Running the 3-phase coolant pump from single phase

As mentioned last time, I can't just run the coolant pump off the same VFD as the spindle due to the wide spindle speed range and I don't plan on buying a VFD just to run the coolant pump. However, it's quite common practice to power 3-phase motors from single phase by providing a sort-of third phase using a motor run capacitor. This seems to require something like 70uF per kW as a rule of thumb. That number comes up when you search for examples, although it's not clear where it comes from, other than perhaps empirical knowledge. I can't be arsed to attempt any form of analysis - I'm just trying to get it to run and not much more and it's far from rocket science.

It seems that once the motor is up and running around base speed you could actually run it perfectly well with just 2 phases. Obviously if you ran it near rated load (100W / 1/8 hp here) on 2 phases it would run hotter but for a 0.1kW motor it seems pretty big, so I suspect it's quite conservatively rated. In this scheme, the capacitor is required to provide a phase angle on the third phase to get the thing moving. Depending which phase you connect this capacitor to determines the direction of rotation. If you could be arsed, the capacitor could then be disconnected and the motor would run on. In this state, it operates just like a rotary phase converter and you could even connect a (3-phase) load across all 3 phases, sort of like an autotransformer. But I can't be arsed, as I've said.

So I found that Maplin keep a range of 250V and 440V motor run capacitors by LCR (seems LCR are still in business) in quite a range of values from 1uF to 20uF. The prices aren't amazing but when you are buying a single example, what do you expect. And it's nice to be able to simply call in and pick one up. Maplin actually have a lot of stock hidden out the back of the shop, so even though it may not be visible on the shelves, they may have stock of what you see on the website - you can check online that they have what you want.

Seemed rude not to go for the 8uF 440V example (for £6), so I bought their single stock item on the way home. Despite being illustrated with flying leads, it actually comes with four 1/4" spade terminals, as described in the text. No problem. And it's nicely insulated, being overmoulded in some form of plastic.

Made up a cable that is long enough to eventually reach from the coolant sump to the control cabinet where I will control it with a SSR via the CNC controller. Found a cable gland to fit the junction box and connected it all up. 

The cable is cable tied to the motor, along with the cap - that's good enough for the likes of me. The spare terminals are protected with a couple of spare insulated crimps. 

There's a 50% chance that when you try it, the motor will rotate the correct direction. Seems that the coolant gods smiled on me and I got lucky first time round. No need to swap one of the cap wires to the other phase.

The bearings are clearly in good shape. It spins easily and silently, so no need for any work on the mechanics. I haven't bothered to test it with any load, eg bucket of water but as the initial starting load will be fairly low (it's a centrifugal pump), I don't expect any problem on that front.

Job done - awaiting refitting. But first I will need to clean out the coolant sump. Hmm...