Motors, Starters & Drive Systems

Electric motors drive much of the equipment used in water and wastewater facilities, including pumps, blowers, mixers, conveyors, chemical-feed systems, and mechanical treatment equipment. Operators do not need to perform electrical repair work unless they are qualified to do so, but they should understand basic motor and starter operation well enough to recognize abnormal conditions and provide useful information to maintenance personnel.

Motor problems can be electrical, mechanical, hydraulic, or control-related. Good troubleshooting begins by observing the entire driven system rather than assuming every trip or current change means the motor itself has failed.

What an Electric Motor Does

An electric motor converts electrical energy into mechanical rotational energy.

The rotating motor shaft can drive equipment such as:

• pumps;
• blowers;
• mixers;
• compressors;
• conveyors;
• chemical-feed equipment.

Common AC Motors

Three-phase alternating-current induction motors are common in water and wastewater facilities.

They are widely used because they are:

• reliable;
• relatively simple;
• available in many horsepower ratings;
• compatible with starters and variable-frequency drives.

Major Motor Components

Basic motor components include:

• stator;
• rotor;
• shaft;
• bearings;
• frame;
• cooling system;
• terminal connections.

Stator

The stator is the stationary part of the motor containing the electrical windings that create a rotating magnetic field.

Rotor

The rotor is the rotating part inside the stator.

The magnetic interaction between stator and rotor produces torque that turns the motor shaft.

Motor Shaft

The motor shaft transfers mechanical energy to the driven equipment.

The connection may use:

• flexible coupling;
• belt drive;
• gear drive;
• direct connection.

Motor Bearings

Motor bearings support the rotating shaft.

Bearing problems can be caused by:

• poor lubrication;
• contamination;
• misalignment;
• excessive belt tension;
• vibration;
• electrical damage;
• normal wear.

Motor Cooling

Motors generate heat during operation.

Cooling may depend on:

• airflow over the motor frame;
• internal fan operation;
• ventilation openings;
• ambient temperature.

Blocked cooling passages can contribute to overheating.

Motor Nameplate

The motor nameplate provides important operating information.

Depending on the motor, it may include:

• horsepower;
• voltage;
• current;
• frequency;
• phase;
• speed;
• service factor;
• insulation class;
• frame information;
• manufacturer data.

Horsepower

Motor horsepower indicates the mechanical power the motor is designed to provide under rated conditions.

The driven equipment should not routinely require more power than the motor can safely provide.

Voltage

The motor must receive the correct electrical voltage for its connection and design.

Abnormal voltage conditions can contribute to:

• overheating;
• poor torque;
• high current;
• motor damage.

Motor Current

Motor current is commonly measured in amperes, or amps.

Current changes with motor load.

Operators may use motor current as a practical indication of whether equipment load has changed.

High Motor Current

Higher than normal current may result from:

• increased hydraulic load;
• mechanical binding;
• bearing problems;
• blocked equipment;
• electrical problems;
• operation beyond the intended equipment range.

Low Motor Current

Lower than normal current may indicate reduced load.

Examples include:

• pump losing prime;
• broken coupling;
• reduced pump flow;
• blower unloading;
• equipment no longer performing normal work.

Frequency

AC motor speed is related to electrical frequency and motor design.

In the United States, standard power frequency is typically 60 Hz.

A VFD can change the frequency supplied to a motor and therefore change motor speed.

Motor Speed

Motor speed is commonly expressed in revolutions per minute, or rpm.

Actual induction-motor speed is normally slightly below synchronous speed because of slip.

Operators generally use the motor or equipment nameplate and control display rather than calculate slip during routine operation.

Three-Phase Power

Large facility motors commonly use three-phase power.

Loss of one phase, sometimes called single-phasing, can cause serious motor problems.

Possible symptoms include:

• failure to start;
• reduced torque;
• high current in remaining phases;
• rapid overheating.

Motor Starter

A motor starter controls electrical power to a motor and typically includes protective functions.

A basic starter may include:

• contactor;
• overload protection;
• control circuit;
• start and stop controls.

