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Renewable Energy Q&A / Re: Max winding temp
« Last post by Pete on April 24, 2022, 03:14:19 am »Hi LH , I came across this explanation that may help.
As the frequency of the drive is reduced the voltage has to be reduced also.
So in your case your motor is rated at 460 volts at 60 hz.
That is a ratio of 7.666.
if the voltage is reduced to 400 as in your case then the frequency needs to be reduced too. that would mean the drive should be putting out 52 HZ.
The motor would then be running at 120 x 52/2 which equals 3120 RPM.
If possible back the VFD frequency back of to 52 hz and keep the voltage at 400 and see if it runs cooler and has the power and torque needed to run the compressor.
Pete
AC motor characteristics require the applied voltage to be proportionally adjusted by the VFD whenever the frequency is changed. E.g., if a motor is designed to operate at 460 Volts at 60 Hz, the applied voltage must be reduced to 230 Volts when the frequency is reduced to 30 Hz. Thus the ratio of volts per hertz must be regulated to a constant value (460/60 = 7.67 in this case). The most common method used for adjusting the motor voltage is called pulse width modulation (PWM). With PWM voltage control, the inverter switches are used to divide the simulated sine-wave output waveform into a series of narrow voltage pulses and modulate the width of the pulses.
With a standard AC across-the-line motor starter, line voltage and frequency are applied to the motor and the speed is solely dependent on the number of motor stator poles. In comparison, a VFD delivers a varying voltage and frequency to the motor, which determines its speed. The higher the frequency sup plied to the motor, the faster it will run. Power applied to the motor through the VFD can make the motor working speed lower than the nameplate base speed, or increase the speed to synchronous speed and higher. Motor manufacturers list the maximum speed at which their motors can safely be worked.
As the frequency of the drive is reduced the voltage has to be reduced also.
So in your case your motor is rated at 460 volts at 60 hz.
That is a ratio of 7.666.
if the voltage is reduced to 400 as in your case then the frequency needs to be reduced too. that would mean the drive should be putting out 52 HZ.
The motor would then be running at 120 x 52/2 which equals 3120 RPM.
If possible back the VFD frequency back of to 52 hz and keep the voltage at 400 and see if it runs cooler and has the power and torque needed to run the compressor.
Pete
AC motor characteristics require the applied voltage to be proportionally adjusted by the VFD whenever the frequency is changed. E.g., if a motor is designed to operate at 460 Volts at 60 Hz, the applied voltage must be reduced to 230 Volts when the frequency is reduced to 30 Hz. Thus the ratio of volts per hertz must be regulated to a constant value (460/60 = 7.67 in this case). The most common method used for adjusting the motor voltage is called pulse width modulation (PWM). With PWM voltage control, the inverter switches are used to divide the simulated sine-wave output waveform into a series of narrow voltage pulses and modulate the width of the pulses.
With a standard AC across-the-line motor starter, line voltage and frequency are applied to the motor and the speed is solely dependent on the number of motor stator poles. In comparison, a VFD delivers a varying voltage and frequency to the motor, which determines its speed. The higher the frequency sup plied to the motor, the faster it will run. Power applied to the motor through the VFD can make the motor working speed lower than the nameplate base speed, or increase the speed to synchronous speed and higher. Motor manufacturers list the maximum speed at which their motors can safely be worked.