Pump Overload Management

General Description:
Overload Management On Single Phase Ac. Pumps
By Ian Jackson, Alian Electronics, April 2003

This document describes various strategies on the operation of A.C pump motors, and how best to protect them from a range of adverse operating conditions.

Typical Operation
The typical single phase pump motors have both a START and RUN winding in order to commence and maintain normal pump rotation. On the circuit below Active and Neutral represent the 240VAC power source. SW1 is the circuit used to start and stop the pump and SW2 is used to manage the operation of the starter capacitor and the START winding of the pump.

Theory Of Operation In Brief
Although the above circuit looks simple, the dynamics of the motor operation are quite complex. The RUN winding is wound to sustain normal operating currents after motor rotation has commenced. If only the RUN winding were activated, the motor would draw considerable current but would not rotate. It would simply hum without moving as no reference has been given for direction of rotation. If the motor were a two or three phase motor, then the phase shifts between each phase would provide a sufficient degree of offset between the different magnetic fields to allow an attraction/repulsion event to take place. Rotation of the commutator in the desired direction would then result.

With a single phase supply the ‘Starter Capacitor’ is used to apply current to the START winding. The capacitor provides a phase shift of enough magnitude to simulate a second phase. This START field, in conjunction with the RUN field, creates an environment for the motor shaft to commence movement in a known direction. Once rotation has commenced the START winding is usually deactivated. The rotation of the commutator will create enough of a magnetic field offset to sustain the rotation with only the RUN winding connected.

Startup Currents
At the instant that power is first applied to the motor (to either winding) the commutator is stationary and no dissipation of electrical energy as kinetic energy is occurring. The motor behaves as though the commutator is ‘stalled’ and current drawn will be typically three times the normal run current.

As soon as rotation commences, the current drain quickly falls. Because of the high current demand, The START winding and its associated capacitor, is only designed to be operated for a short period of time. A mechanism must disconnect this START circuit as soon as valid rotation has commenced. Many motors have a mechanical ‘On Speed’ contact to automatically isolate the START winding when rotation is fast enough. This is difficult to achieve with submersible pumps because of the risk of water contamination in the contact, so other techniques must be used to isolate the winding.

Starter Capacitors
The Starter capacitors are usually rated to allow for 20 ‘normal’ starts in one hour. For most installations this is quite appropriate. However, if a pump is started too often, or the start events last for too long then the capacitor will quickly overheat and will ultimately be destroyed. Symptoms range from slow leaks, to fiery electrolyte sprays and explosive destruction. Stalled rotor conditions effectively place the full mains potential across the low impedance capacitor plates, a condition that can only be absorbed for a few seconds.

The diagram below demonstrates a typical start-up event of a pump as captured with a high current probe on a storage oscilloscope.

Point A shows when power is first applied to a stationary motor.
Initial current is high as the starter capacitor is coupled into the ‘Start’ winding.

At Point B the start current can be seen to falls as the motor begins to rotate.

Point C shows where the START winding and capacitor have been disconnected from the motor.

The Run winding continues to operate.
A short time later the pump was manually halted

'Current Sensing’ Isolation Of Start Windings
Often an electronic current sensor is used to monitor the current being applied to the START winding. The theory is that as power is first applied to the pump, the circuit delivers a very high current to the START winding via a starter capacitor. As rotation takes place a second or so later, the current sensor detects a fall of the start current (typically down to 50%). It uses this event as a trigger to isolate the START winding and the motor will continue to operate on the RUN winding.

This system is often used because of the belief that it will work with a wide range of pumps without having to know too much about them. In reality a current sensing starter circuit should be fine tuned with each model of pump to correctly detect the threshold of rotation.

The biggest problem with this sort of system is apparent when the pump shaft ‘binds’ causing a locked rotor condition. The startup currents through the START winding and capacitor are high, and remain high. The current sensor fails to see the usual fall of start current and continue to deliver power to the START circuit. Eventually the over-current condition may destroy the capacitor and/or trip the supply circuit breaker and remove power from the pump. It is common that the RUN contactor has a motor overload protection, but this is often configured to protect the RUN winding only. Regardless, the slow response time of most overloads would seriously compromise the starter capacitor integrity.

Often the problem with the motor is not catastrophic, that is the motor rotation is sticky or impaired, but it still rotates. This can be the case when the pump shaft is partially bound with an obstruction or the material to be pumped is extremely viscous. As the pump slowly gets harder to start, the capacitor start winding events are extended longer and longer before valid rotation is sensed. With this scenario the starter capacitor is gradually degraded to the point of ‘slow’ destruction. Eventually the capacitor will go open circuit and the motor overloads will keep tripping because with no start circuit, the RUN winding gobbles up large amounts of current without trying to rotate.

Timed Isolation Of Start Windings
A better method of isolating start windings is with precision timing techniques. A trial assessment is made to ascertain a normal time frame for a motor to begin rotation after power is applied. A small margin of around 10% is added to this period, and this value will be used thereafter for all similar installations. Power is applied to the START and RUN windings simultaneously, as per the circuit on the previous page. When the time is up and the motor ought to be rotating, the START circuit is isolated. This ensures the integrity of the Starter Capacitor for all situations. Should the motor slowly deteriorate or stall, the motor overload will do its job and kill the power without having destroyed the capacitor first.

The biggest difficulty with this system is that the optimal time period has to be assessed properly in the first place, then applied with a precision electronic timer. Also the timer period would have to be optimised whenever a different brand of pump is being fitted to an installation, but it remains a much superior method than current sensing only.