Contactor

A contactor is an electrically operated switching device that connects and disconnects motor power.

The contactor is controlled by a lower-power control circuit.

Overload Protection

Motor overload protection helps protect the motor from excessive current over time.

An overload condition can result from:

• mechanical overload;
• blocked equipment;
• bearing failure;
• abnormal voltage;
• phase problem;
• excessive process load.

Overload Trip

When an overload device trips, the motor is stopped or prevented from continuing operation.

The operator should determine why the overload occurred before resetting repeatedly.

Overload Is Not the Same as a Short Circuit

An overload usually involves excessive current associated with equipment load over a period of time.

A short circuit or ground fault is a different electrical fault and is normally handled by protective devices such as breakers or fuses.

Circuit Breakers and Fuses

Breakers and fuses provide electrical protection against fault conditions.

If a breaker trips or fuse opens, qualified personnel should determine the cause before equipment is returned to service.

Across-the-Line Starter

An across-the-line starter applies full line voltage to the motor during startup.

This produces:

• high starting current;
• rapid acceleration;
• high starting torque.

The method is simple but may not be appropriate for every motor or process.

Starting Current

Motor starting current can be several times higher than normal running current.

This condition usually lasts only while the motor accelerates.

Repeated or prolonged starting can increase heating.

Frequent Starting

Excessive motor starts can contribute to:

• motor heating;
• starter wear;
• contactor wear;
• mechanical stress.

Frequent starting may indicate a process-control or equipment-sizing problem.

Soft Starter

A soft starter reduces electrical and mechanical stress during startup by gradually increasing the voltage applied to the motor.

Benefits may include:

• reduced starting current;
• reduced mechanical shock;
• smoother acceleration.

Soft Starter Versus VFD

A soft starter primarily controls startup and sometimes shutdown.

A VFD can continuously control motor speed during operation.

This is an important distinction.

Variable-Frequency Drive

A Variable-Frequency Drive, or VFD, controls motor speed by changing the frequency and voltage supplied to the motor.

VFDs are widely used for:

• pump flow control;
• pressure control;
• blower airflow control;
• tank or wet-well level control;
• energy management.

VFD Speed Control

Increasing VFD frequency generally increases motor speed.

Decreasing frequency generally reduces speed.

For centrifugal equipment such as pumps and fans, speed changes can significantly affect:

• flow;
• head or pressure;
• power demand.

VFD Display Information

A VFD may display:

• frequency;
• speed;
• motor current;
• voltage;
• power;
• fault code;
• run status.

Operators should record fault codes before resetting a drive when practical.

VFD Faults

Common VFD fault categories can include:

• overcurrent;
• overvoltage;
• undervoltage;
• overtemperature;
• motor overload;
• communication fault.

The exact meaning of a fault code depends on the drive manufacturer.

Do Not Repeatedly Reset VFD Faults

A drive fault may be protecting the motor, drive, or process.

Repeated resets without investigation can:

• damage equipment;
• hide the original fault;
• delay proper repair.

Minimum Motor Speed

VFD-controlled equipment may have a minimum allowable speed.

Reasons can include:

• motor cooling;
• minimum pump flow;
• minimum blower airflow;
• lubrication needs;
• process requirements.

Do not assume that operating at extremely low frequency is harmless.

Maximum Motor Speed

Equipment should not be operated above its approved maximum speed.

Excessive speed can create:

• mechanical stress;
• bearing problems;
• excessive pump or blower load;
• unsafe equipment operation.

Belts and Pulleys

Some motors drive equipment through belts and pulleys.

Operators should inspect for:

• belt wear;
• cracks;
• fraying;
• incorrect tension;
• pulley misalignment;
• missing guards.

Loose Belts

Loose belts can cause:

• slippage;
• reduced driven-equipment speed;
• heat;
• belt wear.

Excessive Belt Tension

Belts that are too tight can increase load on:

• motor bearings;
• driven-equipment bearings;
• shafts.

Couplings

Direct-drive equipment often uses a coupling between motor and driven equipment.

