Every Induction motor has given a Nameplate which describe some important aspects about motors. There are standards sets by NEMA for Nameplate marking that following is the Minimum information shall be given on all nameplates of single-phase and polyphase induction motors.
Following are the abbreviations for the same:-
1. Manufacturer's type and frame designation
2. Horsepower or KW output.
3. No. of poles
4. Maximum ambient temperature for which motor is designed.
5. Insulation system designation.
6. RPM at rated load.
8. Number of phases.
9. Rated load current.
11. Power factor
12. Efficiency of Motor
13. Motor Weight
14. Bearing size
There is brief discussion about above abbreviations:-
- Manufacturer Type and Frame Designation:-
Manufacturer must describe their name on the name plate. Frame size of the motor also to be described on motor name plate. Usually manufacturers will give same frame size of motors with same rating so that any manufacturer motor can be replaced with existing motor.
This nameplate block can offer a lot of information if the motor is nearly standard. The frame size sets important mounting dimensions such as:-
(i) Foot hole mounting pattern
(ii) Shaft diameter
(iii) Shaft height
NEMA standards do not set some dimensions that can turn out to be important if the motor must fit into a confined space.
These include maximums of overall height and length, and maximum conduit-box extensions.
The data in the "Frame" block can be hard to interpret when special shafts or mounting configurations are used.
- Horsepower or KW O/P
Motor KW and Horsepower is also recommended to be described on motor name plate so that even a non technical person can easily read and select the motor of desired rating and their load requirement. Shaft horsepower is a measure of the motor's mechanical output rating, its ability to deliver the torque required for the load at rated speed. It is usually given as "HP" on the nameplate.
HP = (Torque) x (speed)/5,250
Here Torque is in lb-ft Speed is in rpm
- No. of Poles
No. of poles will tell about speed of the motor as Speed= 120X Frequency/ No. of poles this is known as synchronous speed. But there is slip also in induction motor which is usually around about 4% in most of induction motors. If Slip is 4% that means induction motor speed is lower by 4%.
More the no. of poles lower will be the speed. So when poles are given speed can be easily calculated.
If there are 4 poles on motor then synchronous speed will be =120X 50/4= 1500 RPM and if there is 4% slip then Induction motor speed will be = 1500- (4/100X1500)= 1440 rpm.
- Maximum ambient temperature for which motor is designed.
The nameplate lists the maximum ambient temperature at which the motor can operate and still be within the tolerance of the insulation class at the maximum temperature rise. It is often called "AMB" on the nameplate and is usually given in degrees C.
- Insulation Class used
Insulation class is also mentioned on Motor name plate and is often abbreviated "INSUL CLASS" on nameplates. Insulation class specifies about thermal tolerance of the motor winding.
Insulation class is a letter designation such as "A," "B," or "F," depending on the winding's ability to survive a given operating temperature for a given life. Insulation classes of a letter deeper into the alphabet perform better.
- RPM of Motor
Although we can calculate motor speed by knowing no. of poles of motor but motor RPM are also described on Motor name plate as slip of motor in not mentioned on Name plate. By knowing RPM of motor and no. of poles we can easily find slip of an Induction motor.
Power varies directly with torque and with speed, for a centrifugal-type load, power varies approximately as the cube of speed - a small speed change produces a much larger change in power requirement. For example, a 1% increase in speed would bring a 3% increase in load: (1.01)3 = 1.03
- Frequency of motor
Motor frequency at which motor will operate or ratings are given should be inscribed on motor name plate. Input frequency is usually 50 or 60 Hz. When there are more than one frequency is given on Motor name plate other parameters that will differ at different input frequencies must be defined on the nameplate.
This represents the number of ac power lines supplying the motor. Single and three-phase are the norms.
Motor Rated load current in amps is given along with motor Horsepower or KW and voltage and frequency. Usually motor rated currents in both star and delta are given on nameplate. Also different Currents according to different voltages should also be given.
The voltage at which the motor is designed to operate is an important parameter.
It is common for manufacturers to nameplate a wide variety of voltages on one motor nameplate.
A common example is a motor wound for 230 and 460 V (230/460 V) but operable on 208 V. This 208-230/460 V motor will have degraded performance at 208 V. Another common misconception is to request a motor rated at network voltage; for example, at 480 V. The NEMA standard is 460 V. The voltage rating assumes that there is voltage drop from the network to the motor terminals. Thus, the 460-V motor is appropriate on a 480-V network.
- Power Factor
Also given on the nameplate as "P.F." or PF," power factor is the ratio of the active power (W) to the apparent power (VA) expressed as a percentage. For an induction motor, power factor also varies with load. The nameplate provides the power factor for the motor at full load.
Active power is the power is the actual power consumed and apparent power has a reactive component. This reactive component is undesirable - the utility company must supply it, but it does not work.
A power factor close to unity is most desirable. But due to air gap power factor is low and usually of 0.80 to 0.91.
- Efficiency of Motor
Efficiency is defined as output power divided by input power expressed as a percentage:
(Output / Input) x 100
There are losses known as Windage losses, Iron losses and Stray losses. This will reduce motor efficiency. NEMA has established the maximum variation allowed. Generally motor efficiency remains between 85% to 95%.
- Motor Weight
Motor weight is specified on motor name plate.
Bearings are the important factor for maintenance in an ac motor. Bearings information is usually given for both the drive-end bearing and the Non driving end.
Some manufacturers use a simplified designation simply indicating the bearing size and type –for example, 6309 for a size 309 ball bearing. This brief information can leave questions like: Is the bearing sealed, shielded, or open? Still, some manufacturers may use special bearings and elect to display their own bearing part numbers on the nameplate. Many special bearings are applied in motors for reasons such as high speed, high temperature, high thrust, or low noise. It pays to understand your motors' bearing requirements.