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Constant Hp Application Of Vfd Motors


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#1 subrao

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Posted 06 July 2009 - 01:18 PM

1.many of vfd suppliers as well as motor suppliers claim the following feature for constant hp loads above rated
synchronous speed

Up to 60hz voltage is increased linearly with frequncy so that vfd-motor develop constant torque and at 60hz rated hp as well. Above the 60hz the voltage is clamped at rated value and the system can deliver constant hp above rated speed.
Unlike a dc motor in an ac motor torque and hence hp varies as square of voltage at a given frequncy. So a 60hz motor if operated at constant volts above 60hz gives torque inverslyvarying with square of speed and hp inversly with speed.
out of above two propositions which is correct?

#2 marke

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Posted 06 July 2009 - 07:10 PM

Hello subrao

Below rated frequency, the flux in the iron must be kept at the rated flux in order to prevent flux saturation of the iron.
To achieve this, the voltage is reduced with frequency so that the V/Hz ratio is kept constant.
As the flux is kept constant, the maximum torque is limited to the same level at all speeds below rated speed.
The power rating reduces with speed (P = T x N)

Above rated speed, it is not possible to increase the voltage above line voltage, so the flux reduces as the speed increases. The V/Hz ratio reduces. The torque reduces and the power remains constant.

Best regards,
Mark.

#3 subrao

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Posted 07 July 2009 - 05:48 AM

QUOTE (marke @ Jul 6 2009, 08:10 PM) <{POST_SNAPBACK}>
Hello subrao

Below rated frequency, the flux in the iron must be kept at the rated flux in order to prevent flux saturation of the iron.
To achieve this, the voltage is reduced with frequency so that the V/Hz ratio is kept constant.
As the flux is kept constant, the maximum torque is limited to the same level at all speeds below rated speed.
The power rating reduces with speed (P = T x N)

Above rated speed, it is not possible to increase the voltage above line voltage, so the flux reduces as the speed increases. The V/Hz ratio reduces. The torque reduces and the power remains constant.

Best regards,
Mark.


Dear Mark,
thank you very much for your prompt reply. However my doubt remains uncleared and is as under.
Torque of a mtor varies as square of applied voltage. power is product of torque and apeed. If frequency is increases voltage should be increased proportinately to main the torque capability.However in case VFD application the voltage is held constan above rated frequenctY. This means torque decreases as inverse square of frequency, Because of speed increase Hp decreases as inverse of frequency.
For example we consider 10hp, 480volts 50hz 1800 rpm motor. the torque of the motor is about 4.2 kg-m,If the frequncy is increased to 120Hz , to maintain torque capability we should apply 960v, But we are keeping voltage constant at 480 V. so torque capability of motor is only 4.2/root2 that is 2.96 kg-m and speed is about 3600 rpm hence hp will be only 5 not 10.
There are contradictory views . Some claim constant hP above rated speeds but some other claim hp decreasing inverse to speed. Which is true? Or both are true with in certain range of speeds above rated one.

Best regards,
subrao

#4 marke

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Posted 07 July 2009 - 10:23 AM

Hello Subrao

The maximum torque that can be produced by a motor is a function of the flux in the iron.
The actual torque produced by a motor in it's normal running mode (low slip) is a function of the flux in the iron and the slip.

Under high slip conditions (while the motor is starting) the torque is a function of the power dissipated in the rotor and this is a function of the current squared at that slip.
Under normal low slip operating conditions, for a given flux, the torque alters with the slip. (voltage remaining constant)

With a VFD, you never operate the motor under high slip conditions, always low slip conditions, so the torque is a function of the flux and slip. As you increase the frequency above line frequency, the voltage can not increase, so the flux reduces. To produce the same torque, the slip must increase, but there is a maximum slip before the motor begins to stall, so the maximum torque reduces with the flux, and therefore reduces with increasing frequency. The maximum power (torque x speed) remains constant above line frequency.
The voltage squared characteristic is true for high slip conditions that are not relevent when using a VFD, so it does not contribute to the torque or power characteristic.

Best regards,
Mark.

#5 subrao

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Posted 16 July 2009 - 10:48 AM

QUOTE (marke @ Jul 7 2009, 10:23 AM) <{POST_SNAPBACK}>
Hello Subrao

The maximum torque that can be produced by a motor is a function of the flux in the iron.
The actual torque produced by a motor in it's normal running mode (low slip) is a function of the flux in the iron and the slip.

Under high slip conditions (while the motor is starting) the torque is a function of the power dissipated in the rotor and this is a function of the current squared at that slip.
Under normal low slip operating conditions, for a given flux, the torque alters with the slip. (voltage remaining constant)

With a VFD, you never operate the motor under high slip conditions, always low slip conditions, so the torque is a function of the flux and slip. As you increase the frequency above line frequency, the voltage can not increase, so the flux reduces. To produce the same torque, the slip must increase, but there is a maximum slip before the motor begins to stall, so the maximum torque reduces with the flux, and therefore reduces with increasing frequency. The maximum power (torque x speed) remains constant above line frequency.
The voltage squared characteristic is true for high slip conditions that are not relevent when using a VFD, so it does not contribute to the torque or power characteristic.

Best regards,
Mark.


Hello Mark,

thank you very much for your clarification ,This problem was bothering me quite a few months and your clarification has cleared my doubts.
Furthur, I conducted an experiment by loadong a 7.5 kw,50Hz motor above 50hz up to 100hz at constant voltage and loaded to constant hp and found that load current sligtly decreases with increase in frequency probably because of reduction in magitizing current at higher frequency.

Best regards
Subrao




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