Selection of AC Electric Motors is Not for Dummies!
Electric motors, both ac and dc types, come in many shapes and sizes. Some are standardized versions for general-purpose applications. Others are intended for specific tasks. In any case, motors should be selected to satisfy the dynamic requirements of the machines on which they are applied (pumps, fans, elevators) without exceeding the rated motor temperature. Transformer voltage calculator is now a day’s available online which helps in calculating it.
So, let’s get started with first understanding the basics of an AC motor (DC motors will be covered in another blog).
There are three basic types of AC motors: Wound Rotor, Squirrel Cage andSynchronous.
The Wound Rotor Motors (Slip-ring Motors). A wound-rotor motor is a type of induction motor where the rotor windings are connected through slip rings to external resistance. Adjusting the resistance allows control of the speed/torque characteristic of the motor. Wound-rotor motors can be started with low inrush current, by inserting high resistance into the rotor circuit; as the motor accelerates, the resistance can be decreased. Compared to a squirrel-cage rotor, the rotor of the slip ring motor has more winding turns; the induced voltage is then higher, and the current lower, than for a squirrel-cage rotor
The Squirrel Cage Motor is By far, the most common type of three phase induction motor is the squirrel cage motor. Squirrel cage motors have higher starting current than the wound rotor motor and require larger protective devices as well as larger starters and power cables.
The Synchronous motor, the rotation of the rotor is synchronized with the frequency of the supply current and the speed remains constant under varying loads, so it is ideal for driving equipment at a constant speed and are used in high precision positioning devices like robots, instrumentation, machines and process control.
The mechanical power rating of a motoris measured in horsepower and is referred to as the motor load.The torque required to drive a machine is used to calculate the required horsepower. The most common equation for power based on torque and rotational speed is: hp = (torque X rpm)/5,250.
The electrical power rating of a motor is in kilowatt (watt x 1000)and is typically used to express the power consumption of electric motors. One HP = 746 watts. One kilowatt is approximately equal to 1.34 horsepower.
Every motor is designed to a specific code depending on the load it’s required to move, and whether the load is a continuous or zero start load. These codes are identified by a letter symbol either A, B, C, D, or E.
Motor Design A: Typical Applications: Fans, blowers, centrifugal pumps and motor-generator sets, etc. where starting torque requirements are undefined and extremely high.
Motor Design B: Typical Applications: Fans, blowers, centrifugal pumps and motor-generator sets, etc. where starting torque requirements are relatively low.
Motor Design C: Typical Applications: Conveyors, crushers, stirring machines, reciprocating pumps and compressors, etc., where starting under load is required.
Motor Design D: Typical Applications: High starting wheels such as punch presses, torque and shears, elevators, extractors, winches, high slip hoists, oil-well pumping and cranes.
Motor Design E: Typical Applications: Fans, blowers, centrifugal pumps and compressors, motor-generator sets, etc. where starting torque requirements are relatively low
The Motor Design Code B is the workhorse of the motor industry and the majority of motors fall into this category.
Duty cycle is a fixed repetitive load pattern over a given period of time .It is categorized into 3 types.
Continuous Operation – Once started, the motor works at a constant load at least until thermal equilibrium is reached. Then that motor may be operated for an unlimited period.
Hydraulic pumps, fans, blowers are typical applications.
Short Time Operation – Once started, the motor works at a constant load for a limited period and thermal equilibrium is not reached. Motor will be started a second time then when its temperature has decreased to room temperature (starting conditions are restored). Typical Applications – Household appliances
Intermittent Periodic Operation – A sequence of identical duty cycles, made up with a time of operation at constant load and a time at rest. When at rest, the motor is not fed. Typical Applications – Lifting motors, Standby Duty. Normally at rest and not fed. Serves as a standby to a designated motor(s), with identical specifications to the designated motor(s) it’s supporting.
Before we move on, some clarification as to how ampere designations such as full load amps, full load current and running load amps are determined.
During design stage of motor selection, calculating the maximum amperes of the motor running at full load is obtained by using the standard motor formulas. This is referred to as running load amps. This is used to determine the starter size and breaker or fuse protection of the motor.
The running load amps are sometimes referred to as Full Load Amps (FLA). However, the full load amp designation is determined when the motor is tested after manufacturing is complete. Each motor will have a unique FLA rating depending on the manufacturer, materials etc.Once determined, this value is then added to the nameplate.
The designer’s goal is to bring the design stage of the motor calculations as close as possible to a similar manufactured FLA nameplate rating. One way this achieved is by using a manufacturer’s motor performance data sheet. From these data sheets the designer can match the power factor and efficiency of the listed motor with his, which is critical to arriving at the correct design values for the motor.
Also, each motor has a service factor designation, ranging from 1.15 to 1.25 depending on the manufacturer’s design.
This being the case, some national codes have added a multiplier of 1.25 to the running load amps to incorporate this added load.
This value is the motor’s Full Load Current value and used to determine the cable size for the motor.
Also, manufacturers added a Service factor which indicates how much over the nameplate rating a motor can be driven without overheating. In other words, multiplying nameplate horsepower by the service factor tells how much the motor can be overloaded without overheating.
Note: A change in NEMA standards for service factors has been brought about because of better insulation. A service factor — once standard for all open motors — is no longer standard above 200 hp motors.
Selection of Electric Motors is an essential part of having a successful design ensuring a long motor life cycle as well as cost effective operation. I have covered a just a small but important part of motor selection. I will cover more on this subject in a future blog. Until then, keep on designing!