Question 18.13: Split-Phase Motor Nameplate Analysis The table below depicts...
Split-Phase Motor Nameplate Analysis
The table below depicts a split-phase motor nameplate. Determine the following quantities using nameplate data:
1. Rated slip
2. Synchronous speed
3. Rated torque
Known Quantities: Nameplate data.
Find: s ; \omega_{S} ; T.
Schematics, Diagrams, Circuits, and Given Data:
\begin{array}{|c|c|} \hline \textbf{Thermal Protected} & \begin{array}{c} \textbf{Split-Phase}\\\textbf{Fan \& Blower Motor}\end{array} \\ \hline \begin{array}{cc|cc} \text{MOD }& 4 \mathrm{k} 800 & \text{HP} & 1 / 3 \\ \\ \hline \text{V} & 115 \\ \hline \text{FR} & 48Y & Hz & 60 \\ \hline \text{INS.CL.} & B & \text{MAX.} & 40~C \\ && \text{AMB }& \end{array} & \begin{array}{c|c} \begin{array}{c} \text{RPM }& 1.725 \\ \\ \hline A. & 5.5 \\ \hline \text{PH} & 1 \\ \hline \text{S. F.} & 1.35 \\ ~&~ \end{array} & \begin{array}{c}\text{KVA }& \text{N} \\ \text{CODE} & \\ \\ \hline \text{DUTY }& \text{CONT.} \\ \hline \text{BRG }& \text{SLEEVE} \\ &\end{array} \end{array} \\ \hline\end{array}
Analysis: An explanation of the nameplate for a typical electric motor was given in Chapter 17. This example focuses on a few specific items of interest in the case of a split-phase motor. As you can see, the nameplate directly indicates the split-phase motor classification. Following the \mathrm{Hz} designation is the phase information. AC systems may have one, two, or three phases. Single-phase and three-phase systems are the most common.
The code letter following “KVA CODE” indicates the locked-rotor kilovolt-amperes per horsepower, as explained in NEMA Motor and Generator Standards, NEMA Publication Number MG1-10.37. The symbol ” \mathrm{N} ” means that this motor has a maximum locked-rotor kilovolt-amperes per horsepower of 12.5. Since the motor is rated at \frac{1}{3} \mathrm{hp}, the maximum locked-rotor kilovolt-amperes is 12.5 / 3=4.167. The maximum locked-rotor amperes at 115 \mathrm{~V} will be 4.167 \mathrm{kVA} / 115 \mathrm{~V}=36.23 \mathrm{~A}.
A large percentage of fractional-horsepower motors are now provided with built-in thermal protection. The use of such protection will also be indicated in the motor nameplate-here, for example, by “THERMAL PROTECTED.”
Bearing is abbreviated as “BRG.” Fractional-horsepower motors normally use one of two types of bearings: sleeve or ball.
A variety of additional information may appear on the nameplate. This may include instructions for connecting the motor to a source of supply, reversing the direction of rotation, lubricating the motor, or operating it safely.
For the machine in this example, the synchronous speed is
n_{s}=1,800 \mathrm{rev} / \mathrm{min}
The slip at rated speed is
s=\frac{n_{s}-n}{n_{s}}=\frac{1,800-1,725}{1,800}=0.042
The power is
P=\frac{1}{3} \mathrm{hp}=\frac{1}{3} \times 746 \frac{\mathrm{W}}{\mathrm{hp}}=248.7 \mathrm{~W}
The rated torque is
T=\frac{K \times P}{n}
where the constant K is given by
\begin{aligned}&K=0.97376 \text { when } T \text { is expressed in meter-kilograms } \\&K=9.549 \text { when } T \text { is expressed in newton-meters } \\&T=9.549 \times \frac{248.7}{1,725}=1.377 \mathrm{~N}-\mathrm{m}\end{aligned}