The speed of the DC motor decrease with an increase of load on the motor. This is not desirable. Therefore, there must be a lesser speed difference between no-load to full-load operations.
In fact, every moving machine has some drop in the speed with load increase. It is desired that the motor speed should be constant throughout the speed range of the motor with an application of the variable load on it.
When the motor runs at no-load its speed is more, and speed drops as the load on the motor increases.
The difference between no-load and full-load speed is called Speed Regulation of the DC motor. The difference in the speed with respect to the full load speed is called the % speed regulation of the DC motor.
Importance of Speed Regulation of DC Motor
The different types of DC motors have different speed regulations. The best speed regulation is the need of the process.
If the motor maintains the nearly constant speed at a variable load, it has good speed regulation. For better process control to achieve better product quality, we use a motor with good speed regulation.
The speed regulation of the motor is also an important criterion for selecting the motor for a particular application.
The permanent magnet DC motor has speed regulation between 10 to 15 %. A speed regulation below 10 % is considered bad speed regulation. The compound dc motor has a speed regulation range of 25%, and the differential compound motor has a speed regulation range of 5%.
Speed of DC Motor
Speed of DC motor is given by the equation shown below.
From equation (3 ), it is clear that the speed of the DC motor depends on the following.
- Back EMF of DC motor( Eb)
- Flux in the motor
If the ratio of E/Φ in DC motor is constant from no load to full load, the regulation of motor is good.
Definition of Speed Regulation of DC Motor
The term speed regulation refers to change in the motor speed with change in the applied load on the motor shaft, provided all other conditions remain unchanged. The change in the speed of the motor from no-load to full load condition is expressed in the percentage of full load speed.
DC Motor Speed Regulation Formula
Speed Regulation formula Per Unit (p.u),
Where:
- Nno-load= Speed of the motor at no load
- Nfull-load= Speed of the motor at full load
A speed regulation of 0.02 p.u. means a 2% drop in speed under full-load conditions.
% Speed Regulation Formula of DC Motor
Example : A DC motor rated RPM is 1500 RPM. At no load its speed is 1500 RPM and, when it is on half load its speed reduces to 1480 RPM. What is the % speed regulation of the DC motor.
% speed Regulation = (Nno-load – Nfull load)/Nfull load x 100
= (1500 – 1480)/1490 x 100
= 20/1490 X 100
= 1.34 %
% speed Regulation = 1.34 %
A motor that runs at a nearly constant speed for all loads below full rated load has good speed regulation. The lower the percentage of regulation, the more constant the speed of the DC motor. Therefore, we prefer the lower % speed regulation of the DC motor.
How Does a DC Shunt Motor Maintain Good Speed Regulation When Load Increases?
A DC shunt motor is known for its excellent speed regulation, even when the load changes. Here’s how it maintains a nearly constant speed when the load increases:
- Load Increases
When mechanical load on the motor increases, the motor tends to slow down. - Speed Decreases
As the load increases, the speed (N) of the motor momentarily decreases. - Back EMF Decreases (Eb ∝ N)
Since back EMF (Eb) is directly proportional to speed, the drop in speed leads to a decrease in Eb. - Armature Current Increases [Ia = (V – Eb)/Ra]
A decrease in Eb causes an increase in armature current (Ia) because the supply voltage (V) remains constant. Ia=(V−Eb)/Ra - Torque Increases (T ∝ Ia)
As torque (T) is directly proportional to armature current (Ia), the motor generates more torque to handle the increased load. - Speed Restores to Set Point
The increase in torque accelerates the motor, and it regains its original speed.
This automatic correction mechanism enables the DC shunt motor to maintain good speed regulation under varying load conditions.
Speed Regulation of Various DC Motor
DC Shunt Motor
The speed regulation of the DC shunt motor is better than all types of DC motor except DC differential compound motor. The speed regulation of the DC shunt motor is in the range of 10-15 %.
DC Series Motor
- The speed regulation of the DC shunt motor is the worst among all types of DC motors.
- The % speed regulation of the motor is more than 40%.
- DC series motor is not suitable for maintaining precise speed control.
DC Cumulative Compound Motor
- The speed regulation of DC cumulative compound motor is better than that of DC series motor and inferior to DC shunt motor.
- The percentage speed regulation of DC cumulative compound motor is in the range of 25 -30 %.
DC Differential Compound Motor
- The speed regulation of the DC differential compound motor is the best among all types of DC motors.
- The percentage speed regulation of the DC differential compound motor is in the range of 2-5 %.
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