Motor Reference

                                       compiled by Mohamed Grissa

                                            for Dr. Charles Burt



This document contains summary information about the following motor topics:

     Motor selection information

     AC induction motors

     Inverter duty motors

     Variable frequency drives

     Motor losses

     Troubleshooting



The information contained in this document was gathered from a variety of online, interview,

and print sources, including NEMA specifications, with support from the California Energy

Commission (CEC) PIER Program.



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                                                  Motor Reference

                         http://www.itrc.org/reports/motor.htm    ITRC Report No. R 11-001



                                       MOTOR SELECTION



Terms like "inverter duty", "inverter ready", and "inverter rated" are marketing terms and are

useless without any backup data.



In order to select a motor, first identify the load torque characteristic over the speed range

intended for operation.  This information will identify the motor load torque requirement, which

can be translated into motor input current. The input current determines the motor heating over

the speed range and the motor manufacturer can identify how far down in speed the motor should

be able to go.  This point is highly variable and is a function of cooling method (TEFC, ODP, or

aux. blower), horsepower (typically, a 250hp TEFC motor is only good for 2/1 where a 5hp

TEFC motor is good for 5/1), and general design (extruded aluminum or cast iron, high or

premium efficient).



                                  Example of inverter duty motor name plate



Once the continuous current and peak current required for the motor have been determined,

select a drive with output current ratings that meet or exceed that level.           A variable torque drive

has only 10% peak over current capacity and a constant torque drive has 50% peak over current

capacity.   One is not necessarily more reliable than the other as long as it is sized to cover the

motor currents as described above.  Drive terms like “variable torque” and “constant torque” are

somewhat misleading; both kinds of drives get used on both kinds of loads.                The terms simply

refer to the amount over current capacity they can produce short-term.



When contacting a dealer, it is good to provide as much information as possible, such as:



Pump Information: Characteristics, dimension, shaft, mounting, etc.  How the motor fits with

          the pump will affect the bearing, efficiency (heat), frame size, etc.

Horsepower: 15-250 HP

RPM Rating:  900-3600

Voltage Rating:  480V

Phase: 3 phases AC

Motor Type: Vertical hollow shaft inverter duty



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                                                  Motor Reference

                         http://www.itrc.org/reports/motor.htm     ITRC Report No. R 11-001



Frame Size: The frame is important because of the weight, the space available and also price.

         The frame selection will depend on the motor size, the pump size and motor mounting.

Mounting

Motor Application:  For instance, pumping water from a canal

Speed Range: low – max.  Specify the highest and lowest RPMs. It is recommended to provide

         a torque vs. speed curve.  Typically, the control operating output frequency range is 2 to

         1 for variable torque type loads. The 2 to 1 speed range is not a consideration issue with

         the control, because all adjustable frequency controls operate over a minimum of 6 to 1

         output frequency range.



Operating Speed Range:  The desired speed range may be difficult to achieve depending on the

         type of application. In general, depending on motor size and load type, very wide ranges

         may require a special motor. Operation at very low speeds, requiring the motor to run

         at very low frequency (below approximately 6 Hz) or very high speeds requiring the

         motor to run at very high frequencies (above 90 Hz) may require a special motor.



         Motor synchronous speed varies directly with the control output frequency. Therefore,

         the frequency required to achieve a desired application speed can be approximated by

         dividing the desired speed by the motor rated speed and then multiplying by the rated

         frequency of the motor. If the minimum or maximum frequencies are near or outside the

         limits mentioned above then the motor manufacturer should be consulted before

         proceeding. Examples of speed ranges are listed below, expressed as a ratio of the motor

         base speed to a minimum speed.



                 Constant and variable torque speed range examples (Base speed = 2500 RPM)



Motor Cooling:  There are two general methods of motor cooling or ventilation:

     1)  Speed dependent: this method is used in totally-enclosed fan cooled or open drip-proof

         motors. The cooling depends on the motor speed since the fan rotation is supplied by the

         motor. A change in motor speed will result in a change in fan speed and this change

         depends on the speed range. Some motors at low speed may have 20 to 50 percent of

         their base speed.

