Whelco Industrial, Ltd.
Industrial Motor & Electrical Apparatus Repair
28210 Cedar Park Boulevard
Perrysburg, OH 43551

AC Motors

AC motors- are the most common motors used in industrial motion control systems as well as in home appliances. They are simple in design, rugged, dependable and relatively inexpensive to make and maintain. They can be connected directly to an AC power source. Although they are easier to design than DC motors, the speed and torque control of AC motors is more complicated and requires a special understanding of the design and characteristics of these motors. Variable speed is achieved through the use of AC drives.

Primary AC motor parts:

Stator – is the fixed outer portion of the motor which houses the rotating magnetic field. Groups of insulated wire are wound into slots of a hollow iron cylinder. Each group of wires (coils) forms an electromagnet when energized by an AC supply. These electromagnets are connected in such a way as to create a rotating magnetic field within the motor.     

Rotor – is the rotating inner part of the motor. The rotor chases the rotating magnetic field of the stator.

1. Squirrel Cage Rotor – consists of a number of copper bars pushed into the rotor slots which are connected by end rings.
2. Wound Rotor – uses insulation covered with wires in the rotor slots connected by slip rings.

AC motor types:

1. Single-Phase Induction Motors – are more common than all the other types put together. They are the least expensive, lowest maintenance motors available. They are used in countless residential and commercial applications. They are typically fractional horsepower units used where three phase power is not available. This motor has only one stator winding and operates on a single-phase power supply. The rotor is always a squirrel cage construction. There is a main winding and an auxiliary winding for starting the motor which is disconnected by a centrifugal switch when the motor reaches about 75% of synchronous speed. Generally used for applications up to about 3/4hp.
   
1. Split-Phase AC Induction Motor – is also known as an Induction Start/Induction Run Motor. It has two windings; a start winding with a small number of turns of fine wire relative to the main winding to create more resistance and therefore a phase shift at start-up. This causes the motor to start turning. With low starting torque these motors are good for applications such as small fans and blowers.
2. Capacitor Start AC Induction Motor – has a capacitor in series with the auxiliary winding which creates a larger phase shift between the auxiliary and main winding for increased starting torque and lower starting current draw. A variation on this motor is the resistance start motor which replaces the capacitor with a resistor. This motor is somewhat less expensive but provides less starting torque. These motors are used for a wide range of belt drive applications such as small conveyors, large blowers, pumps and geared applications.
   
3. Capacitor Run AC Induction Motor- has a capacitor permanently connected in series with the start winding. There is no centrifugal switch so the start winding becomes an auxiliary winding once the motor reaches running speed making it essentially a two phase motor. It does not have the starting boost of a Capacitor Start Motor but does have low starting current which makes it ideal for applications with high on/off cycle rates and fixed loads. They’re considered to be the most reliable of the single-phase motors because there is no centrifugal starting switch. They’re ideal for applications where noise is a factor because they also run very quietly. These motors can be found on fans, blowers with low starting torque and intermittent cycling times, gate operators and garage door openers.
   
4. Capacitor Start/Capacitor Run AC Induction Motor- has both a capacitor in series with the auxiliary winding for high starting torque and also a run type capacitor in series with the auxiliary winding which is enabled after the starting capacitor is switched out of the circuit while the motor is running. This allows for a very high overload torque when required. Due to the inclusion of both start and run capacitors along with a centrifugal switch, these motors are fairly costly. Able to handle applications too demanding for other single phase motors, they can be found on wood working machines, air compressors, high-pressure pumps and other high torque applications from 1 to 10 hp.
   
5. Shaded-Pole AC Induction Motor – has only a main winding and no start winding. Starting is achieved by means of a design that utilizes a copper ring around a portion each of the motor poles. The shaded portion of the windings lags the unshaded portion and the reaction of the two starts the shaft rotating. These motors are simple and inexpensive with the speed controlled merely by varying the voltage. They are considered disposable motors as they are cheaper to make than repair. Overall they have low starting torque and high slip with a running speed 7 to 10% below the synchronous speed- resulting in low efficiency. Primarily they are found in inexpensive multi-speed fans for household use.                                                                          
   
   
2. AC Synchronous Motors – are motors in which AC voltage is applied to the stator and DC voltage is applied to the rotor. The stator construction is identical to a 3-phase Induction motor and the number of stator poles equals the number of rotor poles. The DC is applied to salient poles. Squirrel cage bars (damper windings) allow a Synchronous Motor to start as an Induction Motor. The motor runs at synchronous speed with the rotating stator field. The DC voltage can be applied via slip rings or by rectifying the output of an attached AC alternator. DC excitation to the rotor at near synchronous speeds forms north and south poles which eventually lock into step with the stator. The torque developed is called “pull-in torque”. The applied load causes a lagging power factor due to the rotor falling slightly behind the stator rotating field. Too large a load can cause a break in the attraction between stator and rotor called “pull-out torque”, when this occurs large currents are generated which are bad for the motor. Synchronous Motors provide much more torque at low speeds than Induction Motors.
   
   They can be used for power factor correction in factories by connecting across the supply. The power factor of the motor is varied by adjusting the field excitation and can be made to behave like a capacitor when over excited. Most motor loads are inductive. When rotor current > stator current the motor has the semblance of a capacitor which helps nullify the effects of the other induction motors in the plant and therefore lowers the utility bill. This is known as a synchronous capacitor and usually occurs when the motor is running with no load.
   
   AC Synchronous Motors are capable of very precise, constant speeds. They are typically in the 200- 20,000 hp range and run at speeds of 150- 1800 rpm; or they are fractional hp machines.
   
