Vfd control of sump pump application

[remullis]

I am controlling a very tight window in a 8 foot sump. My low level to keep from cavitating the pump is 44” and my high level is 55”. The problem is inrush of waste water has a surge periodically. I was hoping to get some feedback on possible solutions in reference to rapid response out of the drive. I am using v/hz control and I other than the pidl loop I am attempting to control with decel and accel times. I have about an 8” window for level control before I over flow or cavitate with a unstable water in rush.

 

[Steve Bailey]

It's hard to understand your system from your description. You're trying to maintain the level in an 96-inch sump between 44 and 55 inches? Also, what are you trying to control with accel and decel times?

How fast does the level rise during the high inrush condition? How fast can you drop the level from 55" to 44" when the pump is running at 100% and no water is coming in? How accurately are you measuring the level? Do you get variation in the measurement of the level due to waves generated by the inflow?

I would think proportional control with deadband would be sufficient to the task. Leave the drive's accel and decel times as fast possible without generating an overcurrent fault on acceleration or a bus overvoltage fault during deceleration. Shut the pump off when the level gets within the deadband above 44". Make your speed reference equal to a proportional gain constant times the elevation above 44". The gain constant should be enough to generate maximum speed at some reasonable level below 55". You can make your job easier if there is an influent flow sensor available so you can anticipate the need to speed up the pump rather than reacting when an influent surge generates a rise in level.

 

[remullis]

Sir, the sump is at around 96 depth, but the suction of the pump is at 44 inches. I need to maintain that 44 in order to Ke͏ep the pump from cavitating. I’m only using the proportional gain and integral time in the loop. So, by what you are saying, if I had a flow meter in the line I could use the derivative to anticipate the in rush of water. I can put in a flow meter.

 

[drbitboy]

do you think could you control the level by manually e.g. if you had some sort of speed control knob, 0-100%, in your hand?

can you put some numbers on this? e.g.

• cross-sectional area of sump
• typical inlet flow rate (volume per time)
• max inlet flow rate (volume per time)
• max pump outlet rate (volume per time)
• how fast does level rise on large in-rush (inches per second or inches per minute)
• pump/motor/drive specs (how fast can pump be made to accelerate and decelerate)

 

[remullis]

I don’t have those numbers. I’ll need to get a flow meter in place. The in rush of water is within 4 seconds of over flowing. Within the 8” window.

 

[drbitboy]

So, by what you are saying, if I had a flow meter in the line I could use the derivative to anticipate the in rush of water. I can put in a flow meter.

I think @Steve Bailey is saying P-only, with an offset such that at 44" the pump speed is 0%, and at around 50-54" the pump speed is 100%, and with a PV deadband in the middle where the PID does nothing (does not change speed) when the level (PV) is in that range (@Steve said turn the pump off, but I think he meant turn the PID off i.e. effectively put it into manual).

If that is right (and it may not be), I think it would be possible to implement like this:

--[LIM             ]-----[CPT                         ]---
  [Low limit     49]     [Dest      pump_speed_percent]
  [Test  lvl_inches]     [Expr  (lvl_inches-44) * 12.5]
  [High limit    47]

which models a PID with

• Kc = 12.5%/inch (=100%/8inch)
• Ki = 0
• Kd = 0
• PV = level (inches)
• SP = 48 (halfway between 44 and 52)
  • ±1-inch dead band
• CV = pump speed, 0-100% = E * Kc
• Direct-acting (Error = PV - SP)

P.S. that assumes the pump running at 100% is about as much as the maximum inlet flow rate. There could be instability problems (surging) with that gain.

 

[remullis]

I will try that, great information. I have the stop mode as coast to stop. So I’m interested to see how it reacts using the deadband.

 

[drbitboy]

The nominal dead band I proposed (Low limit of 49, High limit of 47; N.B. low is greater than high!) assumes the normal inlet flow rate is equivalent to that of the pump running at 50% speed; if the typical rate is less (or more), those limits can be shifted, expanded, whatever.

 

[Steve Bailey]

The system model I have in mind is similar to the sump pump in my basement. On my sump pump there is a low level sensor that turns the pump off and a high level sensor that turns it on. Using two sensors minimizes the on/off/on/off cycling that happens when you try to control with only a single level sensor with some hysteresis. But my sump pump doesn't have a VFD, so it only runs at zero or 100% speed.

In the OP's system there is possibility to maintain a more consistent level in the sump by changing the speed of the pump. Adding an influent flow meter allows even tighter control. If you have 50 GPM inflow, use that information to set the pump's speed to produce 50 GPM output. Riding on top of that use the level sensor to increase or decrease the pump's speed as the level rises above or falls below some nominal level.

That all assumes the OP is using a PLC for control. If there is no PLC and he's limited to the VFD's built-in PID function, that's a different story. Remember, in the first post he mentioned adjusting accel and decel rates for control. I still don't get that and he hasn't offered an explanation.

 

[drbitboy]

That all assumes the OP is using a PLC for control. If there is no PLC and he's limited to the VFD's built-in PID function, that's a different story. Remember, in the first post he mentioned adjusting accel and decel rates for control. I still don't get that and he hasn't offered an explanation.

Good point. The PID with I=D=0 may be the best bet; I'm not sure the dead band is critical, and it would be your basement sump pump with "infinite" levels and speeds.