Alternating Current Induction Motor Design
Introduction:Electric motor systems consume 20% of all energy generated in the United States, 57% of all electrical energy, and 70% of electrical energy consumed by industry. Over 1.1 billion motors, of all types, are presently in use in the United States at this time.
Induction motors were invented by Nikola Tesla in 1888 while he was a college student. In the present day, induction motors consume between 90 to 95 percent of the motor energy used in industry.
In the first part of our presentation, we are going to discuss:
- The purpose of induction motors
- Induction motor construction
- Operating principles
- NEMA Designs
- Design E motor discussion
- Motor insulation
- Inverter duty motor construction
The Purpose of Induction Motors
Contrary to popular belief, induction motors consume very little electrical energy. Instead, they convert electrical energy to mechanical torque (energy). Interestingly enough, the only component more efficient than the motor, in a motor system, is the transformer. The mechanical torque that is developed by the electric motor is transferred, via coupling system, to the load.
The electrical energy that is consumed by electric motors is accounted for in losses. There are two basic types of losses, Constant and Variable, both of which develop heat (Figure 1):
Core Losses: A combination of eddy-current and hysterisis losses within the stator core. Accounts for 15 to 25 percent of the overall losses.
Friction and Windage Losses: Mechanical losses which occur due to air movement and bearings. Accounts for 5 to 15 percent of the overall losses.
Stator Losses: The I2R (resistance) losses within the stator windings. Accounts for 25 to 40 percent of the overall losses.
Rotor Losses: The I2R losses within the rotor windings. Accounts for 15 to 25 percent of the overall losses.
Stray Load Losses: All other losses not accounted for, such as leakage. Accounts for 10 to 20 percent of the overall losses.
Induction Motor Construction:
An induction motor consists of three basic components:
Stator: Houses the stator core and windings. The stator core consists of many layers of laminated steel, which is used as a medium for developing magnetic fields. The windings consist of three sets of coils separated by 120 degrees electrical.
Rotor: Also constructed of many layers of laminated steel. The rotor windings consist of bars of copper or aluminum alloy shorted, at either end, with shorting rings.
Endshields: Support the bearings which center the rotor within the stator.
Operating Principles
The basic principle of operation is for a rotating magnetic field to act upon a rotor winding in order to develop mechanical torque.
The stator windings of an induction motor are evenly distributed by 120 degrees electrical. As the three phase current enters the windings, it creates a rotating magnetic field within the air gap (the space between the rotor and stator laminations). The speed that the fields travel around the stator is known as synchronous speed (Ns). As the magnetic field revolves, it cuts the conductors of the rotor winding and generates a
current within that winding. This creates a field which interacts with the air gap field producing a torque. Consequently, the motor starts rotating at a speed N < Ns in the direction of the rotating field.
The speed of the rotating magnetic field can be determined as:
Ns = (120 * f) / p eq. 1
Where Ns is the synchronous speed, f is the line frequency, and p is the number of poles found as:
p = (# of groups of coils) / 3 eq. 2
The number of poles is normally expressed as an even number.
The actual output speed of the rotor is related to the synchronous speed via the slip, or percent slip:
s = (Ns - N) / Ns eq. 3
%s = s * 100 eq. 4
Torque:
By varying the resistance within the rotor bars of a squirrel cage rotor, you can vary the amount of torque developed. By increasing rotor resistance, torque and slip are increased. Decreasing rotor resistance decreases torque and slip.
Motor horsepower is a relation of motor output speed and torque (expressed in lb-ft):
HP = (RPM * Torque) / 5250 eq. 5
The operating torques of an electric motor are defined as (Ref. NEMA MG 1-1993, Part 1, p.12):
Full Load Torque: The full load torque of a motor is the torque necessary to produce its rated horsepower at full-load speed. In pounds at a foot radius, it is equal to the hp times 5250 divided by the full-load speed.
Locked Rotor Torque: The locked-rotor torque of a motor is the minimum torque which will develop at rest for all angular positions of the rotor, with rated voltage applied at rated frequency.
