Select the Best Motor
April 1, 2011
Our society takes for granted that washers will agitate, dryers will spin and furnaces will circulate warm air, often not understanding the sophisticated motor technologies that make these products reliable, quiet and efficient.
As a design engineer, selecting motor and control technologies is not always an easy task. A growing range of motors for specific applications provides a variety of benefits, limitations and price points. So how can OEMs choose the best fit for an application?
The BasicsNormally, motor specifications begin with the fundamentals, including:
Based on this information, it’s ideal to select the simplest motor available to permit fast delivery and to eliminate the cost of special features but, if any application details are overlooked, the chosen motor may fail to perform to the standards of the job. In some cases even specially engineered designs may fail.
It’s important to consider all of the following factors relating to a project, whether it be for a dishwashing, laundry, refrigeration or HVAC application:
Motor Horsepower and Operating Point(s)Start by understanding the device the motor is driving, its load and speed, starting load and duty cycle. The horsepower needs to be determined at the intended operating speed(s). This can be calculated using machine handbook formulas and tables, or from the manufacturers of the driven devices such as fans and blowers, which provide graphs and charts as guides to the load requirements that will achieve a certain output.
OEMs also need to consider any efficiency losses of the unit being driven from bearings, belts, gears or friction. In addition, it’s important to account for the worst case loading such as when the washing machine is full of wet clothes versus dry, or when an air handling system is operating at high external static pressures.
Motor TypeEngineers should pay attention to the types of motor that best meet load requirements and power supply specs for an individual application. Fractional and sub-fractional horsepower motors are classified by electrical types.
Shaded pole motors are simple in mechanical construction and are usually low in price. However, their low efficiency, power factor, dip torque and starting torque typically limit their application to direct drive fan and blower applications and submersible pumps where efficiency is not a concern and price is the main consideration.
Permanent split capacitor motors are widely used when starting torque requirements are not too high and moderate electrical efficiency is needed. They are typically used on fans, blowers, small pumps and gear motors. Like most AC induction motors, permanent split capacitor motors have a peak efficiency that occurs at a point 5 to 10% below synchronous speed that decreases dramatically as you move toward no load speed or locked rotor. They are typically designed for 1 to 5 speeds.
Split phase motors often are used in appliances such as washers and dryers. They have a start and a run winding. They can accommodate larger starting load requirements. Once a certain speed is reached, the start winding drops out by a mechanical switch or relay to run on the lower power run winding.
Capacitor start motors are typically used in compressors and pumps. They have a start and a run winding. However, the additional capacitor, in series with the auxiliary start winding, helps produce a higher starting torque per ampere than the split phase motor. As a result, a capacitor start motor can accommodate larger starting load requirements. Once a certain speed is reached, the start winding drops out, typically using a mechanical switch or relay to run on the lower power run winding.
Variable speed electronically controlled (including permanent magnet) motors include the category known as electronically commutated motors (ECM). The single phase AC power is converted to DC power using a rectifier and filter capacitor bank. The DC power is then converted back to AC power by using power transistors and a microprocessor control. This allows control of the current and frequency to the motor winding, allowing torque and speed control. Since both current and frequency can be controlled, the motor can operate up to the 80% efficiency range for larger fractional horsepower motors. While induction motors can be designed to run almost that efficient at a single speed, these motors maintain high efficiencies across wide speed ranges. The electronic control of the motor allows various input control methods such as 24 VAC, PWM (pulse width modulation or duty cycle control), and others from the user system control board.
Enclosure and Mounting TypeConsidering the environment that the motor will be operating in and the requirements to keep the motor functioning reliably over the design life of the unit is important for OEMs. They should also take into account any required regulatory standards (UL, NEC, GAMA, CE, etc.). For outdoor motors, a totally enclosed motor may be required; it may also have to meet certain water mist or spray requirements.
For indoor motors, an open or partially open motor may be acceptable, depending on whether water may drip in. A dusty environment may require additional protection if there is no filtering of air coming into the unit motor compartment. Many fractional horsepower motors have air flowing over them from the fan or blower they are driving. These are termed air-over motors.
The type of bearing system required is another consideration. Sleeve use is where the shaft is subject to moderate thrust in any direction and may require oiling periodically. Ball use is where high thrust is exerted on the shaft and low maintenance is required. This is often for use in hard-to-re-lube locations.
Further ConsiderationsHow are you intending to mount the motor?
Check with the motor manufacturer in order to use a standard mounting system or bolt pattern. Some typical mounting configurations are: lugs attached to the motor; belly band (customer supplied); resilient or welded to shell base mount; or through bolts from motor face.
What power will be supplied to the motor?
Typical single-phase power runs from 100 through 575 VAC, with a frequency of 50 or 60 Hz. Depending on where the end product will be sold in the world, this will vary greatly. Lower-cost motor designs will try to stay with one voltage and frequency to minimize cost. Three-phase motors also are an option. Electronically controlled motors may allow operation over a wider input voltage and frequency range, reducing the number of models required to serve a spectrum of products.
What type of overload protecting is required?
If none, then no internal motor overload protection may be required if the motor is protected by an external acceptable means. Engineers must be sure to check safety and regulatory codes or requirements.
Automatic or thermal protectors are designed to reset and restart motor after a cooling period. Electronic protection is typically used in fan and pump application , and manual protectors are designed to prevent the motor from becoming reenergized until reset by hand.
What safety and regulatory standards apply to the application?
Many applications have their own regulatory standards with motor specifications. All applicable safety and certifying authorities, along with local, state or other standards should be reviewed.