Special Motor Design

A number of drive applications can only be fully realized with a technically and economically optimized solution that comes with a special motor design.

When designing three-phase asynchronous motors, the fi rst step is defi ning the shape of their magnetic active parts, i.e. the layout of the lamination sheets for both stator and rotor. The sheet dimensions depend to a very high degree on the following parameters:

• Output, and duty type
• Number of poles, and frequency
• Cooling method (surface cooled, internally cooled, water-cooled)
• Voltage (low, medium, high voltage)
• Type of rotor (squirrel cage, slip ring = wound rotor)

It is practical to differentiate between so-called “norm” and “transnorm” motors. Norm motors are three-phase squirrel cage motors according to DIN EN 50 347. In this standard, there are defi ned – amongst others – the main mechanical dimensions for the frame sizes 56 till 315. In addition one frame size may be designed in two different lengths. Thus for each number of poles a motor type range with an output gradation is created. For instance, the output range for surfaced-cooled 4-pole motors goes from 60 W to 132 kW.

For larger motors the frame size gradation is continued in the standard IEC 60 072, i.e. beyond the standard DIN EN 50 347. Therefore these motors are called transnorm motors.

For norm motors, manufacturers have developed optimized lamination designs, providing suffi cient torques for standard applications, and furthermore creating the basis for the realization of requirements regarding a good effi ciency, high power factor, low noise levels, and temperature limits required.

The technical requirements can only be met in an optimum way by using individual lamination designs for each frame size, and at least for each of the pole-pairs, from two to eight poles. However, this necessitates a large number of punching tools and increased efforts in both investments and production.

The use of only one lamination design for several pole-pairs is one option, but compared to individual lamination designs, certain technical values cannot be optimum. As a consequence, the result might show that in a certain effi ciency classifi cation the particular type slips to a lower grade.

For motors up to approx frame size 160 these disadvantages with regard to effi ciency, power factor, and noise levels might be of minor infl uence. For larger motors, however, these disadvantages are often quite signifi cant. In addition, such motors very often cannot be modifi ed to meet higher requirements.

Matching of motor design
For standardized or norm motors, manufacturers make available extensive catalogues showing the important data for standard outputs i.e. current, power factor, and efficiency for duty type S1 (continuous operation) as well as starting current, starting torque, and break-down torque.

These data shown are valid for either 400 V, or for a voltage range of e.g. 380-420 V at 50 Hz. Furthermore, the catalogues also show the mechanical dimensions, weight, and moment of inertia, and provide useful design information. Thus for many applications all data necessary for the solution of drive problems and the selection of suitable motors is provided.

However, there are quite a few requirements which cannot be met by considering standard solutions. In such cases, the motors need to be specially matched using electrical designs different to the standard design, and considering the particular drive problem. Since these special drive problems occur to a high degree within the output range of standard/norm motors, this article is limited to this group of motors.

A modifi cation of the stator winding in norm motors is a relatively simple measure resulting in only minor additional efforts. In case the requirements are different from the standard design the fi rst attempt will be a modifi cation of the stator winding with the same lamination design.

This is suffi cient for instance for different voltages, frequencies, or duty types. The necessary modifi cations of the motor design are the turns of the winding and the diameter of the wire. There is a further differentiation of this modifi cation, if the magnetic relations (iron saturation) remain the same, or are different.

In case the drive requirements can no longer be met with a modification of the stator winding, or only with considerable disadvantages, a change of the rotor lamination design becomes inevitable. The following sections deal with some of such special requirements and describe the matching modifi cations of the rotor lamination design.

Torque-speed modification
Both the shape and the material of the rotor bars have a fi nal infl uence on the torque-speed curve. A relatively common special requirement compared to standard motors is the starting capability at only 80 percent of the rated voltage. While the torque depends on the voltage square, in practice, the torque decreases more due to the saturation of leakage bridges. This situation could still be born by the motor at constant load torque, however, when driving a loaded conveyor belt, or a crusher mill, or a screw compressor the situation deteriorates due to the high breakaway torque at standstill.

This problem can be solved by using a larger norm motor with higher output and higher torque, but in many applications, this solution is not a good choice because of larger dimensions and heavier weight of the motor. Furthermore, the values of starting current and rated current increase. And since these data are determining the engineering of the complete electric installation the result is a price increase for fuses, leads, switches, etc.

The better solution therefore is the extension of the leakage bridge between the upper and the lower bar in the rotor. The motor then develops a suffi cient starting torque. Minor differences in power factor and effi ciency compared to the standard motor can then be accepted. A not suffi cient torque when operating pumps or fans can result if motors are started in star connection to reduce the starting current. These motors develop, compared to the delta connection, less than one-third of the torque and only 70 percent of the synchronous speed.

If then the motor is switched to delta connection, too high a current fl ows despite the start at star connection. The problem can be solved using two modifi cations: fi rst, the turns in the stator winding are reduced (higher saturation); and second, the standard double-bar is replaced by a modifi ed double-bar with lower leakage inductivity.

Temperature control
One of the most vital requirements for electric machines is holding the permissible temperatures at continuous operation. The temperature of the winding and consequently the insulation temperature is a decisive value since it is the most important factor for the lifetime of the insulation.

Industrial motors normally are utilized according to class B, i.e. a limit temperature of 130 °C. Considering a maximum coolant temperature of 40 °C and local temperature differences (hot spots) of 10 K, the winding may reach a temperature rise of 80 K in rated conditions.

