Middleby Marshall PS360, PS570 Low Flame Bypass Orifice, Burner Transformer, Burner Motor Relay

SECTION 3 - SERVICING COMPONENTS

G. Burner Transformer

120V burner blower motors feature a transformer located in the electrical junction box on the burner. In PS570S ovens, AND all ovens with 208/240V burner blower motors, the transformer is located inside the machinery compartment.

The transformer supplies 24V to the burner motor relay (if present) and also to the 24V gas valve through the motor’s centrifugal switch (or air pressure safety switch).

H. Burner Motor Relay

120V burner blower motors feature a 24V time delay relay located in the electrical junction box on the burner. In PS570S ovens, the relay is located on the back wall of the machinery compartment.

Approximately 25 seconds after power is applied to the coil of the relay, the contacts will close and complete a circuit to the burner blower, allowing the motor to operate. This 15-30 second delay is a prepurge for a cold start. When the coil of the relay is warm, the delay is bypassed.

Ovens with 208/240V burner blower motors are not equipped with the burner motor relay, and do not have a prepurge for a cold start.

I. High Flame Solenoid Valve (if so equipped)

Ovens that use an on-off gas regulation system are equipped with a high flame solenoid valve. The valve is located be- tween the combination gas control valve and the burner. The valve operates on 110-120V.

The valve opens when the oven’s temperature falls 2 de- grees Fahrenheit below the set point, and closes when the temperature rises to 2 degrees above the set point. Be- cause of residual heat within the oven, however, the tem- perature swing after preheating is about 5 degrees Fahren- heit above and below the set point.

The solenoid valve can only be positioned fully open (100% gas flow) or fully closed (0% gas flow). While the valve is closed, gas is still supplied to the burner through the by-

pass line that goes around the solenoid valve. This allows the burner to operate in low flame mode while awaiting re- activation.

When the oven has been fully preheated and has main- tained the set point for about an hour with a moderate prod- uct load, the low flame to high flame ratio should be ap- proximately 1 to 1. That is, the oven will be calling for heat approximately half the time. To check this ratio, refer to the HEAT indicator on the temperature controller, which will only be active when the burner is on high flame.

EXCEPTION: Older PS310/314/360 ovens have a tempera- ture-sensing thermocouple mounted at the front of the oven, inside the machinery compartment. On these ovens, the low flame to high flame ratio should be approximately 2 or 3 to 1 (oven calls for heat 1/3 or 1/4 of the time).

IMPORTANT: For proper functioning of the sole- noid valve, the temperature controller MUST be set to the on-off operating mode. Refer to Tem- perature Controller on Page 31.

J.Bypass (Low Flame) Orifice (ovens with a high flame solenoid valve only)

All ovens that use an on-off gas regulation system are equipped with an orifice in the low flame bypass line. See Figure 70.

When the oven is not calling for heat, the solenoid valve is closed, but gas is still supplied to the burner through the bypass line. This allows the burner to operate in low flame mode while awaiting re-activation.

The low flame orifice regulates the gas flow through the bypass line. If the orifice size is too large, the low flame will be too high. In some cases, this will prevent the oven from being able to maintain the set point.

Refer to the chart on Page 91 to determine the proper size for the bypass orifice.

Figure 70

Low Flame (Bypass) Orifice

Bypass line configuration varies depending on the oven model. Refer to the drawings on pages 88-90.

Low flame	Solenoid
(bypass)	valve
orifice

Bypass

line

96