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Home » Inverter VFD’S For Dummies
April 22, 2003
Revised April, 2024
Variable Frequency Drives (VFD’s) are used to vary the speed of an electric motor. They do this by changing the frequency of the electric power going to the motor. Today VFD’s are very economical, to the point that we rarely use belt drives on our screw presses. Surprisingly, as we detail below, they can be used with either a single-phase or three-phase electrical supply. There is nothing better for starting, operating, and protecting a screw press.
In the States, normal electric power is supplied at 60 cycles per second, sometimes called 60 hertz (Hz). At this frequency motors run at 1,800 rpm, 3,600 rpm, 1,200 rpm, or 900 rpm, depending on how they are wound. The number of poles in the winding determines its speed. For example, four-pole motors run at 1,800 rpm, and two-pole motors run 3,600 rpm.
Similarly, in most countries outside of North America, the standard frequency is 50 hertz. There four-pole motors run 1500 rpm; 2-poles, 3000 rpm; 6-poles, 1000 rpm; 8-poles, 750 rpm.
The actual motor speed, as read on the motor nameplate, is a little lower than these theoretical figures because of slippage that occurs.
The speed of the motor changes in direct proportion to the hertz. Thus, a four-pole motor running at 45 hertz will turn 1,350 rpm, and a six-pole 1200-rpm motor at 40 hertz will run 800 rpm. A motor can also be sped up: a four-pole motor running at 90 hertz will turn 2,700 rpm.
VFD’s come with a preset maximum limit, frequently 80 Hz. This is easily changed, and Vincent usually changes it to 120 Hz in our VFD’s. This is handy for finding optimal performance.
When a motor is slowed down, the cooling fan that is mounted on the motor shaft also slows down. Thus, motors sometimes overheat at low speeds like 5 Hz or 10 Hz. The rule we follow is this: Feel the motor and see if you can keep your fingers on it for two seconds. That is about 80C or 180F, which is perfectly acceptable for long term operation. Motors world-wide are all required to meet efficiency standards which makes all of them suitable for operation at low hertz.
VFD’s have a built-in circuit breaker that shuts down the motor if the load gets too high. This provides excellent (the best we know of) electrical protection for a motor and the machine it is driving.
A VFD must be rated for, at least, the horsepower rating of the motor being driven. Frequently we use a VFD which is rated for more horsepower than the motor. This arises not only from using any available VFD, but it also provides for switching to a larger motor.
It is very easy to install a VFD. They almost always work on three-phase power. So, there are four wires coming from the power control panel: white, black and (usually) red power wires and a green ground wire. The three power wires are hooked to the L1, L2 and L3 terminals of the VFD. There are three output terminals, labeled T1, T2 and T3 (sometimes U, V, and W), to which you connect the power wires going to the motor.
When you turn on the motor, it may be running backwards. It is usually easy to change the direction of rotation with the VFD itself. Most VFD’s have a simple toggle command for forward and reverse.
Unfortunately, when the motor is shut down and later restarted, it will again restart running backwards. To correct this permanently it is necessary to switch two of the power leads. This is a little tricky. Simply switching leads at the main circuit breaker in the motor control panel will not work. Instead, it is necessary to switch the leads coming out of the VFD, the ones going to the motor.
More and more we see sensors used to monitor the level in the inlet hopper of the press. This is especially applicable when very abrasive materials (like eggshells, fiberglass, and biofuels) are being dewatered. They are also used to achieve maximum press capacity and dewatering, which is achieved by running with the screw completely covered in the inlet hopper. The signal from the level sensor is incorporated into the VFD to minimize operating wear while, at the same time, maintaining optimum performance.
Amps can be read by toggling the menu button to the amps display. Amps readings are a little peculiar with VFD’s. They are no longer directly in proportion to the power being consumed. So, use them as a reference only. The “load” display on the VFD is usually more useful.
God never intended for water and electricity to mix. It is very easy to fry a VFD, and they are not worth trying to repair when you do. Because of this we lean toward using NEMA 4 VFD’s because of their “waterproof” rating. However, if the VFD is to be mounted within a control panel, the less expensive NEMA 1’s are used.
VFD’s are good only either for high voltage or for low voltage. That is either 208-220-240 volts or 380-480 volts. Be sure you know what voltage you are working with. There are more sophisticated VFD models that work on both voltages, but Vincent does not have any of these in the rental fleet.
Vincent’s popular CP-4 lab presses are supplied with a VFD even though these presses run off wall-socket 110 or 220 single phase power. These VFD’s have an output voltage of 220 volts, and the power is converted from one to three phases. Thus, we put three phase motors on these presses.
The same technique is used for installations where only single phase power is available. A VFD rated for at least twice the motor horsepower is required. With that, the single phase power input will allow variable speed operation of the three phase motor.
Another use for a VFD is called auto-reversing. In some screw press applications, like pressing shrimp or onion waste, the screen of a screw press can become blinded. Running the press in reverse for a few turns will wipe this blinding layer away from the screen, restoring normal operation. So, we set the VFD to automatically run in reverse every few minutes, for a few turns of the screw. Presses can be run 24/7 in this mode without affecting service life of the motor or gearbox.
Vincent keeps a large number of VFD’s available for use with our rental fleet. We can supply VFD’s that are pre-programmed for specific applications like auto-reversing.
For advanced students we offer the following: Basically, with a VFD set below 60 Hz, the motor drops the horsepower output and instead holds constant torque. Above 60 Hz however, the power is limited to the motor nameplate maximum horsepower, which means there is a reduction in torque. In some cases where better performance is obtained at high screw rpm, but there is the problem of tripping out for lack of torque, the motor can be replaced with a higher horsepower high-speed motor. We are glad to provide technical assistance in this situation.
One last characteristic of VFD’s is that many can be set for a maximum power output. We had one case where more power was required to operate a screw press. We had a 30 hp motor on the press. In order to preserve a 1.5 service factor on the gearbox, we were told that the maximum power input should be 37 horsepower. So, we put on a 40 hp motor and we set the VFD for a maximum power output of 37 hp. This solved our problem.
Issue 137-R