Application and Selection of Adjustable
Speed Motors to Optimize Spares

| Abstract | Introduction | Multiple Rating | Base Speeds and Numbers of Motor Poles | Examples of Application Ratings | Case History | Conclusions | Aknowledgement |

Abstract

The application of variable speed AC induction motors to process line requirements in the paper industry continues to increase. In addition to other advantages documented elsewhere, the variable speed AC induction motor applied with an adjustable frequency controller can provide an opportunity to reduce the number of spare motors needed. By taking advantage of the induction motor's capability to be externally reconnected for different Volts per Hz levels combined with the inverter's capability to operate with various "base frequencies," applications even with different base speeds may be covered with a single spare motor.

I. INTRODUCTION

In a "process line" application, whether an annealing line, primary paper making, paper converting, etc. there are numerous individual loads, each of which could conceivably define a unique motor rating. With a hundred or more driven loads, it is common to 4 6 round up" to the next HP rating as part of consolidating ratings. The use of a small number of standardized base speeds by selecting certain gear ratios is also commonly done. Such "common sense" approaches to reducing the number of unique motor ratings are equally appropriate for either AC or DC motor applications. With AC motors, however, there is a further level of consolidation which is not possible with DC machines. The reconnection of external motor leads is a feature of AC induction motors which allows another effective method to combine ratings which can be served by a single motor design, thereby reducing spare motor requirements.

II. MULTIPLE RATING CAPABILITY OF AC INDUCTION MOTORS

Single speed AC Induction Motors are frequently supplied with multiple external leads for various voltage ratings in fixed frequency applications. These multiple leads may be designed to allow either series to parallel reconnections (Figs 1 a,b), or WYE to Delta reconnections (Fig 1 c), or combinations of these. Such arrangements allow flexibility in the utilization of general purpose AC induction motors. Using this technique, the nominal voltage (and current) rating of an AC induction motor can be "adjusted" as shown in Table I.

Note: This type of reconnection should not be confused with the reconnection of multi-speed polyphase induction motors. In the case of multi-speed motors, the reconnection results in a motor with a different number of magnetic poles. And therefore a different synchronous speed at a given frequency.

Fig. 1 Common Winding External Reconnections

Table 1
Voltage and Current Ratings of Common Winding External Connections

When AC induction motors are applied to adjustable frequency power for variable speed applications it is useful to think of these different possible connections as being different "Volts/Hz" arrangements, instead of just different voltage levels. After all, when using an inverter, the base frequency is not typically constrained to be equal to a particular frequency (such as 50 or 60 Hz). If the "base voltage" is considered "fixed" and the "base frequency" is what is adjusted by modifying external connections, then another version of Table I can be constructed as shown in Table II.

Table II
Frequency and Power Ratings of Common Winding External Reconnections

It should be pointed out that Tables I and 11 are not intended to imply that EVERY motor can be reconnected to these alternate ratings. For example, if the voltage is increased, the insulation system needs to be suitable for that voltage level. Or, if the frequency and power were increased, the motor would need to be able to withstand potentially higher core losses and stray load losses compared to the "standard" connection. However, it is quite possible that these "alternate" ratings can be achieved - they just need to be checked out by a motor engineer.

Finally, in applications where a wide constant power speed range is required, it is common to utilize a motor design which does not reach full voltage until well above the "application's base speed." * This results in another possible set of uses of the various "V/Hz" connections for variable speed induction motors. An example of a "series WYE," "series Delta" reconnected motor used for a nominal constant power and a wide constant power speed range application is shown in Figure 3. Note that the torque below base speed for these arrangements is the same, but the constant power speed range goes from 1.33:1 to 4:1 (at the 100 HP rating). The "application base speed" in the example of Figure 3 is 650 RPM, but in the "series delta" connection the motor would not reach full voltage until close to 1150 RPM. This then allows the same size motor to provide the constant power (100 HP) required by the application from 650 to 2600 RPM. The "down side" of using this connection is that this motor requires 169 kVA from the inverter (as opposed to 97 kVA needed if the 100 HP were only required from 650 to 900 RPM).

