Variable Frequency Drive Sizing

Have anyone done much vfd sizing for motors? I know you have to take torque and and horsepower into consideration alot. It appears most of the vfds I have seen tend to have larger conductors feeding it than one would calculate for a motor load without a vfd. Maybe its because the vfd is more oversized, or they used 150% NEC 2008 430.6(C). from the motor tables unlike only 125% sizing for conductors to the motor. If you want to buy VFD, you can go to our offical website: Fasttobuy.com. We provide We carry various VFD manufacturers from Mitsubishi to Delta VFD to provide you with a brand you know and trust. We supply both single-phase and three-phase drives in various voltages to help meet the specifications of your current system.

Any one have an instance where they had to size the vfd? Was the feed to the vfd allowed to be smaller than it would be for the motors itself? How did you size it? 

When a single VFD is utilized to manage numerous motors, VFD sizing and choice turn out to be much more complicated unless all the motors are began simultaneously. With numerous motors connected to 1 VFD, adding the horsepower of every to acquire a total load and choosing the VFD accordingly might not be adequate based on operating circumstances.

1 of much more motors cannot be began up whilst 1 or much more motors are currently operating unless the chosen drive is sufficiently oversized. To illustrate this point, think about the following instance with 3 460 VAC motors connected to 1 VFD.

Two from the motors are rated at five HP having a complete load amp (FLA) rating of six.two amps. The third motor is rated at ten HP with an FLA of 14 amps. If all motors are accelerated, decelerated and run in unison-the sum from the connected motor FLA enables use of a 20 HP drive. But if it had been essential to accelerate and run the five HP motors and after that begin the ten HP, the sum from the FLA would need to be recalculated.

The FLA for every from the five HP motors could be utilized within the calculations, however the locked rotor amps (LRA) for the ten HP would need to be taken into account. The LRA will be the quantity of present drawn by a motor at startup.

Simply because the ten HP motor wouldn’t be accelerated from zero frequency and voltage to its operating situation, it would appear in the drive as a fixed voltage/frequency line starter and would need its complete LRA rating to rapidly accelerate towards the drive’s output frequency.

Therefore, the amp draw around the drive when the ten HP drive is coming on line could be the FLA of every from the five HP motors, plus the LRA of ten HP motor that is 86.five amps. The total amp figure to become utilized for VFD sizing is therefore six.two FLA + six.two FLA + 86.five LRA = 98.9 Amps. This amp load would need upsizing to a 75HP VFD having a minimum continuous output rating of 99 Amps, a 50% improve in VFD size.

Final Recommendation

With modern power electronics and advanced microprocessor technology, Delta's AC Motor Drives are able to efficiently control motor speed, improve machine automation and save energy. Taking advantage of our strong position in power electronics technology, Delta's VFD Series of AC motor Drives has evolved rapidly. Each Drive series is designed to meet specific application needs. Our AC Drives accurately control speed and torque, smoothly handle an increased load, and provide numerous custom control and configuration operating modes. Our AC Motor Drive product line provides a full range of motor control technologies and is used throughout a wide range of industries, to enhance and improve machine automation. 

This guide is intended to become of use for common application sizing and isn't intended to become a complete guide. You will find applications and loads that might need unique sizing and consideration. When you're sizing or specifying a VFD for any application it pays to become conservative and leave some buffer space within the FLA and overload ratings. This really is particularly accurate in case your load is difficult to begin or sees heavy loading throughout operation. When you have any concerns about your application or in sizing a brand new VFD contact and speak to 1 of our application specialists before buying.

Variable Frequency Drive vs. Servo Motor Drive

A VFD (variable frequency drive) is generally used to control a squirrel cage type motor, where both stator and rotor are of a wound type to create the magnetic flux. Servo drives are used to control permanent magnet motors. Permanent magnet motor because they use rare earth magnets in the rotor, create a much higher magnetic flux for their given size. This enables the motor to be able to create more torque in a much smaller rotor and hence motor size. Giving the motor a lower inertia to accelerate and decelerate much more dynamically than that of the asynchronous squirrel cage type motor.


Servo motors are used for getting a constant torque on all the speed ranges. Normal Induction motor torque varies with speed. Servos are normally used with machines for better torque characteristics. Servos are in normally closed loop controlled. Induction motors can be controlled with VFD in vector & vector less control.

