# What happen to Current (I) and Voltage (v) when we increase or decrease the frequency with the variable speed drive (VSD).



## Wardenclyffe (Jan 11, 2019)

,...


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## PreetKamal (May 3, 2021)

AC drive.


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## Peewee0413 (Oct 18, 2012)

PreetKamal said:


> AC drive.


Frequency.......

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## PreetKamal (May 3, 2021)

Peewee0413 said:


> Frequency.......
> 
> Sent from my SM-G998U using Tapatalk
> [/QUOTE
> Frequency????????


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## PreetKamal (May 3, 2021)

Wardenclyffe said:


> ,...


AC Drive.....


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## Peewee0413 (Oct 18, 2012)

PreetKamal said:


> Hi,
> We know when we increase or decrease the frequency with a Variable speed drive (VSD)- speed increases or decreases.
> What happen to Current (I) and Voltage (v) when we increase or decrease the frequency with the variable speed drive (VSD).


Depends on drive set up and torque demand I guess.

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## Wardenclyffe (Jan 11, 2019)

How Pulse Width Modulation in a VFD works


Pulse Width Modulation is the process used by VFDs to invert DC voltage to a variable voltage variable frequency. The VFD's IGBT's are cycled on and off.




www.kebamerica.com


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## PreetKamal (May 3, 2021)

Peewee0413 said:


> Depends on drive set up and torque demand I guess.
> 
> Sent from my SM-G998U using Tapatalk


When we decrease the frequency to decrease the speed of the Fan. What will happen to the current and Voltage at that time?


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## PreetKamal (May 3, 2021)

Wardenclyffe said:


> How Pulse Width Modulation in a VFD works
> 
> 
> Pulse Width Modulation is the process used by VFDs to invert DC voltage to a variable voltage variable frequency. The VFD's IGBT's are cycled on and off.
> ...


When we decrease the frequency to decrease the speed of the Fan. What will happen to the current and Voltage at that time?


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## Wardenclyffe (Jan 11, 2019)

One of the advantages of using a VFD with PWM technology is the ability to control the amount of current going through the motor windings, which when running a rotary industrial motor, translates into controlling the amount of torque at the motor shaft.


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## gpop (May 14, 2018)

Motors are designed with a hertz to voltage ratio so if you reduce the hertz with out reducing the voltage you end up with heat. To make matters worse the fan is also running slower so your ability to remove the extra heat is compromised. (you can buy vfd rated motors that are designed to run at slower speeds)
Vfd's have different modes with the simplest being V/H ratio so a 480v motor at 30 hertz will be running around 240v. At this point the vfd can increase the amps.
If you measure the in going amps verses the out going at reduced speed on the vfd you will see the difference. 

As a motor takes a while to heat up the vfd can ignore the v/h rule for short periods of time which allows it to recover from sudden surges in load and there is a lower voltage limit where the rules have to be broken or it simply would not be able to run.

If you overspeed the motor you simply run out of available voltage past 60 hertz.


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## paulengr (Oct 8, 2017)

Not an easy question, especially when you said FAN.

