Surface Grinder Restoration Complete. With pics!

Identical to the one in the shop but cleaner than ours!

It is surprising that more serious home shops do not have one. It is so useful for many things, we turn ours on in the morning and it is shut off at the end of the day and is most times being used for something. Not just for surfaces, used for turning down punch press pins to size, sharping cutting blades, doing your nails etc.:))
Pierre
 
jgedde said:
Getting back to the VFD, what I do know is single phase input current when the VFD reads 4A, is over 8A since it blew the fuses I used on the incoming line! Since the remaining 4A+ required to blow the fuses is going somewhere external (the VFD runs very cool), I suspect the motor current readout on the CFD isn't the whole picture.

John
Click to expand...

You are correct that the motor current readout on the VFD is not the whole picture as far as total power consumption. There is power consumed inside the VFD to operate the control circuits and, to a greater extent, from heat losses of the output devices (thyristors of some kind such as SCRs, etc.). But also consider this: Even though the input to the VFD and its output are the same voltage (220), one is single phase and the other is three phase.

For the sake of this discussion, since we are looking for the 'lost' 4A+ and not trying to account for every milliwatt, let's disregard any power factor considerations. However, there is going to be some phase shift between voltage and current in both the input and output since we are dealing with a reactive motor load, not a purely resistive load. So, I will use VA (Volts x Amps) as the preferred unit of power rather than Watts, which is only accurate as V x A when the voltage and current are in phase.

The input power in VA can easily be calculated by line Voltage x line Current. However, the output power is not single phase so we must take into account that there are three lines, each having voltage and current supplied to them. The relationship between phase-to-phase (sometimes called leg-to-leg) voltage and phase to ground (or leg-to-ground) voltage in a three phase balanced setup is the square root of three (approximately 1.732). There are three phase systems that have a high leg and it is a higher voltage to ground than the other two legs, so this calculation only applies to a balanced system in which all three legs of the three phase are at equal potentials to ground.

Your output voltage was stated at 220 V, and this is the leg-to-leg voltage. So, dividing 220 by 1.732 yields a leg-to-ground voltage of 127 volts. According to your VFD readout, each of the three legs is carrying 4 Amps. Thus, the output power is calculated by 127V x 4A x 3. This means there is 1524VA being supplied to the motors.

Since I do not know what your actual input current was when you were seeing 4A output on the VFD, I cannot know your true input power required. Let's use the previously calculated 1524VA output power to estimate the input current. Power in equals power out, so the 1524VA output equates to 6.93 amps at 220 volts input. Understanding that we have the earlier mentioned internal VFD losses, the input must be supplying more than 6.93 amps and thus is not far from being over 8 amps.

I do not know if your 8A fuses blew after some time or if they blew immediately upon startup. If they blew immediately, this can be explained by the fact that a motor can draw startup currents as much as ten times their full load current! In order to best protect motor circuits use only time delay type fuses. This avoids the need to have grossly oversized fuses.

Good luck with all.
George
 
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