Servos have a higher bandwidth than VSDs as well as may be controlled at a lot much less than 1 rpm. They preserve the optimum present within the windings utilizing an algorithm that calculates utilizing info from a really higher resolution positional feedback device (frequently a resolver) around the back from the motor. Their response occasions are a lot quicker (as they've extremely little inertia values) They are able to preserve correct speed and, position if a position loop is supplied by a motion controller, to extremely higher accuracy. VSDs have, at very best, an encoder around the motor and a lot reduce bandwidth

In reality a "servo drive" controls a "servo motor" there are many types of servo motor from dc to ac to brushless dc. A VFD cannot control a servo motor and a servo drive cannot control a servo motor. Calling a VFD, even with add on boards, as good as a servo drive is comparing apples to oranges. They are not the same, and not meant to be used for the same type of applications. A VFD can substitute for a servo in non position critical applications, but I would challenge anyone who said that their VFD drive was capable on +/- 1 micron positioning in a CNC environment, that is what Servo drives are designed to do, position. You can take a servo to a desired position and hold it there, without a brake.

Generally speaking, If you want to control speed and tork only use a VFD. But if beside that, you want to control accurate position then you need a servo.

VFD Influence On Induction Motors

An induction motor feels most comfortable when it is supplied from a pure sine voltage source which mostly is the case with a strong commercial supply grid. In a perfect motor there are no harmonics in the flux and the losses are kept low. When a motor is connected to a VFD it will be supplied with a non-sinusoidal voltage, this signal is more like a chopped square voltage. A square shaped signal contains all orders of harmonics.

As these harmonics will induce additional heat losses that may require the induction motor to be de-rated, a margin between maximum output power and nominal-rated output power is required. The required power margin depends upon the application and the supplied equipment. When in doubt contact the local Flygt engineering office for details.

The performance of the VFDs has improved over the years and is still improving, and the out put signal is looking more and more like an ideal sine wave. This implies that a modern VFD with high switching frequency can run with a low or no power margin whatsoever, while an old one might need a margin of 15%. Unfortunately the extensive work needed to develop VFDs' ability to reduce losses in the motor and in the VFD, tends to emphasize other problem areas. VFDs with high switching frequency tend to be more aggressive on the stator insulation. A high switching frequency implies short rise time for the pulses which leads to steep voltage transients in the windings. These transients stress the insulation material. Flygt recommends reinforced stator insulation for voltages 500 V and above.


Here Recommend You Delta VFD


The Delta VFD007B21A VFD-B series is a general purpose NEMA 1 drive and offers V/F, Sensorless Vector and Closed Loop Vector control. With its Constant Torque rating and 0-2000Hz output, the VFD-B is designed to handle most conventional drive applications found in the industrial manufacturing industry. The VFD-B series drives are used in many applications including: HVAC, Compressor, Crane Gantry, Elevator, Escalator, Material Handling, Water/Wastewater, and Woodworking to name a few.


Specifications:

Item Number: VFD007B21A
Manufacturer: Delta Products
Item Category: Drives
Subcategory: AC
Series: VFD-B
Nominal Input VAC: 208;240 Volts AC
Input Range VAC: 200 to 240 Volts AC
HP (CT): 1 Horsepower
Amps (CT): 5 Amps
Input Phase: 3
Operator Controls: Keypad Included
Max. Frequency: 400 Hertz
Braking Type: DC Injection;Dynamic Braking
Motor Control-Max Level: Open Loop Vector (Sensorless Vector)

Sizing Criteria
The data needed to determine the correct size of a
VFD are:
• Motor kVA rating.
• Nominal voltage
• Rated current
• Ratio max. torque/nom. torque

If the ratio between peak torque and nominal torque, Tp/Tn, is greater than 2.9 it might be necessary to choose a larger VFD. There are basically two reasons why a motor can have a ratio greater than 2.9:

1. The motor has a high magnetisation level
2. The motor has been de-rated.

Running Above Nominal Frequency

Sometimes there is a desire to run the pump at frequencies above the nominal commercial supply frequency in order to reach a duty point which would otherwise be impossible. Doing so calls for extra awareness. The shaft power of a pump will increase with the cube of speed according to the affinity laws. Ten percent over-speed will require 33 % more output power. Roughly speaking the temperature will increase by approx. 80%.