Fig. 3 Use of Different Winding Connections to Achieve a Wide (Constant Power) Speed Range

*We are using the term "application base speed" to mean the lowest speed at which the Application requires nominal power (and constant power above that). When the constant power speed range is not too wide, this speed will usually be equal to the motor "base speed" or the speed where the motor reaches full (rated) voltage, although that is often not the case for wide (constant power) speed range applications.

III. BASE SPEEDS AND NUMBER OF MOTOR POLES

In the world of fixed frequency, line power applications, AC induction motors operating on 50 or 60 Hz power are restricted to a set of synchronous speeds as defined by the number of motor poles (Table III).

Table III
No Load (Sychronous) Speeds at 50 and 60 Hz Applied Frequencies

When applying motors to variable frequency (inverter) power supplies, the nominal frequency is no longer constrained to this extent. This allows a choice of various combinations of motor poles and base frequencies to achieve a desired base speed. Having this choice then opens up the possibility to optimize" this selection to achieve improved performance, cost, etc. There is the opportunity to minimize motor size (cost), to maximize power factor, or to maximize breakdown torque (and thereby overload capability and speed range).

While a complete discussion of the optimal choice of motor poles is beyond the scope of this paper, suffice it to say that the choice of pole number is much more a function of the nominal torque rather than the nominal speed of the application. As long as motors of various base speeds are constructed with the same number of magnetic poles, then the opportunity exists to use a common spare (with reconnectable external leads) for applications with different base speeds.

IV. EXAMPLES OF APPLICATION RATINGS WHICH COULD BE SERVED BY (AND THEREFORE "SPARED BY") A SINGLE MOTOR DESIGN

Some specific examples can perhaps better explain the above methodology of reconnection of external motor leads in order to allow flexibility in the application of AC induction motors to variable speed requirements.

Suppose that three of the ratings required by a particular application are as shown below.

(a) 100 HP at 650 RPM, constant torque below base speed, constant power to 900 RPM ...

(b) 175 HP at 1150 RPM, constant torque below base speed, constant power to 1600 RPM ...

(c) 100 HP at 650 RPM, constant torque below base speed, constant power to 2600 RPM.

Since each of these ratings represents about 800 lb-ft of torque below base speed, they inherently require about the same size motor to provide the "below base speed" (constant torque) demand. In order to provide the wide constant HP speed range of item (c), we could choose to wind the motor to reach full voltage closer to 1150 RPM, rather than at 650 RPM. This is also the rating represented earlier in Figure 3. It can also be seen in Figure 4 that by winding the motor to reach full voltage at 1150 RPM, a capability of 175 HP can be provided, with a field weakened (constant power) range of 1150-1600 RPM at 175 HP (which is item b).

If we choose a "Delta" connection for the winding which reaches full voltage at 1150 RPM, then by bringing out 3 more external leads to allow a WYE-Delta reconnection (Figure lc), we can reconnect to a WYE - resulting in full voltage being expected at 650 RPM as desired for item (a).

Fig. 4 Multiple Ratings Served by a Single Reconnectable Motor

IV.CASE HISTORY

This technique of having a single (externally reconnectable) motor design serve the requirements of several applications was employed on a Lab Coater Line at Union Camp in Princeton, N.J. That line was started up in 1992. There was a single motor provided as a spare to five separate applications. This motor was a WYE - Delta reconnectable (6 lead) design.

The five applications which were spared by a single motor are shown in Table IV. There are two ratings which were met through the use of the Delta connection, while the other three utilized a NWE connection.

Table IV
Five Ratings Spared by a Single Externally Recommendable Motor at Union Camp

Fig. 5 Five Ratings Spared by a Single Externally Reconnectable Motor at Union Camp

VI. CONCLUSIONS

As the usage of variable speed AC induction motors continues to increase for "process industries," it is important to apply these motors in an optimized manner. There are many facets to this optimization. The initial cost of an installation is affected by the number of "spare" motors needed. The techniques discussed in this paper for combining application ratings so that a single motor design can be used can help to provide a lower cost installation, due to reducing the number of unique motor designs thereby reducing the number of spare motors needed.

VII. ACKNOWLEDGEMENT

The author wishes to thank Union Camp, Princeton, N.J. for granting permission to use their Lab Line as a case study.

Document B-7099