Why Does an Electric Motor Run Backwards? How to Change the Direction of Rotation

A three-phase motor runs backwards because the phase sequence of the supply does not reach the motor terminals in the expected order; in most cases there is nothing wrong with the motor. The direction of the rotating field is set by the order in which phases L1-L2-L3 are connected to terminals U1-V1-W1. To reverse it, switch off the power and swap any two phases; the third phase stays where it is, and the terminal links are left exactly as they are. All 120 three-phase models in our Buy Electric Motors Online store are reversed in this way.

Reverse rotation usually appears after a change: a new motor is installed, a panel or cable is replaced, work is done on the transformer or main switchboard, the machine is transferred to a generator, or, with plug connections, the socket and plug are wired in a different order. In systems running on a variable speed drive, the direction is set not by the cables but by the drive's direction parameter or direction input.

So when "it ran the right way yesterday and backwards today", look first at the last job done. A motor running in reverse draws the same current and turns at the same speed as in the correct direction; measuring instruments will not reveal the problem. It is the machine that gives it away: the pump does not deliver, the fan moves little air, the auger pushes material back.

How does phase sequence set the rotating field?

In a three-phase supply the voltages are shifted 120° from one another. When the three stator windings are fed in this order, the magnetic field rotates inside the frame and the rotor follows it. At 50 Hz the field turns at 3000 rpm in a 2-pole motor and 1500 rpm in a 4-pole motor; the rotor lags slightly behind. For example, the DRG-112M-2 runs at a rated speed of 2905 rpm and the DRG-112M-4 at 1450 rpm.

When any two phases are swapped, the sequence changes from L1-L2-L3 to L1-L3-L2, the field starts rotating the other way and the rotor follows. Shifting all three phases (L1→L2, L2→L3, L3→L1) does not change the sequence; the motor keeps turning the same way. This is usually why two people on site say "I swapped the phases but nothing changed": one swapped them, the other swapped them back, or all three were shifted at once.

Which direction is the "correct" one?

According to IEC 60034-8, when L1, L2, L3 are connected to U1, V1, W1 respectively, the motor rotates clockwise when viewed from the shaft end. The most common reading error on site is judging direction from the fan cover side: viewed from the rear, the same rotation looks anticlockwise. The direction a machine needs is a property of the machine, not the motor; the arrow on the pump, fan or compressor casing is what counts.

Causes of reverse rotation on site

• A new motor was installed: the cables were connected to the terminals in a different order, or the old motor was already running with two phases swapped.
• The panel or cable was replaced: the colour code order was not kept at the panel output.
• A change on the supply side: after work on the transformer, main switchboard or meter, the site's phase sequence changed; in that case every motor on site runs backwards at once.
• Generator or temporary supply: the generator's phase sequence differs from the mains.
• Plug connection: the socket or plug was wired in a different order; the machine runs correctly on one socket and backwards on another.
• Drive-fed system: the direction parameter, the direction input, or the default setting after a drive replacement.

Note: if one phase is missing, the motor does not run backwards; it fails to start, hums and trips the overload relay. Reverse rotation shows that all three phases are present but in a different order.

What happens to the machine if it runs backwards?

The motor can turn either way; what usually suffers is the driven machine. That is why direction must be verified before the machine is put under load.

With the 2-pole models most often used on pumps and fans (for example the DRG-90L-2 and DRG-100L-2), reverse rotation can go unnoticed for a long time, because the pump still delivers a little. If the pressure gauge reads lower than expected, the first thing to check is the direction of rotation.

How to change the direction of rotation

On a direct-on-line motor

• Stop the motor, switch off the isolator in the panel, lock it and tag it.
• Before opening the terminal box, verify absence of voltage phase to phase and phase to earth.
• Swap only two of the L1, L2, L3 cables (for example those going to U1 and V1). Do not touch the protective earth (PE) conductor.
• Do not change the star or delta links; there is no need to touch them to reverse direction. For the link layout, see our article Motor Terminal Connection: Star and Delta Wiring.
• Close the cover, tighten the gland and check the direction with a short bump.

