YORK YMC2 — MBC - SPEED SIGNAL FAULT

The OptiView control center shows MBC - SPEED SIGNAL FAULT. The manuals also print the message as MBC - Speed Signal Fault. It is a shutdown message of this chiller: the first occurrence is described as a Cycling Shutdown, and the repeated form — the third speed signal cycling fault within a 90-minute window — is described as a Safety Shutdown.

The shutdown is set when the MBC loses the speed signal that is expected from the VSD, or when the speed signal is less than 10 Hz, while the MBC is commanded in Rotation Mode. When the fault sets, the MBC fault contacts open — the control-center manual prints these as the MBC ESD Fault contacts, the other manuals as the MBC Fault contacts. The MBC then remains in Levitation mode with the motor shaft levitated until it is commanded to de-levitate over serial comms or the VSD DC buss power depletes. A 10 second bypass exists when the initial command to Rotation Mode occurs, so fault detection is bypassed for the first 10 seconds after that initial command.

• The MBC loses the speed signal that is expected from the VSD while it is commanded in Rotation Mode.
• The speed signal is less than 10 Hz while the MBC is commanded in Rotation Mode.
• The output line voltage isolation board transformer phase A has failed — the bulletin on output line voltage isolation board symptoms states that if this transformer has failed, this fault will occur.
• For the Safety Shutdown form: the MBC has received the speed signal cycling fault three times in a 90-minute window; the window is started when a MBC Speed Signal cycling fault is received while no window is currently open.

1. Confirm that the MBC was commanded in Rotation Mode when the message set — the fault is only set while the MBC is commanded in Rotation Mode.
2. Check whether a valid speed signal over 10 Hz is being presented — the shutdown releases when a speed greater than 10 Hz is transmitted or the MBC is out of Rotation mode.
3. If the fault occurs, check the output line voltage isolation board transformer phase A — its failure is given as a cause of this fault.
4. Note how often the message has appeared: the third speed signal cycling fault within a 90-minute window sets the Safety Shutdown form.

• Speed signal threshold: the fault is set when the speed signal is less than 10 Hz during Rotation Mode and is released when a speed greater than 10 Hz is transmitted.
• Bypass: a 10 second bypass exists when the initial command to Rotation Mode occurs.
• Safety Shutdown counter: three speed signal cycling faults within a 90-minute window; the window starts when a MBC Speed Signal cycling fault is received while no window is currently open.

The shutdown is released when the transmission is reestablished; the other manuals phrase this as release when the MBC is out of Rotation mode or a speed greater than 10 Hz is transmitted. Once the condition is released, the chiller can be restarted from the panel: The chiller can be started when the condition is released and the OptiView panel CLEAR FAULTS button is pressed. Remember that after the fault sets, the MBC stays in Levitation mode with the motor shaft levitated until it is commanded to de-levitate over serial comms or the VSD DC buss power depletes.