Inverter VFD'S For Dummies

April 22, 2003

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. They work only with three-phase power. Today they are very economical: we recently paid $500 for a 5-hp unit.

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.

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 be sped up, also: a four-pole motor running at 90 hertz will turn 2,700 rpm.

Most VFD's come with a preset limit of 60 Hz. This can be easily changed, and Vincent usually changes it to 120 in our VFD's. This is above the recommended limit, but it is handy for short tests.

When a motor is slowed down, the cooling fan that is mounted on the motor shaft also slows down. Thus, motors have a tendency to overheat at low speeds like 10 Hz or 15 Hz. Feel the motor to see that it is not overheating. A premium efficiency motor will overheat less. Low speeds are fine for a trial, but they may not be suitable for extended operation.

VFD's have a built-in circuit breaker that shuts down the motor if the amps get too high for the speed at which the motor is being run. This provides excellent (the best we know of) electrical protection for a motor and the machine it is driving.

It is best to have a VFD that is rated for more horsepower than the motor being driven. This gives more flexibility. However, where electrical overload protection is deemed important, the rating of the actual motor being driven should be loaded into the VFD. Otherwise the VFD might put out enough power, if called for, to burn up the motor.

It is very easy to install a VFD. They work only 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. 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 restart running backwards again. To correct this permanently it is necessary to switch two of the power leads. It is a little tricky to change the direction of rotation of a motor with a VFD. 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.

Vincent keeps Saftronics VFD's in the rental fleet. Saftronics was selected because of their excellent telephone assistance. Just call 1-800-533-0031, day or night, seven days a week.

Once a VFD is wired up, there may be frustration trying to get the motor to start. The solution usually is to toggle from the Remote to the Local operation, then hit the Start button.

To change the speed (frequency), get into the frequency adjustment display (next to the actual frequency output display). Toggle the speed up or down, then hit the enter button.

Amps can be read by toggling the menu button to the amps display. Amps reading 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.

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. Be sure to have a plastic bag or sheet over the VFD. Protect the VFD from dripping pipes, rain, and wash-down water. If a loaner VFD gets cooked, we ask the customer to pay for the replacement.

VFD's are good for only one voltage, either 208-220-240 volts or 440-480 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 has a loaner VFD that works on household 110-volt single-phase power. Our unit transforms the voltage to 220 volts, and the power is converted from one to three phases. We put a three phase motor on the machine we are driving, usually a laboratory CP-4 press.

For advanced students we offer the following: Basically, with a VFD set below 60 Hz, the motor drops maximum horsepower output and instead holds constant torque. Above 60 Hz, the horsepower is limited to the motor nameplate maximum, which means there is a reduction in torque. Some VFD's can be set for overload trip on either amps or torque; set it on torque for the best overload protection.

Saftronics has a low torque, easy start version (as for a fan or centrifugal pump) which is good for 1.3 of the nameplate kW. The upgrade unit we buy is for high torque, heavy load starting (like for a screw press or hoist); it is good for 1.5 of nameplate kW. For example, the fan type is $10,500 for 100-hp; the screw press type is $12,500. (A third type, the vector drive, is $14,500.) They say that it is okay to use a VFD at 60 Hz plus or minus 25% if you have a premium efficiency motor, while an inverter duty motor gets you up to plus or minus 50%. That would be for a permanent installation, not just a quick trial.

Issue 137