Sylvain Raymond New Member Posts:2  
| 05/07/2014 1:59 PM |
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We would like to use a CR4395 to limit plugging torque on a wound rotor motor. During plugging (commanding the opposing rotating direction) the output relay would drop on overcurent, preventing the secondary accelerating resistors from being shorted (and limiting the counter torque at the motor).
The frequency at the rotor varies from very low frequency at nominal speed to 120Hz at start of plugging. Plugging inhibit is removed as the motor passes through zero speed and the rotor current frequency is approximately 60Hz. Current is high during the reverse plugging phase and decreases as the rotor follows the rotational field of the stator.
We do not need accurate relay setpoint. We just need to make sure the relay stays closed when current is under setpoint for a frequency varying between 0Hz and 120Hz; the relay would normally be opened from 120Hz to 60Hz (with current higher than setpoint) and be closed from 0Hz to 60Hz.
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Tony HodgesCRM Staff  Basic Member Posts:154
| 05/09/2014 9:27 AM |
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Dear Sir,
Could you please clarify this application. Our current sensing relays have a single function, which is tripping when the current is above or below a set-point. Your explanation suggests you are looking for a relay that has two functions. Please explain.
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Sylvain RaymondNew Member Posts:2
| 05/10/2014 2:16 PM |
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Mr. Hodges, sorry if my description seems confusing. We are really looking for a current relay with setpoint output. The relay contact would be closed if the current is under the setpoint and would open if the current is higher than the setpoint.
The frequency of the rotor current is induced from the stator to the rotor.
When the motor runs at nominal speed, the frequency is very low (rotor current frequency is motor slip frequency); we need the current relay contact to stay closed because the current will be under the setpoint.
During plugging, the rotor current frequency is about twice the stator frequency (stator field turns in one direction and the rotor rotates in the other direction); we need the current relay contact to open because the current will be over the setpoint.
The rotor current would be below setpoint (current relay closed) at frequencies from 0 to 60Hz and above setpoint (current relay opened) at frequencies from 60 to 120Hz. The setpoint will be adjusted according to rotor current but will correspond to the rotor being at standstill (motor not rotating - frequency of induced rotor frequency approximately 60Hz).
Hoping the above explanation is clearer.
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Tony HodgesCRM Staff Basic Member Posts:154
| 05/12/2014 9:20 AM |
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Dear Sir,
Thank you for clarifying. So by your description, you expect the current to be over set-point when you have higher frequencies. We can easily manufacture a part that will activate over a set-point. This would be the EH version, and you would use the normally closed contact so that it would break open when current set-point is exceeded as you described. Please note that the current sensing relay is not a true RMS device, and the response will change depending on frequency encountered. We recommend selecting the range such that your expected current level at 60 Hz that is the set-point is set in the middle of the range. The adjustable dial will then give you the freedom to adjust exactly the set-point level that meets your needs. If this is an OEM application and will be repeated many times, we recommend then purchasing a fixed set-point device after the empirical level is determined to improve value and save money.
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