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.