In order for a motor to rotate, a torque must be applied to the rotor. The direction of that torque is what determines the direction of rotation; i.e., clockwise or counterclockwise. In both AC and DC motors, the torque is created by a magnetic field. Among the many styles of AC and DC motors, there are many techniques for creating and maintaining the rotational torque. In all cases, the magnetic field is created by an electric current flowing in coils in the motor. DC Motors are actually complex mechanical and electrical devices; generally, more complex than AC motors. This is because, since DC voltages do not have cyclic variation, DC voltages do not maintain a ongoing inductive coupling between the field and armature. Creating a consistent magnetic field in DC motors requires mechanical complexity that is not necessary in AC Motors.
The rotating part of a motor is called an “armature” in DC motors, and a “rotor” in AC motors. The terms both refer to the same thing, and are often used interchangeably. I mention it in case I lapse from one to the other in the descriptions that follow. Most motors are built with multiple independent field coils and multiple independent armature coils. These separate coils are electrically arranged in pairs or groups. In common discussion, these coils – or windings – are normally discussed for their electrical function, as if the collection of separate coils were a single entity. The distinction between the mechanical and electrical detail only matters when actually discussing actual motor construction. DC motor varieties in particular have complex schemes of the internal interconnection of the windings, depending on speed and starting torque design requirements.
Series-wound DC motor types have their field coil(s) wired in series with the armature coils. Ordinary 12/24/32 volt gasoline and diesel engine starter motors are an example. DC series-wound motors are used in applications where the motor must spin-up against a significant mechanical pre-load. These motors must spin-up to operating speed while powering that mechanical load; operating speed can be highly variable. Series-wound DC motors that are not pre-loaded can and will reach “run-away” rotational speeds that can lead to physical self-destruction.
Parallel-wound DC motor types have their field coil(s) wired in parallel with the armature. These motor will not generally start against a pre-load. These motors are used in applications where load is applied after the motor is spinning at full speed, and/or where operating torque requirements are low. Starting torques are generally low-to-moderate. Fans and blowers might be a useful example.
A hybrid design of the series-wound and parallel-wound DC motor is one where some field coils are wired in series with the armature and others are wired in parallel with the armature in the same motor. This are known as series-parallel motors. These motors have the combined advantage of moderate-to-high starting torque and smooth running characteristics. They do not have the same speed range flexibility of the series-wound motor, and do not run away when unloaded.
In DC motors, a “commutator” is the internal mechanical means through which DC voltage is applied to the armature windings (coils of wire) of the motor. Multiple individual electric coils are assembled around the perimeter of the armature frame. The electrical connections to the individual coils emerge to terminals on the commutator assembly. As the armature rotates, the individual armature coils are sequentially connected and disconnected by carbon-brushes that ride on face of the commutator terminals. The more coils in an armature, the more evenly the motor will run. The torque of the armature is maintained by the offset of the magnetic field of the armature winding and the magnetic field of the field windings. That offset and the strength of the magnetic field determine total torque. On fractional and small horsepower DC Motors, these coils are generally arranged in groups of two. On large and very large HP motors, they are generally arranged in groups of four or more. Some large DC motors have brush racks that are adjustable, which allows torque to be adjustable through a limited range.
DC motors can often reverse rotational direction (run backwards) based on the offset of the magnetic field. DC Motors can also reverse rotational direction if the polarity of the applied voltage is reversed. Generally, reversing voltage polarity is simpler than designing and building an mechanically adjustable brush rack. Adjustable brush racks can be used to vary the speed of rotation of the motor itself; i.e., the more offset, the more torque, and the more torque, the greater the speed.
All motors have specification ratings for “start-current” and “run-current.” The reason is an interesting electrical phenomena that occurs withing the windings of the motor. When a DC motor starts, it accelerates from stand-still (also called “stall,” or “locked-rotor”) to it’s intended rotational speed. At the instant that power is applied, the amount of current that flows in the motor circuit is predicted by Ohm’s Law. However, during the period of armature acceleration, there is a progressively increasing DC voltage “generated” in the armature winding as a result of it’s rotation through the magnetic field of the field coils. This is the exact phenomenon that is desired in the case of a generator. The induced voltage is known as “back EMF.” Back-EMF voltages are opposite in polarity to the voltage that is applied to make the motor turn in the first place. As the rotational speed of the armature increases, the applied voltage and the back EMF voltage “cancel” one another to result in the net current flowing in the armature. That net current is what produces the motor’s operational output torque. This phenomenon explains why DC motors draw many hundreds of amps when starting, referred to as “locked-rotor.” As the armature accelerates in the magnetism of the field windings, and the back-EMF voltage rises, DC current flowing in the armature settles to much lower levels.
