Positive displacement motor speed

In general, the larger lobe profile number ratios of a positive displacement motor, the higher the torque output and the lower the speed (assuming all other design limitations remain the same). 

Rotary speed of the positive displacement motor is directly proportional to flowrate. Torque is directly proportional to pressure. Thus, normal surface instruments can be used to monitor the operation of the motor downhole. 

High torques and low speeds are obtainable with certain positive displacement motor designs, particularly, the higher lobe profiles (see Figure 4-203). 

For this example, a pressure of about 100 psi is required to start the rotor shaft against the internal friction of the rotor moving in the elastomer stator (and the bearings). With constant flowrate, the positive displacement motor will run at or near constant speed. Thus, this 1 2 lobe profile example motor has an... 

As the positive displacement motor is lowered and weight is placed on the motor and thus the bit, the motor speed continues but the differential pressure increases, resulting in an increase in torque and horsepower. As more weight is added to the positive displacement motor and bit, the torque and horsepower will continue to increase with increasing differentiated pressure (i.e., standpipe pressure). The amount of torque and power can be determined by the pressure change at the standpipe at the surface between the unloaded condition and the loaded condition. If too much weight is placed on the motor, the differential pressure limit for the motor will be reached and there will be leakage or a mechanical failure in the motor. 

A 61-in. outside diameter positive displacement motor of a 1 2 lobe profile design (where performance data are given in Table 4-114) has rotor eccentricity of 0.60 in., a reference diameter (rotor shaft diameter) of 2.48 in. and a rotor pitch of 38.0 in. If the pressure drop across the motor is determined to be 500 psi at a circulation flowrate of 350 gal/min with 12.0 Ib/gal, find the torque, rotational speed and the horsepower of the motor. 

Planning for a positive displacement motor run and actually drilling with such a motor is easier than with a turbine motor. This is mainly due to the fact that when a positive displacement motor is being operated, the operator can know the operating torque and rotation speed via surface data. The standpipe pressure will yield the pressure drop through the motor, thus the torque. The circulation flowrate will yield the rotational speed. 

Air (or Gas) Downhole Motors. Some positive displacement mud motors can be operated on unstable foam. In general, these mud motors must be low-torque, high-rotalional-speed motors. Such motors have found limited use in air and gas drilling operations where directional boreholes are required. Recently a downhole turbine motor has been developed specifically for air and gas drilling operations. This downhole pneumatic turbine motor is a high-torque, low-rotational-speed motor. 

For positive-displacement pumps the only variable is the operating speed. The only ways to change the capacity of these pumps are to use a variable-speed driver, use a variable-speed transmission (not usually recommended), or replace a given rpm motor by another. Since most motors run at 1,750 or 3,500 rpm, the last method may be used only if the speed is to be doubled. 

Fig. 3 shows the experimental apparatus. The feed tank had a 50 gallon capacity and was equipped with a variable speed mixer. The feed pump was a flexible impeller, positive-displacement pump to minimize shearing of the feed emulsion. The pumping rate was regulated by a Graham Variable Speed Transmission. Each flotation tank was 11.5 in. ID with 6.5 in. liquid depth maintained by a CE IN-VAL-CO conductometric level controller with a pneumatically actuated control valve in the effluent line. The fourth cell was not equipped with an air inducer. The outer diameter of the air downcomers was 1.5 in. The rotor in each air inducer was a turbine taken from a 2 in. turbine flow meter. Each rotor was belt driven by a 10,000 rpm, 1/30 hp motor and all three motors were governed by the same variable transformer. Another pulley on each rotor shaft was attached to a non-powered belt connecting all three shafts to ensure that each rotor turned at the same speed. 

Smaller rotary positive displacement compressors are controlled with external bypass. Such equipment usually has a built-in relief valve that opens at a pressure short of damaging the equipment, but the external bypass still is necessary for smooth control. Large units may be equipped with turbine or gas engine drives which are speed adjustable. Variable speed gear boxes or belt drives are not satisfactory. Variable speed dc motors also are not useful as compressor drives. Magnetic clutches and hydraulic couplings are used. 

Mixed acid and glycerine are stored externally in separate tanks and are transferred by means of centrifugal and gear pumps to small feed tanks. From the feed tanks the reactants are sent to the nitrator by means of two positive displacement pumps driven by the same electric motor. Two speed variators allow change in the total flow rate as well as the ratio between acid and glycerine. The glycerine feed pipe in the nitrator, fitted with a multi-nozzle distributing head is... 

To make up the difference between the expected expander returned power and the required compressor input power. Meruit designed an air system using a commercially available positive-displacement compressor driven by an electric motor in tandem with Meruit s turbocompressor. Meruit designed a family of compressor and expander wheels with var5dng characteristics (specific speed, dimensional ratios, etc.) representing different compromises in providing performance compatible with the DOE specifications. After extensive evaluation, a compressor wheel was chosen to provide a desired pressure curve over a wide range of mass flows, and a turbine wheel was matched to it. The chosen compressor wheel is intended to trade peak efficiency for extended turndown performance. 

Figure 2. Experimental set-up for slurries. 1, screw feeder 2, rotary drum (lucite) 3, rubber-lined rollers 4,1/4-hp motor (variable speed) 5, slurry collecting tank 6, rotameter 7, positive displacement pump 8, fluid tank V, valve.

Split-Phase Motor. A split-phase motor is a single-phase induction motor equipped with an auxiliary winding, displaced in magnetic position from, and connected in parallel with, the main winding. Note Unless otherwise specified, the auxiliary circuit is assumed to be opened when the motor has attained a predetermined speed. The term split-phase motor, used without qualification, described a motor to be used without impedance other than that offered by the motor windings themselves, other types being separately defined. 

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