Archive for the ‘Motor’ Category

Basic Operation of AC Induction Motors (6) – FINISH

May 2, 2009
Voltage and Current Waveforms
Today’s AC variable speed drive systems (up to 600 Volts and about 1500 HP) are dominated by PWM configurations. The current waveforms seen today (Figure 14) are much closer to the ideal sinusoid, thanks mostly to higher switching rates of transistors. The availability of low switching loss devices has allowed this to occur.

One of the negative aspects of the newer devices is that the low switching loss is typically accompanied by a very short transition time. This short transition between on and off states implies a high dV/dt output of the inverter. The high dV/dt results in capacitively coupled current flow according to Equation 4.

In addition to the capacitively coupled current, the high dV/dt also results in a higher peak voltage (ringup) due to cable-to-load mismatch. Finally, the high dV/dt also results in an instantaneous high voltage across the first windings within the ac motor. A companion paper, “AC Induction Motor Insulation Issues in High dV/dt Environments,” addresses this in greater detail.

Conclusions
AC induction motors are likely to continue to be increasing sources of variable speed rotating power. Their successful use in variable speed applications is a function of the collective understanding of the various parties involved in the specification, design, application, and integration of the system.

Basic Operation of AC Induction Motors (5)

May 2, 2009
Constant Power Operation
The prior discussions regarding voltage boost and field oriented control as a means to maintain motor flux have been presented in regard to “constant torque” operation. This can also he thought of as operation “below base speed” (Figure 10).

Above the speed at which the output voltage of the controller is maximum, the controller can no longer maintain constant flux as speed is increased further (since the voltage cannot be increased to keep pace with the frequency). This is equivalent to where a DC motor begins to be “field weakened” to achieve higher speeds. Both for AC as well as DC machines, voltage (armature voltage for DC) remains constant, so for constant load current, constant output power
is available.
As the frequency supplied to an AC induction motor is increased (with voltage held constant), the resultant “field weakening” causes a reduction in the motor peak torque capability as seen in Figure 11.
This family of curves can alternatively be drawn as speed – power, rather than speed – torque curves (Figure 12). The fact that the peak power decreases as speed is increased by field weakening is the most “inherent” limitation to the “constant power speed range” of an AC induction motor drive.

A technique which is commonly employed to achieve wider speed ranges above base speed (constant power) is to utilize some of the “constant flux” speed range to augment the inherent constant power capability. By selecting a motor winding which does not require full voltage until some speed already into the desired constant power speed range, the constant power speed range can be extended as seen in Figure 13. The plots of Figure 13 show an example where the application demands a constant 100 HP from 650 RPM to 3200 RPM. By utilizing this technique, the motor size does not have to be increased in order to satisfy the wide constant power speed range.

This same technique is also used to extend the constant power speed range of DC systems as well. In the case of DC, it is to avoid commutation limits to the top speed at which constant power can be provided. For both AC and DC systems, the “price” which is paid to use this technique is an oversized source of power (higher kVA inverter or DC supply). Wind and unwind applications, along with machine tool spindles employ this technique quite commonly.

Basic Operation of AC Induction Motors (4)

May 2, 2009
“Field Oriented” Control
In order to obtain even better yet control of AC motor torque, adjustable frequency controls often can make use of a regulation scheme known as “field-oriented” or “vector” control. This technique is intended to control the motor flux, and thereby be able to decompose the AC motor current into “flux producing” and “torque producing” components. These current components can be treated separately (in the control), then recombined to create the actual motor phase currents. This results in a solution to the boost adjustment problem, plus provides much better control of the motor torque – which allows much higher dynamic performance.

One way of looking at field oriented control is that the inverter would like to be able to have the same sort of simple, direct control of both flux and torque that is enjoyed with separately-excited dc motors. With dc motors, the flux level is controlled by simply regulating the field current, while the torque is controlled by regulating the armature current. By using field oriented control, the inverter can treat the ac induction motor as if it had the same sort of independently regulated flux and torque characteristic. When the actual induction motor phase currents are decomposed into flux and torque producing components (in the control, not in the motor), this gives the opportunity to “decouple” these two and achieve better system performance as a result.
In order to accomplish field-oriented control, the controller needs to have an accurate “model” of the motor. Over the last several years a large number of different schemes have been proposed to accomplish the “flux and torque control” desired. Many provide this control without the use of a speed feedback (tachometer) signal. These are typically referred to by the generic term of “sensorless” vector control. Many of today’s techniques also involve some sort of self-tuning at startup in order to obtain information which helps to more accurately model the motor – and thereby produce more optimal control. In addition, there are also techniques by which the models can adaptively adjust to changing conditions, such as the motor temperature going from cold to warm (which impacts the slip).