Limit temperatures of the winding higher than shown in the table can be achieved by using insulation systems of class H or class C. High-temperature motors like special fi re-gas motors or motors for hermetic pumps are for instance equipped with class C insulation for 400 °C.

• Higher coolant temperature
In applications where the coolant temperature is for instance 50 °C, the motor can either be utilized according to class F, or the output has to be reduced by approx. 10 percent. A modifi cation of the motor is normally not necessary.

• Heavy starting conditions
The limit temperatures of antifriction bearings and rotor bars are not reached in standard applications and duty types S1, S2, S3, or S6. Thus these limits are not relevant. However, in case of heavy starting conditions the situation is different. A starting process over a longer time period (with high current, and bad cooling due to low speed) may cause both the permissible winding temperature as well as the permissible rotor temperature to pass above the limits. A starting procedure of more than 15 seconds is normally considered as heavy starting duty. In such cases, it must be checked whether either a standard motor is suffi cient or the motor needs to be modifi ed to meet the requirements.

• Centrifuges/separators
When starting centrifuges, the motors very often need to accelerate heavy masses against very low load torques. A running-up using standard motors on the mains with constant voltage and frequency is in most cases no longer possible due to the high temperature rise. Using a combination of various measures, however, even extreme application problems can be solved. Apart from using either copper or brass bars, very often, the short-circuit rings are reinforced, the connections between bars and rings are welded instead of brazed, and the stator winding is sealed with a silicone mass to increase its heat capacity. With such measures and a star-delta-starting, run-up times of up to 20 minutes are possible.

• Sole protection
A drive requirement very often specifi ed is that the limit temperatures are kept even in case of failure, e.g. at blocked rotor. This can be achieved with features like protective switch, or a direct temperature monitoring of the stator winding e.g. with PTC thermistors. However, whether temperature monitoring of the stator winding allows the rotor to be protected (sole protection) depends on the temporary-thermal behavior of the motor. In order to determine this behavior, the temperature rise at blocked rotor should be measured at the test phase.

Sole protection can be met by using a copper instead of an aluminium rotor. The results are two advantages: copper is capable of storing about 40 percent more heat than aluminium; the permissible temperature limit is 100 K higher. By using a copper rotor, a sole protection of, for instance, 2-pole motors up to frame size 315 at an output of 200 kW is possible. At higher numbers of poles the sole protection is possible even for larger motors.

• Explosion-proof motors
In the fi eld of norm motors, explosion-proof motors in the designs II G 3 EEx nA “nonsparking“, II G 2 EEx e II “increased safety”, and II G 2 EEx d(e) II “fl ameproof enclosure” are considered.

The electrical and mechanical design of non-sparking motors is basically the same as standard motors. However, materials used must not create sparks under normal operating conditions. And certain requirements with regards to leakage paths, material strength (e.g. of fan cover), minimum dimensions (e.g. between fan and fan cover) must be kept. The electrical design of flameproof motors is also identical to standard motors but mechanically, these motors are of special design. The flameproof enclosure needs to be capable of keeping an internal explosion within the enclosure, and not to ignite the surrounding explosive atmosphere. This request is met by using stronger housings and end shields, and at the connecting areas (e.g. between housing and end shields, and end shields and shaft) by keeping the dimensions of the so-called fl ame paths strictly within the limits set for the gas groups and temperature classes.

Increased safety II 2 G EEx e motors have to at least take account of the ignition temperatures in order to guarantee the explosion protection. This is applicable for all motor parts, in particular also for the rotor. Furthermore it applies to all operational conditions including failures like blocked rotor.

The relevant standards also specify that the temperature rise of the winding in rated condition is lower than defi ned for the applicable temperature class. Hence EEx e motors have lower rated outputs than standard motors. The reduction in output is considerable mainly in 2- and 4-pole motors and can be up to 30 percent.

In order to maintain the explosion protection, either protective switches or thermal motor protection, as described in “sole protection”, can be used. An important test is here again the operation at blocked rotor. From the temperature curves and the operational temperature rise the heating-up period – the time until the critical temperature of a motor part, either stator winding or rotor is reached – is calculated. The protection devices have to switch off the motor before reaching this temperature.

These requirements clearly show that EEx e motors need to be designed differently to standard motors. The changes apply to both the stator winding and the rotor. In the latter very often the standard double-bar is replaced by a drop-shape bar or a copper high-bar in order to reduce the temperature rise at blocked rotor.

Beyond standards
The designs of lamination sheets both in norm motors and transnorm motors are the basis for the realization of most requirements with regard to suffi cient torque, good effi ciency, high power factor, low noise levels, and temperature limits. Apart from the standard requirements there are, however, quite a number of special requirements which can only be realized with a technically and economically optimized solution, i.e. with a special motor design.

The selected examples used in this article demonstrate which changes in the motor design can fulfi l special requirements. The modifi cations mentioned are to be understood as possible solutions for changing the operational behaviour of motors into the direction required. However, whether the solution described is suffi cient, or whether further changes and modifi cations become necessary needs to be checked for each individual case.

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Pictorial Summary

Thomas Fladerer is Head of Electrical Calculation of Loher Industrial Motors, Siemens; Prof. Dr.-Ing. Dieter Seifert teaches electrical engineering at the University of Applied Sciences, Regensburg.