The most common problem in sewage, lift stations and the transfer of solids-laden wastewater is clogging. Fibres, rags, wet wipes and solid pieces quickly lock up a standard submersible pump. This is where the grinder submersible pump comes in: a cutting mechanism with blades at the pump inlet grinds the solid waste into small, pumpable pieces, enabling transfer through narrow pipes without clogging. A grinder submersible pump motor has different demands from an ordinary pump motor: high torque, low flow, IP68 submersible protection, thermal and moisture protection, S3 intermittent duty. In this guide we cover all the critical aspects of grinder submersible sewage pump motor selection for clog-free wastewater transfer.

Grinder submersible sewage pump motor and bladed grinding mechanism

How Does a Grinder Submersible Pump Work?

A grinder submersible pump has a grinding (cutting) head consisting of stationary and rotating blades placed in front of the impeller inlet. Wastewater passes through these blades before entering the pump; solids, fibres and bulky pieces are sheared into small particles. The ground wastewater can now be pumped over long distances, even upward, through small-diameter pressure pipes. This largely eliminates the risk of clogging and provides reliable transfer in sewage and lift stations.

Grinder pumps differ from standard submersible drainage and sewage pumps by their grinding capability. We cover standard submersible and sewage pump motor selection in our submersible drainage and sewage pump motor selection article; for the other motors of a water-treatment and wastewater plant, see water treatment and wastewater plant motors. For the general centrifugal pump motor selection logic, the centrifugal pump motor selection: flow and head article forms a foundation.

Where Is a Grinder Pump Used?

Grinder submersible pumps are used in villa and estate sewage systems, pressure sewer systems, lift stations, camps and recreation facilities, hospital and hotel wastewater lines, and rural areas without mains sewerage. Especially in narrow-pipe systems where wastewater must be pumped over long distances or upward, a grinder pump is almost mandatory. You can find the motor side of lift and circulation applications in the in-line and circulation pump motor selection article, and the booster and pressurisation side in the booster pump motor replacement content.

High Torque, Low Flow Characteristic

The most decisive feature in grinder pump motor selection is high torque and relatively low flow. The blades cutting solid waste require high starting and running torque; this means a motor characteristic different from a standard centrifugal pump. In a grinder pump the goal is not large flow but grinding solids and pushing them through a narrow pipe at high pressure. So grinder pumps usually have a high-head, low-to-medium-flow profile.

It is critical for the motor to have sufficient starting torque so the blades can start turning even when caught on a piece. On starting torque and torque class the asynchronous motor torque classes (Design N/H) article guides you, and for motor power calculation the motor power calculation: pump, fan and conveyor content. To correctly establish the flow-head-power match, also see the centrifugal pump motor selection article.

Protection of an IP68 submersible grinder pump motor with thermal and moisture sensors

IP68 Submersible Motor and Sealing

A grinder submersible pump motor operates fully immersed in wastewater; therefore an IP68 protection class is essential. IP68 indicates the motor is designed to maintain its seal continuously underwater at a certain depth. Water entering the motor burns the winding quickly; that is why sealing layers such as a double mechanical seal and an oil chamber are used. The oil chamber between the two mechanical seals serves both as lubrication and as an intermediate barrier against leakage.

We cover what IP protection classes mean and which is needed for which application in our electric motor IP protection class selection (IP55, IP65, IP66) article. In salty and corrosive environments (such as coastal lift stations) the motor's material and protection selection gain extra importance; our seawater and saltwater pump motors and cast iron motor in coastal and marine facilities articles explain this.

Thermal Protection and Moisture Sensor

It is impossible to see from the outside whether water has leaked into a submersible motor; that is why two critical protections are used in grinder submersible pumps:

Thermal protection: Thermal contacts (klixon) or PTC thermistors embedded in the motor winding stop the motor when the winding temperature reaches a dangerous level. In a blade jam, dry running or overload condition the motor heats up quickly; thermal protection saves the winding from burnout. You can find winding temperature monitoring methods in the temperature monitoring with PT100 and PTC thermistor article.

Moisture (leakage) sensor: A moisture sensor placed in the oil chamber or motor cavity gives early warning when water leakage begins through the mechanical seal. This allows maintenance before the winding is damaged and prevents an expensive motor failure. Thermal and moisture protection are the two most important pieces of equipment that determine the life of a submersible motor. For general protection-equipment planning, see the electric motor protection devices content.

Grinding Mechanism and Impeller Type

The heart of a grinder pump is the bladed grinding head. It usually consists of a stationary cutter ring made of hardened stainless steel and a rotating blade; the narrow cutting gap between the two shears and grinds the waste. The material and hardness of the blades determine both the life and the grinding capacity of the pump. Worn blades reduce grinding efficiency over time and increase the risk of clogging; so blade replacement is an important part of the grinder pump's periodic maintenance.