    2)   Speed independent: some types of systems that use this method are totally-enclosed non-

         ventilated, totally-enclosed air-over, blower-cooled, etc. The cooling rate does not change

         with motor speed. This method is effective with motors operated at low speed.

Space Heater:  Electric motors frequently have space heaters installed, at the customer’s

         request, to prevent moisture condensation in the motor when it is not running. In

         applications where the possibility of condensation is not a factor, or where continuous

         operation of the motor prevents the formation of condensation, space heaters are not

         necessary.



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                                                   Motor Reference

                         http://www.itrc.org/reports/motor.htm      ITRC Report No. R 11-001



Enclosure: This will depend on the application and location. If the motor is

         located in a hazardous or corrosive environment, it will require a

         different enclosure than a protected motor inside a room.             A Weather-

         Protected Type I (WPI) machine is a guarded machine with its

         ventilating passages so constructed as to minimize the entrance of

         rain, snow and airborne particles to the electric parts. It is the most

         common type of enclosure for motors used in farms. All the motors

         at the Water Resources Facility have WPI enclosures.



         Other types of protection include:

         Totally Enclosed Fan-Cooled (TEFC) machines are frame-surface cooled totally enclosed

         machine equipped for self-exterior cooling by mean of fans integral with the machine but

         external to the enclosed parts. They are often used in environments that are hazardous,

         harsh, or present a high risk of contact with salt or salty water, like close to the sea.



         Totally Enclosed, Blower Cooled (TEBC) motors are most commonly used for variable

         speed motors combined with variable speed drives of some sort. Sometimes these motors

         are rated as "Inverter duty" or "Vector duty". They are considerably more expensive than

         similarly rated TEFC motors. The motor is constructed with a dust tight, moderately

         sealed enclosure which rejects a degree of water. A constant speed blower pulls air over

         the motor fins to keep the motor cool at all operating speeds.



Different Sensors: Could be for motor or bearings, or temperature monitoring.  One dealer

         stated that these could probably be added for no extra charge. Sensors could help monitor

         and detect some motor problems. Normally, temperature reading and trend could give a

         good idea about the condition of the motor.



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                                                   Motor Reference

                         http://www.itrc.org/reports/motor.htm      ITRC Report No. R 11-001



                                   AC INDUCTION MOTORS



How they work:

       The stator has three windings mechanically spaced 120 degrees apart.

       The three stator windings are driven by a three phase symmetrical voltage set (voltages

           spaced 120 degrees apart). This AC supply voltage creates a rotating magnetic field in

          the stator.

       The rotor consists of conducting bars along the motor axis.                The bars are shorted at both

           ends of the rotor. The rotating magnetic field cuts through the rotor, inducing a voltage

           in the rotor bars. Sequentially, the bars create their own magnetic field.

       The rotor magnetic field will attempt to line up with the stator magnetic lines of force.

           Since the stator magnetic field is rotating the rotor “chases” the stator but stays just

           slightly behind, creating “slip”.

       Theoretical motor speed = (120 * frequency) / (number of poles).

       The actual speed of the motor is just slightly less because of “slip”.

       Increase in inrush current will reduce the slip and increases efficiency.



Inside a motor, the magnetic fields try to align, just as two magnets close to one another will try

to align their magnetic fields. This perpetual effort at alignment causes the motor's rotor to spin.

The strength of the fields and their degree of misalignment make the effort to align, or the

torque, more or less strong.



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                                                  Motor Reference

                         http://www.itrc.org/reports/motor.htm    ITRC Report No. R 11-001



                                 INVERTER DUTY MOTORS



Motor Design



The motors recommended by NEMA MG-1 are Design A and B squirrel-cage motors



The following information is from NEMA MG1-2006 and applies to 60 Hz NEMA Designs A

and B squirrel-cage motors. These motor are rated 5000 horsepower or less at 7200 volts or less.



Design A : a squirrel-cage motor designed to withstand full-voltage starting and developing

locked-rotor torque, pull-up torque, breakdown torque, with locked-rotor current higher than

certain values (see MG1-12.35.1) for 60 Hz and having a slip at rated load of less than 5 percent.



