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Emc Testing With Vfds

Posted by marke, 01 January 2012 · 1,518 views

Most European and Australasian countries require that VFDs are tested to comply with EMC standards.
USA and Asian countries do not have this requirement.

The tests are prescribed as a requirement for CE compliance (Europe) and CTick compliance (Australia and New Zealand).

The laboratory tests require two types of emissions be tested, 1) radiated emissions and 2) conducted emissions.

The major source of problems in the real world, is conducted emissions and these are typically in the region of 20kHz to 10MHz.
The source of the emissions, is the output of the VFD which comprises 6 solid state switches that switch each output between the DC bus rails. These switchews are typically high speed IGBTs which can switch in less than 100nS.
The output waveform is a PWM waveform designed to create a sinusoidal current flow into the motor.

The prescribed conducted noise tests emmulate conducted noise tests for other appliances and use a LISN to measure the noise voltage on each of the input phases. Effectively, this test is testing the noise generated by the rectifiers, but if the output is particularly bad, or the output connections to the motor are not ideal, there can be leakage to the input.
The conducted noise tests cover the frequency range of 150KHz to 1MHz. There are no tests on frequencies below 150KHz.
The primary noise, is generated by the output of the VFD (untested) and this is capacitively coupled to the frame of the motor.
If the frame of the motor is only connected to the frame of the VFD and no other earth paths exist, then there should be little or no leakage out the input of the VFD and it should be relatively easy to pass the conducted noise tests.
Some laboratory tests are made with the motor mounted on an insulated medium to ensure that there are no leakage currents from the motor to the system earth.

One of the best indicators of how noisy a VFD will be in a real world installation, is to measure the peak voltages between the frame of the VFD and the Frame of the motor. If this voltage is high, there will be elevated noise currents flowing from the motor to the system earth.

Voltage measurement is easily achieved by the use of a 100MHz oscilloscope. A 470 Ohm terminating resistor is mounted at the oscilloscope terminals and two cables are used to connect to the frame of the motor and the frame of the VFD. The oscilloscope must be isolated from ground.
Good voltage levels are below 2 volts peak to peak with 10 meters of cable. Bad results can be as high as 160 volts.

Noise voltage levels can be minimised by VFD design, particularly the use of very low impedance decoupling capacitors from both sides of the DC bus to the VFD chassis. Because the frequency for the noise is very high, the conductors from the DC bus to the capacitors and to the chassis, must be as short and fat as possible. - minimum length and maximum surface area for the conductors.
The installation should use properly terminated Screened EMC cable between the motor and the VFD with the screen clamped to the VFD frame using a screened gland or clamp method, and a screened gland inot the motor terminal box.

The screen is used to provide a very lowe impedance earth return at 100KHz, typically the impedance is in the order oof 50 times lower than the impedance of the phase conductor.
An additional ferrite ring around the three output phase conductors can also help to reduce the conducted noise levels.

For more information, see VFDs and EMC

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