Electric motors

Jan 1, 2007 by

Guest Message by Roger Behle, P.E

The squirrel cage induction motor was invented by Nicola Tesla, one of the greatest inventors in modern history.  As he did with so many of his inventions he would dream about how something should work and wake up the next morning and draw and build what he dreamed.

There are many types of motors for all types of applications.   Some applications require higher speed than others.   Others require more torque and more horsepower.   Motors are rated in horsepower starting with fractional values and increasing to thousands of horsepower.   It requires about 746 watts of power to equal one horsepower.   A watt is defined as the power required to maintain a current of one ampere at a pressure of one volt (so a 60 watt lightbulb at 120 volts pulls  half an ampere of current).   This, however, does not include efficiency, the power factor and many other electrical issues needed for the final decision.

Squirrel cage induction motors basically have two components.   They are the stator and the rotor. The stator (or stationary part of the motor) is electrified externally from the utility company’s power.   The portion that turns in a motor is the rotor and it gets its power from the magnetic field generated by the stator.   The combination of frequency and the number of poles in a motor determines the speed.   Sixty cycles per second is normally the frequency that is used by most AC (alternating current) motors.

The following formula determines the speed of the motor.   RPM (revolutions per minute) = 120 x frequency divided by P, where RPM is the desired speed and 120 is a constant.   Frequency is the number of cycles per second generated by the utility company and P is the number of poles in the motor.   Other things to consider in selecting a motor are the torque requirements, service factor and enclosure type.   The National Electrical Manufacturers Association (NEMA) has developed standards for motors that cover torque requirements, enclosure types, and service factor.  These are all covered in NEMA’s motor standard MG-1.   A few of the standard types of motors are Type B (which is a standard squirrel cage induction motor for most applications), Type C (for high torque applications) and Type D (for motor torques that can handle a high slip in starting).   Enclosures for the motors range from open drip-proof (ODP-to protect from the rain) to totally enclosed blower cooled (TEBC) and even explosion proof (XP) motors that are used in a combustible environment.

The service factor is a number which indicates how much above the nameplate rating a motor can be loaded without causing serious degradation, (i.e., a 1.15 S-F can produce 15% greater torque than the 1.0 S-F rating of the same motor).   Most motors have a service factor of 1.0 but some motors are built to incorporate a higher service factor to give the motor additional horsepower for short periods of time.

Let us look at an example of how the speed of a motor is selected.   We have a pumping station that uses a centrifugal pump to move water to a reservoir.   The engineer picks the type of pump that is to be used considering pressure and flow as the two main criteria.   He selects a pump curve that is satisfactory for this application, of course efficiency is part of the selection but not as important.   When the engineer finds a pump for the application, horsepower and the speed are part of the engineer’s selection.   If, for example, the engineer picked 900 rpm as the speed of the pump and it was directly coupled to the motor, the motor would have 8 poles using the formula previously shown and the frequency being 60 cycles.   With this information the engineer can now select the starter and the wire size to feed the motor.   As a rule of thumb each horsepower requires approximately 1.2 amps per horsepower at 480 volts utilizing a 3 phase motor circuit.

Another approach to achieve varying flow rates would be to utilize a Variable Frequency Drive Controller.   This would use the same motor but an AC to DC to AC inverter would be installed between the power source and the motor.   By varying the frequency you could also change the speed from 0 to maximum speed or 900 RPM.  Variable Frequency Drives are commonly used in closed water systems where the pressure is to be maintained, at say 60 psi (pounds per square inch).  A pressure transducer would measure the pressure in the closed water system and input that signal into a controller that would output the signal to the Motor Inverter Controller.   The inverter would change the frequency to the motor and thus the pressure would be maintained with a change in the water demand of the water system.

As the electrical energy rotates in the stator of the motor, the rotor is trying to keep up with it, although it will always lag behind.   As more and more load is applied to the rotor, it will require more energy (watts) to keep up with the stator.  The poles of the motor set up a magnetic field of north and south magnets changing 60 times each second.   The induced magnetic field in the rotor follows with a similar change of north and south poles.   The amount that the rotor lags behind the stator affects the power factor,  i.e. 5 degrees of lag equals a certain amount of power factor reduction.  For large motors, this affects the overall electrical network.   The utility company will assess a surcharge to the customer if the customer’s equipment affects a change to the power factor in the grid.   This can be avoided by using a second type of squirrel cage induction motor.   This is called a synchronous induction motor and additional energy is applied to make the stator and rotor follow exactly together.   This improves the power factor for large induction motors, so synchronous motors are often used.   Another method to improve the power factor is to introduce capacitors to the electrical network.   This will be covered in a later Garagram.

I have designed and installed motors that range from a few horsepower to over 10,000 horsepower. Each of these motor applications carry special design and engineering considerations.   If you have a client that is experiencing some difficulty with their motor applications we can assist with a timely and economical resolution.

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