I was recently doing a first-course lab on Induction Motors. We were asked to plot the torque-slip characteristics of a 4-pole 3 phase induction motor (3.7kW).

I just analyzed the data I plotted from the wattmeter readings (connected as shown below) and the rpm measured using a tachometer.

part of ckt diagram showing wattmeter connection: voltage coil across A of IM and 3-ph N, current coil sees current entering A of IM

Here's the T-slip data I plotted on my spreadsheet.

t-slip from my lab

Data points were obtained by gradually loading a mechanically coupled DC generator.

I don't think my plot looks anything like the t-slip diagrams from my lectures and even Google Images.

Can someone who knows what's going on explain it to me?


Can someone who knows what's going on explain it to me?

It looks pretty normal to me; as you increase torque the motor slips a little more. Maybe the graphical representations you have previously seen look different but are reflected in your diagram. For instance, here's one that looks pretty similar: -

enter image description here

Picture from here.

And your graph appears to be coinciding with "the stable region". But, if you looked at this graph that shows torque against speed you might get confused: -

enter image description here

Taken from a stack exchange question.

In this graph the right hand side of the characteristic is where you graph begins because it shows speed and not slip on the x-axis.

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  • \$\begingroup\$ Yes, you're right. Thanks! \$\endgroup\$ – William R. Ebenezer Mar 23 at 13:00
  • 1
    \$\begingroup\$ Also, keep practicing guitar and piano - you never stop practicing! \$\endgroup\$ – Andy aka Mar 23 at 13:02
  • \$\begingroup\$ Ah, yes, I will! \$\endgroup\$ – William R. Ebenezer Mar 23 at 13:03
  • \$\begingroup\$ BTW I just heard some of your tracks. I liked "So you wanna be a soldier" very much. \$\endgroup\$ – William R. Ebenezer Mar 23 at 13:10
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