overload protection

1. HOW TO SIZE OVERLOAD
THERMAL PROTECTION
JAVID IQBAL SODAGAR
DGM ELECTRICAL

2. The process of determining the correct overload protection to protect a
motor from excessive heating due to mechanical work overloads or
from failure to start is a matter of following all the rules of the NEC®.
Also, each manufacturer has methods to determine proper protection.
Article 430, Part III, determines the parameters of protection. Article 430.32
determines the trip point and the percentage of full-load current when the
overload must open the power circuit to the motor. Article 430.33
determines the overload protection for intermittent or similar non continuous
duty motors. Once you have determined the actual trip percentage
point according to the Code, you multiply the actual motor nameplate
current times the percentage to determine the actual current trip value.

3. Most manufacturers use the system of labeling the overload heaters based
on the nameplate information on the motor.
Each manufacturer uses its own numbering system that corresponds to
the motor starter used. If the motor is a standard rating with a service
factor of 1.15 or greater and the marked temperature rise for the motor
installation is 40°C (104°F) or less, then choose the heater catalog number
that corresponds to the nameplate rating.
This heater will provide a trip point of approximately 125% of the
nameplate rating as required by NEC® Article 430.32(A)(1). However, if the
service factor is less than 1.15, typically 1.0, or the motor has
a marker temperature rise over 40°C (104°F), typically 50°C (122°F), then
the Code requires closer tolerance, and you choose one size smaller from
the manufacturer’s table..

4. The one size smaller will yield 115% protection as required.
In the case of 125% or 115% protection, the Code does allow the electrical
personnel to increase the ratings if the original selection does not allow the
motor to operate. Article 430.32(C) allows us to increase each category by one
size provided the 125% category does not exceed 140%, and the 115% category
does not exceed 130%. All the above situations are based on the fact that
the controller and the motor are in the same ambient temperature.
When sizing the overload heater from a manufacturer’s listing, you must
determine the heater catalog numbers. There are standard methods used
to determine the heater number.
The electrical technician needs to determine whether the motor controller
that contains the overload heater is in the same ambient temperature as the
motor it controls. If the controller is in a higher ambient temperature, that
means the overload sensors are already warmer than the motor, and not as
much heat can be added before they trip and disconnect the motor.
Therefore, choose a heater with a higher number than originally selected.
Conversely, if the control is at a lower ambient temperature than the
motor, select a heater that is one size less than the original choice. The
lower number of the heater produces more heat per ampere of current low
and compensates for the lower ambient temperature.

5. Electronic overload modules are sized according to the nameplate of the
motor and then adjusted from 115% to 140% depending on the
circumstances of the motor nameplate and the conditions relating to the
location of the motor and controller. In the following description, the
overload monitor can also monitor for single phasing of three-phase motors
where one phase fails and the motor continues to run on the other two
phases, now a single-phase supply. The three phase motor is not designed
to operate safely with only one phase and may be damaged if allowed
to run too long. The electronic overload monitor causes the controller to
disconnect the motor.
● Example 3: A 5 hp, 230 V, three-phase motor has a nameplate current of
14.5 A and a service factor of 1.25. The marked temperature rise is 40°C
(104°F). The
controller and the motor are at the same ambient temperature. Use the
heater chart in Figure 13–9.
● Solution: If the controller and motor are at the same ambient
temperature, the motor has a service factor of 1.15 or more, and a
temperature rise of 40°C (104°F) or less, then simply choose 125%
protection and use the nameplate rating of the motor to pick the heater
catalog number H37 for NEMA size 00, 0,1 starters. You need three heaters
for a conventional overload-heater–style starter for a three-phase motor.

6. Example 4: A 5 hp, 230 V motor has a nameplate current of 14.5 A and a
service factor of 1.0. The marked temperature rise is 40°C (104°F). The
controller is in a cooler location than the motor by approximately 10°C
(50°F). Use the heater chart in Figure 13–9.
● Solution: First choose the heater overload as if the controller and the
motor were at the same temperature. In this case, because the service factor
is 1.0, the NEC® requires us to protect the motor at 115% of the nameplate
current.
Use the chart to determine the heater selection at full nameplate
current, and then choose one size smaller to satisfy the 115% requirement.
Now adjust that size based on the ambient temperature of the controller.
Because the controller is already cooler, choose one size smaller than
normal. In this case, the heater is two sizes smaller than would be
indicated by simply using the nameplate current. The heater is H36. If this
does not allow the motor to run normally without overheating, the size
can be adjusted up one size according to Article 430.32(C).

9. Many new motor starters are equipped with electronic overload relays.
These overloads do not depend on the heat generated by the motor current
passing through the heater element, but instead use a current sensor, such
as a current transformer, to measure the current and provide a trip contact
for the controller. This provides an advantage in that the overload can be
adjusted to provide optimum protection without nuisance tripping over a
wider range than that available in a thermal overload .The electronic
overload can also be used to send operating data to a central location for
remote monitoring. Other features include adjustable class of overload
from 10 to 30, current imbalance protection, phase loss protection, phase
reversal protection, and test function and trip indicators (see Figure 13–10).
The class of overload refers to the speed and protection offered by the
motor current monitor
. Class 10-rated overloads are designed for hermetic refrigeration motors and
for quick starting and fast trip on overload. (Trip time is less than 10 seconds,
with six trips per hour.) Class 20 overloads are the most common and are
designed for standard duty motors with normal time versus temperature
curves. (Trip time is less than 20 seconds, with six trips per hour.) Class 30
overloads are used for long-time acceleration motors and extended high-
current overloading. (Trip time is less than 30 seconds, with six trips per
hour.) The trip currents are 125% of the minimum full-load current listed in
the heater tables when the heater is at 40°C (104°F) ambient.