Pull-Up Torque: The pull-up torque of an alternating current motor is the minimum torque developed by the motor during the period of acceleration from rest to the speed at which breakdown torque occurs. For motors which do not have a definite breakdown torque, the pull-up torque is the minimum torque developed up to rated speed.
Breakdown Torque: The breakdown torque of a motor is the maximum torque which it will develop with rated voltage applied at rated frequency, without an abrupt drop in speed.
NEMA Motor Design Classifications:
NEMA defines, in NEMA MG 1-1993, four motor designs dependant upon motor torque during various operating stages:
Design A: Has a high starting current (not restricted), variable locked-rotor torque, high break down torque, and less than 5% slip.
Design B: Known as "general purpose" motors, have medium starting currents (500 -800% of full load nameplate), a medium locked rotor torque, a medium breakdown torque, and less than 5% slip.
Design C: Has a medium starting current, high locked rotor torque (200 - 250% of full load), low breakdown torque (190 - 200% of full load), and less than 5% slip.
Design D: Has a medium starting current, the highest locked rotor torque (275% of full load), no defined breakdown torque, and greater than 5% slip.
Design A and B motors are characterized by relatively low rotor winding resistance. They are typically used in compressors, pumps, fans, grinders, machine tools, etc.
Design C motors are characterized with dual sets of rotor windings. A high resistive rotor winding, on the outer, to introduce a high starting torque, and a low resistive winding, on the inner to allow for a medium breakdown torque. They are typically used on loaded conveyers, pulverizers, piston pumps, etc.
Design D motors are characterized by high resistance rotor windings. They are typically used on cranes, punch presses, etc.
Design E Motor Discussion
The design E motor was specified to meet and international standard promulgated by the International Electrotechnical Commission (IEC). IEC has a standard which is slightly less restrictive on torque and starting current than the Design B motor. The standard allows designs to be optimized for higher efficiency. It was decided to create a new Design E motor which meets both the IEC standard and also an efficiency criterion greater than the standard Design B energy efficient motors.
For most moderate to high utilization application normally calling for a Design A or B motor, the Design E motor should be a better choice. One should be aware of slight performance differences.
Although the NEMA standard allows the same slip (up to 5%) for Designs A, B, and E motors, the range of actual slip of Design E motors is likely to be lower for Designs A and B.
There are a number of considerations which must be observed with Design E motors:
Good efficiency - as much as 2 points above Design B energy efficient.
Less Slip - Design E motors operate closer to synchronous speed.
Lower Starting Torque - May not start "stiff" loads.
High Inrush - As much as 10 times nameplate full load amps.
Availability - Presently low as the standard has just passed.
Starter Availability - Control manufacturers do not have an approved starter developed at this time.
National Electric Code - Has no allowance for higher starting amps. Design E motors will require changes NEC allowances for wire size and feed transformers.
Limited Applications - Low starting torque limits applications to pumps, blowers, and loads not requiring torque to accelerate load up to speed.
Heavier Power Source Required - High amperage and low accelerating torque mean longer starting time and related voltage drops. May cause nuisance tripping of starter of collapse of SCR field with soft starters.
Electric Motor Insulation:
With all this discussion about motor operation, losses, torque curves, and inrush, it is only fitting to review the thermal properties of electrical insulation. In general, when an electric motor operates, it develops heat as a by-product. It is necessary for the insulation that prevents current from going to ground, or conductors to short, to withstand these operating temperatures, as well as mechanical stresses, for a reasonable motor life.
Insulation life can be determined as the length of time at temperature. On average, the thermal life of motor insulation is halved for every increase of operating temperature by 10 degrees centigrade (or doubled, with temperature reduction).
Insulation Class Temperature, oC
A 105
B 130
F 155
H 180
Maximum Temperatures of Common Insulation Classes
There are certain temperature limitations for each insulation class (Table 3) which can be used to determine thermal life of electric motors. Additionally, the number of starts a motor sees will also affect the motor insulation life. These can be found as mechanical stresses and as a result of starting surges.
