The maximum load of a DC motor

The use of (power in watt) = (volt) * (ampere) works great to determine the electrical power requirements. However, it neglects the fact that every system has some amount of loss when energy is transformed from one state to another.

If you're really monitoring a 169 ampere current draw for a machine rated 60 kW at 400 V, then you have one of three conditions:
A) The motor efficiency is extremely low (roughly 88 percent). Note this value SHOULD be in the 93-95 percent range for a good design - and that the efficiency value is more-or-less independent of speed for a DC machine.
B) The motor is overloaded mechanically and is therefore drawing higher-than-normal current. A straight current ratio would tend to indicate roughly 12 percent overload (169 / 150), but that also neglects efficiency. A more realistic overload would be 6 percent, with a nominal machine efficiency of 94.5 percent.
C) Some combination of "low efficiency" and "overload".

As far as the "maximum" power available from a DC machine: the steady state limit is the listed power rating on the nameplate (when operated at rated voltage and speed). This MAY be multiplied by some value greater than one, typically designated as "service factor" or simply "SF". However, operation at the elevated level will incur greater losses and increased thermal stress on the machine - although not necessarily at "injurious" levels.

It should also be noted that - for a motor - the "power" listed on the nameplate is the OUTPUT power measured at the shaft, not the INPUT power measured as electrical quantities (volt and ampere).

Your desire to obtain the maximum load will be defined by your requirements and a specific speed. P(mech) = (Torque{rated} x speed {RPM}) / 9550. So if you are running at anything other than rated speed and you still require the same torque the power consumed less the losses will give you P(mech) or output power. Motors are generally selected to overcome torque issues. Then when you have the torque requirement, the power value is used to select a standard motor. DC motors are now classified as special motors and are normally required when a speed variation is required. Your motor may be over sized (de-rated) to overcome the initial torque required to get it going, thereafter, it may run at a lower torque / power rating.


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