After the grinding head, the wastewater reaches a mostly semi-open or vortex-type impeller. The impeller type affects the head and the passage characteristic. The high-torque motor must have enough power to turn both the blades and the impeller. To assess the pump and system efficiency as a whole, see the real efficiency in a pump system: motor, pump and pipe losses article. Correct motor sizing is critical for both energy and life; for the problems caused by oversizing, see the how loaded should a motor run? correct sizing content.

S3 Intermittent Duty Type

Grinder submersible sewage pumps usually run not continuously but on level control and intermittently. When a sump or tank fills, a float or level sensor starts the pump; when the level drops it stops. This operating profile corresponds to the S3 (intermittent periodic duty) duty type. In S3 the motor runs for a certain time, then stops and cools; the ratio of running time to total cycle time (duty ratio, e.g. 25% or 40%) determines the motor heating.

The effect of duty type on motor selection is large: a motor designed for S3 will overheat if forced into continuous (S1) operation. So the pump motor's duty type must be selected to match the real operating profile of the application. On duty types our electric motor duty type (S1-S6) selection article is a detailed guide. For motor selection in intermittent, precise-flow dosing applications, we cover it in the metering and diaphragm dosing pump motor selection content.

Dry Running and Level Control

It is essential that the submersible pump stays below the water level; when the water runs out the pump runs dry, and both its cooling (the wastewater cools the motor) and the lubrication of the mechanical seal are compromised. So a correct level control must be set up with a float or level sensor. In a pump that switches on and off frequently the number of starts rises; this affects the motor's thermal load and duty ratio. To assess the starting method and inrush current in frequent-start applications, see the starting current in an asynchronous motor (LRA) article.

Installation, Commissioning and Maintenance

Reliable operation of a grinder submersible pump depends on correct installation and commissioning. The pump is lowered into the tank with a guide rail system; this allows the pump to be removed for maintenance without entering the well. During commissioning the motor rotation direction must be checked; a grinder pump running in the wrong direction neither grinds nor delivers the required flow. For rotation direction and phase order checks, the motor rotation direction and phase sequence article guides you, and for general first-start steps the motor commissioning and first-start checklist content.

In periodic maintenance, blade wear, mechanical seal leakage, oil chamber oil level and bearing condition are checked. In a wastewater environment, bearings and sealing carry special importance; the bearing type and life in an asynchronous motor article guides you on this. Because the whole line is affected when a pump stops in a lift station, it is wise to keep a spare pump in critical applications; on this topic see the critical spare motor list and stock planning content. We cover motor selection in hygienic and wastewater environments in the biogas and treatment plant motors article.

Frequently Asked Questions

What is the difference between a grinder pump and a standard sewage (vortex) pump?

Standard sewage (vortex/channel) pumps pass solids of a certain size without grinding; therefore they require wide pipes and a large free passage. A grinder pump grinds the solids with blades into small particles, so it can transfer at high pressure through narrow, long pipes. In places with heavy clogging waste such as fibres, rags and wet wipes, a grinder pump is far more reliable.

What protection is essential on a grinder submersible pump motor?

At minimum thermal protection (embedded thermal contact or PTC) and preferably a moisture/leakage sensor are essential. IP68 sealing is a basic requirement. These protections guard the winding against submersible-motor-specific risks such as blade jam, dry running or seal leakage, and markedly extend motor life.

How should I select a grinder pump motor by duty type?

Sewage and lift applications mostly run on level control, intermittently; this corresponds to the S3 duty type. The motor should be selected for S3 and a suitable duty ratio. If it will run continuously (S1), a motor with higher thermal capacity is needed. Always share the operating profile and starting frequency with your supplier.

Get a Quote

Let us determine the right power, torque, duty type and IP68-protected grinder submersible pump motor for your sewage, lift station or pressure sewer application together. Contact us to plan your clog-free wastewater transfer solution. To get a quote now, visit our contact page or call us at +90 (532) 345 49 86. You can review our entire high-efficiency electric motors range, and for geared solutions take a look at our worm gear reducers category and our homepage.

Purchasing and Selection Checklist

Before ordering a grinder submersible sewage pump motor, verify the following:

  • Have the wastewater content (fibre, rag, solids ratio) and clogging risk been assessed?
  • Are the required flow (L/s or m³/h) and head (m) defined?
  • Are the motor power and starting torque sufficient for bladed grinding?
  • Is the protection class IP68? Are there a double mechanical seal and oil chamber?
  • Are thermal protection (embedded contact/PTC) and a moisture/leakage sensor present?
  • Is the duty type S3 (intermittent), or is continuous operation required?
  • Are level control (float/sensor) and dry-run protection planned?
  • Is the environment corrosive (salty/chemical)? Has material and coating selection been made?

After completing this list, you can obtain a fast and accurate quote with the steps in our information to provide when requesting a quote article.