Siemens induction motors- generators cgii User Manual

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When this method is used on the stator, the stator 
phases may be connected in series or in parallel to 
suit the available power supply if both ends of all 
phases are accessible.  If only three leads are 
brought out of the motor, the current may be 
circulated between one terminal and the other two 
connected together.  If this is done, the temperature 
of the single lead connection must be checked 
frequently, and it is desirable to shift the leads 
occasionally.  Usually 50 to 100% of full load current 
will produce the required temperature. The dc voltage 
required for this current will be 0.25 - 5.0% of the 
normal voltage per phase, and the corresponding 
power will be 0.50 - 3.25% of the rating. 
 
Alternating current can be used on the stators of 
squirrel cage induction motors if the rotors are 
removed.  Alternating current is usually not as easy 
to control as required voltage control, and a.c. 
requires a higher voltage source, approximately 10 to 
30% of the rated winding voltage.  In addition, care 
must be taken that miscellaneous parts adjacent to 
the windings, such as lead studs, core supporting 
member, etc., do no overheat due to induced 
currents and the lack of normal ventilation. 
 

High temperatures. 
May cause damage to insulation. 
Avoid hot spots and radiant type heat 

 

Long life of bearings is assured by maintaining proper 
alignment, belt tension, and lubrication at all times.  
Incorrect alignment of solid and flexible couplings can 
cause excessive load on bearings, and excessive 
vibration and thrust.  Misalignment of belt drives can 
cause thrust or harmful shaft oscillation.  Improper 
alignment of gear drives will produce shock loads and 
may bend the shaft. 
 
Excessive belt tension often causes overheating and 
failure of bearings. Bearings tend to overheat when 
pulley centers are too close, pulley diameters are too 
small, or belt speed is too high. 
 

 
Bearing Construction 
Two types of bearings are employed in induction 
motors.  These are; 
 
1. Antifriction 

bearings. 

2. Sleeve 

bearings. 

 
The type of bearing mounting and supporting 
structure will depend upon the type of bearing. 
 
Bearing housing construction will also vary with the 
type of bearing.  Antifriction bearing motors have one 
piece bearing bracket construction.  Sleeve bearing 
motors have split bearing housings. 
 
Insulated Bearings 
One or both bearings may be insulated to prevent 
shaft currents from pitting bearing surfaces. The 
insulation is located at the joint between the bearing 
housing or bracket and the bearing. Insulated 
bearings are designated by an instruction plate on 
the bearing housing. 
 
Check periodically to be sure the insulation has not 
been weakened or destroyed. 
 
The bearing insulation can be checked using an 
ohmmeter or circuit test light. For sleeve bearing 
motors with one bearing insulated, the shaft must be 
raised a few mils at the non-drive end of the motor so 
that it is not in contact with the bearing and the shaft 
coupling must be parted so that the shaft is not 
grounded through the driven equipment. For motors 
with both bearings insulated, disconnect bearing 
grounding strap before testing for insulation integrity. 
Bearing temperature devices must be disconnected 
and oil rings must not be in contact with both the 
shaft and the bearing.