ECN Forum Forums General Topics of the Trade General Discussion Area Soft Starters on 2 hp water pumps

Typically soft starters are used on larger motors however I am in the need to soft start a 2 hp water pump to lessen the initial in-rush current on typical start up. I am looking af using the Baldor S21CA in particular however my quest is in general in nature. One of the setting is limiting the amps that are drawn by the pump and it is adjusted bu turning it up until the motor starts to turn. my question is does any one know about where the current threshold is when a 2 hp pump underload will start turning? Granted there are too many varibles there for an exact answer. Any takers? Thanx

That is a hard question that I would like to know the answer to. The problem is that it may be different for each head, or it may not as the check valve is there and the motor may get moving before the system begins to pressurize.

But of possible help, I like using the Grundfos SQ/SQE series as they are soft start (the start-up current is equal to the highest value stated on the nameplate). A single phase, 240VAC, 2.25 hp pump draws 11.2 amps at startup according to my understanding.

Maybe this helps.

The biggest issue is going to be getting enough torque to drive the pump under hydraulic load. Lots of electronic motor drives can do this and several can even take a single phase supply and give a 3 phase output so you could also avoid split phase or capacitor start motors. It seems odd to me to need to soft start a 2 hp motor. perhaps some of the line disturbance is related to undersized conductors?

Mike,
Having a soft-starter on any water pump is a good idea, no matter what size we are talking about.
Not so much from an electrical perspective, but from a mechanical one, soft-starters minimise the initial pressure surge created by direct-on-line or even star-delta starters.

I've lost count the number of times I've turned up to faults calls in dairy sheds where couplings have literally blown apart in water lines, this is caused by the hammering effect of running a pump motor up far too fast and the surge of water caused by it.
Using a soft starter is also kinder on the pump impeller to a degree as well.

I suppose the usefulness of a soft-start depends on the specifics of the pump. Whatever the situation, though, you can't do any harm adding one.

Let's take two "pumping" applications, to illustrate the point.

A sump pump typically is pumping into an empty pipe, with very little back-pressure. The total head, in fact, isn't that much. The motor turns at a moderate rate, and the centrifugal impeller doesn't absolutely 'have' to move the water. Also, water has a pretty low viscosity. In this application, using a soft-start would gain little benefit.

At the other end of the scale might be a refrigeration compressor. You're always starting up against full pressure, the motor spins pretty fast, the vanes are 'positive displacement (If they move, the fluid inside MUST also move), and the 'expansion valve' at the end of the run poses a significant restriction for the pump to overcome. For this, a soft-start can make a big difference.

Sparkyinak,
What exactly is this pump feeding, or draughting from?

One is a well pump and the other is a pressure pump. It will be fed with an inverter system so the inverter has limited surge capacity

I presume I can figure the FLA as a maximum but not sure since I have no experience in this area