Speed Control in Electric Motors

Static frequency converters are electronic devices that allow adjusting of the speed of cage induction motors in a wide range with the variable frequency and voltage power supply they provide. When fed with a properly designed frequency converter, stepless and practically lossless speed regulation of a cage induction motor can be made. 
In frequency converters, 2 different methods are used to control the mains voltage coming to the motor by switching; 
PAM (Pulse Amplitude Modulation) and frequently used PWM (Pulse Width Modulation). The alternating voltage coming to the motor is first converted to direct voltage, then the three-phase output voltage is created by slicing the direct voltage in the PWM method and without applying slicing in the PAM method. 
 

Speed Adjustment of Cage Induction Motors

Today, cage induction motors, whose speed is adjusted with frequency converters, are used in all kinds of facilities and equipment where automation is applied. The main benefits of lossless speed regulation over a wide range are energy savings, process, and quality improvement.
Calculations and measurements have shown that the best operating characteristics in the speed ranges encountered in practice are generally obtained with 4-pole induction motors. Therefore, in practice, this number of poles should be preferred. However, when very low or high speeds are required, other pole numbers can be chosen.
The rated motor voltage is normally taken equal to the mains voltage so that when the frequency converter fails, the motor can be fed directly from the mains. Induction motors used with frequency converters are of standard construction, but specially designed motors may be required at higher powers. The common feature of all frequency converters is that the motor losses increase compared to direct supply from the mains. Due to this increase due to the presence of harmonics in voltage and current, an induction motor fed from a frequency converter may not be able to deliver its rated power. 
In practice, it is appropriate to reduce the rated power by 0-20%, following the IEC Recommendation. In the selection of the derating factor for a particular engine, the temperature reserve of that engine should be considered. (See IEC 60034-17: An application guide for cage induction motors fed from frequency converter). 
Due to the high voltage increase rate and the possibility of high instantaneous voltages, the insulation systems of induction motors fed from the frequency converter may be more strained than the supply from the mains. This strain increase depends on the frequency converter frequency and the cable length between the frequency converter and the motor, apart from the leakage reactance of the motor. Therefore, cable length, filter requirement, and, in some cases, the use of special insulation systems are issues that need to be examined. In practice, the cable length should be taken as short as possible, therefore, integrated (wireless direct connection) induction motors with frequency converters included in the Gamak manufacturing program are recommended for variable speed applications. 
This design should be avoided, especially in large motors, as a double cage or deep groove rotor construction causes high harmonic losses. Different cage designs may be more suitable as the motor does not need to be high starting torque when fed from a frequency converter. However, it should be noted that in the event of a frequency converter failure, direct starting is not guaranteed, especially in drives with constant torque, since induction motors with special cage rotors will be fed directly from the network.  On the other hand, an induction motor fed from a frequency converter may generate more noise than a mains supply due to harmonics. This noise can be reduced with proper motor and frequency converter design. 
Another effect of feeding from the frequency converter is that voltages can be induced in the motor shaft. If these voltages rise to significant values, the resulting currents can damage the bearings and cause premature failure. Although this type of failure is rarely encountered, insulation can be foreseen by the drive of the bearings in terms of operational safety.

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