Question: What is the difference between a phase converter and a “Variable Frequency Drive” Why would I choose one over the other?
Let’s first look at what defines each product…
According to Wikipedia: “three-phase induction motors may operate adequately on an unbalanced supply if not heavily loaded. This allows various imperfect techniques to be used. A single-phase motor can drive a three-phase generator, which will produce a high-quality three-phase source but with high cost for apparatus.”
Generally phase converters are divided into three popular methods of control:
- Static conversion techniques in which the motor is run at less than full efficiency mainly on two of the legs of the three-phase motor. Current is sometimes injected into the third leg with a capacitor or transformer arrangements that provide imperfect phase shift. In these systems the motor must be de-rated.
- Rotary phase converters constructed from a three-phase electric motor or generator “idler”. These normally require some kind of starting aid and capacitors to improve phase balance and power factor. This is a two-motor solution. One motor is not connected to a load and produces the three-phase power, the second motor driving the load runs on the power produced.
- A digital phase converter uses a rectifier and inverter to create a single voltage with power electronics, which is added to the two legs of the single-phase source to create three-phase power. Unlike a VFD, it cannot vary the frequency and motor speed. Since phase converters generate one leg that must match the voltage and frequency of the single-phase supply, it does have the advantage of a sine-wave output voltage and excellent voltage balance between the phases.
Benefits of using a Phase Converter
- Installation Simplicity (less wiring)
- Phase converters are the “Plug and Play” option for simple phase conversion
- Operating both Resistive & Inductive Loads
- Only generates three phase power
- Best option for a general type power conversion based on HP or AMP requirements
- Can be used for multiple machines to run at the same time.
This is where terms can be crossed. Also known adjustable-frequency drive, variable-speed drive, AC drive, or micro drive, a VFD is a type of adjustable-speed drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and voltage
The VFD “Speed” controller is a solid state power electronics conversion system consisting of three distinct sub-systems: a rectifier bridge converter, a direct current (DC) link, and an inverter. Voltage Source inverter (VSI) drives are by far the most common type of drives. Most drives are AC-AC drives in that they convert AC line input to AC inverter output. There are other types, but this is the most common.
As you can see the line power is wired directly to the VFD and then the VFD is directly connected to the motor, they are not connected to switches or any front end starters or other devices. The control now takes place either through an operator interface or through “external signal” controls, like separate push buttons and switches to activate certain tasks, like “Start”, “Stop”, “Forward/Reverse” , and an external “potentiometer” to adjust speed.
Types of Drives
Control method in which the stat or currents of a three-phase AC electric motor are identified as two orthogonal components that can be visualized with a vector. One component defines the magnetic flux of the motor and the other the torque, controlled without and external sensor, utilizing an algorithm.
In variable-torque applications suited for Volts-per-Hertz (V/Hz) drive control, AC motor characteristics require that the voltage magnitude of the inverter’s output to the motor be adjusted to match the required load torque in a linear, V/Hz relationship. For example, for 460 V, 60 Hz motors, this linear V/Hz relationship is 460/60 = 7.67 V/Hz. While suitable in wide-ranging applications, V/Hz control is sub-optimal in high-performance applications involving low speed or demanding, dynamic speed regulation, positioning, and reversing load requirements. Some V/Hz control drives can also operate in quadratic V/Hz mode or can even be programmed to suit special multi-point V/Hz paths
(Closed Loop w/encoder feedback)
Similar to the Sensorless Vector, but provides even more accuracy due to utilization of an encoder providing the signal feedback, resulting in an improved percentage of speed regulation, improved speed range at constant torque, and improved minimum speed at 100% torque at % of base speed.
VFD Control Performance
AC drives are used to bring about process and quality improvements in industrial and commercial applications’ acceleration, flow, monitoring, pressure, speed, temperature, tension, and torque.This ease of control makes more complicated applications such as vertical mill, drill press, lathe, even a saw or air compressor the perfect fit for our selection of VFDs.
Fixed-speed loads subject the motor to a high starting torque and to current surges that are up to eight times the full-load current. AC drives instead gradually ramp the motor up to operating speed to lessen mechanical and electrical stress, reducing maintenance and repair costs, and extending the life of the motor and the driven equipment.
Variable-speed drives can also run a motor in specialized patterns to further minimize mechanical and electrical stress. Most VFD manufacturers produce drives that are 115V input capable up to 1 or 1.5HP rated and 208-240V 1 phase input capable up to 3 HP factory rated. If higher horsepower is needed, a VFD can be de-rated (oversized) to allow single phase input and still provide the horsepower rating. This will require your motor FLA rating and sometimes the S.F. (service factor) of the motor depending on the application.
What’s involved in the installation of these devices? In situations like lathes, drill presses and vertical mills, the idea is to wire your power from your breaker box to the outlet or hardwire from the breaker box (utilizing ) the proper breaker size.
Then, you go ahead and wire the VFD for the incoming power line and also directly from the VFD to the motor on the machine. What this means is removing the current motor starters from the circuit. Any and all machine installed switches that are on the equipment will need to be rewired to the terminal boards on the VFD for separate external signal control. They cannot be left on the machine as is and be expected to work.
Benefits of using a VFD
- Motor Speed Controller
- Phase conversion by-product
- Precision Frequency Control
- Soft Starting motors
- Energy Savings
- HVAC, Pumps, CNC, Wood Working, Conveyors, Mixers, Hoists, Cranes, Elevators for performance improvement.
- Building Automation Systems
So what about the difference in products? If you are trying just to connect a machine up for plug and play operation with minimal performance gain in a single phase to three phase conversion. Well your best solution for the above example is a “Phase Converter” since this is a general type power conversion based on HP or AMP requirements. This is best for refrigeration equipment, portable equipment on trailers, freezer’s etc. or when the machine has numerous low voltage controls and internally for the machines control panel.
VFD’S are very well suited , if you have a pump, vertical mill, drill press, lathe, even a saw or air compressor where not only phase conversion is desired, but the desire to control speed of that motor for fine tuning your application, then a VFD is your desired controller. Most VFD manufacturers produce drives that are 115V input capable up to 1 or 1.5HP rated, 208-240V 1 phase input capable up to 3 HP factory rated. If higher horsepower is needed a VFD can be de-rated (oversized) to allow single phase input and still provide the horsepower rating. This will require your motor FLA rating and sometimes the S.F. (service factor) of the motor depending on the application.
What’s involved in the installation of these devices? In situations like Lathes, Drill Press, Vertical Mills Machines, the idea is to wire your power from your breaker box to the outlet or hardwire from the breaker box (utilizing ) the proper breaker size. Then wire to the VFD for the incoming power line, then wire directly from the VFD to the motor on the machine. What this means is removing the current motor starters from the circuit. Any and all machine installed switches that are on the equipment will need to be rewired to the terminal boards on the VFD for separate external signal control, they can’t be left on the machine as is and be expected to work.
This is just a quick summary of what is involved when switching out a motor starter for a VFD to control a motor on one of these machines and the differences on when you would want to use one vs using a phase converter. There is a place for these devices.
Have more questions about incorporating a VFD or another form of motor control? Our tech support agents are here to help! Give us a call at (847) 658-8130 or email us at [email protected] and we’d be happy to help!
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