A Generator Time Delay Loading Relay

It is a good idea to start a generator and allow it to warm up without
any load.  Indeed some generators will not come up to speed with any
significant load attached.  Bogging the generator is not only hard on
the generator, it is also very hard on loads such as air conditioners
which suffer under low voltage, low frequency, high current conditions.
The Onan AJ series generator in my 82 Itasca motor home is an example of
a generator that will not start under significant load.

It is a pain to have to go around and turn off all the major loads before
cranking the generator.  This is particularly annoying when one wants to
start the generator while underway in order to use the roof air conditioner
to aid in cooling the vehicle.  I decided to do something about it.

A system to automatically delay the application of the load until after the
generator is up and running and warm is an appropriate solution.
Fortunately this solution is simply implemented, costs under $50 and
involves only two components.

My solution uses a 30 amp single pole definite purpose contactor intended
for controlling the compressor in a central air conditioner.  A single pole
suffices in my case because my generator generates only 120 volts.  If your
generator is a 120/240 volt unit, use a double pole contactor.

To implement the time delay, a simple, inexpensive time delay module is
used.  This module is a delay-on-make relay.  It is also known as an
anti-short-cycle relay because its most common use is to prevent short
cycling (energizing too soon after turning off) the compressor in an air
conditioner by delaying the power to the compressor by a set number of
minutes.  The relay I selected is a MARS (Motors And Armature Service)
#32391.  This relay can be set for a delay from between 0.1 minutes to
about 8 minutes.  The relay is a 2 terminal device that simply
blocks the passage of current for the set number of minutes. Many companies 
manufacture a compatible relay; the MARS unit was selected because my 
friendly local electric motor repair shop stocks it.  A photo of the relay
is in the file tdr_photo.jpg in this directory.

The 30 amp relay should cost from $25 to $50 depending on whether or not 
you buy it wholesale and the time delay relay should cost from between $12
to $24. In addition to electric motor repair shops, these components can be
had from appliance parts warehouses, HVAC supply houses and HVAC
contractors as well as direct sales operations such as Graingers.

The simple schematic for this device is shown in the file schematic.gif in
this directory. The hot wire from the generator is connected to one side of
the contactor.  The house wiring is connected to the other.  Power is
supplied to the 120 volt contactor coil from the generator lead through the
time delay relay (TDR).

How it works

When the generator is started, 120 volts AC appears on the generator "line"
lead.  This energizes the time delay relay.  After a set amount of time,
the TDR energizes, passing the 120 volts to the contactor coil.  This
energizes the contactor which passes the 120 volts to the house wiring.
When the generator is turned off, as soon as the voltage drops
significantly, the TDR de-energizes, dropping the contactor and cleanly
cutting off the power to the house.  This keeps trash from the slowing
generator from the house wiring.

Installation

The contactor and the relay was mounted in a PVC plastic electrical pull
box, available from Home Depot.  This box is waterproof.  The leads were
brought out through a strain relief and caulked with silicone RTV.  The
entire assembly is waterproof.  The box is mounted in the generator
compartment away from any heat generating sources such as the exhaust.  Set
the TDR to about 30-45 seconds and the job is complete.

Operation

Simply start the generator.  Everything else is automatic.  After the time
delay you set, the house will power up normally.

Legal

This article and graphics copyright 1999 by John De Armond.  All rights
reserved.  Private, personal use permitted.  Other uses only by prior
explicit written permission.  I may be contacted at johngd@bellsouth.net.