Friday, November 5, 2010

Determining Which Motor

Determining Which Motor

Determining which type motor you need may not be an easy task. There are many different types available today. Before you order, there are a number of parameters that need to be addressed. So how can you properly accomplish this? This article is written to assist you in determining which motor is best for your application.
First and foremost you will need to know what voltage source is available in your application. Electric motors can be classified as either AC (Alternating Current) or DC (Direct Current). Alternating current types only run on AC Voltage and direct current types only run on DC Voltage. There is also a universal motor that can run on both AC and DC voltages.
Once you have established which power source you have you will need to determine which style will work for your application. AC motors can be sub-divided into the following: Single Phase Induction, Three Phase Induction, Two Phase Servo, and Hysteresis Synchronous. DC motors can be sub-divided into: Brushless DC, Brush DC, and Stepper types.
Next we need to understand the different characteristics of each type in order to properly match a motor to its application.
A single phase induction motor is connected to a single voltage line. An external capacitor is required to make this motor operate. The different types of single phase induction motors are distinguished by which method they are started. The four basic types are: split phase, capacitor start, permanent split capacitor, and capacitor start/capacitor run.
A split phase motor uses a switching device to disconnect the start winding once the motor gets to 75% of its rated speed. Although this type has a simple design which makes it less expensive for commercial use, it also has low starting torques and high starting currents.
The capacitor start motor is basically a split phase capacitor motor with a capacitor in series with the starting winding to create more starting torque. This motor is more expensive on account of the switching and capacitor requirement.
A permanent split capacitor motor does not have any staring switch. For this type, a capacitor is permanently connected to the starting winding. Since this capacitor is required for continuous use, it does not provide starting power, therefore starting torques are typically low. These motors are not recommended for heavy starting load applications. However, they do have low starting currents, quieter operation, and higher life/reliability, thereby making them a good choice for high cycle rates. They are also the most reliable capacitor motor on account of not having a starting switch. They can also be designed for higher efficiencies and power factor at rated loads.
The capacitor start/capacitor run motor has both a start and run capacitor in the circuit. The start capacitor is switched out once achieving start-up. This type of motor has higher starting, lower loaded currents, and higher efficiency. The drawback is the expense that's required for two capacitors and a switching device. Reliability also plays a factor on account of the switching mechanism.
The three phase induction motor is wound for three phase alternating voltage. These are the simplest and most rugged electric motors available. The motor could be designed for either DELTA or WYE hook-up. This type is designed for continuous use and high starting torques. Motor speed is relatively constant. If three phase voltage is available this is the motor to choose.
Two phase servo motors are used in servo systems, hence the name. They are very sensitive to voltage variations on the control phase. This style requires two voltages in 90 degrees phase shift from each other in order to produce a rotating magnetic field. Servo motors have high torque to inertia ratio, high speed and works well for velocity control applications. Tachometer feedback devices can be supplied with these motors.
Hysteresis synchronous motors are basically induction motors that run at synchronous speed. When your application requires synchronous speeds this is the best choice. These motors can be designed for either single phase or three phase. For single phase voltage a capacitor will be required. Hysteresis synchronous motors develop what's known as pull-out and pull-in torques. Pull-out torque is the amount of torque/load the motor can handle just as it pull out of synchronous speed. Pull-in torque is the amount of torque on the output shaft that allows the motor to pull into synchronism and stay there. Both pull-in and pull out torques are very similar. These motors have low starting currents and low vibration. Since the rotor assembly is made from a cobalt material, which is hard to come by, this style of motor is expensive.
The direct current (DC) motors that are available are brushless DC (BLDC), brush, and stepper motors. When you only have DC voltage available then one of these motors should be used. Brushless DC motors do not have any brushes therefore there aren't any worries of brush wear or sparking. Solid state controls and feedback devises are required for operation. These motors have predicable performance, high starting torques, and are capable of high speeds. Although more power output can be achieved in a smaller package, the electronic controls make this style motor expensive.
Unlike brushless motors, brush DC motors do not require any control electronics. Brush motors use commutator and brushes to generate a magnetic field. Although these motors are usually inexpensive, brush and commutator wear limits their reliability and longevity.
Stepper motors are DC motors that produce incremental steps. If you require shaft positioning to be predicable then stepper motors may be an option. These motors are reliable and low in cost. They are however, limited in its ability to handle large inertia loads.
Once you have determined the voltage and frequency source your system has available you can determine the number of phases and type motor to look at. Next you would need to know the following in order for your motor design engineer to help choose the best motor:
(1) Power Output/Horsepower: The designer will need to know what the rated speed and torque parameter that your system requires.
(2) Frame Size: It is helpful for the designer to know the mechanical constraints in order to properly size the motor.
(3) Duty Cycle/Time rating: The amount of time the motor is operating vs. time it is not is an important criteria when designing the insulation systems of the motor.
(4) Environmental Conditions: It is always important to advise the motor designer what environments the motor will see. This is important so the correct enclosure is determined.
As you can see there are many different types of motors to choose from. There are also many factors used in the choice. By working with a design engineer you can ensure to get the right motor for your application. This is why it is important to seek out a manufacturer before finalizing any systems design.
Donna Kirk is an associate at ARC Systems, Inc., producer of motors and motion control components for over 40 years. ARC's products and services can be viewed at


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