Did you know that improperly protected motors might result in unexpected production downtime, losses, and breakdowns in the machinery? In this NEW series, we unpack the many facets of motor protection. We kick off Part 1 with an overview as to the importance of adequately protecting motors.
Why protect your motors?
The basic idea behind electric motors is pretty simple – supply electricity at one end and a metal rod or an axle at the other end, which will provide you with power to drive a machine. This traditional form of generating electricity has now been in existence for more than 200 years and is now an advanced technical breakthrough in the field of power generation and industrial automation.
The manufacturing industry in today’s market dominates the industrial sphere. In this sector, a major portion of power is consumed to convert raw materials into finished goods. It is a mandatory resource, as over 50% of the work in today’s manufacturing plant is attained by electric motors or new-age servo motors. As per a research study to understand the electricity consumption trends, this percentage will continue to increase to sustain the production rate in the future.
Thus, it’s not really all that surprising that the production and manufacturing sector is dependent on electric motors. It’s essential to ensure that motors and motor starters are adequately protected. If you follow this, then it will provide you with maximum operating time and cut down on any chance of outages.
Another thing to keep in mind is that, over the last few decades, a major amount of resources have been deployed to make the operating systems even more efficient. The control manufacturers are mainly involved in the development of improved and cost-effective motor protection devices.
The role of relays
To ensure motor protection, overload relays are used in circuits to protect motors and motor conductors. This saves the system from any damage caused by prolonged periods of overcurrent circuit conditions. Some of the functions of these relays are:
- It allows harmless, temporary overloads (like in the case of motor starters) without disrupting the circuit.
- They trip and open a circuit if the current is high enough to cause motor damage over a while.
- It can reset once the overload is removed. If the motors are exposed to the increased levels of continuous current and prolonged periods at locked rotor condition, severe damage to the motor and motor circuit conductors might occur.
The motors can be damaged or destroyed under any of the following conditions:
- High or low supply voltage
- Phase unbalance
- Continuous excessive loading
- Single-phasing
- Jam or stall conditions
- Ground/earth faults
- Mechanical failures like seized motor bearing or binding mechanical linkages
In the end, the integration of proper servo motors can turn out to be quite beneficial, as it will ensure that the system of power management is optimised to the fullest. This will ensure that little to no problems are faced by businesses further down the line, ultimately leading to a situation where the goal of proper power management can be accomplished very quickly.
When talking about motor protection , if they are not adequately sized or configured, two possible scenarios can unfold. There are a few cases where they trip continuously and will consume valuable time from your maintenance staff, and in some cases they may not even trip in response to slight under voltage or overload, conditions that are not always evident and which reduce the service life of motors.
To avoid some of the common mistakes while configuring motor protections, the following are the steps one should bear in mind.
1) Under voltage Protections Set Too High
Motors that are operating below their rated voltage might suffer from overheating and have a shorter life cycle. The National Electrical Manufacturers Association (NEMA), they do not recommend operating motors below 90% of their rated voltage for extended periods. Make sure that if an under voltage protection is set too high, it can, and probably will, disconnect the motor when not required.
For instance, three-phase motor if it has a rated voltage of 230V, which means the lowest operating voltage acceptable according to NEMA is 207V (230V x 90%). However, if an adjustable under voltage relay is set at 220V, a 5% reduction in voltage will be enough to disconnect the motor.
2) Thermal Overload Set Incorrectly
The basic requirement for overload protection setting for motors is 125% of their full-load current according to the NEC; however, it makes sure you read the overload relay instructions.
Some manufacturers have the 125% setting built in, which means you must set the overload protection at the motor’s nameplate current.
If the 125% value is not built into the relay, you must set it at the motor’s nameplate current + 25%.
For example, assume you want to protect a motor with 60A of full-load current, and you have an overload relay that can be set from 50A to 100A. If the device already factors in the 125%, you must set it at 60A. If not, the correct setting is 75A (60A + 25%).
If overload protection is set too low, the motor can be disconnected even when operating normally. For example, if the protection device described above came with the dial set at 50A, and it was left that way for a 60A motor, it may not trip immediately if the engine is just lightly loaded which gives the impression that it is working correctly. However, higher motor loads that bring current above 50A will trip the device.
Of course, overload protection shouldn’t be set too high either, since the motor will not be protected adequately from overload. For example, if you add 25% when setting an overload relay that already has the 125% value built in, the actual overload protection value will be 156%, which does not meet the NEC.
3) Magnetic Protection Set Incorrectly
In the circumstances like fault conditions, magnetic protections must disconnect the motor immediately but must allow the inrush current without disconnection. Another thing that one should keep in mind is that if the magnetic protection is fixed, make sure its trip curve allows the inrush current, which can be up to 800% of rated current only. Whereas, if the magnetic protection is adjustable, then set a value so that it will not trip with the inrush current. Make sure that the inrush current is lower, if the motor has a reduced-voltage starter, a solid-state starter or a variable frequency drive.
Resources:
1. DISCOVER Schneider’s Easergy P3 Relay for MV Applications
2. EXPLORE Schneider’s Servo Drives and Motors
3. CHAT to us for a Schneider Electric solution for your facility
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