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TEFC Motors 0. For double shaft extension-Bearing Size ZZ. The noise level of the motors is restricted to the levels specified in IS The motor is said to be in state of vibration if any part of it experiences displacement in any direction. Standard motors comply with normal class of vibration depending on severity as per IS The limits of vibration levels are given below. EFF Level 2 5. Motor terminal voltage transients Fundamental contributors to peak voltages Pulse Rise Time Cable Length between Drive and the Motor Modern drives use power transistors that switch at very high rates.
When such a drive is used with a squirrel cage induction motor, the pulses, in combination with the cable and motor impedance, generate high peak voltages at the motor terminals. These peak voltages are repetitive. They occur continuously and reduce motor insulation system life. Due to space and surface charge creation within the insulation components, the electric stress is not only defined by the instantaneous voltage itself but also by the peak voltages.
When used with drives, maximum repetitive voltage peaks at motor terminals can be 3. For volt motor, these peaks will be of the order of x 3. It is difficult to determine if a particular drive and cable will cause peak voltages in excess of the motor's insulation capability. There are six fundamental issues that determine the amount of peak voltage that will exist at the motor's terminals: pulse rise time, cable length, minimum time between pulses, minimum pulse duration, transition type single or double , and the use of multiple motors.
A certain amount of time is required for the voltage at the drive terminals to transition from low to high. This is called the rise time.
A shorter rise time will cause the peak voltage at the motor's terminals to reach a higher value for a given cable length between the motor and the drive. Distance from the drive to the motor is also important. All motor cables have line-to-line and line-to ground capacitance. Longer the cable, greater the capacitance. Some types of cables, such as shielded cable or cables in metal conduit, have greater capacitance.
Spikes occur at the motor terminals because of the charging current in the cable capacitance. Higher voltage V and higher capacitance long cables result in higher spikes. Voltage spikes caused by long cable lengths can potentially shorten the life of the motor. With modern IGBT drives, the peak voltage begins to occur with a cable length of a few meters and can reach 2 times the control DC bus voltage at a length less than 20 meters. In same cases, however, very long cables in excess of meters, for example can result in a situation where the peak voltage dose not decay quickly enough.
In this case, the peak voltage can be more than 2 times the control DC bus voltage. Operating instructions for motors used with variable frequency drives. Minimum Time between Pulses and Minimum Pulse Duration Transition Type Multiple Motor Switching Frequency Temperature rise An adjustable frequency drive creates, average voltage changes by varying the width of the pulses it produces and the time between them. The peak voltage is potentially at its worst, when time between pulses is at the minimum for the drive and the length of the pulse duration is at the minimum.
The minimum time between pulses is most likely to occur at high peak or high output voltages and during transient conditions, such as acceleration and deceleration.
Minimum pulse width is most likely to occur at low output voltages. If the time between pulses or the minimum pulse duration is less than three times the resonant period of the cable 0. The only way to be sure this condition does not exist in any particular drive is by measuring the pulses directly or by contacting the manufacturer of the drive.
Each of a drive's three output phases is capable of being switched. Generally, only one of the three phases is switched at any given instant. This situation is called a single transition. Some drives will switch two phases simultaneously. This is referred to, as a double transition. The result is a line-to-line polarity reversal with twice the voltage excursion as that of single transition.
This causes higher peak voltage at the motor's terminals. Some drives perform double transitions only during transient conditions such as acceleration and deceleration. Double transitions are generally found in old drives and are not widely used today. The only way to be sure a drive does not perform double transitions is by measuring the pulses directly or by contacting the manufacturer of the drive. If more than one motor is connected to a drive, there can be higher peak voltage due to reflections from each motor.
The situation is made worse when there is a long length of cable between the drive and the common connection of motors. Many PWM drives provide for convenient user adjustment of the switching frequency. This frequency can be adjusted over a range as broad as Hz to 20 kHz. The choice of switching frequency is significant because it defines the number of peak voltages that will be occurring at the motor in a certain amount of time. The higher the switching frequency, the greater the number of peak voltages and their magnitude that will be stressing the motor's insulation system.
When a motor is used with a variable frequency drive supply, it results in higher winding temperature rise as compared to the temperature rise with fundamental sine wave supply. This is due to additional harmonic losses generated due to harmonics present in the output of drive supply. All CGL motors are supplied with class F insulation system and class B temperature rise limits for sine wave supply.
Hence, with VFD supply, temperature rise will be within class F limits. However, all consultants specify that the temperature rise of the motor winding is to be restricted to class B limits, with drive supply, even though the motor is wound with class F insulation system. When we want to meet above condition, the motor needs to be derated. This is to be done at preorder stage. The output earth conductor to be used as equipment earth point for the motor.
Please note, the earthed metal conduit carrying the output power conductors does not provide an adequate earthing for the motor. A separate earth conductor for motor is necessary. The earth conductor of the drive and motor must be separately grounded. These are not to be loop earthed or connected in series.