Electric motors are normally out of sight, but they perform many important functions in residential, commercial and industrial buildings. Electric motors are used to drive air handlers, chillers, furnace blowers, water pumps and elevators, to name a few examples. For this reason, it is important to keep them in optimal operating conditions, and an important aspect of safe motor operation is providing them with reliable electrical protections.
Three-phase electric motors are normally used for mid- or large-sized equipment, and they are characterized by operating with three live conductors. These motors require specialized protections due to their connection. In smaller applications, such as fractional horsepower fans, single-phase electric motors are normally used. These use either two live conductors, or a live conductor and a neutral conductor.
Motor faults are almost always disruptive because key equipment is taken out of operation. Also consider that electric motors are expensive, so it is in the best interest of property management companies to avoid replacements. In a few words, adequate motor protection pays off in the long run.
This article will provide an overview of the main electrical issues that threaten motors, and how to provide protection against them. If you are planning a major building renovation, it represents a good opportunity to upgrade motor equipment and improve electrical protections.
Protection Against Major Electrical Faults
Some faults cause the electrical current of a motor to increase drastically to many times its normal value. These faults must be disconnected immediately, or they cause irreversible damage to the motor, while being capable of starting a fire. The electrical current can reach extremely high values under the following circumstances:
- Contact between a live conductor and a neutral conductor.
- Contact between a live conductor and a grounded surface or conductor.
- Contact between two or more live conductors.
These faults cause what is called a short circuit in electrical engineering: a low-resistance connection between two points at different voltage values, which causes an extremely high current - often with magnitude in the thousands of amperes.
Motor protections use a magnetic tripping mechanism to clear these dangerous faults. The short circuit current induces a strong magnetic field through an electromagnet, which in turn forces a contact to open, disconnecting the motor from its power supply.
Electric motors draw a high inrush current when they start, which can reach values up to 800% of the motor’s rated current. This is normal behavior and it only lasts for brief moments while the motor starts rotating, so it does not count as a fault. Motor protections must be sized to allow the inrush current, while clearing anything higher.
Protection Against Motor Overload
An overload is a less drastic type of fault where the motor is drawing current above its rated value, but only by a slight margin. However, overloads cause electric motors to heat up excessively, which shortens their service life.
Unlike short circuit protection, which is instantaneous, overload protection has a built-in time delay. This allows short-duration currents above the rated value that are normal in some types of equipment, such as the inrush current when a motor starts. In fact, if the motor fails to start for any reason, such as an obstacle locking the shaft or excessive load, the overload protection activates and disconnects the motor.
Overload protection acts faster as the current magnitude increases, which means a current slightly above the rated value may take minutes to be cleared, while a drastic overcurrent may be cleared in seconds. Currents that are high enough to activate the short-circuit protection described in the previous section cause immediate disconnection.
Overload protection can be either thermal or electronic. When it is thermal, it is based on a contact that expands with heat: more current causes more heat, and more heat causes a quicker disconnection. On the other hand, electronic overload protection uses a programmed response with the same behavior as the thermal protection, and the current is measured to determine how fast the contact will open.
Protection Against Voltage Changes
Motors have a rated voltage value, but they normally tolerate a variation of 10% above and below this value. However, the operating voltage should be as close as possible to the rated value to ensure optimal performance. When the voltage supplied to a motor is too low or too high, it draws a higher current, which overheats its windings and shortens its service life.
There are special overvoltage and undervoltage relays that measure the voltage being supplied to the motor, and disconnect the power supply if the voltage falls outside the acceptable range. These relays are often adjustable, so it is important to establish a reasonable voltage range:
- A range that is too narrow above and below the rated voltage may result in unwanted motor disconnections, since even minor voltage variations result in disconnection.
- However, a range that is too broad does not provide adequate protection because the voltage is allowed to change drastically.
Protection Against Phase Loss and Phase Imbalance
Single-phase motors shut down when one of their conductors is disconnected, even if the other remains, since there is no path for electric current. On the other hand, three-phase motors can continue operating with only two phase conductors, but this is extremely demanding for the motor and shortens its service life. Keep in mind that interrupting the voltage to any phase conductor has the same effect as disconnecting the conductor - the motor is left with a voltage supply in only two of its three terminals.
A related issue is phase imbalance, which occurs when the voltage supplied to a three-phase motor varies drastically between phases. Phase imbalance causes the motor to produce more noise and heat, while decreasing its efficiency and service life. Ideally, phase imbalance should be kept below 2% for optimal motor performance.
Phase monitors are devices that track the phase voltages of a motor, and can be integrated with a contactor to disconnect the motor in the event of excessive imbalance or phase loss.
Adequate motor protection can be considered an investment, since it prevents damage to expensive equipment. It also makes building interiors safer, since electrical faults with the potential to start a fire are cleared before they have a chance to do so. By working with qualified electrical design engineers, you can make sure all your electrical motors are specified with reliable protections and installed according to the NYC Electrical Code.
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