---|---|---|
| 2 | 3000 rpm | 2850 a 2900 rpm | Pumps, compressors, angle grinders |
| 4 | 1500 rpm | 1420 a 1450 rpm | The most common: machine tools, fans |
| 6 | 1000 rpm | 940 a 960 rpm | Conveyor belts, mixers |
| 8 | 750 rpm | 700 a 720 rpm | Slow, high-torque loads |
A 4-pole motor at 50 Hz turning at 1440 rpm at rated load:
- ns = 120 × 50/4 = 1500 rpm
- s = (1500 − 1440)/1500 = 0.04 = 4 %
- If the motor is rated 5.5 kW, its rated torque is T = 9550 · P(kW)/n = 9550 × 5.5/1440 = 36.5 N·m
The slip grows with the load: at no load it is under 1 % and at full load it is between 3 and 6 %. Measuring the speed with a tachometer is therefore a quick way to estimate how heavily loaded a motor is.
02The nameplate and the connection
Everything you need to connect and protect a motor is on its nameplate. Reading it correctly avoids most workshop mistakes.
| Nameplate item | What it means |
|---|---|
| 380/660 V — Δ/Y | On a 380 V supply it is connected in delta, and on a 660 V one, in wye. This is the most common case in Argentina. |
| 220/380 V — Δ/Y | On 380 V it goes in wye (star). If it were connected in delta, each winding would receive 380 V instead of 220 V and burn out. |
| kW or HP | Mechanical power at the shaft, not what it draws. 1 HP = 736 W. |
| A | Rated current at full load. It is the value used to set the thermal overload relay. |
| cos φ | Power factor at full load; at no load it is much worse (AC power). |
| rpm | Speed at full load: from it you can deduce the number of poles and the slip. |
| IP 55, clase F | Degree of protection against dust and water, and thermal class of the insulation. |
| S1, S3 40 % | Duty type: continuous or intermittent with its duty cycle. |
The three windings are brought out to a terminal block with six terminals: U1-V1-W1 (starts) and U2-V2-W2 (ends), arranged so that the connection is made with three links:
- Wye: U2, V2 and W2 are joined together with two horizontal links, and the supply enters at U1, V1, W1.
- Delta: three vertical or diagonal links join U1-W2, V1-U2 and W1-V2, and the supply enters at the same three terminals.
The physical layout of the terminal block is designed so that both connections can be made with the same three links, just by changing their position. That detail is what makes wye-delta starting with contactors possible.
03The starting problem
At the instant of starting, the rotor is stationary: the slip is 100 %, the motor behaves almost like a transformer with a shorted secondary and draws 6 to 8 times its rated current for a few seconds. This causes voltage dips throughout the installation—the lights flicker—and sudden mechanical stress.
| Method | Starting current | Starting torque | When it is used |
|---|---|---|---|
| Direct | 6 a 8 · In | 100 % | Small motors (up to about 5.5 kW) or loads that demand full torque from the start. |
| Wye-delta | 2 to 2.7 · In | 33 % | Medium-size motors that start unloaded or with a light load: centrifugal pumps, fans, compressors with unloading. |
| Soft starter | Adjustable, 2 to 4 · In | Adjustable | Conveyors and pumps where avoiding mechanical shock and water hammer matters. |
| Variable-frequency drive | ≈ 1.5 · In | Up to 150 % | When speed control is also needed. Today it is the most widespread solution. |
| Autotransformer | Depends on the tap | Proportional | Large motors; rarely used since electronic starters appeared. |
This is the limit of wye-delta starting and the cause of most of its problems: if the load demands more than 33 % of the rated torque, the motor does not accelerate in wye, it stays at half speed, and when switched to delta the current shoots up again as in a direct start. That is why this method is for loads that start light, and why the changeover is made when the motor has already reached 80 or 90 % of its speed.
- Power: motor protection circuit breaker or fuses + thermal overload relay, and the three contactors KM1 (mains), KM2 (delta) and KM3 (wye). KM3 joins the three winding ends together.
- Control: start and stop pushbuttons with latching (seal-in), a timer that decides when to switch over, and the interlock between KM2 and KM3, which must never close together: it would be a dead short between phases.
