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Inverters no longer break the bank!

There are many electric motor applications where an inverter (or variable speed drive/VSD) is better than fixed speed/direct on line, with price as the only drawback. Traditionally inverters have been too expensive for many uses, but as technology advances the price of inverters continues to fall which opens up the possibility of using them for many more applications. For an overview of the most common motor starting methods, see the bottom of this blog.

One of these use cases is waste water systems. A UK manufacturer, Invertek, are challenging expectations on quality, lead times and cost prices and we love a market challenger 😁 so we've spent time helping develop pump specific programs for their inverters.

To give context to this, a twin pump control panel with (2) inverters is £1, 332.82 + delivery + VAT. So while it's more expensive than a DOL panel, the price difference is much closer than it has been in the past.

Here's some of the advantages of using inverter drives for starting wastewater pumps:

  1. Reduced water hammer - ramping the inverter up slowly prevents sudden pressure being exerted on the discharge pipework, which over time can cause damage ultimately leading to the pipework blowing apart.

  2. Reduced energy consumption - by setting the inverters at the speed they need to run rather than at full speed then the energy consumption savings can be huge.

  3. Greater control - the built-in PLC provides opportunities for greater control, such as speeding the pump up when the inflow is higher, or dry run detection.

  4. Ability to convert single phase to three phase - single phase wastewater pumps are usually more prone to failure, due to the difference in motor design compared to three phase, but an inverter can convert a single phase incoming supply into a three phase outgoing supply (not all three phase pumps can accept a three phase 230V input, so you'll need to check on this with the pump manufacturer).

  5. Facility for an emergency stop - these inverters have built in STO terminals which can be used for a Cat 0 emergency stop, saving the addition of a costly safety relay if an emergency stop is required.

  6. More data - the inverters can be configured to provide a large amount of data via a serial connection to a central BMS system.

so consider inverter drives for your next sewage or stormwater pump application, your system will work better and last longer, and the extra cost won't break the bank!

Motor starting methods:

  • DOL - direct on line, means when the contactor is told to engage 100% of the power is sent to the pump. The benefit of this is simplicity, simple give the pump power and it runs at full speed, remove the power and the pump stops. The drawback to this is the wear on the system, including the pump and pipework. Going from no flow to 100% power in a very short space of time can create 'water hammer, which can burn the motor out over time, break valves and cause fractures at joints.

  • Star/delta start - this is where 66% or 2/3 of the power is applied to the pump on start then moves over to 100% power after a period of time, often around 5 seconds. This is more expensive as it requires 3 contactors per motor as opposed to DOL which requires 1 contactor per pump. There is also some complexity in calculating the correct thermal overload - if the pump is 10A FLC, then times this by 0.58, so 5.8A and this is what the thermal overload should be rated to. A super common mistake is seeing a 10A pump and a 4-6A overload and assuming the overload is 'wrong'. The big benefit of star/delta is reduced water hammer so it's much less likely to damage the motor, pipework, etc.

  • Soft start - soft starts work in a similar method to star/delta, essentially give the motor a reduced amount of power on start up, then increase this once the pump is running. The downside is they don't provide any more control over the motor to star/delta but are more expensive plus they're only marginally cheaper than an inverter with full motor control.

  • Inverters - inverters control the exact speed, or frequency, by adjusting the power supplied to the motor. Inverters have parameters that can be set such as ramp up time, ramp down time, automatic reversing the pump if high amperage is detected (de-ragging mode), and a host of others giving fine tuning


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