   
3. Stepper Motors – are used for very precise controlled positioning of loads. They move in discreet steps. The number of poles on the stator never equals the number of poles on the rotor. Voltage “pulses” are applied to the windings of the motor stator. The speed of the motor is referred to as the “stepping rate” and is limited by the inertia of the rotor and the amount of load. Power is applied to one set of coils in an amount that keeps the motor aligned at one point called the “holding torque”. Stepper motors are generally open-looped as opposed to relying on a feedback device for positioning.
   
   1. Permanent Magnet type – has a rotor comprised of magnets with permanent north-south poles.
      
   2. Variable Reluctance – has a toothed steel rotor that is attracted to the   stator poles.
      
   3. Hybrid Stepper – has two toothed soft iron rotors with permanent magnets in between.
      
      Stepper motors have a low starting current and can achieve a low speed without gear reduction by merely adjusting the pulse rate. They have maximum torque at low pulse rates and are available in a wide range of step angles. Multiple motors can be synchronized from the same source. They tend to oscillate somewhat due to inertia and their motion is not entirely smooth. Computer disc drives are examples of Stepper Motors.
      
      
4. Three (3) Phase Motors – are the standard for industrial use. They are simple, low-priced, rugged and easy to maintain. They are somewhat constant speed but can be made variable speed through the use of AC Drives. Before the development of variable speed AC Drives the DC motor was the motor of choice in industrial applications, however due to their cost of construction, the AC motor has become the motor of choice where AC power is available.
   
   These motors are self-starting and use no capacitor, start winding, centrifugal switch or other starting device. Induction motors at rest appear just like a short circuited transformer. If connected to the full supply voltage they will draw a very high current known as Locked Rotor Current (LRC) and also will produce a torque known as the Locked Rotor Torque (LRT). These are a function of the terminal voltage of the motor and the motor design. As the motor accelerates, both the current and torque of the motor will tend to alter with the rotor speed if the voltage is kept constant. The starting current of a motor with fixed voltage will drop very slowly as the motor accelerates and the general trend is for high current until the motor has reached nearly full speed. These motors produce medium to high degrees of starting torque. The starting torque will drop a little to the minimum or “pull-up torque” initially, then as the motor continues to accelerate it rises to the maximum torque or “pull out torque” at nearly full speed. At synchronous speed torque drops to near zero. The Power capabilities and efficiency in Three Phase Motors exceeds that of their single phase counterparts. They utilize a fixed wire wound stator and rotor:
   
   1. Squirrel Cage Rotor motor– has a rotor constructed of copper, aluminum or alloy bars pushed into the rotor slots and permanently short circuited at both ends by end rings. The rotor slots are not exactly parallel to the shaft to help reduce the locking tendency of the rotor when the number of rotor poles = stator poles, and also to make the motor run more quietly by reducing magnetic hum. Almost 90% of induction motors have squirrel cage rotors due to their simple, rugged construction. They cost less and can start heavier loads than their single-phase counterparts.  
      
   2. Wound Rotor motor – is constructed of insulated rotor slots that are covered with wire and connected by slip rings. The windings are not short circuited together which is helpful in adding external resistors and contactors. The maximum torque of these motors is directly proportional to the rotor resistance. Rotor resistance can be increased by adding external resistance via the slip rings. Brushes are used to connect the resistors to the slip rings. A particularly high resistance can result in maximum torque occurring at almost zero speed resulting in very low starting current.  As the motor accelerates, the value of the resistance can be reduced to suit the load requirement. Once the motor reaches base speed the external resistors can be removed from the rotor circuit resulting in the motor working like a standard induction motor. Slip ring motors need regular maintenance to the slip rings and brush assemblies which are a cost not applicable to Squirrel Cage Motors. Modifying the speed/torque curve by altering the rotor resistors, you can alter the speed at which the motor will drive a particular load down to about 50% of the synchronous speed. When run at speeds less than 50% of the motor synchronous speed the motor exhibits very poor efficiency due to higher power dissipation in the rotor resistances. This type of motor is ideal for very high inertia loads which need to accelerate to full speed in minimum time with minimum current draw.     
      
   3. Permanent Magnet Rotor motor – is constructed of magnetic material bonded to the rotor core. A resi-glas banding or protective coating is usually applied to the rotor to protect the magnets from chips, cracks or corrosion. Permanent Magnet Rotors are the construction of choice for Three Phase Servo Motors due to their unique characteristics. The number of poles on the rotor must be matched by the same number of magnetic poles in the stator. If the rotor is rotated in the motor through the use of a separate source it will generate a voltage in the stator windings caused by the magnetic flux between stator and rotor magnets, in effect acting like an AC alternator with three voltage outputs 120 degrees apart. If a DC voltage is applied to the stator windings the rotor magnetic poles will lock in position with the stator poles of opposite polarity.
      
      The stator field in Three Phase Motors moves at synchronous speed which is synchronized to the input frequency. By increasing the number of poles in the stator the synchronous speed of the motor can be reduced; only one pole pair is energized at a time: Ns = (120 x input frequency)/ number of poles.
      1. For a 6 pole motor running at 60Hz: (120 x 60)/6 = 1200 rpm.
      2. For a 12 pole motor running at 60Hz: (120 x 60)/12 = 600 rpm.

By reversing any two of the leads on a 3 Phase Motor you can reverse the motor’s rotation. 3 Phase Motors are widely used in industrial and commercial applications such as grinders, lathes, drill presses, pumps, compressors, conveyors, printing equipment, farm equipment, electronic cooling, robotics and other mechanical duty applications.