Help me with some math

[Ian Barley]

I need some help with electrical math. I know that there are some experts watching here so I throw myself on your mercy.

I have the potential to bid (good old ebay) for a big thicknesser. This is a 5hp, 415V, three phase machine which I will be running through a phase converter/transformer from a 230V single phase supply.

My problem is that my electrical circuits are not all that special. Basically the whole caboodle runs off a 63amp breaker (15KVA??) and my main circuit is a 32amp ring, which I would hope to run this machine off.

Now, I know that I should have been paying attention in physics class - but I didn't. I think that a 3hp single phase motor draws about 2.25KW @ 230V - give or take thats 10amps

I guess that this means that a 5hp motor running in the single phase will draw about 3.75KW - give or take thats 16 amps.

Now I know that the phase converter will not be 100% effecient but I am also assuming that it will output abou 70-80% of what it has as input. Does that mean that this 5HP machine will require a braker of about 20amps at 240V?

The more I think about this the dumber I feel but I really don't want to end up with a 1200lb boat anchor by buying a machine that I cannot run - help!!

[Dean Baumgartner]

Ian,
You also have to add in motor efficiency of about 80% for standard motors. This gets your 3 hp 3x746watts/hp = 2238/.8 efficiency = 2797 watts at 230v = 12.1 amps. For comparison the nameplate on the 3 hp motor on my new saw says 13.2 amps. or about 74% efficiency. So using these numbers the 5 hp saw would be 5x746/.74/230= 22 amps plus whatever the phase convertor needs.


Dean

[Bill Turpin]

Three phase watts (actual work done) is amps (from motor plate) x volts x 1.73. Fiqure the watts @415v. Refigure them @ the voltage your frequency drive is going to output. Will the motor run at this voltage? The documentation on the drive should tell you the input amps for the necessary output watts. The normal math at this point does not work because the drive has to create the third hot leg. Sometimes there are significant losses. Single phase watts is amps x volts. If this old machine is very old the motor is probably over sized and will never run at full load. However rust, wear, dull blades, lack of lubrication, etc can over come this safety factor.

What is the control voltage of the contactors in this machine? Will you use the built in controls? Can they be reduced to the new voltage?

Bill in WNC mountains

[Ian Barley]

Ian,
You also have to add in motor efficiency of about 80% for standard motors. This gets your 3 hp 3x746watts/hp = 2238/.8 efficiency = 2797 watts at 230v = 12.1 amps. For comparison the nameplate on the 3 hp motor on my new saw says 13.2 amps. or about 74% efficiency. So using these numbers the 5 hp saw would be 5x746/.74/230= 22 amps plus whatever the phase convertor needs.


Dean

Thanks for the response. I have done some digging around on the phase converter side and the figures you quote come up about right with what they say.

They have a 5.5hp (max) rotary converter and quote the single phase draw as 25amps, which is within the bounds of experimental accuracy on your numbers.

I thought I had it about figured but all the different measurments used wit elctrictrickery always get me confused.

Bill - I think that you may be right about the safety factor built into the machine quality. It is a 20inch (500mm) thicknesser made by Wadkin. They spec it for a 20inch width of cut and I rarely have anything over 8in so it should find an easy life in my shop. All I gotta hope now is that nobody else wants it.


Thanks both for your replies