There is however, a limit to what we can squeeze out of the motor at over-speed. Maximum torque of the motor will drop as a function 1/F when running above nominal frequency. This is due to the fact that the VFD output voltage has reached its full value at nominal frequency and cannot be further increased. The area above nominal frequency is denoted as the field weakening range. The motor will be overloaded and drop out if the VFD can't support it with a voltage that corresponds to that needed by the torque. In reality the VFDs' over-current protection will trip after a short while if we try to run the pump too far into the field-weakening range. Running above nominal frequency is not recommended, but if required, use the following guidelines:

• Check rated power. Shaft power will increase to the power of three according to affinity laws.
• Check that the VFD is dimensioned for the load increase. Current is higher than nominal rated current (for nominal frequency) in this case.
• Change "Base frequency" of the VFD. Base frequency is the frequency where the VFD output voltage is the same as supplied nominal line voltage.

If possible, select a machine designed for a higher frequency. When running a pump designed for 50 Hz operation above nominal speed, select a 60 Hz motor.

NPSH-required increases, according to the affinity laws, when running above nominal frequency. Always check that NPSH-available is greater than NPSH-required in order to avoid cavitation.

How to wire 3 phase motor to VFD

Motors usually come in two different types, single or three phase. The number of phases on the motor is determined by how the motor is wound. It is easy to find out how many phases your motor has by looking at one of two factors.

The block diagram below shows a typical VFD installation. This diagram shows the wires that supply power to the Variable Frequency Drive, the wires that provide voltage from the VFD to the motor, and all the necessary input and output signals that the VFD needs for operation. From the diagram one can see that the power source for the VFD is provided at terminals R, S, and T by 3-phase AC voltage. The value of this voltage can be 208, 240, or 480 volts. The 3-phase voltage is converted to DC voltage in the rectifier section on the VFD where six diodes are connected as a 3-phase full-wave bridge rectifier. On larger VFDs the diodes can be replaced with silicon-controlled rectifiers (SCRs).



The "rule" is basic 1 phase / 3 phase math. Power in a 1 phase circuit is V * A * pf. Power in a 3 phase circuit is V * A * pf * 1.732 (sq. root of 3). So the unique situation of a VFD in this case is that the MOTOR is using the power at that 1.732 value, yet the SUPPLY is not, so the power drawn by the VFD from the supply is 1.732 x the power used by the motor. Test it out using 1HP.

1HP = 746W

Amps for 746W in a 230V 1 phase system is 746 / 230 * .8pf = 4.05A

Amps for 746W in 230V 3 phase (the motor) is 746 / 230 * .8pf * 1.732 = 2.34A

4.05 / 2.34 = 1.732

So a 3 phase 1HP Delta VFD probably has components rated for 2.34A, but when connected to a 1 phase source, the input will draw 4.05A from the supply. If the diodes used in the rectifier section are not capable of taking 4.05A through them, they fry. The ripple issue is also important, thats why we round up to 2.0 (50% derate) instead of 1.732 just to allow for extra capacitors that will come with the larger size.

Many small VFDs however are using components on the rectifier side and the DC link that are so cheap that they can afford to oversize them without much cost. So up to 3HP (typically), they don't need derating. But those are usually the ones that SAY they can have 1 phase or 3 phase input. If they don't expressly say it, then they may not have the oversized components and you run a risk of frying the rectifier. In addition if you don't have enough capacitance to smooth out the extra ripple in the DC link, you can end up damaging the transistors on the output side. The net effect is the same to you; the magic smoke is released and it is never worth trying to shove it back in.

All VFDs can convert single phase to 3 phase. But beyond around 3HP at 230V, you have to double the size of the VFD. So for your 7.5HP motor, you will have to use a 15HP VFD. Also consider this; a 7.5HP 230V 3 phase motor will be around 22A FLC. That means it will be drawing 38A from the single phase line when fully loaded. But to use the 15HP VFD you have another problem with meeting the NEC. You are required to size the service for the VFD at 125% of the VFD's maximum current rating, not the motor's. So looking at an average 15HP VFD, it's rated for 46A so the circuit to feed it must be at least 57.5A and the nearest size is going to be 60A. So keep that in mind; you will have to run a 60A circuit breaker and cables to a VFD for a 7.5HP motor.