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.

Variable Frequency Drive Fundamentals

AC Motor Speed - The speed of an AC induction motor depends upon two factors:

1) The number of motor poles
2) The frequency of the applied power.

Inverter Drives - An inverter is an electronic power unit for generating AC power. By using an inverter type AC drive, the speed of a conventional AC motor* can be varied through a wide speed range from zero through the base (60 Hz) speed and above (often to 90 or 120 hertz).

Voltage and Frequency Relationship - When the frequency applied to an induction motor is reduced, the applied voltage must also be reduced to limit the current drawn by the motor at reduced frequencies.(The inductive reactance of an AC magnetic circuit is directly proportional to the frequency according to the formula XL = 2fL. Where: = 3.14, f = frequency in hertz, and L= inductive reactance in Henrys.)

Variable speed AC drives will preserve a continuous volts/hertz partnership from 0 - 60 Hertz. To get a 460 motor this ratio is 7.six volts/Hz. To calculate this ratio divide the motor voltage by 60 Hz. At low frequencies the voltage will probably be low, because the frequency increases the voltage will improve. (Note: this ratio might be varied somewhat to alter the motor overall performance traits such a supplying a low finish increase to enhance beginning torque.)

Depending on the type of AC Drive, the microprocessor control adjusts the output voltage waveform, by one of several methods, to simultaneously change the voltage and frequency to maintain the constant volts/hertz ratio throughout the 0 - 60 Hz range. On most AC variable speed drives the voltage is held constant above the 60 hertz frequency. The diagram below illustrates this voltage/frequency relationship.

Inverter Duty Motors - Initially standard AC motors were employed on inverter drives. Most motor manufacturers now offer Inverter Duty Motors which provide improved performance and reliability when used in Variable Frequency Applications. These special motors have insulation designed to withstand the steep wave front voltage impressed by the VFD waveform, and are redesigned to run smoother and cooler on inverter power supplies.



Sensorless Open and Closed Loop Vector AC Drive. The VFD-B series represents Delta's NEMA1 general purpose AC drive. The VFD-B series drive is rated to provide constant torque, featuring open and closed loop vector control. Delta offers an optional 2000 Hz high speed output that can be factory programmed at the customer's request.

Specifications

    Output frequency 0.1 ~ 400 Hz
    Adjustable V/f curve and vector control
    Master/Auxiliary and 1st / 2nd frequency source selectable
    16-step speed control and 15-step preset speed process control
    Built-in PID feedback control
    Auto torque boost & slip compensation
    Built-in MODBUS communication, baud rate up to 38400 bps
    Support communication module (DN-02, LN-01, PD-01)

Applications

Air conditioners for large buildings; woodcarving machine; punching machine; wastewater treatment systems; crane drive and swivel; washing machine; vertical stamping machines; compressor; elevator; escalator; circular loom; flat knitting machine; pasta machine; four-sided woodworking planer, etc.

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.

How to measure stepper motor

It's hard to stay up to date when you are floating in a sea of technical jargon. However, it's imperative that engineers know the terminology associated with the areas in which they work. And that's especially true if their assignments take them outside their engineering discipline. For example, a mechanical engineer specifying a stepmotor should understand the associated mechanical and electrical terms.
Stepper motors need the right current if they are to work correctly. Without it, the motors can overheat, miss steps, and even freeze in their tracks. Yet the one electrical specification that most confuses all engineers, from the recent graduate through seasoned veteran, is the rating for stepmotor current. No doubt this happens because stepmotor-current ratings come in many forms such as amps/phase, amps RMS, average current, and even amps peak current.

Basics of stepper motor torque

Stepper motor holding torque is one of the main specs of any stepper motor. It is a simple indication of the "strength" of the nema 23 stepper motors.

Stepper motor torque is usually measured in oz/in or ounces per inch. The picture above shows what that measurement means, and a method of actually measuring it. If the motor can HOLD a weight of 100oz on a 1 inch radius pulley it is said to have a stationary "holding torque" of 100oz/in and is therefore sold as a 100oz/in motor.

The picture above shows the same 100oz/in motor but with a more sensible measuring system. Increasing the pulley radius gives greater leverage for the weight, so we can use a smaller weight, and also gives a lever length (10") that is easier to make and will measure more accurately.

As with all leverage ratios increasing the lever length means decreasing the weight accordingly, so instead of 100oz/1" we use 10oz/10" at 10:1, (the result is still 100oz/in).

Measuring stepper motor torque

To measure the holding torque the leadshine m542 does not need to rotate, so the pulley can be replaced with any simple lever.

I used a plastic ruler. The lever distance was 25cm and used the hole that was already on the plastic ruler. Balancing the lever with a simple counterweight can be done using a piece of string. This also compensates for the weight of the measuring cup. I used a flat plastic food cup and 3 strands of fine wire. Everything was glued together in seconds using hot melt glue which can be easily "broken apart" afterwards.