Design A motors are generally of little concern and the motors are well suited for variable speed

operation, exhibiting low slip and high efficiency. The potentially higher breakdown torque of a

Design A motor will extend its constant horsepower speed range beyond that achievable by a

Design B motor. However, caution should be used when applying Design A motors in by-pass

operation, as their high locked-rotor current can increase starter, thermal overload, and short

circuit protection device sizing. Design A motors may also suffer greater thermal and mechanical

stress than other designs when started across-the-line. Design A motors with very low slip may

also exhibit instability under lightly loaded conditions.



Design B : a squirrel-cage motor designed to withstand full-voltage starting and developing

locked, breakdown, and pull-up torques adequate for several general applications, drawing

locked-rotor current not to exceed certain values (see MG1-12.35.1) for 60 Hz and having a slip

at rated load of less than 5 percent.



Design B motors are applied in variable torque, constant torque, and constant horsepower

applications. Adjustable frequency control algorithms are generally optimized to the speed-

torque-current characteristics of Design B motors. They exhibit good efficiency and low slip, and

are suitable for across the-line starting in bypass mode. Design B motors with very low slip may

also exhibit instability under lightly loaded conditions.



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                                                   Motor Reference

                          http://www.itrc.org/reports/motor.htm     ITRC Report No. R 11-001



Why are Inverter Duty Motors Recommended?



Pulse Width Modulated (PWM) inverters change the rms value by turning the controlled value

ON and OFF at a relatively high frequency while varying the voltage pulse width. Sketches of

the voltage applied and resultant current are shown in the figure below.



                                Voltage (l.) and line current (r.) from a PWM ASD

                  Source: IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 33, NO. 2, MARCH/APRIL 1997



The spikes in voltage resulting from the PWM stress the motor winding and insulation, resulting

in early failure. Standard motor and even premium efficiency motor are not built to withstand

this type of electrical stress.



The spikes in voltage cause an electrical stress and an increase in motor temperature. The

increase in motor temperature is due to an increase in current and in non-Fundamental current

which is an increase in variable losses, core losses and spray losses.



Inverter duty motors are built to withstand the stress caused by the change in frequency. Today

the only specifications related to inverter duty motors are NEMA MG1, Parts 30 and 31. They

outline motor capabilities when used with VFDs for motors rated 600 volts or less. MG1-

30.2.2.8 requires that standard motors utilize an insulation system able to endure repeated

voltage peaks of up to 1000 volts with rise times of 2 or more microseconds. MG1-31.4.4.2

defines an inverter duty motor as having an insulation system able to withstand peaks of 3.1

times rated voltage with rise times of 0.1 or more microsecond.



The new inverter duty motors are premium efficiency motors with inverter duty insulation. The

inverter duty motors are different from standard motors due to several factors:

     1.  The premium efficient winding is created using vacuum-impregnated windings to

         eliminate air pockets in the system that inflate as the temperature increases, causing the

         wire to collapse.

     2.  The magnet wire is coated with high dielectric strength film to allow it to resist fast rising

         time pulses.

     3.  Maximized wire size reduces the wire resistance and increases efficiency.

     4.  Some manufacturers add phase and ground protection using stronger dielectric materials

         or applying sleeving over the first turn at the line end of the motor.

     5.  The insulating material has a higher temperature rating.

     6.  Rotor bars are modified to increase surface area and decrease slip.



In other words, an inverter duty motor is as efficient a motor as any premium efficient motor but

is built to better withstand electric stress.



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                                                 Motor Reference

                        http://www.itrc.org/reports/motor.htm    ITRC Report No. R 11-001



                            VARIABLE FREQUENCY DRIVES



An inverter duty motor is controlled by a Variable Frequency Drive (VFD), also called a

Variable Speed Drive (VSD). The VFDs available in the market are not all the same and do not

all use the same technology.      Computers and processors improve and advance every day, and as

a result VFD technology advances as well. Unfortunately, motor technology does not advance at

the same speed as these controllers. As a result, there are some problems created by the

controller such as voltage spikes. On the other hand, the advances in VFD technology result in

better control and better efficiency.  Some losses are caused by voltage spikes but the current

used to drive the load is reduced because the controller is reacting better to the change in load.

