by Tom Bedford BSc, CTech, CCP, CEA
The main function of a motor starter is to provide an easy way to turn motors on and off as needed. However, they can fulfill more advanced roles based on the project requirement. The ability to regulate voltage, turn themselves back on and reverse a motor make them key pieces of machinery in any industrial or commercial setting. Motor starters might be simple at first glance, but they are also some of the most reliable tools in the market.
To prevent nuisance call-backs and frustrated clients, the commissioning provider (CxP) needs a fundamental knowledge of motor starting devices. This knowledge extends not only to their function, but also to who on site is responsible to ensure these devices are operating as intended.
Motor starting devices go by many names, although the functions are similar. Some common, interchangeable names include magnetic starters, hand-off-auto or HOA. Variable Frequency Drives (VFD) are now commonly used as excellent motor starting devices. Properly installed and set-up, a motor starting device provides reliable and safe protection for any overload situation.
First and foremost, a motor starter is a safety device. Starting a motor manually every single time is not only tedious, but presents an unnecessary risk. Motor starters provide a simple alternative for operators to activate motors from a distance, as well as to power them down. A motor starter, motor controller, or electric motor starter is a device designed to allow to turn a motor On and Off, not unlike a relay. These small devices are deceptively simple. While at their core they fulfill the role of a switch, they are vital to handle any motor in an industrial or commercial setting.
How Starters Work
We need to look at the two main components all motor start devices include: the overload protection circuit and a magnetic contactor.
The HOA is the most basic component of a control system and is used to control the action of a single device or load. As the name suggests, the motor can be operated in three different modes. "Hand mode" is continuous manual operation of the motor; "Off mode" de-energizes the load and allows for temporary shut down of the load; and "Auto mode" allows the load to be controlled by an external means such as pressure or flow switch or building automation.
Managing Overload
Alongside the HOA device is an overload device. The overload is typically a bimetallic device that allows the motor circuit to be energized in the event of a high sustained motor current. This condition exists when you have one or more of the following: locked rotor, undersized motor, frequent starts, loss of phase or over under multitude, bearings or shaft misalignment, poor ventilation, and high ambient temperature.
The thermal overload relay is designed to protect the motor or other load from damage in the event of a short circuit, or being overloaded and overheating. Overload relays are generally compensated for by ambient temperature and the trip setting is often adjustable within a relatively narrow range. The simplest overload relay is activated by heat caused from high current flowing through the overload and over a bimetallic strip. A bimetallic strip is a band of two different metals attached to each other where each metal has a different coefficient of thermal expansion. As this bimetallic strip heats up, one metal will expand faster than the other and cause the assembly to curve. When it gets hot enough the curvature will be enough for the contacts in the overload to become separated. Since the overload has a contact wired into the control circuit of the contactor, this effectively breaks the circuit and de-energizes the system. Once the bimetallic strip cools, it will straighten out and allow the circuit to close again.
Older overload relays are available with fixed temperature trip points using bimetallic strips. These are commonly called "heaters" and are specific for each trip point or current. Newer overload relays are available with electronic solid-state control, and can be used for multiple motor functions.
Overload relays may be set for four different operational modes.
- Manual Reset Only – where an operator must physically press the reset button to restart the system. This setting is commonly used for safety reasons to ensure the system will not restart on its own.
- Automatic Reset Only – where as soon as the bimetallic strip cools, the system will automatically restart. This is useful when the system is in a remote location, making it difficult to manually restart a load and automatic restart is unlikely to create a dangerous condition. This position is seldom used.
- Manual Reset / Stop – Similar to Manual Reset Only, but allows the button to be used to manually stop the system. This is useful for simple systems where a separate off switch is not necessary.
- Automatic Reset / Stop – Similar to Automatic Reset Only, but allows the button to be used to manually stop the system. This is useful for simple systems where a separate on/off switch is not necessary.
Typically, the electrical specification contains wording similar to "Confirm motor name plate data with motor starter heater overloads, setting of overload devices and sizing of any circuit fuses." This task is a simple check that supplied equipment matches the duty expected.
Additionally, the mechanical specification contains wording similar to "Post starting run motor up to maximum speed, check amp draw. Check and set overloads." This task is required because the Test, Adjust and Balance (TAB) technician may adjust loads to perform at or near full load amps as indicated on the motor nameplate.
This precise wording puts the responsibility for set-up of a motor overload right in the laps of two separate individuals. This is great…two checks to see if a motor is working.
Unfortunately, this simple task is seldom completed which leads to the frustration of project team members. When either of these tasks are incomplete, the motor does not have overload protection. The CxP needs to ensure this safety device is set as required.
Caption: The setting of the overload device is adjusted by the installing electrician to match the full load amp draw of the motor times the motor service factor. After TAB is completed, the TAB technician verifies the setting once again.
The adjustable range of a correctly sized overload setting will cover the possible motor amperage rating under varying load conditions to the maximum setting.
Knowing the function and set-up of an overload is very important for the CxP. As indicated previously, nuisance calls and equipment damage can be prevented. At the end of the day, this makes for a happy client!