Normally a motor can be locked by shorting its phases.
When the motor is not connected and we short all the the three phase terminals (tie them together) and try to rotate the shaft it offers lot of opposition.
Why is this the case?
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1\$\begingroup\$ Lenz law? Induction? \$\endgroup\$ – winny Feb 5 at 10:14
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Shorting the phases is creating a low resistance load which makes the motor work hard. This is the braking opposition you notice.
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\$\begingroup\$ Even a brushed motor does this. When you short its two terminals and spin its shaft, then the resulting back EMF opposes the spinning. \$\endgroup\$ – Camille Goudeseune Apr 10 at 18:33
