This may be a little basic for some. Sorry in advance...

I would like to use a large 3ph motor to perform a fast, controlled, stop.

The rated motor torque is 850Nm @ 242A.

My inverter claims to be able to deliver a short duration current of 460A.

Calculating the torque constant KT = 3.51Nm/A (from the rated figures 850/242 = 3.51)

Could I realistically expect 460A x 3.51Nm/A = 1615Nm of reverse torque for a short duration?

Or in fact, like when a motor is starting would the ratio of current to torque be a non-linear relationship?

(The motor would be running at normal load and speed and then attempt to decelerate as fast as possible.)

  • \$\begingroup\$ What kind of motor? \$\endgroup\$
    – Eric
    Commented Mar 20, 2015 at 12:10
  • \$\begingroup\$ Since KT is the torque constant it's supposed to be constant, not linear with current but independent of it. As JonRB says, however, if the magnetic circuit saturates you may see Kt decrease, i.e. torque increase less than expected. This is a question for the motor manufacturer if his datasheet doesn't cover performance above rated current. \$\endgroup\$
    – user16324
    Commented Mar 20, 2015 at 14:10

1 Answer 1


Is kt constant (torque relationship linear) with current?

If it is an air core machine then yes, Kt will stay constant (and thus torque will have a linear relationship to current) as you will not saturate air (but equally kt will be low)

For machines using some form of permeable material (iron...) Then no it is not as the B-H curve is not linear and thus kt is not constant.

However... It comes downto the actual machine design. Was it designed to work relatively deep into saturation and as such the difference between 10% and 90% current is measurable then further current will show a significant drop in incremental torque.

Was the machine a 'lazy design' and it operates around the origin (and thus 10% and 90% are very similar...) ... The decrease on kt due to more current might be negligible enough that you can still consider it constant (within tolerances)

  • \$\begingroup\$ Thanks - so many lectures from years ago come flooding back... I am not sure about the core yet. The motor has some pretty poor documentation. It sounds like I need to contact the motor manufacturer and get a performance graph. \$\endgroup\$
    – Muddler
    Commented Mar 20, 2015 at 15:02

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