- The timer includes a pause of a few tens of milliseconds between opening KM3 and closing KM2, to give the arc time to extinguish.
The components of these circuits are covered in electrical installations.
7.5 kW motor, 380 V, cos φ 0.85, efficiency 0.88 → rated current 15.2 A.
- Direct start: 6.5 × 15.2 ≈ 99 A for 2 to 5 seconds.
- Wye-delta start: one third, about 33 A.
- The protection is chosen accordingly: with a direct start, a D-curve breaker; with wye-delta a C-curve is enough.
04Reversing
All it takes is to swap any two phases to reverse the sequence and with it the direction of the rotating field. In a control panel this is done with two contactors whose outputs are crossed.
If both contactors were to close at the same time, two phases would be joined: a dead short circuit. That is why two overlapping interlocks are used:
- Electrical: the NC contact of each contactor is in series with the coil of the other.
- Mechanical: a physical piece between the two contactors prevents both from closing, even if someone makes a mistake in the wiring.
In addition, the reversal should never be made with the motor turning at full speed without going through a stop: plugging (counter-current braking) produces currents larger than those of starting.
05Single-phase motors
With a single phase there is no rotating field but a pulsating one, which does not start by itself: it pushes to one side and to the other equally. All single-phase motors are, at heart, the same solution to that problem: artificially creating a second, phase-shifted phase.
| Type | How it obtains the phase shift | Where it is found |
|---|---|---|
| Split-phase | An auxiliary winding with fewer turns and more resistance, disconnected by a centrifugal switch at 75 % of speed | Small pumps, old washing machines. Modest starting torque. |
| Capacitor-start | A large electrolytic capacitor (100 to 300 µF) in series with the auxiliary winding, which is taken out of the circuit once the motor has started | Compressors, well pumps. Good starting torque. |
| Permanent-split capacitor | A small polypropylene capacitor (4 to 16 µF) that stays connected at all times | Ceiling fans, exhaust fans. Smooth, quiet running, low starting torque. |
| Two capacitors | One for starting and one permanent | The best of both: air conditioners, compressors. |
| Shaded pole | A shorted turn around part of the pole, which delays the flux in that region | Very small motors: microwave oven fans, drain pumps. Cheap and low efficiency. |
| Universal (with commutator) | Not asynchronous: runs on AC and DC | Angle grinders, drills, blenders. High speed and power for their size, but with brushes that wear out. |
The starting one is electrolytic, high-capacitance and rated for intermittent duty: if it stays connected during normal running, it explodes. The permanent one is polypropylene, low-capacitance and designed to work on AC all the time. Replacing one with the other is a frequent mistake in repairs, and it always ends badly.
They are also the most common reason a single-phase motor hums and does not start: an open capacitor or a dirty centrifugal switch. It is easy to recognize, because the motor starts if you help it by turning it by hand—and that, besides, is dangerous.
06Protection and typical faults
A three-phase motor that loses one phase keeps running if it was already running, but the two remaining phases draw an enormous current and the motor burns out within minutes. If it was stopped, it will not start: it only hums. The thermal overload relay—which is sensitive to imbalance—and dedicated phase-loss relays exist for this.
The current stays at the starting value indefinitely. The protection has to act within seconds: hence the trip classes of the thermal overload relay (class 10, 20, 30, according to how many seconds of starting current it tolerates).
Dry bearings, an overtight belt, excessive load. The motor draws more current, heats up and the thermal relay trips. Before resetting, find the mechanical cause.
It is measured with a megohmmeter between windings and frame: below 1 MΩ the motor is on its way to failing. Moisture and conductive dust are the usual causes, which is why motors that have been in storage are dried out before being put into service.
07In the lab
With several motors from the workshop, read the nameplates and decide in each case whether on 380 V it goes in wye or in delta. Check the continuity of the three windings with the ohmmeter and set up the links on the terminal block. Before energizing, check the insulation to frame with the megohmmeter.
Measure the speed with a tachometer at no load and under load (friction brake or a generator loaded with lamps). Calculate the slip in each case and plot speed against load. Measure the current at the same time: you can see how they rise together.