Basic Operation of AC Induction Motors (3)

May 2, 2009
Adjustable Frequency, Variable Speed Operation
For steady-state (as opposed to starting) operation, AC induction motors offer a reasonably linear torque per amp and high power factor characteristic. This is seen in Figure 5 as the part of the speed torque curve between “breakdown RPM” and “synchronous (no load) RPM.” It is this portion of the AC induction motor range of operation within which adjustable frequency drives function.

By varying both the frequency and voltage supplied to an AC motor, the controller can cause the motor to operate on a continuum of speed torque curves which allows operation in the “linear” region between breakdown and synchronous speeds (Figure 6).
This then allows the motor to operate near its optimal torque per amp or maximum efficiency point for a given load and speed.
As long as the motor flux is maintained constant while the frequency and voltage are varied, the basic “shape” of the speed torque curve will remain unchanged. The motor flux is proportional to the internal “counter-emf” divided by the frequency of that generated voltage. This can be described as:
where F is the motor flux,

Eg is the internally generated voltage due to motor rotation, f is the stator frequency, and k is a motor constant related to the winding turns, etc.
The flux paths for a four pole configuration are as seen in Figure 7 (for an instant in time). This pattern rotates at an effective speed given by Equation 1.
The motor counter-emf (Eg) can also be thought of as the voltage across the magnetizing reactance (Xm) in the equivalent circuit of Figure 3.
Maintaining constant flux while the speed (frequency) is varied can then be seen as requiring constant ratio of Eg / f (or constant Im).
Since Eg is a motor internal voltage, this needs to be related to the terminal voltage of the motor. From the AC motor equivalent circuit, it can been seen that the voltage drops across the stator resistance and leakage reactance represent the difference between Eg and the terminal voltage Vt.
If a controller were to maintain a constant ratio of TERMINAL voltage to frequency (Vt / f), rather than Eg / f, this would result in a noticeably decreasing flux level at lower speeds (frequencies).
The curves of Figure 8 demonstrate the effect of this failure to maintain the motor flux. It can be seen that the peak value of torque falls off at the reduced flux levels. In fact, the peak torque is approximately proportional to the square of the flux level, so the drop-off can be significant. The torque per amp is also (directly) proportional to the motor flux, so increased current draw for a given load (torque) will also result from reduced flux.
As a means to improve the system characteristics (beyond the curves of Figure 8), controllers often compensate for the difference between Vt and Eg in order to select the correct voltage for a given frequency. This compensation is often referred to as “voltage boost.” Since the major detrimental effect of constant Vt / f is at low voltages, low frequencies (low speeds), the voltage drop across the stator leakage reactance is usually ignored (as the impedance of an inductor is proportional to frequency). This leaves the drop across the stator resistance as the major source of a discrepancy between Vt and Eg at these low speeds.
Many controllers use a value of voltage boost which compensates for the IR drop of the stator at a current equal to the motor full load amps.

Vb is the per phase (line-to-neutral) voltage boost,
Rl is the per phase stator resistance,
IFL is the motor full load current.
This would result in a voltage versus frequency characteristic as shown in Figure 9. A weakness in this technique of boosting voltage is that the value of Vb is only “correct” for a single value of load current. If the full load current is used to set the voltage boost, then the motor will be overfluxed for lighter loads, and underfluxed for overload conditions. Depending on the low speed performance required by a given application, this may or may not be a problem.
It is now common to provide a “more intelligent” voltage boost function in many controllers. This can provide a closer to optimal operating condition at low speeds, resulting in better low speed torque delivery from the system.

