Not All Electric Motors Are Standard

First Quality ~ First Time

Apparatus Repair & Engineering, Inc.

A.R.&E. is the Premier Electric Motor Sales, Service, and Repair facility in the quad-state region of Maryland, Pennsylvania, West Virginia, and Virginia. This business began in 1927, and we are proud to continue the efforts of the founding partners who have served the local Commercial and Industrial markets over these many years.

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Special Electric Motors

While there are numerous "specialty" motors designed, manufactured, and in use today, this topic page is here to discuss only a few of the more common types that you may run into in the span of your working career. We won't get into great detail but will give you some general information so you won't be totally in the dark if you're asked about one of them.

Traction Motor

A "Traction Motor" is a series DC motor that has very high starting torque and is used heavily in the railroad industry. It is the type of motor that powers the locomotives we see every day. The "Series DC Motor" is discussed in detail on our DC motor page and I would request that you use this link to access that topic, should you wish to read about it's characteristics and operating parameters.

Synchronous Motor

Using one of my favorite sources of technical dictionaries, the following descriptive line comes from©:

"A synchronous electric motor is an AC motor in which, in it's normal operating state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral number of AC cycles."

A standard induction motor, requires "slip" to produce the torque for powering loads. So the induction motor will have a nameplate speed (Full Load Speed) of something less than the synchronous. Something like 1740 RPM for a standard 1800 RPM motor. The Synchronous Motor runs at true "synchronous" speed. We can calculate that "synchronous" speed if we know a particular characteristic about the motor. We need to know the "number of electrical POLES" in the motor. Motors are designed with electrical poles in pairs; i.e. 2, 4, 6, 8, etc. The formula for calculating speed of a motor having 6 poles and operating on a 60 Hz power supply is:

Synchronous RPM = (120 x Power Line Frequency) / Number of Poles
RPM = (120 x 60) / 6
RPM = 7200 / 6
RPM = 1200

So the "synchronous speed of this particular motor is 1200 RPM. If it is an "induction" motor, it's full load RPM will be somewhere around 1140 RPM (accounting for the slip). If it is a "synchronous" motor, it will run at the full "1200 RPM".

Synchronous motors can be "self starting" but they don't have much starting torque. The method for starting is a "steel" or "copper" ring around the diameter of the rotor. This ring is called an "amortisseur" winding (although it's not really a "winding"). It is simply a path to conduct current and therefore create a magnetic field. This magnetic field is sufficient to get the rotor turning and allow it to build speed. As the motor comes up to speed, at some point close to "synchronous" speed, the power supply to the "stator" field coils is energized and the full field power is functional. When this happens, the motor "locks" into synchronization.

Synchronous motors are used in applications where steady speed is required and in the case of LARGE HP motors... slow speeds. And due to their excellent power factor characteristics, a totally UNLOADED synchronous motor may be run in an industrial plant to improve the overall power factor of the total power system. They are also used on pumps, compressors, and paper mills.

Repulsion Start-Induction Run Motor

The repulsion-induction motor can be called a sophisticated version of a repulsion-start motor. Approximately the same overall objective is achieved, but the "repulsion-start" motor comes with a short-circuiting centrifugal mechanism. And, other than the commutator and short-circuited brushes there is no switching process involved in this hybrid, as it accelerates. In addition, the brushes are never lifted from contact with the commutator. The brushes, commutator, and the commutated armature winding are arranged much like that of a repulsion motor. But deeply embedded in the armature iron is a squirrel-cage type winding with short-circuiting rings welded at each end.

During the starting process, the initial slip frequency is equal to that of the power line. Since the squirrel-cage winding has deliberately been made highly inductive, its reactance deters the flow of short-circuit current. Under these conditions, the squirrel cage contributes very little torque as long as the motor speed remains at the low end of the synchronous speed range. So, initially, the rotation of the armature is produced by the torque developed in the commutated armature winding. This works well, placing the repulsion motor high on the list in the torque department, and makes it capable of very high starting torque.

As the armature accelerates, the frequency of the current induced in the squirrel cage decreases. This results in less inductive reactance and, therefore, greater torque-producing current. At the same time, the repulsion-motor torque is decreasing. Somewhere in the vicinity of 80 percent of synchronous speed, induction-motor action begins to predominate. Therefore, the speed-regulation curve departs from what it would be for a repulsion motor and assumes the flatter characteristic generally associated with induction motors. An exception is the speed range "above" synchronous speed. But, how can an induction motor perform in this speed range?

