skotl
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Got excited there until I realised I had misread what you wrote - thought Tesco were now doing VFDs - would put Aldi and Lidl to shame!
- Thread starter carl0s
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Got excited there until I realised I had misread what you wrote - thought Tesco were now doing VFDs - would put Aldi and Lidl to shame!
A 3kw motor is the same rating (3kw) regardless of whether its 1ph or 3ph.
The difference is that the current required to run is all on 1ph on 1ph, but spread over 3 phases on 3ph.
At 4.5A per kw (230vac ph-n) on 1ph its 13.5A while on 3ph its 4.5A per phase.
1 Hp is actually 746 Watts.... but its a nice rule of thumb to consider 1hp to equal 1kva, as this allows for the motor power factor (about 0.84) & efficiency (about 92%) in the mix without messing about.... so a 3hp motor is basically 3kva (so 13.5A running on 1ph, 4.5A per ph on 3ph).
The kicker on 1ph motors is starting multiple inrush demand.... maybe 7x the rated motor load (maybe 4-5x for a 3ph motor). With a VFD though that can drop to maybe 1.3-1.5 x the running load.
On that basis, the inrush current off a 1ph supply of a 3hp motor (depending on what the motor is driving) could be 13.5A × 1.5 = 20 Amps ramping up to start it.
That's what I think anyway. A clampmeter would answer the real demand.
A 3hp compressor motor does indeed peak at 20A on start up according to my moving coil clamp meter. Soft start on VFD can reduce this quite a bit.
What doesn't help is that that 1HP is now quoted as 746 watts, which is a conversion enforced in 1992, so not electrical at all (thankyou Europe.) A kw also equals 1.34bhp from an IC engine.
The actual max load power consumption of a 1hp motor varies according to design and the quality of materials used. LINKY
They do not apply to single motors or vfd as they are components and not a product.
Agreed. The startup & max load can also vary considerably depending upon what a motor is driving (e.g. a compressor with an unloader valve is starting under very little load, where if it was driving say a table saw or a large drum planer, then the peaks can be considerably higher for the same nominally rated motor.
A vfd may be a component, but it's ramp-up & "soft-start" effect reduces transient impact loads considerably.
(I used to size generators for backup on water pumping stations, for running sawmills & allsorts - & where the pumps tended to be VSD, a lot of other stuff was SD, soft-start, DOL motors - and you'd see big variances. For the VSD stuff it wasn't the motor start transients per se that was determining generator size - it was the load schedule & very much the THD that determined what generator kva was required).
hermetic
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Rating a motor in watts is slighly disingenous! Watts are the input power, and are dissipated by the motor, HP is the output power, therefore a motor that dissipates 746 Watts will not produce 1Hp unless it is 100% efficient. The inrush current for a single phase 3Hp motor is more than 13A but due to the fusing factor of the 13A cartridge fuse, as long as the motor starts off load and promptly, the fuse will not blow. 3 phase motors startand run more efficiently at lower current for the same output power.
Merry Christmas.
Phil
Merry Christmas.
Phil
