Motor Controls Troubleshooting of Electric Motors

Motor Controls & Troubleshooting of Electric Motors

Introduction

to Motor Controls

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Devices that pass power to the motor

1.

Relays

2.

Contactors

3.

Starters

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The contacts are controlled by a coil in the control circuit

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The starting relay is only in the circuit for a short period of time

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The type of motor control device is determined by the size of the motor

Run--load and LockedRun Locked-rotor current 

Run load amperage (RLA)

1. 2.

Also called full load amperage (FLA) Current drawn while the motor is running Locked rotor amperage (LRA) Amperage drawn at startstart-up Approximately five times the RLA Both currents must be considered when selecting a control device

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1. 2. 

Relays     

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Relay Uses a magnetic coil to close one or more sets of contacts. Cannot be repaired ±± considered a throw a away away device. device. Used for light duty applications Can be used as a pilot duty relay (A relay used for switching loads such as another relay or solenoid valve coils. The pilot duty relay contacts are located in a second control circuit. Pilot duty relays are rated in volt volt-amperes amperes (VA).) The contacts must be able to handle the current draw of the load Can have normallynormally-open open (NO) contacts, normallynormally-closed closed (NC) contacts, or both normally open and normally closed contacts

Relays, Contactors, &

Starters

Contactor 

Larger versions of the relay

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Has moveable and stationary contacts

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Holding coils of different voltage ratings are available

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Can have one or more sets of contacts

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Some are equipped with auxiliary contacts

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Use exact replacement whenever possible

Motor Starters 

Described as a contactor with overload protection built in

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Can be rebuilt Should be replaced when the become pitted

1.

2.

Heaters and holding coils can also be replaced

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As the contacts become pitted the resistance increases

Motor Protection 

Because of the cost of electric motors they need to be protected from damage

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Fuses protect the entire circuit, not the

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components Motors can operate at a slightly higher current for a short period of time

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Very small motors do not have overload protection

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The larger the motor, the more elaborate the protection should be Motor protection can be inherent (internal) or external

Inherent (Internal)

Motor Protection

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Internal

thermal overloads, OR

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Thermally activated bimetal snap disc

External Motor Protection 

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Devices that pass power to the holding coil of the starter or contactor The device can open its contacts when a current overload deenergize occurs, causing the holding coil to deThe trip point and type of overload protection is determined by the manufacturer The overload protection takes the service factor into consideration Phase monitoring devices that shut down power if leg is lost to prevent single phasing three phase motors

Temperature ± Sensing Devices 

1. 2. 3. 4. 

1. 2. 3.

Bi-metal element BiHeated by a device called a heater The heater is in series with the load and senses the load current The bimetal warps when it becomes too warm The bimetal interrupts the circuit feeding the holding coil of the starter Solder pot Uses a low low--melting solder The solder melts when excessive heat is generated The excessive heat is generated from an over overcurrent condition

Magnetic Overload Devices 

Not attached to the starter

Not affected by ambient temperature  Very accurate overload device 

Restarting the Motor 

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If a motor shuts off on a safety, it should not be immediately restarted The cause for the overload must be found The motor must be given enough time to cool down Many control circuits have manual resets to prevent automatic restarting Some controls reset only after a prepre-determied time period Time delay devices prevent the unit from short cycling

Electric Motor Troubleshooting 

Problems are either mechanical or

electrical  Mechanical problems often appear to be electrical problems 

Technicians must be able to diagnose the problems correctly

Mechanical Motor Problems 

Normally occur in the bearings or drive

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connection Bearing Failure (tight or worn)

1.

Lack of lubrication

2.

Excessive grit in the bearing

3.

Over--tight belts Over

Removing Drive Assemblies 

Pulley, coupling, or fan wheel must be removed from a motor shaft with care

1.

The fit can be very tight The assembly must not be damaged Special pulley pullers can be used

2. 3. 

Set screws are tightened to the motor shaft

1.

Set screw is made of hardened steel Tightened to the flat portion of the shaft

2. 