Information related to one of the VFDs used by the ITRC is presented below in order to illustrate

some of the technologies that are now available. The VFD presented is built for water pumping,

so some features have been added to make more suited for pump control.



VLT 8000 Aqua (used by the ITRC in its WDF):

This AFD (Adjustable Frequency Drive) features an inverter control system called VVCplus

(Voltage Vector Control). VVCplus controls an induction motor by energizing it with a variable



frequency and a voltage that matches it. If the motor load is changed, the magnetization of the

motor changes as well, and so does its speed. Consequently, the motor current is measured

continuously and the actual voltage required and slip of the motor are calculated from a motor

model. Motor frequency and voltage are adjusted to ensure that the motor operating point

remains optimal under varying conditions.



AMA (Automatic Motor Adaptation): This feature in the VLT 8000 Aqua measures main motor

parameters and standstill; it automatically optimizes operation between the driver and the motor

by reading and checking the values without spinning the motor.            VVCplus uses AMA to measure



static values of stator resistance and inductance. This data is provided to the motor model, which

serves to calculate no-load values for the load compensator and the voltage vector generator.

      Optimizes motor performance

      Improves star capabilities

      Compensates for motor cable variances



AEO (Automatic Energy Optimization): This feature ensures that the relationship between

voltage and frequency is always optimum for the motor’s load. Thus, it doesn’t provide a

constant voltage/frequency ratio. In order to automatically provide the correct voltage at any

operating frequency and load, the driver must continuously monitor the motor’s status and

respond to any changes. The VVCplus control algorithm is central to this. Current is monitored on



all three motor phases so that both the real and reactive components for motor current are known

at all times. The combination of the AEO and AMA results in automatically maintaining a peak

motor efficiency under all conditions.

      Minimizes energy consumption

      Maximizes motor efficiency by controlling the motor magnetization current

      Reduces motor noise

      Simplifies commissioning

      Improves load shock handling

      Improves handling of fast reference change



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                                                   Motor Reference

                         http://www.itrc.org/reports/motor.htm      ITRC Report No. R 11-001



Recommendation: There are four different insulation materials. The choice of insulation

depends on the maximum expected windings temperature. If the expected temperature is close to

one insulation class it is better to select the next higher insulation class for the motor winding.

The rule of thumb is that a winding temperature higher than the insulation rating temperature

reduces the motor life expectancy. On the other hand, a lower winding temperature than the

insulation rating increases the motor life expectancy.



                                        Insulation classification MG1-1.66



                                 Insulation Class           Temperature Rating

                                         A                          105° C

                                         B                          130° C

                                          F                         155° C

                                         H                          180° C



NEMA Application Guide for AC Adjustable Speed Drive Systems



Non-Fundamental Currents (NEMA 5.2.1.2): Distortion of the motor currents varies inversely

with switching frequency because of the low pass filtering effect of the leakage inductances of

the motor windings. The higher the switching frequency the lower the total distortion and the

better the current waveform, up to a point. As switching frequency is increased higher and

higher, distortion of the motor currents about their zero crossings caused by the switch deadband

(intentionally built-in time delay between upper and lower switch conduction) becomes

significant. Usually, however, tradeoffs between current distortion and switching loss are such

that little is to be gained above approximately 5 kHz.



Motor temperature is a function of both cooling and the magnitude of heat producing losses in

the motor. These losses are increased, when compared to operation on line power, because of the

current distortion. The non-fundamental currents contribute very little to useful torque, but do

increase several components of motor losses. Core losses are increased due to eddy currents and

hysteresis. Rotor conductor losses are increased due to high frequency surface losses. The high

frequency component also adds to the total rms current and thus the I2R loss in the stator

conductors. The magnitude of this increase in losses depends on the switching frequency of the

control and the motor design characteristics.



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                         http://www.itrc.org/reports/motor.htm     ITRC Report No. R 11-001



                                           MOTOR LOSSES



Power losses are the fraction of input energy converted to heat instead of being used to drive the

load. These losses are divided into two groups: fixed and variable losses.



