10HP VFD

A VFD drive supplied from a 3 phase system will have a bank of rectifiers at the input feeding a capacitor bank. The capacitor bank serves two purposes --- to filter some of the input ripple from the rectifiers, and to absorb some energy during deceleration. Rectifier-capacitor input filters produce non-sinusoidal pulses of current in the input lines. This means the RMS input current is higher than if the same energy was transferred from a DC source. If you take a VFD drive designed for a 3 phase input and use it on a single phase system, then the diodes and the filter bank may be under sized. This is why you may need a 3 times larger VFD. It is not because of the DC to AC conversion electronics, but because of the input diodes and capacitors.

VFDs typically run on single phase power and convert it to three phase power to the motor. Three phase is preferred for speed control as it is easier to implement and maintains near 100% torque at low speeds Bottom line. Buy a VFD, connect it to a 3 phase motor, plug it into 120 or 240 single phase outlet and you are good to go. BTW, I have a couple of Rob's units, one for my KMG and another for the 9 inch disk grinder. They are top quality and easy to set up and if you do have a problem Rob is always there to help.

One feature that is present in the Mitsubishi VFD and not present in cheaper units that I have installed is a braking resistor. These are used in applications where you want to use the motor as a brake to stop the machine or control the speed of a descending load. The bigger the resistor, the quicker the stop can be programmed in. You can eliminate coasting, even under significant load, when using a resistor. A unit without this resistor built in is quite a bit smaller in size and less expensive. Often an external resistor can be added to the cheaper units if you decide you want to add one later, but at least you aren't paying for it when you don't need it.

However, when you rectify single-phase power, the output voltage of the rectifier drops to zero 120 times a second. The task of filtering this to provide pure DC is much more difficult. The standard rule-of-thumb is to derate the VFD by 50% when operating from a single-phase source. Thus, if you want to run a 5HP motor, you need a VFD rated for 10HP. I would not recommend operation of any VFD on single-phase power unless such usage is proper according to the manufacturer. If not discussed in the owner's manual, you can always contact the maker's technical support department and ask for assistance.

The challenge of this custom variable frequency drive design was to maintain constant airflow by changing motor speed as the filter changed from “no load” to “100% load”. The custom VFD design had to fit into an existing control panel with a height limitation of less than 2.5 inches. Fast start-up (<5 seconds) and intelligent set point return are some of the programmed features of this unique custom VFD design.

1hp VFD blows one of the fuses on the line side of the drive and then continues to operate on single phase supply. The fuse size that keeps blowing is a 6A fuse. This motor is a 9-lead motor which originally was wired incorrectly, which we thought was the problem, however now we have confirmed that the motor is wired correctly and it is still blowing this same phase fuse every time. I wouldn’t suspect the fuses are too small because after the fuse blows and the drive is running on single phase supply we would expect the current on the two remaining phases to increase and thus blow the other two fuses if they were marginal.

On a simpler note, a VFD can be used to replace a starter/contactor in a three phase machine. VFDs provide a soft start feature and also provide excellent motor protection circuits thereby negating the use of heaters and other clunker overload relays. Overall, VFDs are pretty nice and more folks are going to them. On the down side, they are not easy to work on and contain lots of specialized electronics making service an issue. There are 60 or so year old phase converters still running today with nothing more than grease and a new set of bearings.

The only problem is the small VFDs are only rated for about 3 HP, but check the amp draw on the motor, you might be able to squeak by on the amp rating, and it would also give you a lot more speed control. You might want to look at the insulation rating on the motor. Also, with heavy use, it needs to be an F rated insulation on the windings. The rating will be on the data plate.

Variable frequency drive (VFD) systems make use of a motor, operator interface and drive controller assembly. Motors used in VFD systems are normally three-phase induction styles. Single phase motors or synchronous may be used. The torque and speed of the motor is controlled by changing the input voltage and frequency.