The Ohmic Resistance test allows to identify:
- Permanent Short-circuits inside the windings
- Open Circuits
- Windings with wrong number of turns.
The method to measure in-used is the classic 4-wire Kelvin method. A new terminal board model guarantees that the “four wires” are maintained until physical contact with the terminal wires of the stator takes place.
Another major innovation is the following: before each resistance test, two very important operations are performed to ensure excellent efficiency and duration of the contacts of the relays involved in the measurement:
1. an “electrical contact-cleaning” operation is activated;
2. the contact resistance of the measurement circuit is measured. In case of error, relay activation and the corresponding “electrical contact-cleaning” operation is repeated for three times. If the error persists, the malfunction is signalled.
The EDC Testing Systems has got a specific software procedure to establish the efficiency of the contacts of each relay at all times. These new functions considerably reduce problems associated to the wearing of the relays and “occasional false contacts” caused by micro particles of dust deposited on the contacts.
With every 10°C variation of temperature, the resistance of a winding varies of approx. 4%. So, it’s very important that the Ohmic resistance measurement is adjusted in relation of the winding initial temperature; in other words, the value measured is automatically adjusted to produce the resistance value the winding would have at Reference Temperature (normally 20°C or 25°C).
Three options are available for the temperature adjustment:
1. Enable the temperature value of the stator under test through the keyboard.
2. A temperature probe situated in the rear of the rack and detects the room temperature. The Ohmic resistance measurement is adjusted by assuming that the stator temperature is the same as room temperature.
3. An infrared temperature probe detects the real temperature of the stator without contact.
It is possible to set the Reference Temperature value, normally 20°C or 25°C as well the wire material (copper or aluminium).