The end of the motor shaft should not be hammered

Removing Drive Assemblies

Belt Tension 

Over--tightened can cause motor failure Over

and or bearing damage  A belt tension gauge should be used

Pulley Alignment 

Drive and driven pulleys should be

properly aligned  Drive mechanisms can become damaged 

Belt life can be reduced

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Belts can skip off the pulleys

Electrical Problems 

Easy to diagnose

Motor may smell burned  An open motor winding 

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A short circuit from the winding to ground

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A short circuit from winding to winding

Open Windings 

Can be found with an ohmmeter

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There should be a measurable resistance between the common, start, and run terminals

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A reading of infinite resistance is an indication that the winding is open

Shorted Motor Windings   

Caused when winding insulation develops a hole An alternative current path is created This path reduces the resistance of the winding

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The current flow through the winding will increase

1. 2.

Motor overload may trip Circuit breaker may trip

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Short in the run winding

1. 2.

Motor may start Motor will draw excessive current

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Short in the start winding

1. 2.

Motor may not start If it does start it will draw excessive current on startstart-up

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Three--phase motor windings should all have the same resistance Three

Short Circuit to Ground (Frame) 

There should be no detectable circuit from a winding to ground

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Motors should be checked for grounds using a good quality ground ohmmeter

1.

The meter should be set to its highest setting Megohmmeters (megger) can detect 10,000,000 ±ohm ±ohm range

2.

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Dirty motors may show resistance from winding to ground if the atmosphere is damp

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Hermetic ground compressors may show a slight

1.

The oil may be dirty Liquid refrigerant in the compressor worsens the condition A suction line drier may help clean the oil

2. 3.

Single--Phase Motor Starting Single Problems 

Fairly easy to troubleshoot

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Check for full power to motor

1.

Low voltage Loose connection Undersized wire Low voltage from electrical panel Measure voltage at the motor terminals Voltage must be within 10% of rated voltage

2. 3. 4. 5. 6.

Open Motors 

Checked with voltage supplied to both

windings  Start windings is removed when motor reaches 75% of rated speed 

Uses centrifugal switch

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The opening and closing of the switch can be heard

CSCR Motors 

Have the start winding energized all

the time  The start capacitor stays in the circuit whenever the motor is energized

Symptoms of Electric Motor Starting Problems 

Motor hums and then shuts off

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Motor runs for a short period and shuts off

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Motor will not try to start

Types of Problems to Consider 

Motor mechanical problem

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Electrical problem

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Motor circuit problem

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Motor load problem

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If

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motor and load turn freely, technician should check motor windings If

motor hums and does not start, starting components should be checked

Checking Capacitors 

Capacitors can be checked with an analog

ohmmeter  Ohmmeter leads placed across a charged capacitor can cause damage to the meter

Procedure for Checking Capacitors     

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Turn power off to the and remove capacitor lead Short across a charged capacitor Set ohmmeter to the R X 10 scale Touch meter leads to capacitor terminals A good capacitor would show the ohmmeter needle sweeping up to zero (0) ohms value and begin to fall back toward infinity Internal shorted capacitors ± needle will sweep to (0) ohms value and fall back toward infinity If the ohmmeter needle will not rise at all, switch the meter leads on the capacitor and look for a needle deflection that rises and returns to infinity No meter scale reading indicates an open capacitor If the capacitor has a bleed resistor, the ohmmeter will sweep to 0 ohms and fall back to the resistance value of the bleed resistor.

Identification

of Capacitors

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Run Capacitors

1. 3.

Metal case Oil filled If the capacitor swells, it should be replaced

4.

Have identified terminals which should be connected to the start terminals

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Start capacitors

1.

Dry tape Encased in plastic (bakelite) Older capacitors should be encased in paper

2.

2. 3.

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Over current can cause the ³vent´ at the top to bulge If the capacitor pops it should be replaced

Wiring and Connectors 

Wiring must be in good condition

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Loose connections

1.

Can cause oxidation

2.

Oxidation causes an increase in resistance

3.

Resistance creates heat

4.

Could result in low voltage at the motor

5.

Low voltage at the motor can result in higher motor amperage draws

Troubleshooting Hermetic Motors 

Can only be checked electrically from the outside

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Can have the same problems as open motors

1.

Grounded circuit Open circuit Short circuit

2. 3.

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Starting components must also be checked