Why do VFD driven Motors fail ?

It is quite common to hear VFD driven Motors failing prematurely, This happens due to combination of Inductive reactance of rotor and stator coils and the problem is aggravated if the cable length is long.

It is the behaviour of the coils to oppose the rise in voltage and change in current.?a AC sine wave is applied to the motor the change in current is gradual, the motors reaction is also gradual. The inductance reactance of the motor simply delays the change of current through the motor, making it lag behind the applied voltage.

When a fast rise rising pulse, (as in the case of VFD) is applied to the stator coils of the AC induction motor, the back voltage generated by the motor coils can cause the voltage pulse to overshoot the Voltage that is applied to the VFD. This overshoot interacts with the inductance of the motor cables ,and the capacitance of the motor cables and the voltage to oscillate. The peak voltage caused by the oscillation is the major point of concern. If this voltage gets to be too?high the motor the voltage range of the motor rating, it can cause the motor insulation to break down, there by shorting the windings.

A number of measurements used to describe the shape and common characteristic of the PWM pulse acting at the motor.

# Peak Voltage (V peak) determines the stress, that is imposed on the motors stator insulation.

#Rise time?This is the time it takes for each pulse to rise form 10% to 90% and is measured in Micro seconds, the shorter the rise time, the greater is the stress that will apply to the motors insulation. A pulse with a rise time of 0.1 microsecs is generally considered to be a very fast rise time.

The other is the dv/dt the rate of rise of the pulse voltage. This is calculated by considering the yellow rectangle above and dividing the change in voltage (dv) by the change in time 9 (dt).

For a given motor voltage , the insulation stress increases as the rise time becomes shorter, or the dv /dt value becomes larger.

But this is not the only problem the additional problem is the length of the cable which aggravates the problem. It is the combination of both these problems that makes the motors susceptible to insulation failure.

The length of the power cable between the VFD and the motor also has an impact?on the peak voltage at the motor. It is always better to keep the cable as short as possible and practical.

Much of the energy that cause the voltage to overshoot and the oscillation of the PWM pulses is from energy that is stored in electro magnetic fields in the motor cable. The longer the motor cable the greater the stored energy. It is the combination and interaction of the dv/dt (rise time)of the pwm pulse?and the motor cable length that determines the peak voltage at the motor.

The other factor that determine the reliability of the motor is Size of the motor, the voltage rating ( higher the rating the better)

PWM switching frequency: When a very high frequency is employed with a very long cable , the pwm pulses can interact and build on each other, in this instance the peak voltage at the motor can be more than twice the Dc bus voltage . This is what damages the motor.

WAVEFORMS Motor Protection from Voltage Spikes and Stresses:

? Output Reactor (~50mtrs’ Motor Cable Length) - Reduces voltage peaks

? dv/dt Filter (~150mtrs’ Motor Cable Length) - Reduces voltage peaks and some high frequencies

? Sine Wave Filter (>350mtrs’ Motor Cable Length) - Near sinusoidal waveform, Larger | More Costly | More Efficiency Loss

WAVEFORMS delivers innovative, ideal, durable, robust and cost effective solutions to improve your power?quality, reliability and efficiency.

Please feel free to call or mail us.

FOR MORE DETAILS

www.waveforms.co.in

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kebblog

Technical applied service

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colmac coil