Sizing Motor Overload Protection

There are several types of devices that can be used to provide overload protection and the sizing procedure can vary depending on the type of device used.

C It is important to keep differences in the procedures separate and understood well so as not to

install overloads that do not provide adequate protection to the motor.

• The simplest and most straightforward sizing procedures for motor overload protection are

applied when sizing overload relays using the cover of the motor starter, control center, or manufacturer’s catalog.

• The National Electrical Code specifies methods to calculate the maximum size motor

overload protection for specific motors if a manufacturers chart is not available.  Installations relying on fuses and circuit breakers as back-up overload protection must be calculated using the NEC method.

NEC Calculations

The NEC in general requires the maximum size overload device be set to open at 115% or 125% of the motor’s full-load current rating, depending upon the service factor and/or temperature rise of the motor.  There are however, exceptions.

• For motors rated 40EC with a Service Factor of 1.15 or greater, 125% of the motors FLA is

used to calculate the maximum size device for overload protection.

• For motors rated greater than 40EC or unmarked, 115% of the motors FLA is used to

calculate the maximum size device regardless of the motor’s Service Factor.

• If use of the previous size rules results in the motor tripping off line during starting, the

device can be increased to a maximum of 140% of the motors FLA.

Example:

Find the maximum size overload device to provide overload protection to a 3 phase, 230 Volt, 10 horsepower motor with FLA of 28 amps if:

Ambient Temp = 40EC, S.F.=1.15: 28 amps X 125% = 35 amps Ambient Temp = 40EC, S.F.=1.00: 28 amps X 115% = 32.2 amps Ambient Temp = 50EC, S.F.=1.15 28 amps X 115% = 32.2 amps Ambient Temp = 50EC, S.F.=1.00 28 amps X 115% = 32.2 amps

If use of the size calculated results in the motor tripping off line when started, the overload device may be increased to a maximum of:

Maximum size allowable:

28 amps X 140% = 39.2 amps

Electronic Overloads

Electronic overloads sense the load current and the heating effect on the motor is computed.  If an overload condition exists, the sensing circuit interrupts the power circuit.

C The tripping current can be adjusted to suit the particular application.

C Electronic overloads often perform additional protective functions such as ground fault and

phase loss protection.

Fuses

Fuses have limited application as the primary means of overload protection for motors but can be effectively used to provide back up overload protection.

• Single-element fuses are not designed to provide overload protection.

C Their basic function is to protect against short circuits and ground faults.

C If sized to provide overload protection, they would blow when the motor starts due to high

motor inrush current.

• Dual-element fuses can provide motor overload protection, but they have to be replaced

when they blow which can be a disadvantage.

• There is a risk of single-phasing damage to the motor when only one fuse blows unless

single-phase protection is provided.

Thermal Overload Relays

A thermal overload relay is an electro-mechanical relay that is operated by heat developed in the relay.

C When the level of current in a circuit reaches a preset value, the increased temperature opens

a set of contacts.

C The increased temperature opens the contacts through a bimetallic strip or by melting an

alloy that activates a mechanism that opens the contacts.

C Two types include melting alloy and the bi-metallic strip.

Melting-Alloy Thermal Overload Relays:

These are probably the most popular type of overload protection.

C The motor current passes through a small

heater winding and under overload conditions, the heat causes a special solder to melt allowing a ratchet wheel to spin thus opening the control circuit contacts.

C Must be reset by hand operation

C Heater coil and solder pot in one unit —

non-tamperable

Bimetallic Thermal Overload Relays:

This design uses a bimetal strip associated with a current-carrying heater coil.

C When an overload occurs,

the heat causes the bimetal to deflect and actuate a tripping mechanism which opens a set of contacts in the control circuit interrupting power to the coil and opening the power contacts.

C Most relays are adjustable

over a range from 85% to 115% of their value.

C They are available with ambient compensation.  An ambient compensated devices’ trip point

is not affected by ambient temperature and performs consistently at the same value of current.

Motor Overload Protection

Motors larger than 1 horsepower must be provided separate motor overload protection devices.

C The most common devices typically used include:

1)  magnetic or thermal overload devices 2)  electronic overload relays 3)  fuses

Magnetic & Thermal Overloads

Overload devices are usually located in the motor’s starter and connected in series with the motors electrical supply circuit and can be operated by either magnetic or thermal action.

C The same amount of current passes through the overload relay and the motor.

C If the current or heat through the overload device is higher than the device’s rating, it trips

and shuts down the electric power to the motor.

Magnetic Overload Relays

A magnetic overload relay is an electro-mechanical relay operated by the current flow in a circuit.

C When the level of current in the circuit reaches a preset value, the increased magnetic field

opens a set of contacts.

• Electromagnetic overload relays

operate on the magnetic action of the load current flowing through a coil.

C When the load current becomes

too high, a plunger is pulled up into the coil interrupting the circuit.

C The tripping current is adjusted by altering the initial position of the plunger with respect to

the coil.

Instantaneous Trip Circuit Breakers

Instantaneous trip circuit breakers respond to immediate (almost instantaneous) values of current from a short circuit, ground fault, or locked rotor current.

C This type of circuit breaker will

never trip from a slow heat buildup due to motor windings overheating.

• A stuck bearing or a blanket of

lint covering the inlets and outlets of the motor’s enclosure will cause the motor to overheat and damage the windings.

• The National Electrical Code

allows instantaneous trip circuit breakers to be sized to a maximum of 800% of a motors FLA value.

• They are used where time-delay fuses set at five times their ratings or circuit breakers at

three times their rating will not hold the starting current of a motor.

Some instantaneous trip circuit breakers have adjustable trip settings.  The instantaneous trip ratings of an instantaneous trip circuit breaker can be adjusted above the locked-rotor current of a motor to allow the motor to start and come up to its running speed.