When a motor starts, there is a high current surge (as previously described). In the case of Design B motors, this averages between 500 to 800% of the nameplate current. There is also a tremendous amount of heat developed within the rotor as the rotor current and frequency is, initially, very high. This heat also develops within the stator windings.
In addition to the heat developed due to startup, there is one major mechanical stress during startup. As the surge occurs in the windings, they flex inwards towards the rotor. This causes stress to the insulation at the points on the windings that flex (usually at the point where the windings leave the slots).
Both of these mean there are a limited number of starts per hour. These limits are general, the motor manufacturer must be contacted ( or it will be in their literature) for actual number of allowable starts per hour. this table also assumes a Design B motordriving a low inertia drive at rated voltage and frequency. Stress on the motor can be reduced, increasing the number of starts per hour, when using some type of "soft start" mechanism (autotransformer, part-winding, electronic soft-start, etc.).\
ServiceFactor Insulation Temperature Class B Class F
1.0/1.15 Ambient 40C 40C
1 Allowable Rise 80C 105C
1 Operating Limit 120C 145C
1.15 Allowable Rise 90C 115C
1.15 Operating Limit 130C 155C
Temperature Limitations
Energy Efficient Electric Motors
The Energy Policy Act of 1992 (EPACT) directs manufacturers to manufacture only energy efficient motors beyond October 24, 1997 for the following: (All motors which)
1.General Purpose
2. Design B
3. Foot Mounted
4. Horizontal Mounted
5. T-Frame
6. 1 to 200 hp
7. 3600, 1800, and 1200 RPM
8. Special and definite purpose motor exemption
To meet NEMA MG1-1993 table 12.10 efficiency values. The method for testing for these efficiency values must be traceable back to IEEE Std. 112 Test type B.
Energy efficient motors are really just better motors, when all things are considered. In general, they use about 30% more lamination steel, 20% more copper, and 10% more aluminum. The new lamination steel has about a third of the losses than the steel that is commonly used in standard efficient motors.
As a result of fewer losses in the energy efficient motors, there is less heat generated. On average, the temperature rise is reduced by 10 degrees centigrade, which has the added benefit of increasing insulation life. However, there are several ways in which the higher efficiency is obtained which has some adverse effects:
- Longer rotor and core stacks - narrows the rotor - Reduces air friction, but also decreases power factor of the motor (more core steel to energize - kVAR).
- Smaller fans - reduces air friction - the temperature rise returns to standard efficient values.
- Larger wire - Reduces I2R , stator losses - Increases starting surge (half - cycle spike) from 10 to 14 times, for standard efficient, to 16 to 20 times, for energy efficient. This may cause nuisance tripping.
In general, energy efficient motors can cost as much as 15% more than standard efficient motors. The benefit, however, is that the energy efficient motor can pay for itself when compared to a standard efficient motor.
$ = 0.746 * hp * L * C * T (100/Es -100/Ee)
where hp = motor hp, L = load, C = $/kWh, T= number of hours per year, Es = Standard efficient value, and Ee = Energy efficient value Eq. 5
Inverter Duty Motors
Inverter duty motors are specially designed to withstand the new challenges presented by the use of inverters. There are a number of ways to designate motors "inverter duty," however, several things must exist as a minimum:
- Class F insulation - to withstand the higher heat generated by non-sinusoidal current from the drive.
- Phase insulation - Insulation between phases is a must to avoid "flashover" between phases from current surges.
- Layered Conductors - To reduce turn to turn potential between conductors.
- Solid varnish system - to reduce partial discharge and corona damage.
- Tight machine tolerances and good air gap concentricity - to reduce shaft currents and resulting bearing damage.
A proper inverter duty motor will have special rotor bar construction designed to withstand variations in airgap flux densities and rotor harmonics. Additionally, the first few turns of wire may be insulated to better withstand standing waves which occur due to the faster rise times in modern inverter technology.
Caution: Some manufacturers may only de-rate motors. This is done by reducing the motor by (about) 25%. Therefore, a 10 hp motor may be rated as a 7.5 hp motor. It should be noted, also, that an inverter application does not always require an inverter duty motor. The old motor or an energy efficient motor may be sufficient for the application.
