Middleby Marshall PS360, PS570, PS200, PS220, PS224 PS310 manual 144, PS555 ovens

SECTION 3 - SERVICING COMPONENTS

C. PS555 ovens

1.Description of heating system

All PS555 electric ovens use a variable pulse system for heater activation.

In response to the millivolt signal from the thermocouple, the temperature controller sends a 4-20mA signal to a controller module. This module contains an amplifer board and two solid state relays. The amplifier board receives the 4-20mA signal and in turn activates the relays. The relays then activate the heating elements.

The relays operate on a fixed cycle time. Within each cycle, the relays activate the heating elements at 100% for a length of time that is proportional to the signal from the amplifier board.

When the oven is switched on, heat demand is at its peak and the elements are activated for 100% of each cycle. As the oven’s temperature approaches the set point, the relays reduce the length of time that the heat- ers are activated wihin each cycle. After the oven is fully preheated, the heaters are usually activated for about 30% of each cycle. This system allows the ac- tual oven temperature can remain constant, with NO temperature swing.

IMPORTANT: The PS555 electric oven uses a variable pulse system that is designed to emu- late the PID gas operating mode. Because of this, the temperature controller MUST be set to the PID operating mode.

Temperature controller P/N 46837 (inc. with Service Kit 47321) includes a dedicated vari- able pulse mode intended for the PS536 elec- tric oven. This mode is NOT used on the PS555 electric oven. If you are using controller P/N 46837, be sure to set the controller for the PID operating mode.

Refer to Temperature Controller on Page 30.

Heaters are arranged into four assemblies, each of which contains three elements. Each pair of assem- blies (left and right) share a controller and a 50A circuit breaker block.

NOTE: All control wire used on this oven is AWM rated at 221°F/105°C. DO NOT replace with lower rated wire.

2. Part number reference

	Part No.	Description
43337		Heating element assembly, 208V
44525		Heating element assembly, 230V
44526		Heating element assembly, 380V
44527		Heating element assembly, 480V
44914		Controller module, 208/230/240V ovens
44568		Controller module, 380/480V ovens
35018		Circuit breaker block, 50A, 3 pole

3.Supply

Ovens have a 208V, 220-240V, 380V or 480VAC elec- trical supply. The control current of the oven operates at 208/240V.

208V and 240V ovens use a 3-wire system (4-wire in- cluding the ground connection). The control circuit op- erates at line voltage.

380V and 480V ovens use a 4-wire system (5-wire in- cluding the ground connection). The 4th (neutral) wire is utilized to split the incoming 3 phase legs and obtain a separate control circuit at 208/240V.

4.Heater specifications

•PS555 electric ovens use 12 heating elements in 4 assemblies (3 elements per assembly). Elements CANNOT be removed from the assembly. If one element is not functioning properly, the entire as- sembly should be replaced.

•208V elements are used for ovens with a 208V supply.

•240 volt elements are used for ovens with a 220V, 230V or 240V supply.

•380V elements are used for ovens with a 380V supply.

•480V elements are used for ovens with a 416V or 480V supply.

•All heaters are connected in Delta.

•Heaters are grouped into four assemblies, each of which contains three elements. Elements CAN- NOT be removed from the assembly.

•Each element has a cold (unheated) end next to the connecting leg. 12”/0.3m leads are connected to the cold ends for easy connection to the termi- nal blocks on the back wall of the oven.

•Individual element electrical data

Volts	Watts	Ohms	Connection
208V	2667W	16.2W	Parallel
220V	2241W	21.6W	Parallel
230V	2449W	21.6W	Parallel
240V	2667W	21.6W	Parallel
380V	2667W	54.1W	Parallel
416V	2003W	86.4W	Parallel
480V	2667W	86.4W	Parallel

5.Checking the Heating Elements

The most common problems encountered with electric heaters are:

•Open or bad connections.

•Open, shorted or grounded heating elements.

To troubleshoot the heating elements, disconnect all leads at the terminal blocks on the back wall. Then, check each element with a digital multimeter set to Ohms (W). Correct ratings are shown in the table in the preceding section. Zero reading on the ohm scale indicates an open or shorted element.

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