Fixed Losses



Fixed losses are constant regardless of load, which is why small motors have higher losses as a

percentage of input power than large motors. It is not accurate to say that fixed losses are always

constant but assumed so, and this assumption will not create a significant error. Fixed losses

include mechanical friction losses (brush friction, air friction, bearing friction or windage) and

magnetic core losses (hysteresis and eddy current).



Core Losses: Core losses are the combination of hysteresis losses and eddy current losses.                  These

losses vary with the load current on the motor, speed variation, and condition of operation.

These losses are considered constant; however, they are considered constant and any variations

are accounted for under stray load losses.



Mechanical Losses: Friction of the moving parts (such as fan blades) causes losses of energy. As

in core losses, these mechanical losses are considered constant from no load to full load.



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                            http://www.itrc.org/reports/motor.htm         ITRC Report No. R 11-001



 Variable Losses



Variable losses include losses that vary with the current drawn by the motor; they are called

                       2

copper losses or I R. They also include stray load losses.



Winding Losses: In a 3 phase induction motor, the voltage is applied directly to the primary

winding. An induced current by the primary winding magnetic field flows in the rotor or

secondary winding. Generally, the secondary and primary windings are made of copper or

aluminum. The winding’s wire resistance and the motor drawn current is the cause of the

variable losses. The winding’s resistance varies with the temperature, load, uneven sharing of

current among conductors, nature of the wires and other similar factors.



Stray Losses: All the losses that are not accounted for; this includes any variation in core and

mechanical losses.



                 Significant causes of variation in the performance of different types of component



                                   transistor  diode        IC   resistor  capacitor             inductor   relay



              temperature               X          X        X          X              X              X             X



                  aging                 X                              X              X                            X



                radiation               X          X        X                                                        



            vibration/shock                                            X              X              X             X



                humidity                                               X              X                              



                    life                                               X              X                              



            electrical stress           X          X                                  X                              



                                                                         X: Significantly affected by environment



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                                                            Motor Reference

                              http://www.itrc.org/reports/motor.htm            ITRC Report No. R 11-001



                                     MOTOR TROUBLESHOOTING



 Temperature Related Life‐Shortening Factors

  PROBLEMS               SYMPTOMS                                  CURES

                         Overload Tripping

                                                                   Correct power supply or match motor to actual power supply voltage

  Low Voltage            High current

                                                                   rating.

                         Short motor life

                         Overload Tripping

                                                                   Correct power supply or match motor to actual power supply voltage

  High Voltage           High current

                                                                   rating.

                         Short motor life

  Unbalanced             Unbalanced phase currents

                                                                   Determine why voltages are unbalanced and correct.

 Voltage                 Overload tripping

                         Overload Tripping

                                                                   Determine reason for overload. Increase motor size or decrease load

  Overload               High current

                                                                   speed.

                         Short motor life

                                                                   * Rewind motor to higher class of insulation. Oversize motor to

  High Ambient

                         Short motor life                          reduce temperature rise. Ventilate area to reduce ambient

 Temperatures

                                                                   temperature.

  Blocked                Short motor life

                                                                   Clean lint and debris from air passageways or use proper motor

 Ventilation             Runs hot

                                                                   enclosure for application.

                         Amperage o.k.

                                                                   ** Use a reduced voltage starting method. Upgrade class of

  Frequent Starts        Short motor life

                                                                   insulation.

                                                                   Oversize motor frame.

  High Inertia           Short motor life

                                                                   Use higher class of insulation

  Loads                  Overload tripping during starting

                                                                   ** Use a reduced voltage starting method.

  * Bearing lubrication must also be matched to high operating temperature.

  **Reduced voltage starting method and motor characteristics must be matched to the load requirement.



Replacements for Failed Motors



When motors fail, they must be repaired or replaced quickly to avoid lost production. Often—

with some advanced planning—it is economically attractive to replace these motors rather than

incur the expense of rewinding.



Though common practice is to replace failed motors below 20 horsepower and repair those

above 20 horsepower, replacing all failed motors up to 50 hp is almost always economic.

Replacing larger failed motors is also often cost-effective, depending on how heavily the motor

is used. Because it reduces capital costs, the return from upgrading to a one size smaller energy

efficient replacement instead of rewinding is even more attractive than replacement with a same

sized energy efficient motor.