Basic Operation of AC Induction Motors (2)

May 2, 2009
Speed / Torque Curves

As an AC induction motor is started, the values of resistance and reactance offered by the motor (or seen by the power source) will vary. At the instant of applying power to a stopped motor, the magnetic field is rotating much faster than the (stationary) rotor. This implies 100% slip, so R2/s is minimized. As a result, the current drawn at starting (locked rotor) conditions is quite high. Also, it is common to design rotor slots which have dramatically different impedance at high slip (say 60 Hz for starting) versus at normal running where slip is typically in the range of 0.2 – 2 Hz. This changes the values of both X2 and R2 from starting to running conditions.

As a motor accelerates to speed from a standstill, the changing impedances result in a unique characteristic developed torque and current drawn during the time of acceleration. Depending on the design of the motor, a torque / current characteristic such as one of those shown in Figure 4 would typically result. The NEMA Design B motor is considered the most “general purpose” of these characteristic shapes, with Design C and D typically used for more “difficult to start” loads. Table 2 gives some ranges of characteristics for integral HP, 1200
and 1800 RPM motors.

As can be seen from all of these speed / torque / current curves, the current drawn by an AC motor in accelerating a load up to speed can be dramatically higher than the nominal running current. At the same time, the developed torque (during acceleration) may in some cases be less than the rated full load torque. Various methods exist to control the starting current drawn by an AC motor but the torque per amp seen during (fixed frequency) starting is always much lower than at running conditions.
The nature of an AC induction motor’s acceleration to running speed is such that it can impose high stresses on both the stator and the rotor. The high current draw also stresses the upstream power system, including cabling, transformers, switchgear, etc. For this reason, there is often significant effort made to “control” AC motor starting and acceleration – both in terms of motor design as well as application.

Basic Operation of AC Induction Motors (1)

May 2, 2009
Terminology and Equivalent Circuits

Before trying to understand the operation of AC induction motors on adjustable-frequency power (variable-speed), it will be useful to briefly review the basic fixed-frequency (constant speed) operation of AC induction motors. The fundamental electromagnetic components are the stator and rotor.
Examples of typical laminations which comprise the basic magnetic path in the stator and rotor are shown in Figure 1. In the most common configuration, the stator has three interconnected phase windings, and the rotor winding is a set of short circuited bars known as a “squirrel cage.” A wound stator and an aluminum die cast (squirrel cage) rotor are seen in Figure 2.

With balanced three phase voltages applied to the windings of the stator, balanced currents flow in the three interconnected phase windings. These currents produce a magnetic field which can be thought of as “rotating” within the stator at a speed given by Equation 1.

N1 = 120 x f /P (1)
N1 = rotational speed of stator magnetic field in RPM (synchronous speed)
f = frequency of the stator current in Hz
P = number of motor magnetic poles



For various numbers of motors poles, Table 1 shows the synchronous speeds based on 60 Hz and 50 Hz frequencies.
The natural tendency is for the rotor to “follow” the rotating magnetic field, and at no-load the rotor will turn at a speed virtually equal to Nl. Any difference in the rotational speed of the magnetic field and that of the rotor will result in a voltage being induced in the rotor squirrel cage winding. The resultant rotor current interacts with the magnetic field to produce torque. The difference in rotor mechanical speed versus magnetic field rotational speed is what is known as “slip.”
The equivalent circuit for an AC induction motor can help visualize some of the motor characteristics.
Figure 3 shows a typical equivalent circuit for AC induction motors. The variable resistor “R2/s”
represents the way slip causes increased current and corresponding increased torque. The greater the slip, the lower this value of resistance, and the more current is going to flow in this branch of the circuit. When the slip is virtually zero at a “no-load” condition, this resistor is seen to be a very high value. As a result, the current can be thought of as all going through the “XM” or magnetizing branch of the circuit.

AC Induction Motor Design (8) – FINISH

May 2, 2009
10. 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 o 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.

AC Induction Motor Design (7)

May 2, 2009

9. 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)

- General Purpose
- Design B
- Foot Mounted
- Horizontal Mounted
- T-Frame
- 1 to 200 hp
- 3600, 1800, and 1200 RPM
- 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

AC Induction Motor Design (6)

May 2, 2009
8. 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).

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 (Figure 4). 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 motor driving 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.).



AC Induction Motor Design (5)

May 2, 2009
7. 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 to 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.


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