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What could happen if 0.5 hp motor is connected to 7 hp VFD with the motor current parameter set to the maximum value of the VFD? let us say 14 Amp.

How could that affect the motor?

The current rating of the motor is about 1A.

Would that affect the acceleration and the deceleration of the motor?

Would that in some circumstances overload the VFD?

EDIT:

What are the side effects of connecting a low power ac motor to a high power VFD without taking so much attention to the motor current parameter of the VFD?

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  • \$\begingroup\$ You are mixing current and power, so we can't tell how 14 A relates to either 0.5 hp or 7 hp. \$\endgroup\$ – Olin Lathrop Mar 13 '17 at 17:39
  • \$\begingroup\$ WHat is the current rating of the motor? \$\endgroup\$ – Trevor_G Mar 13 '17 at 17:51
  • \$\begingroup\$ Assuming the max voltage out form the VFD is the same as the motor rating, and nothing else weird is going on... the VFD will just not work as hard. If, in some mode the VFD can supply higher voltages and currents than the motor can handle, especially during locked rotor tests, you will fry the motor. \$\endgroup\$ – Trevor_G Mar 13 '17 at 18:03
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The maximum current you set on a VFD is the current "limit". When you hit that limit the voltage will droop (foldback) to prevent more current being drawn and the motor rapidly overheating.

Will the 0.5 HP motor draw more current if you are using it within its ratings ...NO
......except that you could potentially overload the motor if you apply more physical load. If you increase the physical load, then a motor draws more current (at a given voltage and frequency) but eventually the motor will stall, then all the input power is dissipated as heat.

The most important characteristics of a VFD are the voltage output and frequency for any particular motor. If you have a 0.5HP motor specified at 230 volts, then the maximum load current would be defined by the voltage and the phase impedance of the windings (load). If you set the current limit high for the motor and apply larger physical loads you would expect larger currents to flow and the motor to heat up. Whether you damage the motor over time would be unknown and subject to the construction of the motor.

If you have a 7 HP VFD, then obviously you can set the current limit to a higher value.

Since the voltage output of the VFD is regulated at 220 - 230 V for example, the 0.5 HP motor won't be 'forced' to take any more current than it requires due to the applied physical load.

Would being able to set a higher current limit provide better performance for the 0.5 HP motor .....very unlikely .... you might just possibly see a more rapid acceleration under load ...because the motor impedance is lower, but you are potentially exceeding the performance/load characteristics of the motor. The downside to overload is heat. Whether that damages your spindle (I assume that's what you are driving) is unknown.

Though targeted at larger motor applications, this might help you with VFD sizing.

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The most undesirable side effect to connecting a motor to a VFD that has a higher rating without setting the motor current rating to correspond to the actual rating of the motor is that the VFD will not be able to provide overload protection to the motor.

If the VFD has a sensorless vector design, the proper motor parameter setting will allow the VFD to accurately estimate the motor performance and adjust the motor voltage dynamically to produce the maximum possible torque per ampere. It can also adjust the frequency slightly for any given speed setting reducing the speed change that would otherwise occur as a result of load change.

Increasing the current limit setting above the normal maximum current limit setting for a given size motor would allow the motor to operate at higher point on the torque vs. speed curve, but without a means to detect where on the curve the motor is operating, the motor would likely slip beyond the pull-out point and stall. That would cause rapid motor overheating.

With a greatly oversized VFD, it would be possible to start the motor by direct-on-line connection (DOL) rather than using the VFD's acceleration feature. With most motors, that would not result if faster acceleration since much of the DOL torque vs. speed curve lies below the torque that can be obtained with a sensorless vector VFD that is properly "tuned" to the connected motor.

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  • \$\begingroup\$ Good point on the OL protection. In a lot of VFDs you will not be able to turn the OL protection parameter down low enough, requiring an external OL relay. Generally, I recommend that a VFD not be sized at more than 200% of a motor FLC when using any kind of Vector Control because your working current as seen by the current sensing components becomes so low at low speeds that you may end up at a point where naturally occurring tolerances in the devices begin to cause instability in maintaining the motor model. But if operating in scalar mode (V/Hz), it makes no difference at all. \$\endgroup\$ – JRaef Mar 14 '17 at 2:08

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