It’s still true that induction motors can only approach synchronous speed. The existence of a super-synchronous speed range in the repulsion-induction motor is due to the fact that the repulsion-motor characteristics continue to exert influence even at zero torque demand. Thus, if the shaft of this motor is spinning freely, without any external load, the torque developed by the commutated-armature winding boosts the speed above its synchronous value. However, the extent of this action is limited because of counter-torque developed by the squirrel-cage winding—it functions as an induction generator above synchronous speed. The unique behavior of this machine stems from the fact that the inductive reactance responsible for the interchange of motor characteristics does not display the abrupt and positive action of an electrical switch.

By the same rationale, inductive motor action is present down to zero speed. In deed, there is always an interchange of energy between the two windings. This coupling between the windings makes the starting torque slightly less than that obtained in the switch-type repulsion-start motor. However, the power factor and the commutation tend to be improved by the presence of the two windings.

Phase Converter

A phase converter is a device that converts electric power provided as single-phase to multiple phase or vice versa. The majority of phase converters are used to produce three-phase electric power from a single-phase source, thus allowing the operation of three-phase equipment at a site that only has single-phase electrical service. Phase converters are used where three-phase service is not available from the utility, or is too costly to install due to a remote location.

There are basically two types of "Phase Converter" in general use today. The "Static" or "Solid State" converter, and the "Rotary Phase Converter". This topic is going to talk about the "concept" of Phase Conversion so we'll touch on both the Static and Rotary devices.

Before we do that, however, it is noteworthy to mention that in today's world, the VFD (Variable Frequency Drive) has been applied successfully as a Phase Converter. In most instances, it is possible to power a standard VFD with single-phase power, and use the generated three-phase power as one would any other three-phase power supply. It becomes necessary to "oversize" the VFD due to the main power components, but it has been done, and very successfully I might add. Current inrush on the load side needs to be considered but that's pretty easy to handle.

Back to the remaining 2 converters...! In either type of phase converter, the theory of operation is that the conversion device is powered by the single-phase supply available. From that power, it is manipulated and "shifted" so as to create a "third" phase that is basically 120° offset from the other phase, just like a three-phase system. The original single-phase power is then wired, along with the "manufactured" phase, to the three-phase load. And it works! In the two types, the "static" converter uses only capacitors to "shift" the phase and create the new 3rd leg, and in the rotary type, it is similar to a generator but also uses capacitors as the shifting mechanism. In the case of the rotary design, it looks like a motor but has no shaft extension!

There are a couple things to watch out for but if the load is reasonably steady, it works pretty well. It has been found that when the load becomes TOO light, the manufactured leg "appears" to disappear and what looks like a "single-phase" condition is created. With this issue, the three-phase motor may burn up if the overload protection does not take it off line. So applying the correct size converter is critical. Don't buy "more than you need"! In this case, "more is NOT better".

Stepper Motor

A Stepper Motor is a device that when fed a digital signal (usually a square-wave pulse), rotates the shaft of the motor a specified number of degrees of rotation, based on a number of design factors. The motor is a rather simple device and inexpensive by most standards. The critical and most important portion of the system is the Drive and Driver circuit. Stepper motor performance is strongly dependent on the driver circuit. The stepper motor is basically a "brushless DC motor.

And while we attempt to offer technical information on this website, the design and application of stepper motors is best left to the manufacturers and their design engineers. A.R.&E. would be happy to partner with you and an appropriate manufacturer of motor and drive for your next project.

In the meantime, advantages of a stepper motor system include:

  • Low cost for control achieved
  • High torque at startup and low speeds
  • Ruggedness
  • Simplicity of construction
  • Open loop operation
  • Low maintenance
  • Will work in any environment
  • High reliability
  • Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3–5% of a step and this error is non-cumulative from one step to the next.
  • Excellent response to starting/stopping/reversing.
  • Very reliable since there are no contact brushes in the motor. Therefore, the life of the motor is simply dependent on the life of the bearing.
  • It is possible to achieve very low-speed synchronous rotation with a load that is directly coupled to the shaft.

Servo Motor

A servomotor is a rotary actuator or linear actuator that allows for precise control of angular or linear position, velocity and acceleration. It consists of a suitable motor coupled to a sensor for position feedback. It also requires a relatively sophisticated controller, often a dedicated module designed specifically for use with servomotors. For these reasons, the Servomotor is often referred to as a "Premium Performance" alternative to a stepper motor system.

An advantage of the Servomotor system is that the system "knows" it's location when it is energized, based on the feedback system. In a stepper motor system, since there is no feedback device, the controller must move the motor to a "specific" or "home" position when it is started up in order to know where it's located. The better servomotors use rotary encoders as the feedback device which offers excellent results.

The ultimate servomotor package will offer the customer a "Brushless AC Motor" having permanent magnet fields. When this motor is coupled with a digital controller and encoder, the customer can be assured of precise operation and long life.

Contact A.R.&E. for additional information and assistance in applying a "Servo" system to your next project.

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