Selecting Proper servo motors Key to Optimum System

March 1, 2000
Generally, application dictates the required motor characteristics which in turn establishes the controller. This will help you better understand how the differences and similarities between various servo motors

Not only are the number of servo motor types expanding, but some designs have multiple names. This article, which covers the the most frequently specified designs, goes back to motor basics to help you specify the best servo motor for your application.

Wound-field, brush-type dc

For many years, the wound-field, brush-type dc motor was the only servo motor. Not surprisingly, it is still the basic design of choice for many applications, plus its design is the foundation for newer and more widely used motors.

As with most motors, the material for the parts that carry most of the magnetic lines of flux are made from laminated steel. This is used for two reasons:

• Offers an economical method for making parts of various stack lengths that must contain openings for the wires.
• Minimizes hysteresis and eddy current losses. Steel thickness is important to motor efficiency, because these losses depend on the thickness (t) of the steel to a power of about 1.6 (t1.6).

Construction of this motor is no exception.

The rotor (also frequently termed “armature” for dc machines) contains laminations stamped from silicon steel, typically M19. Each lamination is typically 0.016 to 0.018-in. thick. The laminations are bonded together to form a stack with a specific length for the motor rating. The stack is pressed on a shaft that has a keyway or spline. A commutator, consisting of copper segments arranged in a circular pattern, is also pressed onto the shaft. The number of commutator segments is the same as the number of teeth on the rotor.

Windings made of insulated magnet wire are inserted into the openings in the rotor and the ends are connected to the copper commutator bars.

The stator also is made with laminations bonded together to form a stack. Interestingly, the number of teeth in the stator is typically less than the teeth in the rotor. The stator may have four teeth and the rotor, nine, because the stator will be wound to form distinct (salient) north and south magnetic poles.

To hold these parts together, endbells (also called endshields) contain bearings to support the rotor and mount on the stator. In addition, the endbells hold the brushes that ride on the copper commutator to enable current to flow through the rotor.

One of the primary advantages of this motor design is its linear operation. Torque is proportional to rotor current and speed to rotor voltage, if the stator field current is constant. Also, the controllers and power amplifiers for these motors are relatively simple and generally understood. For some machine-tool and winder applications, it is desirable to weaken the shunt field (reduce the voltage to the field) to enable the motor to operate above base speed, but with a lessened torque capability. Such operation is usually called “constant horsepower operation.”

On the downside, brush wear requires periodic inspection and replacement. Enclosures for explosive and other special environments are large and costly in relation to other motor designs. The rotor windings are also a limiting factor. If speeds are more than about 7,000 rpm, then you should consider one of the brushless motors that are discussed later.

Permanent-magnet, brush-type dc motor

A direct spin-off from the wound-field motor, this newer design replaces the stator windings with permanent magnets, Figure 1. Generally, for equivalent ratings, these PM motors are smaller than wound-field units. How much smaller depends on the magnet materials. The smaller the motor, the higher the cost, because the expensive rare-earth magnets have more magnetic strength and are smaller than do the less costly magnets.

The rotor (armature), from a construction standpoint, is basically the same as the wound-field motors.

The stator, however, is made with permanent magnets mounted on solid steel (rather than laminations) to conduct the magnetic lines of flux from one magnetic pole to another.

The absence of a wound field is both beneficial and constraining. The PM design eliminates the need for a power supply for the wound field. However, this eliminates the ability to control field strength and obtain a speed that is more than base speed. To overcome this constraint, many engineers just specify a larger motor that can deliver the maximum speed but will have torque to spare. This may still be more economical than using a wound-field motor.

The PM brush-type have most of the same constraints — speed limitations, brushes to maintain, and special enclosure limitations — as do wound-field dc designs. But for some the PM design is adequate.

Permanent-magnet, brushless dc motor (BLDC)

Know by a variety of names — brushless dc, BLDC, and ac servo, Figure 2 — this motor is the most often specified servo motor for servo-positioning applications. Moreover, it is used for generalpurpose applications often without a position feedback device.

Permanent magnets are mounted on the rotor rather than on the stator. For applications requiring fast response — such as robots and copiers — the magnets are mounted on laminations that have cut-outs to reduce rotor inertia. By contrast, machine tool applications and others require speed stability, so the material is left in the rotor to maintain the rotor inertia.

The stator resembles that of a conventional ac induction motor with slots punched in steel laminations. Insulated magnet wire with additional insulation is inserted in the slots to form three-phase stator windings that produces a flux that the permanent magnets on the rotor follow, thus turning the shaft.

There are two basic types of stator winding patterns. One is for trapezoidal current wave forms from the controller, and the other is for controllers that produce sinusoidal current wave forms. It is difficult to determine which is which by looking at it.

There are several reason for the two designs: A controller that produces trapezoidal wave forms costs less than those that produce sinusoidal wave forms. However, sinusoidal controllers and motors will produce more even shaft rotation, whereas the trapezoidal will have minor torque pulses. The magnitude of the difference depends on many parameters including motor rating, rotor inertia, and specific controller characteristics.

BLDC motors are gaining acceptance because when equipped with an accurate speed feedback device and quality amplifier, they can deliver consistent speed regulation, high speeds (over 18,000 rpm depending upon the motor rating), reliable operation (No brushes to maintain.), and small package per rating because the windings that produce the heat are on the stator where the heat can be easily dissipated. This is sometimes called inside- out construction. Also, BLDC motors lack brushes riding on a commutator; as such they are suitable for clean-room applications.

With IP68 sealing, BLDC motors can be used in flammable liquid and gas environments.

As with all devices, there are some caveats. One is the operating temperature of the motor including the ambient temperature and winding temperature. Some magnets can lose some or all of their magnetic energy, depending on the specific magnet material, if they exceed certain temperatures, Check with your supplier if the ambient temperature may exceed the traditional 40 C (104 F).

Temperatures also affect feedback device capabilities. Encoders are typically limited to about 100 to 120 C. This value results from the total heat at the unit including heat produced by the motor.

AC induction

Within the last few years, new controllers that expand the capabilities of ac-induction motors have been introduced, Figure 3. Some controllers enhance motor performance to equal that of dc wound-field motors. Therefore, acinduction motors are now powering web processing machines, machine tool spindles, extruders, traction drives, and servo-positioning devices.

For inverter duty, choose motors designed for this service and equipped with the proper feedback device and separately powered blower for cooling if the motor will be operated at full torque below about 60% of base speed.

DC pancake motor

Also known as a coreless motor and as a disk armature, the dc pancake motor resembles a pancake with a shaft through it. This is the fastest responding motor available but one with the lowest torque ratings. Because it is such a special device, it will be covered in a later article.

View Associated Figure

Victor Feinberg is director of engineering for API Gettys Inc., Racine, Wis.

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