As long as the pump motor is much smaller than the main motor and the VFD is rated for more than the combines power of both yes you can connect the coolant pump in parallel with a switch. The main motor must be permanently connected to the VFD. If the pump motor is wired star (415 V) and the main motor has been changed to delta (240) I would initally leave the pump wired star and se if it produces enough flow for your use. Leaving it star will reduce the additional load on the VFD with the pump running. Make sure you use a 3 pole switch. While a 2 pole will work with normal 50Hz mains leavine one pole permanently connected to a VFD will cause an imbalance fo the high frequency PWM signal of the VFD. This can cause incorrect operation. damage to motor or drive and increased intereference.

Stienmetz is also an option see https://www.js-technik.de/en/faq/detail/groupid/4/id/75

Robert.

Always worth putting figures on subjective statements like 'worked a treat'. When measured, Steinmetz connected motors perform significantly less well compared with running on a 3-phase supply:

• At least 30% less power output
• At least 70% less torque
• 2-pole motors perform extra poorly, possibly turning not at all

When they work Steinmetz connected motors are a convenient compromise, not the best possible engineering solution. The most likely gotcha is low torque, so more likely to perform acceptably on Suds pumps and similar applications that don't need lots of turning power.

If Steinmetz connections worked well in all circumstances there would be no market for rotary converters and VFDs. More sophisticated approaches to 3-phase come with significant advantages, VFD's being extra tasty with speed control, torque boost, soft-start, and efficiency savings. But usually limited to one motor per VFD. On the other hand, if it works, simplifying a major rewire job by adding a small capacitor to an existing motor is fit for purpose and value for money. Just don't expect miracles!

Dave

Hello Dave,

Sounds as if you are in one of your "be careful moods" Absolutely no one is talking about large motors and the deficiency of the Steinmetz system re reduced torque and power! A suds pump motor is way overated for the job it does. Your warnings of grief are most certainly over the top. If emgeee can use such a motor connected WITHOUT the Steinmetz capacitor and without reconfiguring the voltage from 415 to 240 volts Then your Jerimiah predictions are totally invalid.

I certainly would not try to emulate emgee, but a correct value Steinmetz capacitor and rewiring to 240 V is a totally logical thing to do without any drawbacks.

If I were suggesting the Steinmetz system for a 5 HP lathe motor, then your comments would have validity. Please don't put people off from modding a suds pump in the same way.

Andrew.

If you are using the VFD just to generate 50Hz three phase then JFDI* and feed the suds pumpfrom the VFD as well.

If you are using the VFD for variable speed purposes, then use a capacitor.

* Just Flipping Do It!

Edited By Mark Rand on 08/06/2023 17:28:53

Hello Michael,

If you are going to supply 240V Ac to your pump, it may be better to rewire the motor to a star configuration rather than the 440 V delta which it is at the moment. Be careful in selecting a capacitor. The usual motor start capacitors are electrolytic and won't last long before they go bang in a Steinmetz circuit. The type you need are non electrolytic types and rated at 400 V DC or near. These are the types found in Motor run capacitor circuits. Or in fluorescent lamp units (or any power factor correction usage, where it is connected across the mains)'

The 2 microfarad capacitor that I used was selected after a little experimenting. I wanted the 3 phases to be near enough equal in voltage and at almost 120 degree phase difference. A 2 microfarad capacitor will work on your suds pump , but it may not give the optimum results of equal phase voltage and 120 degree phase separation. Not that it is all that important. It should give you an operational pump running on 240 V Ac.

Andrew.

I plead not guilty M'lud!

After providing numbers showing the actual limitations of Steinmetz , I finished my post with:

On the other hand, if it works, simplifying a major rewire job by adding a small capacitor to an existing motor is fit for purpose and value for money. Just don't expect miracles! Is that really a Jeremiad?

All engineering is conditional, and most of it can be expressed numerically. Bridges, ropes, aircraft and ships have weight limits. Car batteries only contain so many ampere-hours, internet connections have bandwidth and latency restrictions. Threads have to be cut accurately or they won't fit. It's useful to understand the underlying rules.

In engineering, guesswork, hearsay, optimism and random experience are all low value compared with knowledge. It's why the forum is wonderful: others know stuff I don't. When something doesn't work, we can establish why not and what to do about it. Sometimes the answer is a lemon.

Dave

Thanks Duncan,

Must be old age, the motor should be wired for 240 Volts i.e. delta connected. goodness knows what I was thinking of.

Andrew.