Opportunities to specify energy-efficient motors may exist if the customer is:

       Designing new facilities

       Modifying existing installations or processes

       Procuring pre-packaged equipment or systems with electric motor components

       Considering rewinding failed motors

       Replacing oversized (underloaded) motors

       Implementing an energy management or preventative maintenance program

       Able to obtain utility rebates



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                                                  Motor Reference

                         http://www.itrc.org/reports/motor.htm     ITRC Report No. R 11-001



Bearing Failure



Major causes of bearing failures include misalignment of the motor and load, vibration, incorrect

lubrication, excessive radial or axial loading, lubricant contamination or inadequate maintenance.

With a motor controlled by a VFD there is a less frequent cause of motor failure, which is

electrical current that goes through the bearing. The solutions for these failures may include:

      Good pump and motor installation

      Good motor and pump selection

      Use the lubricant type and amount indicated by the motor company

      Space heater to reduce moisture in the lubricant

      Proper maintenance and data record

      Good electric grounding



It is difficult to say if motors or the pump bearings fail more often. It all depends on the

installation. There are different causes of pump bearing failure such as misalignment, resonance,

improper greasing, excessive speed or load, or electrical problems from the VFD in case the

shaft current is grounded through the pump.



The use of insulating bearing is not widely recommended because the problem will be shifted

somewhere else. If the path of the current is blocked the current will try to find an alternative

path, which will cause the problem to go from one location to another.              One solution is to control

the path of the current by installing shaft grounding ring to drain the current (voltage) to the

ground. However, the best solution is to control the problem from the source by minimizing the

high frequency pulses by installing a filter between the VFD and the motor.



                                                                         Irrigation Training and Research Center

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                                               Motor Reference

                       http://www.itrc.org/reports/motor.htm   ITRC Report No. R 11-001



                               ELECTRONIC REFERENCES



Bearing:



“Bearing Failure: Causes and Cures.” Barden Precision Bearings



Gray, Will, and Don Macdonald.        “A practical guide to understanding bearing damage related to

PWM drives.” IEEE, 1998.



“Inverter-Driven induction Motors Shaft and Bearing Current Solutions.” Baldor Electric

Company



 “Motor Shaft Voltages and Bearing Currents Under PWM Inverter Operation.”                GAMBICA and

REMA Technical Guide, 2006.



Motors:



“AC Motor Selection and Application Guide.” GE Industrial Systems.



“Advanced Vacuum Pressure Impregnation (VPI) System.” Solidstate Controls, Inc.



“Application Guide For AC Adjustable Speed Drive Systems.” NEMA Standards Publication,

2001.



“Determining Electric Motor Load Efficiency.” U.S. Department of Energy



“Efficiency Improvement for AC Electric Motors.” PG&E Energy Efficiency Information, 1997.



“Information Guide for General Purpose Industrial AC Small and Medium Squirrel-cage

Induction Motor Standards.” NEMA Standards Publication, 2002



“Installation Guide for Power Drive Systems.” GAMBICA and REMA Technical Guide, 2006.



 “Motor Insulation Voltage Stresses Under PWM Inverter Operation.”  GAMBICA and REMA

Technical Guide, 2006.



“NEMA Three Phase AC Horizontal Motor.”             U.S. Electrical Motors, 1999.



“Product Data sheet: Vertical Holloshaft, WPI.” US Motors.



“Quick Engineering Facts.” US Motors.



“Motors Type Designations.” US Motors.



“Product Data Sheet of Inverter Duty-Vertical Holloshaft” US Motors.



“US Motor Sensors and Thermal Protection.” US Motors.



“Variable Speed Driven Pumps-Best Practice Guide.” GAMBICA



Cowern, Edward H. “ Bolder Motors and Drives.” Baldor Electric Company, 1999.



Les Manz. “Applying Adjustable-Speed Drives to Three-Phase Induction NEMA Frame

Motors.” IEEE Transactions on Industry Applications 33. 2 (1997): 402-407.



                                                                     Irrigation Training and Research Center

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