On a star-delta started motor

In a star-delta circuit six cables go to the motor. Swapping two phases only on the U1-V1-W1 side while leaving the W2-U2-V2 side unchanged is wrong: when switching to delta, both ends of one winding end up on the same phase and the winding is fed incorrectly. The safest way is to make the change on the supply side of the panel, i.e. at the input that feeds both the main contactor and the delta contactor. That way both lead groups move to the correct sequence at the same time.

On a drive-fed system

On a motor running from a drive, direction is changed by parameter (direction of rotation / forward-reverse) or by the direction input. Swapping two cables at the drive's motor output also reverses the motor, but then the direction shown on the drive display no longer matches the real direction, which misleads maintenance staff. On pumps and fans, keeping the drive's reverse lock enabled, where available, prevents reverse running from a wrong command.

Contactors and starting current on a reversing motor

On machines that must run in both directions, such as conveyors, doors and hoists, a two-contactor forward-reverse circuit is used: one contactor feeds the phases straight, the other with two phases swapped. The contactors must have both electrical and mechanical interlocking; if both close at the same time, two phases are short-circuited. When reversing, wait for the motor to stop; never give a reverse command while it is turning. Reversing a running motor (plugging) is harder on it than a normal start.

Every reversal is a new start. The table below shows the approximate starting current of some models in our store, calculated from rated current and starting current ratio (In × Is/In). You can check contactor and overload relay selection against these values with our overload relay, fuse and contactor selection tool; for currents of other powers, see the motor current chart.

Example: for the 4 kW, 1500 rpm DRG-112M-4, 8.37 A × 7.8 ≈ 65 A. On a machine that reverses many times per hour, these current pulses heat the motor; in such duty, reversing should be done with a drive.

Direction checklist before commissioning

• Find the arrow on the machine casing; if there is none, consult the machine manufacturer's documentation.
• If possible, disconnect the coupling or belt and test the motor without load.
• If you have a phase sequence tester, check the supply sequence beforehand; use a phase sequence relay on machines such as compressors.
• Mark the shaft end with chalk or tape; make the first start a one-second bump.
• Note whether you are reading the direction from the shaft side or the fan cover side.
• Never run a pump dry or a compressor backwards for long.
• Once the direction is correct, photograph the wiring and add it to the panel documentation.
• All work in the terminal box and the panel must be done by a qualified electrician with the power switched off and locked out.

Suitable motors for pumps, fans and conveyors

For pumps and fans 2-pole motors are the usual choice; for geared conveyors, 4-pole motors. All powers and speeds are listed among the 120 models in our online store.

• DRG-90L-2 – 2.2 kW – 3000 rpm – frame 90L, cast iron
• DRG-100L-2 – 3 kW – 3000 rpm – frame 100L, cast iron
• DRG-112M-2 – 4 kW – 3000 rpm – frame 112M, cast iron
• DRG-132S2-2 – 7.5 kW – 3000 rpm – frame 132S, cast iron
• DRG-112M-4 – 4 kW – 1500 rpm – frame 112M, cast iron
• DRG-160M-4 – 11 kW – 1500 rpm – frame 160M, cast iron

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Frequently Asked Questions

How do you change the direction of a three-phase motor?

After switching off and locking out the power, swap any two of the three phase cables. The links and the earth connection stay unchanged. In a star-delta circuit, the change is made at the panel supply.

Why does a motor suddenly start running backwards?

A motor does not change direction on its own; something has changed in between. Work on the panel, cable, plug and socket, generator or supply side may have altered the phase sequence. If every motor on site runs backwards at once, the cause is on the supply side.

Will a motor running backwards burn out?

The motor draws the same current in both directions; running in reverse does not burn it out by itself. The real risk is to the machine: pumps, compressors and augers can be damaged, and if the pump is overloaded the motor may also draw excessive current.

Does a motor rotate clockwise?

A motor with L1-L2-L3 connected in that order to U1-V1-W1 rotates clockwise when viewed from the shaft end. Viewed from the fan cover side, the same rotation appears reversed.

If you would like help choosing the right power and speed for your application, contact us: +90 (542) 666 11 11, [email protected] or our contact page. You can browse the models directly on the Buy Electric Motors Online page.