Required Features
The National Electrical Code (NEC) requires that every motor circuit include provisions for:
- Starting and stopping;
- Overload protection;
- Disconnecting means; and
- Short circuit protection.
The first two are functions of the motor starter, or motor controller as termed in the NEC.
Starting And Stopping
The first purpose of a motor controller is to start and stop the equipment. A magnetic contactor provides the traditional and still most common start-stop control - a set of three contacts actuated by a solenoid coil, which permits the high voltage and current to the motor to be switched by controlling a low-voltage, low-current circuit to the coil. Contactors must be designed not only to continuously carry the full load current (FLA) of the motor, but also to make and break the locked rotor current (LRA), which may be six to eight times the FLA.The National Electrical Manufacturer's Association (NEMA) has established standard sizes for motor contactors based on motor voltage and horsepower (hp) (Figure 1).
Often in a project, the designer will need to adjust the hp rating of a piece of equipment during construction and will ask the electrical engineer if this will have cost impact. The two factors involved in the answer are usually the required conductor size and the required starter size. If the starter must be changed, there will usually be a cost increase, and perhaps a requirement for more space in the motor control center.
Overload Protection
Protection must be provided to prevent motor thermal damage due to high currents caused by mechanical overloads. In many small single-phase motors, this is provided by an internal thermal switch embedded in the winding, which opens the current circuit when the temperature exceeds a certain level. These motors are referred to as internally protected, and do not require separate protection in the controller. Most three-phase motors, however, operate at current levels for which this approach is impractical.Overload relays in motor controllers are connected in the motor current circuit between the contactor and the motor. They are provided with three current-sensing elements, one per phase, and a control contact, which is wired to open the contactor. Relay operation includes an inverse time characteristic; the higher the magnitude of an overload, the shorter the time delay before opening the contactor. For most applications, the level above which the relay senses an overload is 115% of FLA, and the relay will hold LRA for at least 10 sec to accommodate starting of high-inertia loads.
Traditionally, overload relays simulate the heating effect in the motor windings by passing the motor current through resistive elements in the relay. Heat generated either melts a small bead of solder, releasing a spring-loaded mechanism to open the control contact, or acts upon a bimetallic element (similar to that in a thermostat) to open the contact.
Once the relay trips, the mechanism must be manually reset to close the control contact and allow the motor to run. This is the function of the "reset" pushbutton provided on the front of enclosed motor controllers. Resistive elements in this type of relay are calibrated for specific current ranges and are selected in the field to correspond to the actual motor nameplate current. The common expression "changing the heaters" refers to adjusting the overload trip level by replacing these resistive elements.
Overload relays with heaters are being supplanted by relays with electronic sensing. Electronic relay setpoints are adjustable over a wider current range, eliminating the need to change components in the field. They can also incorporate additional functions such as phase unbalance or single-phasing protection.