Centrifugal fans obey fan affinity laws. Torque varies with the square of speed, power with the cube of speed. Mechanically for a given speed, HP = torque (ft-lbs) x RPM / 5252 where the 5252 is a conversion factor for units. Electrically from a motor point of view and this actually applies to all motors, not just AC induction, if you look at a motor equivalent circuit, there are two electrical paths and two currents. One is the flux current responsible for creating a magnetic field. The other is the air gap power and responsible for torque. There are of course losses in both paths but they are relatively small. You need about 10-15% of the power going into the flux path. In a DC, synchronous, or some specialized AC motors you can independently power each of these current paths but in the AC induction motor they are tied together so you almost can't vary one without the other. You can easily measure these two currents though. Since the flux does NO work, it is pure reactive power. That means the flux current is 90 degrees lagging the voltage and has a power factor of 0. On the other hand the air gap power is all torque, all power, and this is pure kilowatts and draws current at 0 degrees lagging, 1.0 power factor. Purists would argue that I'm ignoring some losses which amount to maybe a few percent of the total power draw but in rough numbers this is correct. Either way pay attention to the fact that except at the lower end (speed < 15% of base speed), to a rough approximation, current IS torque. So if we have a constant torque load (NOT FANS) and we cut the speed in half, the horsepower is going to be cut in half. Electrically speaking power is proportional to Volts x Amps. Knowing that current is torque Amps can't change so if we are cutting the power in half by cutting frequency in half, voltage MUST also be cut in half. So until we get to low speeds where flux begins to be a major factor, we can approximately say that we want to keep the Volts / Hertz ratio CONSTANT for any motor. So for a 60 Hz, 460 V motor the V/Hz ratio is 7.67 at every speed. So we want 460 V at 60 Hz, and 230 V at 30 Hz. So far this is for an open loop or Volts/Hertz mode VFD. So with a constant load like a conveyor belt we don't expect the current to change much at all at any speed. Not so with a fan though. So if we cut the speed in half according to the fan affinity laws torque is going to be 1/4 so the current is going to drop to 25% of the value at full speed. There are of course losses and other factors involved but to a first approximation, this is what we expect to happen. Also this is with a straight linear V/Hz VFD. Many drives offer what is called "quadratic" mode. In this mode instead of adjusting the V/Hz mode linearly they recognize that torque is reduced so to save a little energy they follow a quadratic or "square of frequency" style curve as well so this causes the motor current to stay closer to linear as the VFD slows down because it reduces the voltage along a quadratic curve. Other tricks can be used like monitoring the current and dynamically varying the voltage to try to compensate a little. A motor that is running at say 230 V, 30 Hz but is basically running at or near no load is pulling almost pure flux current that is just going to waste so as a further optimization there are some VFD's with some very basic algorithms that attempt to do some kind of power savings by reducing the voltage if the current is low.

This ultimately brings us to vector control, whether of the encoder controlled or flux vector variety. In this mode the VFD monitors the current and/or an encoder. An encoder is optional if we don't want tight absolute speed/torque control to the 3rd decimal place and/or we don't need to control it below about 1 Hz (all the way to stopped). The VFD uses this data to calculate what the speed, torque, and flux actually is. To a first approximation we know that a I said earlier if we look at the current and we split it into real current (0 degrees lagging) and reactive current (90 degrees lagging) which is simple vector math, we know exactly how much torque current and flux current the motor is drawing. There are similar calculations to measure speed from this information. The only problem is that at ZERO speed there is no "leading/lagging" since it's all DC and getting close to zero speed, we can no longer accurately measure what is going on. So at that point the VFD needs a little help in making measurements in the form of an encoder which allows it to measure the precise rotor position to within a fraction of a degree. So in flux vector mode generally speaking we need to pick to control either speed or torque. If we do speed control, the VFD measures the speed and varies the torque to control speed. In torque control mode, we do the opposite. Either way this gives us a desired torque which we can directly translate into a torque current knowing detailed information about the motor (name plate data plus tuning). Based on the torque required we also know how much flux we need. Adding the two currents together determines the current required. Then another control loop varies the output voltage to make the correct current happen. Now notice that one thing is missing here...I never brought up frequency even once. That's because in vector mode we are outputting pulses of voltage to the motor to command it to move. We are operating at a level where we are pulsing the poles on and off and dragging the rotor towards the energized pole. We no longer look at it as a simple Volts/Hertz mode. Inside the VFD it is looking at angular acceleration. The drive settings are telling it how fast to turn (speed command) for instance and the drive responds by accelerating the rotor up to speed and maintaining a certain amount of torque to hold that speed. So where is voltage in all of this? It's what we ultimately use to control the motor and from the drive's point of view it is outputting one of 7 different firing patterns, either all positive/negative, or connecting one of the 3 cables to the negative DC link and one of the other 3 cables to the positive DC link to give us one of 7 possible outputs.

So the current in vector mode is approximately the same as it is in Volts/Hertz mode, if we measure it with a true RMS current meter. It varies with the square of the speed with a fan because the torque with the fan varies with the square of the speed. But the voltage is now dependent mostly on what the system needs and we no longer over/under flux the motor. So the voltage too is going to be varying approximately the same way with the square of the speed.

I hope this makes some sense and explains why the question you asked is not an easy one to answer.


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