Inspect for:

• wear;
• damage;
• loose hardware;
• misalignment;
• missing guards.

Alignment

Poor alignment can increase:

• vibration;
• bearing load;
• coupling wear;
• seal wear;
• motor load.

Flexible couplings do not eliminate the requirement for proper alignment.

Gear Drives

Gearboxes may be used to change:

• speed;
• torque;
• direction of rotation.

Operators may monitor:

• oil level;
• oil condition;
• temperature;
• noise;
• vibration;
• leakage.

Gearbox Overheating

Possible causes include:

• incorrect lubricant;
• low lubricant level;
• excessive load;
• bearing damage;
• internal wear.

Motor Temperature

Motor temperature should be compared with normal operating conditions.

Overheating can result from:

• overload;
• poor ventilation;
• high ambient temperature;
• frequent starting;
• electrical problems;
• bearing problems.

Motor Vibration

Abnormal motor vibration may result from:

• misalignment;
• imbalance;
• bearing wear;
• loose mounting;
• driven-equipment vibration;
• coupling problems.

Motor Noise

New or unusual motor noise may indicate:

• bearing problems;
• loose parts;
• electrical problems;
• mechanical contact.

Motor Trips Only During High Process Load

If a motor trips only when process demand is high, investigate whether the driven equipment load is increasing.

Examples include:

• pump operating at excessive flow;
• mixer encountering heavier solids;
• conveyor overloaded;
• blower working against unusual system resistance.

Motor Trips Immediately on Start

Possible causes include:

• electrical fault;
• locked rotor;
• jammed driven equipment;
• incorrect starter or protection condition;
• serious mechanical binding.

Do not repeatedly attempt restart without investigation.

Motor Starts but Equipment Does Not Move

Possible causes include:

• broken coupling;
• slipping belt;
• failed gearbox;
• disconnected shaft;
• mechanical damage.

Motor Current as a Troubleshooting Tool

Motor current should be evaluated with equipment performance.

Examples:

• high current plus low speed may indicate overload;
• low current plus no pump flow may indicate loss of prime or broken coupling;
• normal current plus abnormal process data may indicate an instrument or process problem.

Phase Current Comparison

Qualified electrical personnel may compare current among phases to identify imbalance.

Significant phase imbalance can contribute to motor heating and poor performance.

Operators should report abnormal readings rather than perform electrical work beyond their qualifications.

Electrical Enclosures

Control panels and motor-control centers should remain:

• closed;
• dry;
• clean;
• protected from unauthorized access.

Water intrusion, condensation, dust, and corrosion can damage electrical equipment.

Motor Control Center

A Motor Control Center, or MCC, may contain:

• motor starters;
• breakers;
• overload devices;
• control components;
• VFDs.

Access and electrical work should follow facility electrical safety procedures.

Electrical Safety

Electrical equipment can expose workers to shock, arc-flash, burn, and fire hazards.

Operators should:

• keep panel doors closed when required;
• report damaged electrical equipment;
• avoid exposed energized components;
• follow lockout/tagout requirements;
• leave electrical testing and repair to qualified personnel when required.

Lockout/Tagout

Stopping a motor from a control switch does not isolate hazardous electrical or mechanical energy.

Before servicing driven equipment, required lockout/tagout procedures must be followed.

Automatic Restart

Motors controlled automatically can restart because of:

• level change;
• pressure change;
• timer;
• PLC logic;
• remote command.

Automatic startup must be considered before maintenance.

Power Restoration

After a power outage, equipment may:

• restart automatically;
• remain stopped;
• restart in sequence;
• require manual reset.

Operators should know the designed response of critical equipment.

Restart Sequence

Large facilities may stagger motor restarts after power restoration.

This can help reduce:

• electrical demand surge;
• hydraulic transients;
• simultaneous equipment loading.

Standby Equipment

Standby motors and driven equipment should be exercised periodically according to facility procedures.

A standby unit that has not operated for a long time may develop:

• bearing problems;
• corrosion;
• stuck equipment;
• electrical problems.

