Classic AC motors: Concatenation of induction motors

. The concatenation principle of operating induction motors is best described in older texts on electrical engineering. The actual word is not a technical term, per Se, but simply demotes things or events arranged in a series. Concatenation pertains to an old technique of connecting induction motors. It’s relevant to know that the technique was largely used with traction applications of electric motors, especially the railroads. It might soon enough assume relevancy again as interest in electric vehicles as alternative to cars propelled by gasoline and diesel engines increases.

The basic idea underlying concatenation is to enable more than one speed to be obtained when using the induction motor(s) for vehicle propulsion. This can be accomplished by changing the frequency, changing the number of poles, or by causing the slip between the rotor and the rotating electromagnetic field from the stator to change. Until recently, there was no practical way to change the frequency. Changing the number of poles is workable for ceiling fans, but tends to seriously compromise efficiency when used in a traction vehicle. Inasmuch as the immediate source of energy in electric vehicles is the battery, motor efficiency is of primary importance if accept able range between charging is to be realized. Causing rotor slip can be advantageously used, but not by simply loading down the shaft of a single induction motor.

In concatenation, primary reliance for speed change results from a unique use of change in sup and the resultant change infrequency. This is brought about by the use of two induction motors coupled to the same shaft. At least one of these induction motors must be of the wound-rotor type. The other can be a squirrel-cage induction motor. In order for the set to be self-starting, polyphase motors have generally been employed in this scheme. The basic arrangement is shown in FIG. 28. The simplest use of this technique makes use of two motors with identical numbers of poles. This, however, is not a requisite—motors with unequal poles can be used to yield a more desirable selection of speeds. An added feature of concatenation is the ease of reversing rotation by electrical switching of the phase connections of the AC line, or of the machine interconnections.

FIG. 28 Basic setup for concatenation of induction motors. By appropriate selection of the interconnections, it’s possible to obtain either three forward and one-reverse speed, or three reverse and one forward-speed. Otherwise, actual speed governed by the number of poles in the motors and by the line frequency.

In the ensuing discussion of this technique, keep in mind that during its heyday, there were no solid-state frequency changers such as cyclo-converters or three-phase inverters. Rather than relegate concatenation into obsolescence, it’s likely that the newer solid-state circuits can be applied in mutuality to produce enhanced performance and extended operational flexibility to electric-vehicle propulsion.

In determining what speeds are available from a setup such as that of FIG. 28, it’s convenient to deal with synchronous speeds. These will be approximate in the sense that induction motors don’t actually run at synchronous speed, but always slightly lower. Thus, a four-pole induction motor operating from a 60-Hz source does not rotate the synchronous speed of 1800 RPM, but rather at a slower rate, say 1750 RPM. Synchronous speed is the rotational rate of the electromagnetic field set up by the stator windings. Ideally, the mechanical rotation of the rotor should also be at this synchronous speed. However, it’s the nature of the induction motor that zero torque would be developed if the rotor actually attained synchronous speed. Be cause the operating speed is quite close to the synchronous speed, it’s not uncommon to identify a 60-Hz four-pole induction motor as an 1800-RPM motor, a 2-pole induction motor operating from 60 Hz as a 3600-RPM motor, etc.

For purposes of more exact determination of speed, the synchronous speed and the rotor rotation speed give rise to a third quantity, the slip. Slip is expressed as a percentage, and is established as follows:

S = [100 (synchronous speed — rotor speed] / synchronous speed

At standstill, slip is 100 percent and the frequency of the current induced in the rotor is the same as the AC supply frequency. If the rotor could attain synchronous speed, its induced frequency would be zero. Inasmuch as the slip in the convention ally operating induction motor is low, say two or three percent, the frequency of the current induced in the rotor is relatively low. Thus, if a 60-Hz induction motor operates with a slip of 2.5 percent, the frequency of the rotor current would only be 2.5 percent x 60 or 1.5 Hz. Arid, during startup and acceleration, the rotor frequency would change progressively from 60 Hz to 1.5 Hz.

Contemplation of the above facts reveals the induction motor as an inherent frequency changer with an “internal” frequency quite different from the AC frequency applied to the stator windings! The idea naturally presents itself that a second induction motor powered from the current induced in the rotor of the first motor would tend to run at another (lower) speed. Indeed, the two motors depicted in this “series” or concatenated relationship in FIG. 28 must exhibit unusual speed characteristics. Inasmuch as the two motors share a common shaft, it’s not easy, by in tuition alone, to ascertain the shaft speed(s) of this interesting arrangement.

It turns out that, to a good practical approximation, the shaft speed of such a motor combination is equal to that of a fictitious third motor having either the sum or difference of poles in the actual motors. Generally, this allows the choice of four different speeds, with one choice being a reverse-rotation speed. How might this work out in an example? Assume the AC line frequency is 60 Hz.

Let one of the two shaft-coupled induction motors have four poles, while its companion has eight poles. The synchronous speed of the four-pole motor is 180-degree RPM. The synchronous speed of the eight-pole motor is 900 RPM. These, then, are two of the four approximate no-lead speeds that are available. The speeds are said to be approximate in the sense that induction motors operate close to, but not at, synchronous speed. Additionally, an approximate speed will be available corresponding to a twelve-pole induction motor (the “fictitious” motor alluded to). Such a twelve- pole motor would have a synchronous speed of 600 RPM.

Yet, another speed is available by connecting the stators so that the motors tend to rotate in opposite directions. Under this condition, you have a four-pole “fictitious” motor turning at 1800 RPM in the reverse direction. Summing up, by merely changing the stator connections you obtain at least four shaft speeds. (“At least” four, because the first three speeds and the fourth speed could also have been traded, making eight speed situation in all).

It can be appreciated that, depending on the selection of the motors, a variety of speeds can be provided. Also, with modern variable-frequency solid-state inverters, one isn’t restricted to 60 Hz and the speed range of the combination can be made continuous over a wide range. Also, motor “B” in FIG. 28 can be a simpler squirrel-cage induction motor rather than the wound-rotor type shown. The possible advantage of the wound-rotor machine over the squirrel-cage type lies in its higher starting torque.

In the event this technique is adopted for electric vehicles, it’s likely that the two induction motors will be constructed as a single machine. One purpose of its use will be to eliminate need for a mechanical transmission.

Incidentally, in those speed situations in which the resultant shaft speed is due to the two motors tending to turn in opposite directions, the concatenated motors are never self-starting. The shaft must already be turning in the desired direction, even for a brief instant. This can readily be taken care of by the switching arrangement in the motor leads.