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Inspiration for this week’s topic came from a LinkedIn post I saw that had something very WRONG when it comes to this topic. It was incorrectly stated that Manual Motor Protections (MMPs) provide short circuit protection in fan array applications. The article did not realize/acknowledge that VSD short circuit sensing is faster, and the VSD is what provides short circuit protection. This is an important concept to understand for critical applications in order to minimize downtime.
Here is some background In an HVAC fan array application, a single VSD can run multiple motors at once. However, a single VSD cannot provide appropriate overload protection to each motor. This is because it sees the “total” ampere draw from all the motors combined – and it does not know how much current each individual motor is drawing.
To solve this challenge, individual motor overload protection should be added for each motor.
A Manual Motor Protector (MMP) is an ideal device to use for overload protection. A MMP is one device (per motor) that eliminates the need for an overload/contactor or fuses. MMPs can have auxiliary contacts indicating open/closes status. These contacts are excellent for monitoring status, i.e. BAS throwing an alarm when it sees an MMP open on a 2x2 HVAC fan array application.
MMPs protect the motor from overload conditions. But when they are applied on the output of a VSD, they are NOT the device protecting against low impedance short circuit type conditions. This is because the VSD’s sensing is much faster and will fail to protect itself before the MMP opens.
There is a common misunderstanding that the MMPs will always open if the motor “shorts,” but the VSD sensing is so much faster than how a MMP operates, that the VSD will fault first.
In that fan array example above, a short circuit condition on a single motor would bring the entire array offline. For critical fan array applications, consider designing one VSD per motor, or one VSD for a smaller number of motors instead of all the motors. This eliminates the VSD as a single point of failure.