Trend Motor Data

Useful trend data include:

• current;
• run hours;
• starts;
• temperature;
• vibration;
• VFD frequency;
• power.

Gradual changes can reveal developing problems before failure.

Common Motor and Drive Problems

Operators may encounter:

• motor will not start;
• motor starts and trips;
• high current;
• low current;
• overheating;
• vibration;
• unusual noise;
• VFD fault;
• belt slippage;
• coupling damage;
• gearbox overheating.

A Practical Motor-Will-Not-Start Check

1. Confirm the correct equipment.
2. Check control mode.
3. Review alarms and trips.
4. Verify process permissives.
5. Confirm that a start command exists.
6. Check whether the motor is electrically available.
7. Check whether the driven equipment is mechanically free where appropriate and safe.
8. Escalate electrical testing to qualified personnel.

A Practical Motor-Overload Check

1. Review the overload or VFD fault indication.
2. Check motor current history.
3. Review driven-equipment load.
4. Check for blockage or mechanical binding.
5. Review bearings and vibration.
6. Check recent process changes.
7. Check whether equipment is operating outside its normal range.
8. Have qualified personnel evaluate electrical causes when needed.

Do Not Diagnose Only from the Motor

A motor trip may be caused by the equipment being driven.

Examples include:

• blocked pump;
• jammed conveyor;
• failed bearing;
• overloaded mixer;
• high blower load.

Motor and process data should be considered together.

Common Motor, Starter, and Drive Mistakes

• Repeatedly resetting overloads or VFD faults without investigation.
• Assuming every high-current condition is an electrical failure.
• Ignoring motor cooling and ventilation.
• Ignoring belt or coupling problems.
• Operating with damaged guards.
• Assuming a soft starter and VFD perform the same function.
• Operating VFD-controlled equipment below approved minimum speed.
• Ignoring unusual vibration or bearing temperature.
• Failing to record VFD fault codes before resetting.
• Performing electrical work without proper qualification.
• Assuming equipment cannot restart automatically.
• Using a control switch as a substitute for lockout/tagout.

A Practical Operator Inspection

1. Confirm equipment status and control mode.
2. Review active alarms and VFD faults.
3. Review motor current.
4. Review motor speed or VFD frequency where applicable.
5. Listen for abnormal noise.
6. Observe vibration.
7. Check motor temperature where appropriate.
8. Inspect cooling openings.
9. Inspect belts, couplings, and guards where applicable.
10. Check gearbox oil and condition where applicable.
11. Compare readings with normal historical operation.
12. Document abnormal conditions.

What to Remember for the Exam

• Electric motors convert electrical energy into mechanical rotational energy.
• Three-phase induction motors are common in water and wastewater facilities.
• Important motor nameplate information includes horsepower, voltage, current, frequency, phase, and speed.
• Motor current changes with mechanical load.
• High current can result from hydraulic, mechanical, or electrical problems.
• Low current can indicate reduced mechanical load or equipment not performing normal work.
• A motor starter controls power to the motor and commonly includes overload protection.
• A contactor is an electrically operated switching device.
• Motor overload protection responds to excessive load conditions and should not be repeatedly reset without investigation.
• Across-the-line starting applies full line voltage during startup.
• A soft starter reduces startup stress but does not normally provide continuous speed control.
• A VFD controls motor speed by changing electrical frequency and voltage.
• VFD fault codes should be recorded and investigated rather than repeatedly reset.
• Minimum and maximum motor speeds may be limited by equipment and process requirements.
• Loose belts can slip, while excessive belt tension can overload bearings.
• Flexible couplings do not eliminate the need for proper alignment.
• Motor overheating can result from overload, poor cooling, frequent starts, electrical problems, or bearing problems.
• Loss of a phase can cause serious three-phase motor overheating and performance problems.
• Motor and driven-equipment conditions should be evaluated together during troubleshooting.
• Automatic equipment may restart without warning when process conditions change.
• A stop button or control switch is not a substitute for required lockout/tagout.
• Electrical testing and repair should be performed by personnel qualified for the work.