The carbon brushes that ride on commutator terminals, and the commutator terminals themselves, experience mechanical wear and electric arching. Brushes require periodic replacement. The most common failure is that the motor will not start to rotate. In gasoline and diesel engine starter motor applications, the starter solenoid will be heard to pick, or click, but the starter motor will not start to turn. The larger symptom is, “the engine doesn’t crank.” In this case, the starter motor brushes may be worn to the point that one or more is not quite in contact with the commutator. An emergency action – worth trying – is to firmly tap the starter motor with a hammer or similar heavy object. Don’t beat it to death, just give it a few firm taps. Then, retry. The taps may re-set the brushes in their rack, and allow the starter motor to spin-up. Assuming the engine does start, make arrangements for immediate servicing of the starter motor. This procedure, if it works, will only work a couple of times!
Carbon brushes are widely available as generic maintenance items at many hardware, auto parts and specialty shops. Carbon brushes for small power tools can be obtained from the tool manufacturer, usually at premium pricing. Commutator re-surfacing is sometimes required, although at much longer service intervals than brush replacement. Commutator re-surfacing will require dis-assembly of the motor, and machine tooling. This service is generally available at any alternator/generator specialty shop.
All AC motors act like transformers. Compared to DC motors, AC motors are electrically and mechanically rather simple devices. Three-phase AC motors are by far the simplest of all. The very nature of three-phase AC creates a rotating magnetic field in a motor armature. The AC current in the armature induces a voltage in the rotating field. These motors will reverse rotational direction (clockwise or counterclockwise) if any two of the three-phases are reversed. This is usually accomplished with a controller, not physical re-wiring of the machine.
Small single-phase AC motors (split-phase) do not have/need commutators or slip rings, and depend entirely on this transformer-like induction to create the magnetic fields that result in torque and therefore, rotation. The field coil is built with a pre-set physical offset from the armature coil, and thus, the induced magnetic field causes rotational torque. These motors will not “run backwards.” The torque characteristic of a split phase AC Motor is determined by the mechanical offset the designer establishes between the armature coils and the rotor coils. Generally, these motors do not generate high starting torque. These motors start slowly, and bog down against load. These motors are cheap to build. They are a real workhorse when torque-matched to the correct applications. Mechanically overloading these motors simply stalls them.
Fractional and small horsepower motors that have to start against light to moderate pre-loads are made with a second field coil that is only connected during starting. When starting most shop tool motors under no-load, you can sometimes hear a centrifugal switch kick in (or out, when it’s spinning down). That switch connects and disconnects the start coil, which has a larger physical offset and will also draw more current than the steady-state running current draw.
A very common design for applications requiring moderate to high start and run torque in single-phase motors is the use of capacitors to electrically “adjust” the phase-offset timing of the current component flowing in the motor winding. This is advanced AC theory, but basically, Ohm’s law is slightly modified in all AC circuits. In Ohm’s Law, the resistive factor in AC circuits is really “impedance.” Impedance has both inductive and capacitive components. When a capacitor (or two, one for the start winding and one for the run winding) is added in series to a motor winding, that capacitor change the fundamentally inductive character of the motor winding. Capacitors change when, during the AC sine-wave cycle, the AC current flows with respect to the applied AC voltage. (AC current is *only* in-phase with the applied AC voltage in resistive loads, like light bulbs. This is the concept of “power factor.”) Suffice it to say, capacitors modify the otherwise mechanical offset of the magnetic field, and thus, the torque characteristics of the motor. This is particularly common in air conditioning compressors, where the AC motor must start against a moderate to high static pre-load back pressures. Capacitor-start, capacitor-run motors are used where high starting torques are needed.
The most common failure of fractional and small horsepower AC motors is seizing of the motor bearings. This results in the motor overheating, and slow or no rotation. The best prevention is to follow the manufacturer recommendation for lubricating the bearings. Do not over-lubricate these bearings. Use only lubricating oils recommended by the manufacturer. Some oils get sticky as they age, and will only aggravate the problem. These motors almost always can be refurbished by an AC motor repair specialty shop. Such shops are widely available in most communities. Replacement units can be ordered from manufacturers, assuming the OEM part is still available when you need it, but almost certainly at premium prices. Generic replacement motors are also available, but can require custom mechanical fabrication at the job-site to install.
There is a special case of motor commonly called a “Universal” motor. These motors run on both AC and DC. They employ a commutator, and have very high starting torques for their size. They operate across a wide and variable speed range. The penalty they pay is they draw a lot of current proportional to their size, and generate a great deal of heat for their size in operation. They are rated for intermittent duty-cycle applications, not for continuous use. They are commonly used in household and kitchen appliance and hand-tool applications. Think blender, vacuum cleaner and electric drill applications.