## Contorl The Motor Speed With Variable Frequency Drive

A Variable Frequency Drive (VFD), sometimes referred to as a Variable Speed Drive (VSD), is a piece of equipment that regulates the speed and rotational force of an electric motor. By controlling the speed at which your applications operate, your business can save on energy costs and significantly reduce the amount of energy being consumed.

A **Delta VFD** may enhance the user’s profitability by improving the process, which in turn produces a fast return on investment (ROI). Process improvements may come from better:

Speed control;

Flow control;

Pressure control;

Temperature control;

Tension control;

Torque control;

Monitoring quality; and

Acceleration/deceleration control.

Let’s examine some motor equations to understand how this works.

The governing formula for the no-load (synchronous speed) of an alternating current (ac) motor in revolutions per minute (rpm) is as follows:

rpm = (ac frequency, or Hz) x (60 sec/min) / (No. of motor pole pairs)

Since rpm are in units of minutes (rev/min), and frequency is in units of seconds (cycles/sec), seconds are converted to minutes by multiplying by 60 seconds/minute. The equation can be rewritten as:

rpm = (Hz) x (60 sec/min) / (No. of motor poles/2)

If both the denominator and numerator of the above equation are then multiplied by 2 and written without units, the equation becomes:

rpm = (Hz) x (120) / (No. of motor poles)

This really is the simplest type of the equation. The much more poles the motor has, the slower the RPM. Conversely, the fewer poles the motor has, the quicker the RPM. Also, because the electrical frequency decreases, the motor?ˉs speed (rpm) will reduce, and, as frequency increases, the motor’s speed increases.

Because it’s simpler to let electronic devices alter the frequency from the voltage coming to a motor than it’s to alter the amount of poles within the motor, **vfd110b23a** have gained growing recognition within the HVACR business. Numerous refrigeration, air conditioning, and heat pump compressors these days can employ electronic VFDs to differ the frequency feeding their electric motors.

VFDs function by converting the motor’s ac input to direct present (dc). In the direct present, the VFD will produce a simulated ac signal at varying frequencies (see Figure 1 above). The microprocessor controlling the VFD turns on and off the waveforms?ˉ good or unfavorable half. A greater output voltage materializes in the energy device remaining on longer. Conversely, the shorter the energy device is on, the reduce the output voltage will probably be. The switch frequency will be the speed at which the energy device switches on and off. Much more heat is generated within the energy device because the switch frequency increases, but there’s now much more resolution and smoothness within the output waveform.

As talked about earlier, motor speed is straight proportional towards the electrical frequency utilized by the motor. Consequently, by varying the frequency towards the compressor?ˉs motor, the compressor’s motor speed (and, therefore, the compressor capacity) may be controlled.

Usually, the VFD also features a backlit liquid crystal show (LCD) that shows a number of motor operational parameters which are totally programmable by the user. Solid-state devices just like the silicon controlled rectifier, triac, and insulated gate bipolar junction transistor have permitted the variable-frequency drive to turn out to be the technique of option for AC motor speed manage.

## How do you calculate the RPM of a 3 phase VFD motor

Variable frequency drives (VFDs) and electric motors are strange companions: The **Delta VFD Drive** is a static device, delicate, intolerant of wide variations in environmental conditions; extremely adjustable and controllable by microprocessors; capable of being monitored and controlled from remote locations; and a product of modern electronic engineering and precision—the beauty.

Calculating RPM for a three phase VFD is relatively simple… AC Three Phase Induction Motor RPM is determined by the formula:

RPM = (120 * Frequency) / # of poles in the motor

Since the number of poles of a three phase induction motor is established when it is manufactured, the only way to change the speed of the motor is to change the Frequency.

For Example: A four pole three phase VFD when operated at 60 Hz will be very close to 1800 RPM(synchronous speed). The rated full load speed will be less than synchronous speed by the value of “Slip”. A four pole three phase induction motor with a rated full load speed of 1750 has a slip rating of 2.7%. By formula:

((Synchronous Speed – Rated Full Load Speed) / (Synchronous Speed)) * 100% = Slip Rating

((1800 RPM -1750 RPM) / 1800 RPM) * 100% = (50 RPM / 1800 RPM) * 100%

(50 RPM / 1800 RPM) * 100% = .027 * 100%

.027 * 100% = 2.7%

Slip Rating = 2.7%

When using this information and the above formulas the running speed of an AC three phase VFD can be calculated at any input frequency. So how fast will a four pole three phase **YASKAWA VFD** run when operated at 45 Hz? The three phase VFD has a Full Load RPM rating on the nameplate of 1760 RPM.

RPM = (Frequency * 120) / # of poles in the motor

RPM = (45Hz * 120) / 4

RPM = 1350

Next, we calculate the Slip Rating:

((Synchronous Speed – Rated Full Load Speed) / (Synchronous Speed)) * 100% = Slip Rating

((1800 RPM – 1760 RPM) / (1800 RPM)) * 100% = (40 RPM / 1800 RPM) * 100%

(40 RPM / 1800 RPM) * 100% = .022 * 100%

Slip Rating = 2.2%

Instead of using a percentage, we will convert the Slip Rating into how many RPMs actually slip using the following formula:

RPM Slip = RPM * Slip Rating

RPM Slip = (1350 * .022) = 27.7 RPM

RPM Slip = 27.7 RPM

So full load RPM of this motor at 45 Hz will be calculated as such:

Full Load RPM = RPM – RPM Slip

Full Load RPM = 1350 RPM – 27.7 RPM

Full Load RPM = 1322.3 RPM

When selecting a three phase motor, the number of poles is chosen to achieve the speed of rotation that you require. Here are two tables, one for a 50 Hz power supply and one for a 60 Hz power supply:

The formula is n = 60 x f /p where n = synchronous speed; f = supply frequency & p = pairs of poles per phase. The actual running speed is the synchronous speed minus the slip speed.

For a 50 Hz three phase supply:

2 poles or 1 pair of poles = 3,000 RPM (minus the slip speed = about 2,750 RPM or 6 -7% n)

4 poles or 2 pairs of poles = 1,500 RPM

6 poles or 3 pairs of poles = 1,000 RPM

8 poles or 4 pairs of poles = 750 RPM

10 poles or 5 pairs of poles = 600 RPM

12 poles or 6 pairs of poles = 500 RPM

16 poles or 8 pairs of poles = 375 RPM

For a 60 Hz three phase supply:

2 poles or 1 pair of poles = 3,600 RPM (minus the slip speed = about 2,750 RPM or 6 -7% n)

4 poles or 2 pairs of poles = 1,800 RPM

6 poles or 3 pairs of poles = 1,200 RPM

8 poles or 4 pairs of poles = 900 RPM

10 poles or 5 pairs of poles = 720 RPM

12 poles or 6 pairs of poles = 600 RPM

16 poles or 8 pairs of poles = 450 RPM

To figure out the amount of poles, you are able to study the information plate straight or calculate it in the RPM stated around the information plate or you are able to count the coils and divide by three (poles per phase) or by six (pairs of poles per phase). Exactly where the energy from the induction motor is continuous, the torque increases in the price that the speed decreases.

Using the advent of variable frequency drive (VFD), you are able to have any frequency / rated volts you want. I frequently see name plates with issues like 575VAC, 42.five Hz and so on. When these ”specials” are produced I generally see six pole machines – but that might be just a manufacturer’s preference.

## 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 **vfd004el43a** 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 **vfd110b43a** 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 **vfd015b43a** 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 **vfd110b43a**. 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.