# Soft Starting Slip Ring Motors

21 replies to this topic

### #21 jraef

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Posted 22 August 2005 - 08:25 PM

No response. How typical.
"He's not dead, he's just pinin' for the fjords!"

### #22 marke

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Posted 22 August 2005 - 09:30 PM

I think that we can reasonable assume that it is using a form of cycloconverter algorithm to create a series of subharmonics and rely on these subharmonics to develop the toprque at very low speeds. This type of operation has certainly been around for a while and there are inherent potential issues with subharmonic currents on the supply plus the losses in the motor. Because the driving supply is still at line frequency, the cycloconvertor dois not eliminate the fundemental torque field and so slip losses are still very evident.

For those that are unsure as to what the cycloconverter principle is, it is a means of controlling the SCRs so that effectively a lower frequency is built up by a series of pulses of the fundemental frequency.

Under normal operation, we turn on all SCRs for a portion of each cycle.
If we now turn on the SCRs following a particular pattern, we get a sub harmonic of the supply. for example:
cycle 1
phase a + SCR ON; phase b - SCR ON; phase c - CSR ON

cycle 2
phase a -SCR ON; phase b + SCR ON; phase c - SCR ON

cycle 3
phase a - SCR ON; phase b - SCR ON; phase c + SCR ON

cycle 4
phase a + SCR ON; phase b - SCR ON; phase c - CSR ON

cycle 5
phase a -SCR ON; phase b + SCR ON; phase c - SCR ON

cycle 6
phase a - SCR ON; phase b - SCR ON; phase c + SCR ON

This simplistic representation creates a rotating magnetic field at one third of line frequency. There will also be a strong rotating field at 50Hz.

The current on the supply will be a series of pulses, not a continuous current flow.

If 18 SCRs are employed instead of 6, there is a much smoother current flow and more frequency options are possible.

This technology is available in a number of soft starters as a low speed jog mode.

Best regards,

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