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\$– winnyFeb 5, 2019 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\$ Might want to add something about generators. \$\endgroup\$– DKNguyenMar 8, 2019 at 14:53
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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\$ Apr 10, 2019 at 18:33