With a clamp meter that has a MAX or inrush function, measure the direct-start peak and compare it with the rated current on the nameplate. Then repeat with wye-delta starting and check the reduction to one third.
Wire the complete circuit with three contactors, a timer, a thermal overload relay and pushbuttons. Test first without the motor, checking the sequence and the interlock with indicator lamps. Only once the sequence is verified, connect the motor. Measure the changeover time and adjust it to the load.
On a permanent-split capacitor motor, measure the capacitance with a capacitance meter and compare it with what the motor says. Then disconnect the capacitor and check that the motor hums without starting. Reconnect it and reverse the connections of the auxiliary winding: the motor turns the other way. This is how you reverse the rotation of a single-phase motor.
08Common mistakes
| Symptom | Usual cause |
|---|---|
| The motor hums but does not start (three-phase) | A phase is missing: blown fuse, loose contact or a stuck contactor pole. |
| The motor hums but does not start (single-phase) | Open starting capacitor or a dirty or stuck centrifugal switch. |
| Starts with difficulty and the thermal relay trips | A load that demands torque is connected in wye: in wye the torque is one third. Or excessive mechanical load. |
| Burned out when connected | Wrong connection: a 220/380 motor connected in delta on 380 V receives √3 times the voltage it should. |
| Turns the wrong way | Reversed phase sequence. Swap any two conductors. |
| The speed is lower than the nameplate speed | This is normal: the nameplate gives the speed under load, already including slip. If it is much lower, the motor is overloaded. |
| The protection trips when switching to delta | Premature changeover: the motor was still slow. Lengthen the timer setting. |
| Runs hot even though the current is normal | Blocked ventilation, high ambient temperature, or intermittent-duty rating used as continuous. |
| The drive trips on overcurrent at start | Acceleration ramp too short for the inertia of the load. |
09Self-assessment
What is the synchronous speed of a 6-pole motor at 50 Hz?
ns = 120 × 50/6 = 1000 rpm. Its actual speed under load will be around 950 rpm.
A 4-pole motor turns at 1455 rpm: what is its slip?
ns = 1500 rpm; s = (1500 − 1455)/1500 = 0.03 = 3 %.
Why does an asynchronous motor never reach the speed of the field?
Because if it did, there would be no relative motion between the field and the rotor bars: no currents would be induced and the force would vanish. The motor needs to lag behind in order to produce torque.
A motor reads “380/660 V — Δ/Y.” How is it connected on an Argentine supply?
In delta, because the supply is 380 V between phases. The wye connection of that motor is intended for 660 V supplies, and it is also the one used temporarily during wye-delta starting.
Why does wye-delta starting reduce the current to one third?
Because in wye each winding receives the line voltage divided by √3, which reduces its current by that factor, and in addition the line current is equal to the phase current. The combined effect is a factor of 3 in the line current. The torque, which goes with the square of the voltage, also falls to one third.
In which cases is wye-delta starting not suitable?
When the load demands more than 33 % of the rated torque to start: the motor does not accelerate in wye and on switching it produces a peak equal to that of a direct start. It is also not suitable if the motor does not have all six terminals accessible.
What direct-start current can be expected from a motor with a 10 A rating?
Between 60 and 80 A for a few seconds. That is why the magnetic protection must be a C or D curve: a B curve would trip on every start.
Why does a single-phase motor not start by itself?
Because with a single winding the field is pulsating and not rotating: it pushes equally in both directions. A second winding with a phase-shifted current is needed—by resistance, by capacitor or by shading coil—to create a field that rotates.
What happens if a phase is lost on a running three-phase motor?
It keeps turning with less torque, but the two remaining phases draw a current far above the rated value and the winding burns out within a few minutes. Stopped, it will not start: it only hums. That is why the thermal overload relay is sensitive to phase imbalance.
How do you reverse the rotation of a single-phase capacitor motor?
By reversing the connections of the auxiliary winding relative to the main one (not those of the supply, which would change nothing). In a three-phase motor, on the other hand, it is enough to swap two phases.