Motor starting & protection
With so many different types of overloads and their options, how do I choose the right protection for my motor?
The purpose of the overload is to protect the motor from overheating. Overheating can result from numerous reasons. The more advanced the overload, the more possibilities for overheating it caters for. Another distinction between overloads is the method by which they monitor the heat of the motor and their accuracy. The decision on which overload to use comes from considering a number of issues such as budget, the replacement cost of the motor should it fail, and the opportunity cost or downtime cost while the motor is out of action. Quite often the biggest contributor is the last one - the downtime cost. Simply put, the choice of overload is 'horses for courses". The bigger the dollars involved, the more sophisticated the motor protection should be. The Sprecher + Schuh range of overloads cater for all industry requirements.
I know the kW rating of my motor, how do I choose the starting components?
Before choosing the starting components, you need to decide on the starting method. DOL (direct on line) is the most common as it is the simplest and cheapest method. NHP provides selection guides in the Part A Catalogue to assist here. However, because of the effect of the large starting current when the DOL starting method is used, other starting methods are used. Also, the application will determine which starting method is more appropriate as each has their own pros and con's. NHP Part A Catalogue includes a table which provides assistance with which starting method to choose.
Should I use a fuse or circuit breaker to protect my motor circuit?
The role of a fuse or circuit breaker is to protect the downstream cable from overloads and short circuits. In motor starting circuits, their main role is short circuit protection. The fuse and circuit breaker limit the level of fault current that the other starting devices and the motor see as the fault capacity of these devices is generally much less. The decision regarding whether to choose a fuse or circuit breaker depends on the application. Circuit breakers have the advantage that they are easy to reset and break all three phases in fault conditions. Fuses provide good fault rating capacities at higher voltages and do not produce exhaust gases. A further consideration is that the Sprecher + Schuh range of circuit breakers are designed specifically for motor starting so that for general applications they provide overload and short circuit protection reducing the main starting components by one device.
Applications
I have a large motor and think the starting current will be too big for the site. What can I do?
There are numerous methods for controlling the starting current for large motors. The key is to ensure that enough torque is provided by the starting method to ensure the motor gets up to speed as quickly as possible. In NHP Catalogue Part A, a table provides suggestions on what starting method is most appropriate. It is common to use soft starters to reduce the starting current.
My motor drives a conveyor and it needs to start without a jolt, what can I do?
The jolting that occurs when starting a motor can be overcome by changing the starting method. A soft starter or Variable Speed Drive, VSD are two such options. VSDs are generally costs prohibitive if used only to control the acceleration at start up and so a soft starter is a good choice. Sprecher + Schuh provide a range of soft starter which are easy to set up and connect easily with the other starting devices.
The ambient temperature condition where my motor is installed is high? Is this a problem?
Overheating is the most common reason for motor failure. Motors operating in a high ambient will eventually fail unless adequately managed. An advanced motor protection relay such as Sprecher + Schuh's CET5 provides extended temperature monitoring and reporting functions to assist in this regard. Further, forced ventilation or air-conditioning are other options to ensure that the motor does not overheat.
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