Motor Controls Troubleshooting of Electric Motors

7/29/2019 Motor Controls Troubleshooting of Electric Motors

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Motor Controls &

Troubleshooting of ElectricMotors


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Introduction to Motor Controls

Devices that pass power to the motor

1. Relays

2. Contactors

3. Starters

The contacts are controlled by a coil in the control circuit

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

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


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Run-load and Locked-rotor current

Run load amperage (RLA)

1. Also called full load amperage (FLA)

2. Current drawn while the motor is running

Locked rotor amperage (LRA)

1. Amperage drawn at start-up

2. Approximately five times the RLA

Both currents must be considered when selecting a control

device


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Relays

Relay

Uses a magnetic coil to close one or more sets of contacts.

Cannot be repaired considered a throw a away 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-amperes (VA).)

The contacts must be able to handle the current draw of the load

Can have normally-open (NO) contacts, normally-closed (NC) contacts, or both normally open

and normally closed contacts


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Relays, Contactors, & Starters
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Contactor

Larger versions of the relay

Has moveable and stationary contacts

Holding coils of different voltage ratings are available

Can have one or more sets of contacts

Some are equipped with auxiliary contacts

Use exact replacement whenever possible


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Motor Starters

Described as a contactor with overload protection built in

Can be rebuilt

1. Should be replaced when the become pitted

2. Heaters and holding coils can also be replaced As the contacts become pitted the resistance increases


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Motor Protection

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

damage Fuses protect the entire circuit, not the components

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

Very small motors do not have overload protection

The larger the motor, the more elaborate the protection should be

Motor protection can be inherent (internal) or external


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Inherent (Internal) Motor Protection

Internal thermal overloads, OR

Thermally activated bimetal snap disc


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External Motor Protection

Devices that pass power to the holding coil of the starter or contactor

The device can open its contacts when a current overload occurs, causingthe holding coil to de-energize

The 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


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Temperature Sensing Devices

Bi-metal element

1. Heated by a device called a heater

2. The heater is in series with the load and senses the load current

3. The bimetal warps when it becomes too warm

4. The bimetal interrupts the circuit feeding the holding coil of the starter

Solder pot

1. Uses a low-melting solder

2. The solder melts when excessive heat is generated

3. The excessive heat is generated from an over-current condition


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Magnetic Overload Devices

Not attached to the starter

Not affected by ambient temperature

Very accurate overload device


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Restarting the Motor

If a motor shuts off on a safety, it should not beimmediately restarted

The cause for the overload must be found

The motor must be given enough time to cooldown

Many control circuits have manual resets to

prevent automatic restarting Some controls reset only after a pre-determied

time period

Time delay devices prevent the unit from short

cycling


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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


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Mechanical Motor Problems

Normally occur in the bearings or drive connection

Bearing Failure (tight or worn)

1. Lack of lubrication

2. Excessive grit in the bearing

3. Over-tight belts


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Removing Drive Assemblies

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

shaft with care

1. The fit can be very tight

2. The assembly must not be damaged

3. Special pulley pullers can be used

Set screws are tightened to the motor shaft

1. Set screw is made of hardened steel

2. Tightened to the flat portion of the shaft

The end of the motor shaft should not be hammered


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Removing Drive Assemblies


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Belt Tension

Over-tightened can cause motor failure and or bearing damage

A belt tension gauge should be used


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Pulley Alignment

Drive and driven pulleys should be properly aligned

Drive mechanisms can become damaged

Belt life can be reduced

Belts can skip off the pulleys


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Electrical Problems

Easy to diagnose

Motor may smell burned

An open motor winding

A short circuit from the winding to ground

A short circuit from winding to winding


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Open Windings

Can be found with an ohmmeter

There should be a measurable resistance between the common,

start, and run terminals

A reading of infinite resistance is an indication that the winding

is open


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Shorted Motor Windings

Caused when winding insulation develops a hole

An alternative current path is created

This path reduces the resistance of the winding

The current flow through the winding will increase

1. Motor overload may trip

2. Circuit breaker may trip

Short in the run winding

1. Motor may start

2. Motor will draw excessive current

Short in the start winding

1. Motor may not start

2. If it does start it will draw excessive current on start-up

Three-phase motor windings should all have the same resistance

h d


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Short Circuit to Ground (Frame)

There should be no detectable circuit from a winding to ground

Motors should be checked for grounds using a good quality ground

ohmmeter

1. The meter should be set to its highest setting

2. Megohmmeters (megger) can detect 10,000,000 ohm range

Dirty motors may show resistance from winding to ground if the

atmosphere is damp

Hermetic compressors may show a slight ground1. The oil may be dirty

2. Liquid refrigerant in the compressor worsens the condition

3. A suction line drier may help clean the oil


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Single-Phase Motor Starting Problems

Fairly easy to troubleshoot

Check for full power to motor

1. Low voltage

2. Loose connection

3. Undersized wire

4. Low voltage from electrical panel

5. Measure voltage at the motor terminals

6. Voltage must be within 10% of rated voltage


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Open Motors

Checked with voltage supplied to both windings

Start windings is removed when motor reaches 75% of rated

speed

Uses centrifugal switch

The opening and closing of the switch can be heard


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CSCR Motors

Have the start winding energized all the time

The start capacitor stays in the circuit whenever the

motor is energized


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Symptoms of Electric Motor Starting Problems

Motor hums and then shuts off

Motor runs for a short period and shuts off

Motor will not try to start


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Types of Problems to Consider

Motor mechanical problem

Electrical problem

Motor circuit problem

Motor load problem

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

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


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Checking Capacitors

Capacitors can be checked with an analog ohmmeter

Ohmmeter leads placed across a charged capacitor can cause

damage to the meter


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Procedure for Checking Capacitors

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.


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Identification of Capacitors

Run Capacitors

1. Metal case

2. Oil filled

3. If the capacitor swells, it should be replaced

4. Have identified terminals which should be connected to the start terminals

Start capacitors1. Dry tape

2. Encased in plastic (bakelite)

3. Older capacitors should be encased in paper

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


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Wiring and Connectors

Wiring must be in good condition

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 motor5. Low voltage at the motor can result in higher motor amperage draws


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Troubleshooting Hermetic Motors

Can only be checked electrically from the outside

Can have the same problems as open motors

1. Grounded circuit

2. Open circuit

3. Short circuit

Starting components must also be checked