GENERAL APPLICATION CONSIDERATIONS
Regenerative operation and braking
All motors are inherently capable of regenerative operation, but in drives the basic power converter as used for the ‘bottom of the range’ version will not normally be capable of continuous regenerative operation. The cost of providing for fully regenerative operation is usually considerable, and users should always ask the question ‘do I really need it?’
In most cases it is not the recovery of energy for its own sake, which is of prime concern, but rather the need to achieve a specified dynamic performance. Where rapid reversal is called for, for example, kinetic energy has to be removed quickly, and, as discussed in the previous section, this implies that the energy is either returned to the supply (regenerative operation) or dissipated (usually in a braking resistor).
An important point to bear in mind is that a non-regenerative drive will have an asymmetrical transient speed response, so that when a higher speed is demanded, the extra kinetic energy can be provided quickly, but if a lower speed is demanded, the drive can do no better than reduce the torque to zero and allow the speed to coast down.
Duty cycle and rating
This is a complex matter, which in essence reflects the fact that whereas all motors are governed by a thermal (temperature rise) limitation, there are different patterns of operation which can lead to the same ultimate temperature rise.
Broadly speaking the procedure is to choose the motor on the basis of the r.m.s. of the power cycle, on the assumption that the losses (and therefore the temperature rise) vary with the square of the load. This is a reasonable approximation for most motors, especially if the variation in power is due to variations in load torque at an essentially constant speed, as is often the case, and the thermal time-constant of the motor is long compared with the period of the loading cycle. (The thermal time-constant has the same significance as it does in relation to any first- order linear system, e.g. an R/C circuit. If the motor is started from ambient temperature and run at a constant load, it takes typically four or five time-constants to reach its steady operating temperature.) Thermal time-constants vary from more than an hour for the largest motors (e.g. in a steel mill) through tens of minutes for medium power machines
down to minutes for fractional horsepower motors and seconds for small stepping motors.
To illustrate the estimation of rating when the load varies periodically, suppose a mains-fed cage induction motor is required to run at a power of 4 kW for 2 min, followed by 2 min running light, then 2 min at 2 kW, then 2 min running light, this 8-min pattern is repeated continuously. To choose an appropriate power rating we need to find the r.m.s. power, which means exactly what it says, i.e. it is the square root of the mean (average) of the square of the power. The variation of power is shown in the upper part of Figure 11.6, which has been drawn on the basis that when running light the power is negligible. The ‘power squared’ is shown in the lower part of the figure.
The average power is 1.5 kW, the average of the power squared is 5 kW2, and the r.m.s. power is therefore p5 kW, i.e. 2.24 kW. A motor that is continuously rated at 2.24 kW would therefore be suitable for this application, provided of course that it is capable of meeting the overload torque associated with the 4 kW period. The motor must therefore be able to deliver a torque that is greater than the continuous rated torque by a factor of 4/2.25, i.e. 178%: this would be within the capability of most general-purpose induction motors.
Motor suppliers are accustomed to recommending the best type of motor for a given pattern of operation, and they will typically classify the duty type in one of eight standard categories, which cover the most commonly encountered modes of operation. As far as rating is concerned the most common classifications are maximum continuous rating, where the motor is capable of operating for an unlimited period, and short time rating, where the motor can only be operated for a limited time (typically 10, 30 or 60 min) starting from ambient temperature.
Enclosures and cooling
There is clearly a world of difference between the harsh environment faced by a winch motor on the deck of an ocean-going ship, and the comparative comfort enjoyed by a motor driving the drum of an office photocopier. The former must be protected against the ingress of rain and seawater, while the latter can rely on a dry and largely dust-free atmosphere.
Classifying the extremely diverse range of environments poses a potential problem, but fortunately this is one area where international standards have been agreed and are widely used. The International Electrotechnical Committee (IEC) standards for motor enclosures are now almost universal and take the form of a classification number prefixed by the letters IP, and followed by two digits. The first digit indicates the protection level against ingress of solid particles ranging from 1 (solid bodies greater than 50-mm diameter) to 5 (dust), while the second relates to the level of protection against ingress of water ranging from 1 (dripping water) through 5 (jets of water) to 8 (submersible). A zero in either the first or second digit indicates no protection.
Methods of motor cooling have also been classified and the more common arrangements are indicated by the letters IC followed by two digits, the first of which indicates the cooling arrangement (e.g. 4 indicates cooling through the surface of the frame of the motor) while the second shows how the cooling circuit power is provided (e.g. 1 indicates motor-driven fan).
Dimensional standards
Standardisation is improving in this area, though it remains far from universal. Such matters as shaft diameter, centre height, mounting arrangements, terminal box position and overall dimensions are fairly closely defined for the mainstream motors (induction, d.c.) over a wide size range, but standardisation is relatively poor at the low-power end because so many motors are tailor-made for specific applications.
Supply interaction and harmonics
Most converter-fed drives cause distortion of the mains voltage which can upset other sensitive equipment, particularly in the immediate vicin- ity of the installation. There are some drives that are equipped with ‘front-end’ conditioning (whereby the current drawn from the mains is forced to approximate closely to a sinewave at unity power-factor), but this increases the cost of the power-electronics and is limited to small- and medium-power drives. With more and larger drives being installed the problem of mains distortion is increasing, and supply authorities therefore react by imposing increasingly stringent statutory limits governing what is allowable.
The usual pattern is for the supply authority to specify the maximum amplitude and spectrum of the harmonic currents at various levels in the power system. If the proposed installation exceeds these limits, appropriate filter circuits must be connected in parallel with the installation. These can be costly, and their design is far from simple because the electrical characteristics of the supply system need to be known in advance in order to avoid unwanted resonance phenomena. Users need to be alert to the potential problem, and to ensure that the drive supplier takes responsibility for handling it.