Perhaps you've just installed a drop-in replacement motor, or maybe you're installing a new power train. You turn the electric motor on and ... its's rotating in the wrong direction! What the heck is going on? Is there something you can do to reverse my electric motor?
The answer is yes -- most of the time there is. The first step to figuring out how to fix your rotation problems is to establish whether it is a AC or DC motor. From there, the solution depends on exactly what type of motor you are working with.
AC Induction Motor
If you have an AC induction motor, you need to establish whether it is a three phase or single phase motor before making an attempt to change the direction of rotation.
Three-phase AC induction motors are the most commonly used type of motor in industrial applications. This is primarily because they are highly efficient and, relative to single-phase, they cost loss.
Three-phase AC motors have a rotating magnetic field that causes the rotor to spin in a specific direction. If the motor is running the wrong direction, that means it is not in the correct phase sequence. This happens to be an easy fix: all you need to do is swap any two the power leads to flip/reverse the magnetic field, and the most common practice is to switch lines 1 and 3. Once this is done, the motor should be running in the right direction. If you have more than 3 leads, it may take a bit more. Consider looking at the connection diagrams supplied with the unit.
Single-phase AC motors only have one voltage waveform applied to the motor. They are not as efficient as their three-phase counterparts, but are still widely used.Like the three-phase induction motors, the direction of the rotating magnetic field is what determines which way the motor runs.
However, single-phase AC induction motors are a bit more complicated to fix if they are running in the wrong direction. To change/reverse that direction, you need to reverse the polarity of the starting winding.
You can find directions from the manufacturer on how to do that for your specific -- unless your motor is labeled non-reversible. In that case, its not that the polarity of the starting winding can't be changed but the wires you need to access are housed within the motor. Unless you really know your way around an AC motor, it's best to leave this task to the professionals.
There are three basic types of DC motors: shunt wound, series wound, and compound wound. While their direction can be reversed in a fairly straightforward manner, its best to know what type of DC motor you're working with before you get started.
Shunt wound motors
In a DC shunt wound motor (or just DC shunt motor) the field windings are shunted (connected in parallel) to the armature winding. Because of this, the armature and field winding are exposed to the same supply voltage and part of the current passing through the field winding and the other part through the armature winding. The magnetic field flux in these motors is practically constant, which is why they are referred to as constant flux motors, and they are able to regulate their own speed so that it is almost constant.
Series wound motors
Series wound DC motors, as the name implies, have the field windings and armature winding connected internally in series so that they both receive the same current. As a result of this design, the field windings in these motors receive more current than they do in other types of DC motors.
What makes these motors special is the high torque than can produce. This high torque makes them useful as a starter motor, often running for just a short period of time. Unlike the DC shunt wound motor, the series wound motor is unable to regulate its own speed.
Compound wound motors
A compound wound DC motor combines the design of both shunt and series wound DC motors into one. The result is good speed regulation and a high starting torque. However, the speed is not quite as well regulated as a shunt wound motor, nor is the torque quite as high as a series wound motor.
There are two basic types of compound wound DC motors: long shunt compound wound and short shunt compound wound. The long shunt motor has the shunt field winding connecting in parallel across the armature and series field coil. In this case, the speed regulation is better.
The short shunt motor is a little different: the shunt field winding is connected in parallel across only the armature winding. In addition, the series field coil receives the entire supply current before it is split into the shunt and armature field currents. This results in better starting torque.
Fixing rotation problems for DC motors
DC motors, like AC motors, can be configured to turn in either direction. Their direction can be easily controlled by inverting the polarity of the applied armature voltage by reversing the armature leads. This works with shunt, series, and compound wound DC motors.
On the other hand, you can also reverse the field leads but this is risky: doing so can affect the stability of your DC motor.
Having an electric motor running in the wrong direction isn't the end of the world. For DC motors, changing direction simply involves reversing the armature leads. For an AC three-phase motor, you need to swap any two of the power leads (usually 1 and 3 are chosen), while for a single-phase motor you will need to refer to the manufacturer instructions or seek the assistance of a certified electric motor technician. We just happen to know of an electric motor repair shop that can help.